LOOP & FRAME AERIALS
and ANTENNA TUNING UNITS
A Loop or Frame aerial is a wonderful tool to assist long wave and medium wave reception and, indeed, absolutely essential for serious long distance reception (DX-ing). Fortunately a loop aerial is extremely easy and very cheap to construct, you may even have most of the parts required in your junk box. I offer a few pointers to the construction of loop aerials below.
For good Short Wave reception long 'random wire' aerial really is required to dig those distant stations out of the ether. To effectively couple such an aerial to a radio a matching unit called an ATU (Antenna Tuning Unit) can be extremely helpful. An ATU is relatively straightforward to construct and uses simple parts that are quite easy to obtain.
> Go to the ATUs page for a few pointers.
A loop aerial is extremely helpful when trying to receive long distance stations, not only will it dramatically 'boost' the signal received compared to using a portable radio's internal ferrite rod aerial because a loop aerial is much bigger than a ferrite rod, but it also has two other very useful properties: Directivity and Selectivity. Directivity is very useful in that it can often be used to 'null out' an interfering station and selectivity is useful to overcome overloading of the radio's 'front end' as the loop will tune very sharply to the required frequency will rejecting all others.
When using a communications receiver a frame aerial might be more convenient than installing a 'long wire' antenna. Although a frame aerial might not collect as large a signal as a really long wire, the directional properties are very useful for nulling out interference from unwanted stations.
A loop can be made for Medium Wave and Long Wave and can be of almost any size you wish, although it must be small enough to fit in your listening room! The bigger the area of the loop the more signal it will collect, the portable loop described below is around 40 cm in diameter and is probably the smallest size worth considering to be effective and useful.
Traditionally loop aerials have been made on large frames about 1 meter square for use with communications receivers and is essentially just one long piece of thin 'hook-up' wire wound approximately 8 to10 times around a wooden (or other non metallic) frame. The frame can be somewhere between about 50cm square and 1m square - the dimensions are not especially critical but some experimentation will be required to find the exact number of turns required for a particular size of frame so that the desired tuning range will be obtained. - As the size of the frame is increased the inductance of the windings will increase, therefore it may be found that slightly fewer turns will be needed for correct coverage. For example for the Medium Wave band a 1m square frame may only need - say - eight turns on the main winding compared with ten turns on a smaller frame.
The bigger the area of the frame, the larger the signal pick-up will be.
The wire ends of this main winding are simply connected across pretty much any standard medium wave tuning capacitor with a value of something about 300pF or 500pF or similar. Again this is not especially critical, though like the number of turns of wire it will affect the tuning range. Therefore if a very low value capacitor is used - say 150pF or 200pF - then an additional turn or more on the main winding may be required to enable the lower frequencies to be tuned. Even with a 300pF tuning capacitor it will probably be necessary to include an additional fixed capacitor - connected in parallel with the tuning capacitor - that can be switched in and out of circuit. This will enable the tuned circuit to resonate at lower frequencies and is shown in the diagrams below.
470pF may be a good value to try for the fixed capacitor, but experimentation with different values may be necessary to suit the particular value of tuning capacitor and size of frame being used.
This loop of wire and tuning capacitor form a simple 'Tuned Circuit" which can be tuned across the Medium Wave band using the variable tuning capacitor.
MW and LW Frame Aerial
1 meter square is probably the largest size that would normally be considered practical. for a frame aerial that is used in the home, but even this may be too large in some domestic circumstances. This Long Wave loop is only 55cm wide and 150cm high and is more easily accommodated in a small 'box room'.
Some Helpful Wiring Diagrams:
Applicable to all styles, shapes and sizes of loop aerial:
Basic Wiring of a Medium Wave Loop Aerial
Using a dual gang tuning capacitor to tune to lower frequencies
Increasing the low frequency tuning ability by adding
a switchable fixed 470pF capacitor
Increasing the low frequency tuning ability by
adding an additional switchable tuning capacitor
Shown Above: A close up photo showing the 'control box' and the joint of the broom handles that for the frame. The loop windings which are first taken to a tag-strip and soldered in place before the wires are taken into the control box, this help keep the windings taught. The circuit for this loop is the same as for the portable loop above except for the addition of a variable 1k resistor across the coupled output to act as a simple attenuator.
Diagram of Long Wave Loop
As shown above: For a size that might be more easily accommodated in a smaller listening room, a frame aerial can be taller than it is wide - for example between 120cm and 150cm tall and 50cm to 80cm wide.
Tuning and Operation
Find a very weak station that is almost inaudible. Couple the radio to the antenna and rotating the tuning capacitor on the frame aerial: As the resonance of the loop aerial approaches the frequency of the radio station the recovered audio will rapidly become louder and clearer until it reaches a peak. The peak of loudness will be at the exact point of resonance of the aerial - i.e. if tuned to a weak radio station on, say, 1566 Kilohertz when the audio is at its loudest the aerial will also be tuned (resonated) at 1566kHz.
The automatic gain control (a.g.c.) circuits of most radios can mask these peaking effects somewhat, but it should still be quite obvious when the point of resonance is achieved.
Two separate frame aerials will be needed to cover both Long Wave (150 to 280 kHz) and Medium Wave (510 to 1611 kHz).
With the medium wave aerial the band wave may possibly need to be covered in two parts by either having 2 tuning capacitors wired in parallel (depending on their value), or by having one tuning capacitor (of about 500pF) and a fixed capacitor that can be switched in to circuit (wired in parallel) to cover the lower part of the band.
To get the correct coverage the trick is to set the loops aerial's tuning capacitor to its minimum value (vanes open) and tune the receiver to the top of the medium wave band (1611 kHz in Europe and 1705 kHz in North America) then adjust the number of turns of wire wound around the frame until the top of the band can be tuned in and 'peaked up' by the aerial's tuning control. Since the may not be a broadcast station present it will be necessary to listen for a peak in the overall noise level.
Important: Use as many turns of wire on the loop that will still allow the top of the band to be tuned in. Don't remove a turn unless really necessary since the more turns of wire there are on the loop the better the pick up will be.
Once that is done check how far down the medium wave band the aerial will resonate.It may only tune down to 700kHz or 800 kHz for example. If that's the case then a fixed value capacitor will need to be added that can be switched into circuit - connected in parallel with the tuning capacitor. The value will vary depending upon what value tuning capacitor is being used, the size of the frame and the number of turns on the loop, but maybe somewhere between 200pF and 600pF (ish). With careful experimentation the exact value can be established that will allow a particular loop to tune down to 510 kHz - the lowest end of the medium wave band.
A Portable MW Loop Aerial
With smaller loops and a small portable radio with a built in ferrite rod ("Loopstick") aerial, it is very easy to place the radio inside the loop to obtain the best Inductive Coupling between loop aerial and the radio's own aerial. Rotating the loop and radio together for best reception by making use of the aerial's directional properties.
Above: Photo of a frame aerial sent in by an MDS975 reader
- A very fine example!
AN INCONSPICUOUS LOOP AERIAL
An Interesting Loop Aerial Idea by Alberto San Juan who writes:
I like very much your web page (radio section), being very helpful to me. I also have a Lowe HF-150 and have installed my loop antenna around the small chest of drawers next to my desk, as you can see from the photograph below.
This method of construction has many advantