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ANTENNAS ( AERIALS ) 3 FELIX
SCERRI VK4FUQ DISCUSSES
HIS ANTENNA EXPERIENCES
Delta Loop Antennas | To err is human | History Of The Cubical Quad One Wavelength Quad Optimization & Evaluations | Unwanted Mast / Antenna Interactions Radiation Angle | More Loop Experiments | The Effects Of A Metal Mast | Delta Loop update The Use Of A Non Conductive PVC Mast | Baluns | The End Of The Loop Saga? | (NEW!) 20 Metre antenna comparisons Balun_Discussions | More and W8JI | Metal Poles & The Quad Loop My Renewed Love Affair With The Quad Loop Antenna. A recent chain of events has led me back to the beginning in a sense! Going back to the 'early' days (the mid to late 1980's), I was a huge fan of one wavelength loop antennas like the Delta Loop and the like, however over the years the simple half wave inverted V has become my favourite antenna for HF use for various reasons. Recently however a number of unusual events has seen me re- erect my old fixed wire Quad loop for 20 m. It is currently working very well. It is an antenna with some endearing features and virtues. I suppose that many Amateurs would know of the story of the Quad's beginnings at the broadcast station HCJB in Ecuador where owing to high transmitter power and high altitude, the yagi antenna in use was slowly destroying itself due to destructive coronal discharge and arcing. One of the stations engineers, Clarence Moore W9LZX thus conceived the idea of a 'pulled open' folded dipole with no 'ends' as a possible solution. It worked! A little later on Moore realised that this 'Quad' antenna was also a pretty good antenna and thus the Quad mystique was born! The original version of the Quad as used at HCJB was a two element design, reflector and driven element, and subsequent HF designs also included directors. Each element contains approximately one wavelength of wire in a closed loop except for the driven element. The parasitic elements are tuned in a similar fashion to yagi antenna parasitic elements. At this QTH I use one single loop resonant midband on 20 m as a bidirectional radiator (in and out of the loop). I use the formula, length in feet = 1005/ frequency which appears to be sufficiently accurate. A check with a loosely coupled GDO at the feedpoint can be used to check the resonant frequency. Even on its own it is an excellent antenna. Why is it such a good antenna? Over the years despite the true excellence of the Quad design, many 'myths' regarding the operation of the Quad have abound. One of these involves the inherent gain of the Quad element. I have seen written in many older but well regarded antenna texts a quoted a figure of 2 db 'inherent' gain over a dipole for the Quad element. This is incorrect. Whilst a single Quad element has some slight gain over a dipole it is probably closer to 1 db, not 2 db. Regardless of the real truth it is a very efficient radiating system. Its general radiation pattern is similar to a horizontal dipole but with deeper nulls off the ends*.
In my own installation, my loop
is
erected as a diamond shaped loop hung off a 33 ft pipe mast.
It
'looks' to the North and South and is fed at the bottom, horizontally
polarized with high quality 300 ohm feeder as a tuned line, and of
course a good quality 4:1 balun and ATU is necessary. One
aspect of
the Quad Loop's operation that I find particularly valuable is its low
noise 'receive' profile. As a person who has been plagued at
this QTH
by RF noise of many kinds, but mainly power line sizzle over many
years, the low noise profile of the loop is much appreciated and the
apparent cancellation of power line sizzle has to be heard to be
believed!. It is much 'quieter' than dipole type
antennas. Although
operating well out of its resonant frequency range, it is also an
excellent low noise receive antenna for general short wave listening
applications, where the low noise pickup is also evident and
appreciated. Practical considerations: As one wavelength of wire is used in any given loop design, available space is a limiting factor and indeed 20 m is the only band where available space allows its construction at this QTH. It does require a fair bit of space. People who successfully build large rotatable Quad arrays have my complete respect! Thankfully though the diamond shape is easily supported off a simple pipe mast and installation is quite straightforward assuming sufficient space is available. I used 2.5 mm plastic covered 'earth' wire to make up my 20 m Quad Loop and it works fine. According to various references the actual loop 'gain' is somewhat dependent on loop shape, with a circle giving the highest gain and the 'delta' triangle shape the lowest. The square or related diamond shape is not too far off the circle in terms of inherent maximized gain. That's nice to know! The Quad Loop in operation: I guess this part gets a little subjective but there is something a little unique about even a single Quad Loop, in my opinion. As was mentioned earlier the low noise receive pick up profile is a major plus, in my opinion. Interestingly enough this aspect hasn't been mentioned too much in the antenna books based on my observations. The experience that has 'led me back' to the loop was the realisation of matters pertaining to directivity. Although a simple dipole, inverted V or Quad Loop is 'broadish' in its radiation pattern there are nulls off the ends which are actually somewhat more pronounced with a Quad Loop. Thanks to some compass checks I finally realised why my inverted V was not performing as well as expected. It was not actually pointing in the correct intended direction. It was in fact almost in the nulls of the antenna. Earlier evaluations with my Quad Loop suffered from the same positional flaw, and owing to the deeper nulls of the Quad Loop 'off the ends', this actually showed the Quad Loop up worse than the inverted V! This basic error took many years to realise! D'oh, as a certain TV cartoon character might exclaim! In actual operation the Quad Loop is one of those highly dependable and consistent performing antennas 'putting out' a signal much better than it should, based on the minimal complexity of the design! S meter readings are always well up on the meter and I feel this is a HF antenna ideal for QRP HF duties. A highly recommended HF antenna! 73, Felix vk4fuq. 20/2/08. 'To Err Is Human' or 'When One Can't See the Forest for the Trees!' Yes there is an aspect of 'humble pie' to this story but I have decided to tell it anyway, a simple story of not being able to 'see the forest for the trees' or the inability to realize a very basic long term error! For some years I have had weekly skeds with an Amateur friend of mine in vk3 on 20 m. Over the years we have had many excellent skeds with good signals in both directions despite both of us using basic single element antennas. However in recent times this 'path' has become poor and many of our weekly contacts have been essentially non- existent! As a consequence I have been giving much thought to improving my 20 m antenna set-up. One recent Saturday after lunch whilst having a doze and staring at the Amateur Radio Map of the World in front of me, it occurred to me that perhaps my inverted V dipole was not really 'looking' at the vk3 area at all, as I had previously thought that it was. Well, it wasn't! It was firing essentially to the SE and NW from this location in Northern vk4 and was skirting the edge of the vk3 area, a long way away from the intended target area in vk3, confirmed by a couple of basic compass bearing checks! I quickly ventured down to the backyard and reorientated the direction of the inverted V to 'look' due south and due north. Before I did so I noted the signal strength of Radio Australia on 15.240 MHz from the Shepparton transmitter with the 20 db attenuator in circuit as a 'reference' signal . It was 5 db over S9. When I returned to the shack a few minutes later after repositioning my inverted V, the Radio Australia signal was now at 15 db over S9, a definite improvement! I sat there for a few minutes as the significance of this sank in. In a flash of inspiration I decided to put my wire 20 m Quad loop back up again as now in this 'new' position it was completely clear of obstructions unlike its previous position in the same location as the inverted V in its original position when it was used for comparison testing with my inverted V, where it had a frequent unfortunate tendency to get caught up in our family's clothes hoist! The Quad loop was hauled up the mast in a few minutes ('Murphy' was obviously away for the day), and I did more receive tests. Radio Australia was still at around 15 over S9 on the Quad loop. I made a quick phone call to my friend in vk3 and arranged a contact on 20 m. The result: Despite poor and disturbed propagation (a 'K' index of 5), a good solid contact at the 100 watt SSB power level with no problems resulted with the S meter indication at both ends 'well up' on the meter throughout the lengthy contact. I was quite happy, as one might expect! The whole incident has been a real eye opener as I had not properly appreciated the importance of correct aiming, even of a basic single element antenna. One does not ordinarily think of a single element antenna as a 'directive array'. An 'array', perhaps not, but 'directive', yes! I had always assumed that a single element antenna's radiation pattern was broad enough to be relatively non critical! Well, yes it is but there is a limit! The Quad loop actually does have a slightly sharper pattern with deeper nulls 'off the ends' than a dipole making optimised positioning even more important. In the time since that fateful day I have rethought many aspects of this general situation and why I had not properly appreciated it before. My other antennas for HF are separate inverted V dipoles for 40 m and 80 m. As they are physically quite long, in this QTH they fit in the backyard pretty well as dictated by the available space. For this reason they run essentially in the N to NE and S to SW directions, firing broadside to the wires, yet they give excellent coverage to the desired vk coverage area as evidenced by my weekly 40 m Sunday morning WIA news broadcasts. This, despite being positioned poorly and firing mostly E (out to sea) and W, in theory anyway. Why? Well, it is worth remembering that inverted V antennas tend to have a somewhat more omnidirectional pattern than true horizontal dipoles, and combined with the fact that dipoles on low HF frequencies and close to real 'Earth' tend to exacerbate this tendency! A big blob of radio frequency energy going just about everywhere and up is the main result. 20 m is a band where perhaps these 'omnidirectionalising' effects are substantially absent, making more precise positioning more important and practically mandatory. Well, it certainly works out that way in practice at this QTH! Well my 20 m Quad loop is still up and whilst I'm still not totally convinced that it is noticeably better than my half wave inverted V for various reasons, such as 'effective height' considerations, it is working very well in practice and I have to concede that it does have some other unique virtues such as excellent noise cancellation on 20 m and also when used as a general coverage 'receive' antenna which as a keen SWL, I find a most desirable quality! Especially on the lower short wave frequencies and the AM medium wave broadcast band this noise cancellation and the resultant improvement in 'listening' S/N ratio has to be heard to be believed! For this reason as well as its good performance on 20 m, I think I might leave it up! I admit to having a considerable aversion to noise of all kinds! Although in some ways I'm a little annoyed at how silly this error was, thankfully the antenna still worked quite well despite its non- optimum positioning into the vk3 area. Sometimes I think that it might be preferable if things simply didn't work at all when things aren't right rather than 'sort of working, after a fashion'. But perhaps I shouldn't be too annoyed as this sort of thing happens in life all the time, or so I'm told. Hi. Felix Scerri vk4fuq. 6/7//2/08. * * * * *
History of The Cubical Quad As reported by W6SAI and W2LX in "All About Cubical Quad Antennas", Radio Publications Inc. 1972 In the year 1939 a group of radio engineers from the United States travelled to the South American republic of Ecuador to install and maintain the Missionary Radio Station HCJB, at Quito, high in the Andes mountains. Designed to operate in the 25 metre short-wave broadcast band with a carrier power of 10,000 modulated watts, the mission of HCJB was to transmit the Gospel to the Northern Hemisphere, and to tell of the missionary work in the wilds of Ecuador. To insure the best possible reception of HCJB in the United States a gigantic four element parasitic beam was designed, built and erected with great effort and centered upon the heartland of North America. The enthusiasm of the engineers that greeted the first transmission of Radio HCJB was dampened after a few days of operation of the station when it became apparent that the four element beam was slowly being destroyed by an unusual combination of circumstances that were not under the control of the worried staff of the station. It was true that the big beam imparted a real "punch" to the signal of HCJB and that listener reports in the path of the beam were high in praise of the signal from Quito. This result had been expected. Totally unexpected, however, was the effect of operating the high-Q beam antenna in the thin evening air of Quito. Situated at 10,000 feet altitude in the Andes, the beam antenna reacted in a strange way to the mountain atmosphere. Gigantic corona discharges sprang full-blown from the tips of the driven element and directors, standing out in mid-air and burning with a wicked hiss and crackle. The heavy industrial aluminum tubing used for the elements of the doomed beam glowed with the heat of the arc and turned incandescent at the tips. Large molten chunks of aluminum dropped to the ground as the inexorable fire slowly consumed the antenna. The corona discharges were so loud and so intense that they could be seen and heard singing and burning a quarter-mile away from the station. The music and programs of HCJB could be clearly heard through the quiet night air of the city as the r-f energy gave fuel to the crowns of fire clinging to the tips of the antenna elements. The joyful tones of studio music were transformed into a dirge of doom for the station unless an immediate solution to the problem could be found. It fell to the lot of Clarence C. Moore, W9LZX, one of the engineers of HCJB to tackle this problem. It was obvious to him that the easily ionized air at the two mile elevation of Quito could not withstand the high voltage potentials developed at the tips of the beam elements. The awe-inspiring (to the natives) corona discharges would probably disappear if it were possible to operate HCJB at a sea level location. This, however, was impossible. The die was cast, and HCJB was permanently settled in Quito. What to do? Moore attacked the problem with his usual energy. He achieved a partial solution by placing six-inch diameter copper balls obtained from sewage flush tanks on the tips of each element. An immediate reduction in corona trouble was noted, but the copper orbs detuned the beam, and still permitted a nasty corona to spring forth on the element tips in damp weather. Clearly the solution to the problem lay in some new, different approach to the antenna installation. The whole future of HCJB and the Evangelistic effort seemed to hinge upon the solution of the antenna problem. The station could not be moved, and the use of a high-gain beam antenna to battle the interference in the crowded 25 metre international short-wave broadcast band was mandatory. It was distressingly apparent to Moore that the crux of the matter was at hand. The Birth of the Quad In the words of W9LZX, the idea of the Quad antenna slowly unfolded to him, almost as a Divine inspiration. "We took about one hundred pounds of engineering reference books with us on our short vacation to Posoraja, Ecuador during the summer of 1942, detrmined that with the help of God we could solve our problem. There on the floor of our bamboo cottage we spread open all the reference books we had brought with us and worked for hours on basic antenna design. Our prayers must have been answered, for gradually as we worked the vision of a quad-shaped antenna gradually grew with the new concept of a loop antenna having no ends to the elements, and combining relatively high transmitting impedance and high gain." A Quad antenna with reflector was hastily built and erected at HCJB in the place of the charred four element beam. Warily, the crew of tired builders watched the new antenna through the long operating hours of the station. The vigil continued during the evening hours as the jungle exhaled its moisture collected during the hot daylight hours. The tension of the onlookers grew as a film of dew collected on the antenna wires and structure, but not once did the new Quad antenna flash over or break into a deadly corona flame, even with the full modulated power of the Missionary station applied to the wires. The problem of corona discharge seemed to be solved for all time. The new Quad antenna distinguished itself in a short time with the listeners of HCJB. Reports flooded the station, attesting to the efficiency of the simple antenna and the strength of the signal. In his spare time, Moore built a second Quad antenna, this one to be used in the 20 metre band at his ham station, HC1JB, in Quito. At a later date, after Moore had returned to the United States, he applied for a patent covering the new antenna. the fact that the Quad-type antenna radiated perpendicular to the plane of the loop was deemed by the Patent Office to be of sufficient importance to permit the issuance of a patent to Clarence C. Moore covering the so-called Cubical Quad antenna. To understand the characteristics of the antenna, it is convenient to borrow the description of the Quad element given by W9LZX - "a pulled-open folded dipole." This interesting account is taken from William I. Orr's book, "All about Cubical Quad Antennas". In it, technical details are addressed at length. However, there are only two pages devoted to the four element, full sized quad. Orr calls it the "Monster Quad". We know why. One Wavelength Quad Loop Optimisation. Initial Investigations: In previous articles I've written on the single Quad loop antenna I mentioned possible concerns about the 'effective height' of loop antennas compared to dipole or inverted V types of antennas. This is an unfortunate fact! Sadly this has been confirmed by many on air contact signal strength observations between my Diamond Quad loop antenna and a test half wave inverted V dipole cut for 20 m at the same top mast height over recent months. On 'transmit' anyway, the simple inverted V dipole was the superior antenna by several 'S' points! Not wishing to give up on my beloved Quad loop I've been embarking on a long series of trials, tests and evaluations all aimed at improving the general 'transmit' capabilities of the loop antenna at 'lowish' mast heights. These observations are ongoing, but some improvement has indeed been achieved. The modifications involve shifting the feedpoint from the bottom to the top of the loop in an attempt to optimise the loop 'effective height', along with the inclusion of a home brew 1:1 choke (or current mode) balun at the feedpoint, whilst feeding the antenna with 300 ohm balanced feedline as a 'tuned' line. In the past I have been somewhat impressed with the quasi- magical properties of these choke baluns in 'forcing' equal currents into a load and this, combined with 'top' feeding seems to have improved the general 'transmit' performance of the loop possibly through improved 'decoupling' of the feedline from the actual 'radiating system' (the loop). The use of a choke balun may seem unnecessary when balanced feedline is being used, however the choke balun is useful in compensating for 'incidental' unbalance and asymmetry in the antenna system and I suspect this is also the reason for the improved noise cancellation noted. Although in this new feedpoint position the loop polarisation remains horizontal, I have noted an unmistakeable reduction in noise pick up over an already low noise receive pickup as mentioned a moment ago. These so called 'choke' or 'current mode' baluns have many interesting applications in unconventional duties. Apart from this application in conjunction with 'balanced' feedline I have also used them in the past to feed a 'groundplane' antenna being fed with coax, another application where it might be thought that the use of a balun (or 'unun') was unnecessary, however its use was necessary in order to stop the braid of the coax interacting with the resonant radial system used. Further Investigations: This investigation just keeps getting more interesting! It has often been my personal observation that one discovery then makes other things possible, and as such has been the case with this investigation. One slightly disturbing thing noted with 'top' feeding through a 1:1 choke balun was that under 'some' conditions power line noise pick up was actually quite bad depending on local weather conditions, indicating that a particular local power pole mast probably had faulty or electrically 'leaky' pole 'hardware'. This noise pick up made reception difficult, yet at other times the noise pick up was very low as was expected with loops generally. Further thinking inspired me to try one other experiment, that of feeding the loop at one side resulting in nominally 'vertical' loop polarisation. I had tried this in the past with slightly disappointing and generally inconclusive results. Overall there was no real improvement compared to feeding at the bottom as originally arranged (horizontal polarisation), along with other strange observations such as a significant narrowing of the SWR bandwidth and detectable mast 'interaction' despite using the same length of feedline and all else being the same, when fed in one side and vertically polarised!  Quad loop for 20m and side feed
position with choke balun
However this time there was a difference, feeding through the 1:1 choke balun. So I tried it, and the difference was massive! Despite the loop now being 'vertically' polarised, the general power line noise pick up was much reduced under all conditions, making reception of even weak signals easy and a pleasure! DX performance seems improved, indicating a lower radiation angle in this feed position and most interesting of all, the 'narrowed' SWR bandwidth and the mast 'interaction' noted on previous occasions did not materialise. All of these changes seem entirely due to the inclusion of the 1:1 choke balun, thus showing the great value of its inclusion. These devices have quasi - magical properties! Overall I am very happy with these improvements. Actually, I am quite delighted.  Quad loop for 20m and side feed
position with choke balun
The choke balun itself couldn't be easier to make! I made mine simply by winding about ten turns of 1.25 mm enamelled winding wire as a 'tight bifiliar' winding on a small piece of ferrite rod leaving a small space between each adjacent bifiliar turn ('super glue' type adhesives help keep things in place!) This type of balun is not 'polarity conscious' as such, simply connect the feedline to one end and the antenna feedpoint to the other. I simply covered the balun in self amalgamating tape to provide some protection for the winding, but as is obvious in the photos that is how it is mounted in the air along with a little silicon sealant applied to all connections. 73. Felix Scerri vk4fuq. 26/4/09. Evaluations
For the sake of 'completeness' it occurred to me that perhaps I should do one more experiment with my Quad loop, shifting to feedpoint position back to the original bottom (horizontally polarised) position, this time feeding through a 1:1 choke balun. I am currently evaluating this configuration although a number of different things are already apparent. The operational SWR is definitely widest in this feedpoint position although the SWR bandwidth widened in all feedpoint positions tried, due to the use of the 1:1 choke balun. Noise pick up (again due to the effect of the 1:1 choke balun) is again impressively low, but ongoing evaluations over a longer time frame will indicate the best feedpoint position for optimal noise cancellation. It is worth remembering that every QTH (location) may give differing results in this respect as every QTH is highly individual along with their respective noise sources! The one very obvious result of this series of experiments is the great value of the feedpoint 1:1 choke balun regardless of whether coax or (tuned line) balanced feeder is being used. There is little or no doubt that it's addition has been highly beneficial regardless of the feedpoint position used, in terms of reduced noise pick up and generally improved antenna efficiency both on transmit and receive within the 20 m band. Allied with this, I've noticed that the resonance of this antenna has become more 'apparent' after the inclusion of the 1:1 choke balun. Tuning on either side of the 20 m band it is now more readily noted that the general noise pick up has become (just noticeably) greater, something that I had not noticed previously, although the loop remains 'low noise' on receive. From a speculative viewpoint this appears to be additional confirmatory evidence that the choke balun is very effective in 'decoupling' the feedline from the antenna, allowing the loop to function as intended as a truly resonant and independent antenna element. As an aside, it is interesting to observe that baluns do not appear to have been regularly used with Quad antennas. From memory I can only recall seeing one Quad article in the past that recommended the use of a feedpoint balun. I have often seen the recommended use with 50 ohm coax, of a quarter wave matching section of 75 ohm coax as a means of providing an impedance 'step up' to the nominal 120 ohm resonant impedance of a Quad loop, but no balun. Given the improvement I've personally witnessed through the use of a feedpoint choke 1:1 balun, perhaps it's time we did so more routinely! Conclusion: This has been a fascinating exercise! Clearly the performance of a Quad loop antenna is affected (to a greater or lesser extent) by its feedpoint position. The main alteration here is polarisation and this has implications on several things, namely noise pickup and signal radiation angle. As touched on earlier, every QTH (location) is a little different and therefore so will be the results. If one is in the mood to investigate the possibilities then a wire Quad loop with insulators at each corner is probably the 'way to go', as then the loop's polarisation can be shifted easily by the simple exercise of physically rotating the loop. The other important conclusion to come out of this research as mentioned earlier, has been the great value of a feedpoint 1:1 choke balun. I have praised these devices numerous times. The reduction in general noise pick up with the balun in use is unmistakeable and most welcome, and in addition the loop just seems to 'work better' regardless of the feedpoint position with a feedpoint 1:1 choke balun in use. 29/04/09 - 2/5/09 * * * * * * * Since the last update to this discussion, although I had wanted to go no further with my research, indeed this has happened 'helped along' by the discovery of an interesting article on the so called 'hybrid' balun. Before I discuss this interesting balun, a couple of words on my Quad Loop. I have settled on 'side feed' that is, vertical polarisation. In this QTH vertical polarisation gives me at least two and a half S points less power line noise pickup than horizontal polarisation, and sometimes the difference is even more dramatic! Yes I know that seems to go against 'conventional thinking' on the subject of noise pick up but at least at this QTH, it is a fact! Other QTH's, of course may be the opposite! Back to the 'hybrid' balun. Information on this unique type of balun can be found at http://www.arrl.org/qex/2005/qx9roos.pdf It is simply a series connected 1:1 choke (current mode) balun with a voltage mode 4:1 voltage balun. The combination is said to represent 'the best of both worlds', as it were. My extensive testing with this type of balun appears to confirm the claims for this type of balun when used with a station ATU. So much so that I no longer recommend the use of a separate feedpoint 1:1 choke balun, at least when the loop is being fed with balanced line as a 'tuned feeder'. I now advocate the use of this 'hybrid' balun in conjunction with a good station ATU with direct feed to the loop feedpoint with balanced line as a 'tuned feeder'. Extensive on air testing with this type of balun being fed from my T match tuner has been excellent in practical use with very low apparent loss and excellent current balance at the antenna load. The really good (and easy) thing about this type of balun is that it can be easily 'retrofited' to existing voltage mode only 4:1 balun arrangements that are normally incorporated into commercially made ATU's. On other little changes, another interesting thing I'm evaluating at the moment is the placement of a half wave 'counterpoise' beneath my Quad loop at around 1.5 metres above the ground, as a means of reducing potential 'earth loss'. This experiment follows on from another interesting article I've also recently read in an old copy of the Australian 'Amateur Radio' magazine (November 1996). The original article used this half wave counterpoise below a dipole for 160 m however I see no reason why it cannot be used on other (higher) frequencies as well. Certainly the counterpoise does have an effect of the loop as noted by a detectable 'tightening' in the SWR bandwidth with the counterpoise in place as explained in the original article. Evaluation of this counterpoise is ongoing. It is my thought that with vertical polarisation of the loop, earth loss may become more of an issue so this counterpoise may have real value in reducing earth losses and improving overall antenna efficiency. Digressing on to the performance of loops in general terms, also in recent times it has become apparent that on some ionospheric paths the loop may be 'too good' given the inherent lower angle tendencies of loops compared to dipoles, inverted V's and the like especially at lower heights, where a higher overall radiation angle may indeed be advantageous over a more 'local' path eg across vk (Australia). The various graphs supplied by the IPS in Australia are somewhat enlightening as ionospheric propagation is indeed a complex thing.....interesting though, a bit like life itself! http://www.ips.gov.au/HF_Systems/1/1/1 73. Felix vk4fuq. 01/07/09 Unwanted Mast / Antenna Interaction This is an issue that I consider very much underestimated in its possible negative impact on antenna performance. In previous installations of this article I have commented on interaction between the mast (about 33 ft in length) and my Quad loop antenna, especially when the loop has been vertically polarised. As 33 ft is essentially a 'worse case situation' (a resonant half wave length), I have frequently wondered if the loop performance has been compromised by a metal pipe mast of this length and if so, what are possible ways around it? I did not set out the make a pipe mast of this particular length, but sadly things just turned out that way!  Earth stake at base of 33 foot pipe
mast holding up Quad Loop 002
Very recently I've been giving the whole matter a great deal of thought and a solution was found through a rather unrelated matter, that of lightning protection! In the end I drove a five foot length of a copper plated steel electrician's earth stake into the ground near to the base of the metal pipe mast and electrically bonded the two together with a short length of braid material. After driving the earth stake into the ground it was determined that although 'in the ground' through a piece of water pipe cemented in the ground, no measurable electrical connection to earth was found at the mast, possibly exacerbating the coupling between the loop and mast as a 'floating' half wave element. In the time since installation of the earth stake, 20 m contacts have indicated clear improvement in strength, sometimes dramatically. To be perfectly honest, an exact analysis of the mechanism involved is difficult to come up with, but purely on the basis of signal reports there is no doubt that things have definitely improved. A bit of a revelation actually.  ARRL Guy Wire Interaction Chart
Interestingly enough in my
very old copy of the ARRL Antenna Book,
there is a very interesting table on 'guy wire' lengths to avoid in the
various Amateur Radio bands and the length of my (electrically
floating) metal pipe mast was right in the middle of the range to avoid
in the 20 m Amateur Radio band. No, I'm not surprised that there
was undesirable interaction between the mast and loop antenna!
Vertical polarisation would be the worst possible case, but horizontal
polarisation is still not ideal all things considered. Even more
interesting in this table is the statement that 'grounded' wires will
exhibit resonance at odd multiples of a quarter wavelength',
effectively, in my case anyway, shifting the resonance out of the 20 m
band, as I understand it anyway. This is certainly consistent
with my own observations of improved performance.Possibly a loop antenna is more badly affected by mast interaction as wire exists in all geographical directions despite 'defined' polarisation unlike say, a dipole or similar antenna. Be that as it may be, I love my 20 m Quad loop antenna to bits....and now it's even better. The possibility of antenna/ mast interaction is definitely a point to watch on your own antenna installation, and as an aside I now (hopefully) have better lightning protection! 73 Felix vk4fuq. 17/09/09 * * * * * * * *
In a brand new discovery allied with the general subject of unwanted interaction, a series of interesting observations and 'chance' discoveries in this shack and QTH have made me aware of yet more examples of unwanted interaction, this time between different antennas! Specifically between my 20 m Quad loop and 'one' leg of my 80 m dipole which 'runs past' my quad loop although not at a particularly close distance, but close enough it would seem judging by my observations! It would appear that (powerline) noise is being picked up and 're-radiated' by one leg of my 80 m dipole (fed with balanced line directly as a 'tuned line') onto my quad loop causing a large increase in 'apparent' noise pickup on the loop. This was proven whilst listening on 20 m with the loop and simply grounding one particular leg of the 80 m feedline here in my radio shack. It is quite bizarre but simply amazing to observe an at least 2 S point drop in noise pick up on the loop when one leg of the 80 m dipole is grounded with no negative effects on received general signal strength! It makes general listening a lot easier as one might expect, and as an added bonus the noise blanker then works more effectively at reducing the remaining noise! Why this is happening is difficult to explain but the effect is quite real yet and despite the obvious 'coupling' between the two separate antennas there is no apparent shift in the transmit SWR or other deleterious effects on the loop's general performance when one leg of the 80 m dipole is grounded to reduce the noise pickup. Strange but very welcome! Antennas....don't you love them? With antennas, the general possibilities are seemingly endless. Yes! 73 Felix vk4fuq 15/10/09. Radiation Angle - The forgotten factor in antenna performance Over the last few months as a consequence of various evaluations with different one wavelength single loop antennas, I've come to realize that there is one very important, yet seemingly forgotten or at least underestimated factor in overall antenna performance, that of the antenna radiation angle. Everybody seems to be 'chasing' antenna 'gain' as a matter of some priority whilst my most recent research suggests that optimised radiation angle for the path desired is at least as important as 'gain' if not more so. The analogy I use to define antenna gain is that of bright spotlight with higher gain being represented by a brighter spotlight. In a way I see this as somewhat of a 'brute force' solution. Obviously a brighter spotlight will be seen at a greater distance however my observations indicate that optimised antenna radiation angle is a more elegant approach and if combined with appreciable antenna gain, the results can be most impressive indeed! One of the really nice things about one wavelength loop antennas is that polarisation, and as a direct consequence radiation angle can be changed at will by a simple change of feedpoint position even with the loop at low physical height. This does not seem to have been much realised in the past as the 'traditional' way to feed a Quad element was always at the bottom and as a consequence, horizontally polarised and a reasonably high radiation angle. In contrast, a simple horizontal dipole type of antenna needs to be placed high above the ground in order to reduce the radiation angle when horizontally polarised (the usual situation). For very low angle performance from a yagi on 20 m, I have seen written that the optimum height is roughly 1.5 wavelengths (105 ft or thereabouts). Considerable mast and tower engineering is needed as a result! An optimised one wavelength loop at low height can easily produce low angle vertically polarised radiation at around 20 degrees, a much easier engineering achievement! The type of actual polarisation of sky wave propagation through ionospheric refraction is essentially unimportant given the polarisation scattering that inevitably takes place, however what is important is low angle take off. My long time interest in general short wave listening has shown me the value of low angle radiation at least 'on receive' as when listening on the loop up on the 19 metre broadcast band, some greater distance stations are much better copy on the loop when comparing a 20 m inverted V dipole and a Delta Loop vertically polarised for low angle radiation both 'looking' in the same directions, (time signal station WWVH in Hawaii and the BBC World Service relayed from somewhere in Asia, on 15.310 MHz) as shown in the audio file. 'Reciprocity' applies on transmit applications as well. Allied with this is the realization that for much inter VK working, at least for the higher HF frequencies, low radiation angles are actually a distinct disadvantage, resulting in poorer apparent signal levels. This neatly explains why Quad loops used at this QTH in the past have been frequently 'less than ideal' for more local working. The are also other relevant factors of course, but too low a radiation angle is the predominant one in my opinion. Indeed I have found that for local working on the higher HF bands, traditional (bottom feed) horizontal loop polarisation resulting in higher radiation angles is preferred. I am presently working relay switching of alternative feedpoints in order to optimise working conditions. The possibilities are amazing. Listen to the BBC World Service audio file Felix vk4fuq 19/11/09 More Loop Experiments - October - November 2009 I've recently embarked on a whole new series of loop experiments, this time with a Delta Loop (3 sided equilateral triangle) one wavelength loop. In actual fact Delta Loops were my first loop antennas many years ago and a number of interesting articles recently found on the 'net have been most inspiring. Indeed my experiments with my Delta Loop have made me realize how very little I knew! My Delta Loop is of the 'apex up' variety and my experiments have centred around the possibilities afforded by altered feed position. Although the SM5JAB article is based on a loop designed for 30 m, my loop scaled for 20 m as shown in reference 2 confirms the practical observations in the SM5JAB article. Switching feedpoint is essentially like using a completely different antenna! The low angle/ DX performance with vertical polarisation at this '1/4 wavelength feedpoint position' is extremely impressive to the detriment of more 'local' working where higher radiation angles are more suited (around the general VK area, for example). Effective height appears not to be overly important in this configuration, making the Delta Loop an excellent performer at lowish mast heights. Interestingly enough, for reasons that I cannot quite understand my vertically polarised Delta Loop outperforms my Quad Loop when also vertically polarised in low angle/ DX coverage! Why, I don't know although I suspect that the 'quarter wavelength' feedpoint position possibly results in a more defined low angle vertically polarised lobe. Whatever the true reason, it works superbly! Specifically for DX working, this simple antenna is very highly recommended! Essentially a Delta Loop follows the same length formula as all one wavelength loops in general of 1005/ f = length in feet, but a loosely coupled GDO or antenna analyser can be used to check for the resonant frequency. I am using balanced feder all the way to the feedpoint as a tuned line using a 'hybrid balun' and ATU for matching down to 50 ohms 'unbalanced'. This feedline system works very well. References: http://www.isy.liu.se/~mj/HAM/ANT/nabla.html http://www.thebrowns.fsnet.co.uk/20mdelta.htm I have to admit that the last month or so of intensive loop research has been rather exciting, almost like those early days in 1985 when I first gained my 'full call' Amateur Radio licence, and discovering the magic of one wavelength loops for the first time! This latest research dealing with altered feedpoint position and the implications on radiation angle has been incredibly enlightening! I've since placed my diamond Quad loop back up vertically polarised (side feed), and I'm quite gratified to say that the low angle/ DX results I've obtained have been identical to my vertically polarised Delta Loop and if anything the diamond Quad loop should have 'slightly' more gain by virtue of slightly increased internal enclosed area. Another interesting change has been the re-inclusion of a feedpoint 1:1 choke balun. In the past after discovering the so called 'hybrid balun', I no longer recommended a feedpoint balun however I now think that its inclusion is a good idea, even with balanced feedline feed as a 'tuned' line. Certainly the use of a feedpoint 1:1 choke balun is recommended in the excellent SM5JAB Delta Loop article, and after placing it once again in both my earlier Delta Loop and present diamond Quad Loop, a very significant widening (without any additional loss) in SWR bandwidth was noted indicating much improved decoupling between the feedline and the loop antenna itself. I am using a simple ferrite rod cored 4:1 'voltage' balun in my shack following my T match ATU in conjunction with the 1:1 choke balun at the feedpoint of the loop and fed with high quality balanced 300 ohm as a 'tuned line'. This overall feedline system works very well with very little loss. If using all coax feed with a 'quarter wavelength' matching section of 75 ohm coax then the 1:1 choke balun is mandatory! I think I've finalised things now, more or less (maybe). In general terms vertically polarising my 20 m loop has been a bit of a revelation to be perfectly honest, now that I've finally worked out and realised the implications on radiation angle! I am hearing just so much more compared to previous horizontal polarisation (and high angle) radiation, especially during the night time hours where previously the band would appear quite 'dead'. Although 'closer in' working may be less than optimum with vertical polarisation, it probably isn't quite as bad as I first thought, at least here in vk! I don't feel that I'm losing out much at all with vertical loop polarisation! 73 Felix v k4fuq 23/10 & 27/11 2009. The Effects of a Metal Mast or a (very) Stupid Mistake! Although I consider myself quite ok with HF antennas, very recently I realised a basic (very stupid) long term error I'd made that prevented optimum operation from my favoured HF antenna type, one wavelength wire loops. In recent years they just never seemed to perform as well as they should have and despite many, many investigations the mystery was never actually solved and I guess the expression D'oh, might apply now! Very recently as a general test, a simple half wave 'sloping' dipole was erected for 20 m and this simple antenna performed much better than any of my various loop antennas. I simply had to work out why and slowly but surely, and with a little guesswork, the true reason was finally revealed! My 34 ft metal pipe mast! All of my loops were symmetrically 'hung off' this mast and although over the years I had often wondered if this arrangement might not be a good idea especially given that the length of the metal mast was very close to the resonant half wave on 20 m and ran right through the active radiating area of the loop, I had considered the possibility of undesirable interaction, but then discounted it! In any case, the loops seemed to work well enough, but my 'gut feeling' was that they should have been better. My original Delta Loop constructed back in the 1980's was mounted 'between' two metal masts! The experience with my sloping dipole now made me sure that that something very basic was seriously amiss. On a hunch, and using the 'Sherlock Holmes' approach to fault finding, the metal mast was considered the most likely suspect, so after my 40 m WIA Amateur Radio news broadcast one Sunday morning recently, I was inspired to shift things around to take the metal pipe mast out of the radiating area of the loop as much as I could and see if that improved things. Space was restricted, but I managed to put up my pre- existing one wavelength Delta Loop mounted 'on its side' as it were, strung up between my 34 ft metal pipe mast and a shorter 15 ft metal pipe mast and fed at the junction of the side mounted two sloping sides, giving I think, vertical polarisation. A general immediate improvement was noted and signals reports, both transmitted and on receive, were much better than previously (and better than the sloping dipole). Several other distinct changes were noted and it seemed to me that the mast was interacting very adversely with the loop in several ways. Firstly, by direct absorption of transmitted RF. Secondly, pattern distortion, and thirdly increased noise pick up. In the time following since the repositioning of my Delta Loop, power line noise pick up (always a long term problem on any of my loops at this QTH, in whatever 'shape' they were), is now markedly reduced and my HF transceiver's 'noise blanker' actually works when it did nothing before! The pattern distortion aspect is interesting as in the past I had noted that vertical loop polarisation was best for DX and horizontal loop polarisation was best 'locally'. Now the Delta Loop performs equally well on both DX and local signals, despite being nominally vertically polarised. I have to think about that one! In the past, a friend had presented me with some IPS data that suggested that low angle radiation was best for 'all' paths. As the loop is now working so well vertically polarised, perhaps this is another indication that the Delta Loop is now working as it should, as vertically polarised loops inherently produce excellent low angle radiation! Needless to say, I am quite delighted at this turn of events! Perhaps I should have realised my error years ago, however a recent internet search found only one article (out of many found), expressly suggesting that metal within the radiating area of a loop antenna was a bad idea! Well, I've certainly learned my lesson! I suspect that the self resonant length of my mast was also a big factor, however the lesson is, whenever erecting a one wavelength loop antenna, keep any or appreciable lengths of metal out of the area of the loop, completely if possible. Then you'll see how good a one wavelength loop antenna really is! I cannot emphasise this point strongly enough! 73 Felix vk4fuq. 30/12/2010.  20m
Delta Loop offset from metal mast -
Felix Scerri VK4FUQ
The photograph above is of the reconfigured
Delta Loop. I wish it was a little 'neater' and a bit more
'separation' but it's much better regardless, and an article from one
of my antenna books that I've read a thousand times, but somehow I
never noticed the note about a 'non metallic pole'!
Felix vk4fuq.  Delta Loop -
Practical Wire Antennas DELTA LOOP UPDATE - February 2011 Sometimes the most unexpected things happen and 'great discoveries' happen by accident. This is one of them (more or less)! In the previous installment [see articles below] of the ongoing loop adventure I mentioned the improvement arising from removing the metal mast from the internal radiating area of my full wavelength Delta Loop. Well a few weeks ago our general location was badly damaged by a very large tropical cyclone (Severe Tropical Cyclone Yasi). http://en.wikipedia.org/wiki/Tropical_Cyclone_Yasi Apart from all the other damage, most of my Ham Radio masts and antennas were either badly damaged or destroyed in the 250 plus kph wind gusts. I've since rebuilt most of my antennas and masts (one to go!), but the really interesting thing is that at least for my 20 m Delta Loop, I've had to shorten the mast quite a bit due to available undamaged mast material, from around 34 feet to about 25 to 27 feet, and believe it or not my Delta Loop is now working much better as a result! The improvement on 'all' signals (local and DX) is unmistakable! In the past I have often read the anecdotal comment that HF Delta Loops seem to work better at lower heights. I can't really explain this phenomenon as observed at this QTH (location), but it is real. When I look out my window and observe how low the antenna looks now with the lowest corner only a couple of feet above the ground, it seems ridiculously low, but it does work very well indeed! Perhaps that previous longer (resonant length) metal mast, although now 'outside' of the loop, was still sufficiently close to and adversely interacting with the loop, I don't really know! It is my feeling that this observed result may not necessarily happen in others' QTH, but it does give hope to those with only limited height available. I've been very pleasantly surprised! As things stand now, I might even be able to put up a fixed parasitic element for a bit more 'gain', but even as a single element antenna it couldn't be any better in terms of general performance! 73 Felix vk4fuq 19-24 Feb 2011. And now it's even better! I guess I really should put an end to these ongoing loop experiments, however I've just made another change which is most interesting. Yes, another change in feedpoint position, now to the bottom corner, giving I think a mixture of low angle vertically polarised radiation but also an appreciable high(er) angle horizontally polarised component as well. At least at this QTH this 'corner' feed position appears to give the best overall result between 'local and DX' coverage, in fact DX coverage seems essentially unchanged, however 'local' coverage within the general VK / ZL area is markedly improved. This is not without some penalty though. Corner feeding does introduce a strange asymmetry in the 'sides' of the polar pattern of the Delta Loop but this appears to be of little real disadvantage in practice. One other little change relates to feeding the antenna. I use the 'hybrid' voltage/ choke balun design as mentioned previously, however after some recent reading and online discussion on this type of balun it was suggested the two individual baluns are better interchanged in their positions. This does make sense to me as the 1:1 choke balun is then kept to lower impedances which is preferred. Anyway, I have tried this and I think it is better. One more thing to try anyway. 73 Felix vk4fuq 15 03 2011. More: Further to the interesting 'possibilities' afforded by differing feedpoint position, my friend Mick SM5JAB has run several antenna modelling simulations for me on the 'on its side' 20 m Delta Loop I use at this QTH showing that with differing feedpoint position, the same physical antenna becomes very different! I use the lower corner feed position giving horizontal polarisation with high angle radiation for 'local' working around Australia (vk) with excellent results, and my other feed position giving vertical polarisation with low angle radiation for purely DX working. The same results are achieved with the more traditional apex up or down Delta Loop by feeding in the appropriate position, either 1/4 wave along one of the sloping sides for low angle vertically polarised radiation, or the middle of the horizontal side or (equivalent position), for horizontally polarised high angle radiation.     vk4fuq. 17 05 2011 The Use Of A Non Conductive PVC Mast: Further 20 m Delta Loop experiments involving a new mast! Well it's been quite a while since my last installment and this latest round of experiments was prompted by a desire to install a new non metallic mast to hold up my 20 m Delta loop in order to finally conclusively determine if a metallic mast was as detrimental to the loop's performance as I had suspected. So, just this last Monday morning after my usual early morning 50 minute walk (oh, the things one needs to do to manage type 2 diabetes!), my walking partner (a fellow local Radio Ham), and I decided to erect a mast entirely built of sturdy PVC pipe. I had absolutely no experience with this material, however my friend comes from a engineering/ tradesman background and he assured me that such a structure was entirely feasible. So off we went to a local hardware store bought two lengths of PVC pipe, PVC glue and luckily, transport was provided free for me to this residence only a relatively short distance away! The two lengths were quickly glued together making an overall height of about forty feet. We knew that some guying would be required, however we had seriously underestimated this aspect and our first attempt to raise with new PVC pipe mast was a disaster. We needed to cut the mast back to about thirty one feet (a more manageable length), and still slightly higher than the old metallic mast and we decided to enlist the assistance of another adult helper and obtain more guy rope. The 'erection' was then set for the following morning. The following morning arrived and our improved preparedness proved effective, and the mast was indeed successfully raised and once the guying was completed, it looked a substantial and solid enough assembly, but that guying is critical! The Delta Loop was successfully installed and experiments and observations began. I was secretly hoping for a large improvement in general performance.  VK4FUQ Felix Scerri's New PVC Mast
Well, the results so far have been interesting and experiments are ongoing, however it is quite apparent that the metal mast was indeed affecting the electrical performance of the loop! The first thing I noticed was the SWR bandwidth had broadened very considerably, being only about 1.35:1 at both band edges (through the station ATU and direct balanced feed), instead of rising steeply at the band edges with the loop vertically polarised at the 1/4 wavelength feed position on the loop with the metal mast, (as previously discussed elsewhere in this page). However testing over a few days indicated that the general level of performance was similar to previous! Welle that got me thinking and further experiments today involving past ideas has yielded positive results. Readers may recall my past experiments involving the use of a feedpoint current mode or 'choke' balun as a means of providing effective feedline/ antenna decoupling. So early this morning I made up a simple 1:1 choke balun with ten bifilar turns on a short length of ordinary 'loopstick' ferrite material wound with 1.25 mm copper winding wire and I installed it at the feedpoint. I've tended to change my mind frequently on the value of this feedpoint balun however after installing it, I noticed something quite interesting that suggests that it is indeed acting to effectively 'decouple' the feedline from the antenna. The measured SWR with the balun in circuit do not noticeably change, however it was apparent that the ATU 'tuning' had become much less critical and 'touchy', suggesting a generally lower Q situation inferring that the 'effect' of the feedline was greatly reduced! Further evidence of greatly reduced feedline 'effect' was observed when a 'birdie' evident on my main HF transceiver, produced by another receiver in in this shack was much lower in strength than usual. It is thought that feedline 'unbalance' was mostly picking up this annoying 'birdie'. This also suggests improved feedline 'balance' as well as effective 'decoupling'. As of now, things look promising! More to follow! 73 Felix VK4FUQ 14th October 2011. BALUNS: A Rethink, Observations and Unexpected Results! This update might also be entitled 'Was I wrong? Maybe', as recent experiments and observations have yielded very unexpected results! As I mentioned in the last instalment, I've never been quite sure about the use of a feedpoint 1:1 choke balun with balanced feedlines. As of now, I say don't use one! As an experiment I installed feedpoint 1:1 choke baluns on all my HF dipoles as well, and general testing indicated poorer signal reports and in conversation with a friend knowledgeable in transmission line matters, he also suggested that the use of a feedpoint 1:1 choke balun with balanced line was ill advised and 'lossy'. I still cannot see why this might be the case, but the observations seem to confirm this. So don't use one, (at least with balanced line)! Interestingly enough, further investigations with an ATU 'hybrid balun' as originally designed and mentioned in earlier instalments, with the 1:1 choke balun on the 'output side', appears to work extremely well and signal reports are good. More interesting were the observations and comparisons made between a simple 4:1 voltage balun and this 4:1 'hybrid balun' showing a full 3 'S' point reduction in received (power line) noise in favour of the 4:1 hybrid balun, suggesting massively improved feedline balance and common mode noise rejection. This result was totally unexpected! This 'hybrid balun' gets my define tick of approval. 73. Felix vk4fuq. 2nd November 2011. The End Of The Whole Loop Saga? Hopefully this is the end of the whole Loop saga, but I really love my 20 m Quad loop again! This week I had a real breakthrough. In a previous instalment I mentioned incorrect positioning of my loop, well even after 'repositioning' it, it still wasn't right! Another unexpectedly poor signal report very recently forced me to check this again, thankfully. I have a shocking sense of direction, even with a compass in hand! I even enlisted the help of an older 'bushie' friend who is very familiar with correctly using a compass, and he confirmed that I now have my loop firing North and South (broadside directions to the 'line' of the loop)! And what a difference it makes. Now when I call a station, I now get answered....every time, unlike previously. Analysis of compass bearings indicate that I was firing in 'null' directions and this leads me to another interesting observation. In tech school years ago we were taught that a 'three decibel level difference was about the minimum discernable' level change. Don't you believe it! It all depends on prevailing signal levels. A three decibel level difference can (and does) make a potentially massive difference. Nulls in the 'ends' of a full wave loop when horizontally polarised can be up to 12 decibels. Aiming that loop correctly 'is' very important. So now I have my 20m Quad loop up (in a slightly different position in my backyard),and 'looking' truly North and South fed with balanced line as a 'tuned line' direct into my shack and into a 4:1 hybrid balun and T match ATU, and I couldn't be happier. No, it's not a beam and it's not rotatable, but for what I want I couldn't be happier! 73 Felix VK4FUQ 09/03/2012. Fascinating observations with two 20 m antennas or the value of comparing two antennas side by side G'day all, I had a fascinating afternoon recently comparing two separate 20 m antennas. Readers will know of my constant experiments with simple 20 m antennas, so on this afternoon I put up two separate antennas. My beloved 20 m Quad loop (horizontally polarised), looking North and South at a top height of 28 feet (quite low in absolute terms), and a separate 20 m vertical ground plane with the base at about 15 feet above ground with two elevated quarter wave resonant radials with a 1:1 choke balun at the feedpoint. Two elevated radials are all you need, not three or four, as long as they are some distance off the earth, or ground loss will be a problem. There is almost no eccentricity in the radiation pattern despite the use of only two in-line radials. Both antennas were fed with balanced line as tuned lines and are sufficiently distant from each other to avoid interaction. Both antennas were fed into two separate receivers which were reasonably well matched on receive with closely matched receiver sensitivity and S meter calibrations. I listened to various signals, both local and DX, during the IARU contest on 20 m between 0400 and 0600 hours UTC. The results on receive were absolutely fascinating and it is great when theory agrees with the observed reality! Across VK (I am in Northern VK4) the Quad loop, in the broadside directions, was the clear winner with very low noise on receive and about 2 to 3 S points better than the vertical. However on all DX signals (America and Europe), the vertical was very superior by on average 3 to 5 S points! With the vertical the noise level was slightly higher (S2), but not obtrusive. These results were absolutely amazing to observe. The S meters don't lie! There seems to be an interesting transition zone around the ZL region, as the Quad loop and the vertical were approximately equal. Conclusions: The Quad loop was/is clearly the best antenna for working across VK in the broadside directions. Into VK6 (nulls on the loop), the vertical was about 1 S point better (varying somewhat). The books say that a low Quad loop horizontally polarised radiates at very high angles and I agree. Although the higher radiation angles are clearly better for local working across VK. The radiation angle of the vertical ground plane is low and DX performance is superb, the expected result. All tests were receive tests, but I would expect similar results for transmission too. Regards, Felix VK4FUQ 20/07/2012 Thanks Felix, I myself have been comparing my antennas on RX over the last couple of days. I have been comparing my Inverted L, a small fan dipole that I have in the loft for 10m and 6m and a Delta Loop that I have recently hung up. The loop's wire is about 16 metres in length, but I also have another wire handy that is 12 metres in length to form a smaller loop for the 24 MHz band that I can easily hoist in to position. The Inverted L has the addition of an extra sloping wire element tuned for 17 metres. I have mainly done RX comparisons on 20m, 17m and the local 11 metre CB band. It's quite amazing how switching between antennas can produce entirely different results. For example while monitoring the local CB band (which isn't in itself very edifying!! ) I could hear strong signals on the horizontal 10m dipole in the loft, but switching to the Delta Loop produced an entirely different transmission on the same channel and almost equally strong signal. This is a product of the delta loop being fed at the bottom corner which should give polarisation somewhere between horizontal and vertical (mixed) and its position fires roughly north east / south west, whereas the dipole is horizontally polarised and fires east / west. Similar, but less spectacular, differences have been observed on 20m and 17m. I'd say that overall the loops antennas are better compared to the Inverted L for their particular bands, but differences in directionality will sometimes favour the Inverted L in both cases. I installed the loop to try to improve access to the higher frequency bands. The horizontal fan dipole in the loft for 10m and 6m while being extremely efficient does suffer with quite high noise pick up and I fear that using 100 watts may cause EMC issues for neighbours - anyway it's a useful and cheap aerial! As for my loop experiments then I am quite encouraged so far. While the 80/40 Inverted L tunes up easily on 20m, 15m and 10m, with 20m being fine, results are not especially amazing on 15 and 10 - acceptable when conditions are above average, but not as good as dedicated antennas. This prompted me to try some loop antennas. While loops are supposed to be single band antennas, I have read that a number of people have used them as multi-banders with some success, so I thought I'd try one out for 17metres and upwards and the other for 12 metres. Initial results are good and I am certainly tempted to leave the loop as a permanent fixture, but while I don't find either the 12m or the 16m loop to be noticeably quieter than the Inverted L, they are, however, quieter than the loft dipole for obvious reasons! All very interesting! 73, Mike. M0MTJ 20/07/2012 Balun Discussions
The other morning after feeling a bit negative about aspects of balun performance, I started this thread on the QRZ.COM technical forum. http://forums.qrz.com/showthread.php?357490-Are-we-all-being-fooled-about-baluns The whole thread is definitely worth reading! I started out questioning whether the 'current mode' baluns were really any good, and by the end of the thread I had more or less changed my mind after running a couple of impromptu tests. The so called 'hybrid' balun that I have mentioned before is actually quite remarkable in the way it reduces noise pick up over a standard 4:1 voltage balun. That 1:1 choke (current mode) balun in the hybrid assembly does the magic! It makes one realise that in the 'real world' things are never even remotely close to the perfect, but the hybrid balun has a quasi magical way of bringing things closer to the ideal through enforcing equal currents and choking off 'common mode noise'. As I mentioned in one of the posts my dipoles/inverted V dipoles are now as 'quiet' as my beloved loops, thanks to the hybrid balun! Others have also noted large reductions in 'apparent' noise pick up when using a current mode balun/line isolator. In many cases the transmission line is picking up more noise than the antenna and this is greatly reduced through proper 'balancing'. It is all quite involved when one thinks about it. Here are two another quite interesting and though provoking articles. http://audiosystemsgroup.com/RFI-Ham.pdf and http://www.yccc.org/Articles/W1HIS/CommonModeChokesW1HIS2006Apr06.pdf 73 Felix VK4FUQ 15/0812 More thoughts, and W8JI In recent times, things have been ‘coming together’ as it were, when it comes to antennas and the jigsaw puzzle is starting to come together nicely. It appears to me that a lot of quite important yet slightly ‘intangible’ things are now being seen for their real importance. Things like feedline unbalance and the generally unrecognised results of this like increased noise pickup which are often ‘blamed’ on the antenna itself (incorrectly) when noise picked up on the feedline is actually (mostly) responsible! Current mode (i.e. choke) baluns are close to magical in what they can do in improving feedline (current) balance. In recent times I have proven this conclusively to myself. In the past I have always regarded loops are having this ‘magical’ reduced noise pickup advantage, however simple dipole type antennas if feed through a well-constructed choke balun/feedline system, whether it be coax or balanced feedline, with a choke balun positioned as appropriate (at the antenna feedpoint when using coax and at the shack/ATU end if using balanced feedline), are equally good in this respect and do have the advantage of inherently greater ‘effective height’ which is always helpful! Proper attention to the ‘little’ things often does make a very large difference, even when one may not think so! This website has an enormous amount of very useful and interesting information. Click on the ‘antenna’ tab and you might be reading for weeks! http://www.w8ji.com 73 Felix VK4FUQ 18/09/2012 Metal Poles and The Quad Loop After more work and experiments I have concluded that the reason for the apparent degraded ‘transmit’ loop performance that I have frequently reported in these pages is the way that I used a metal mast as the supporting structure. I had actually identified this earlier, however a recent opportunity to re-mount my single Quad loop in ‘between’ two metal masts (at not very great height), transformed the performance of my loop. The loop is still ‘looking’ in the same directions, by the way! Given that I am somewhat physically disabled, it was a challenge to successfully do this by myself but I persevered and succeeded! Thankfully aluminium makes for a light and manageable mast, but it is expensive stuff! I now have negligible noise pick up, compared to a solid and constant S7 as previously mounted ‘around’ my main metal mast, and the loop is now both radiating and receiving very well. Mounting a loop in ‘diamond’ fashion off a metal mast is very convenient, but it seems to mess up antenna performance. An interesting thing is that modelling done by others for me using EZNEC showed very little negative effect of a metal mast running through the middle of a full wave loop. However, practical results appeared to show that the modelling was wrong or perhaps incorrectly applied. I am still using balanced ‘tuned’ feedline with a hybrid balun at the shack end which I think all helps, but the ultimate lesson is to use two masts and keep any metal out of the loop! Yes, it complicates assembly but that is a small price to pay for excellent antenna performance. 73 Felix Scerri VK4FUQ 08/10/2012. Although the Delta Loop has slightly less ‘gain’ than a Quad Loop, I’ve found the Delta Loop every bit as good as the Quad loop in practice, and is much easier to mount, requiring only one ‘high’ mast at one end and an approximately ‘half size’ mast at the other ‘apex’ end.  I am actually surprised that this usual arrangement is not known of and/or used more as its ease of mounting is a very significant advantage, and of course feeding at the ‘apex’ end will yield vertical polarisation. For ‘local working’ with horizontal polarisation, fed at the lower corner, works very well and of course all metal masts are kept outside of the boundary of the loop. Felix VK4FUQ 12/01/2013.  Antennas 1 | Antennas 2 | Antennas 4 | Antennas 5 | Antennas 6 Top of page ^ |
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Mike Smith - MDS975.co.uk © 2003 - 2013
M0MTJ
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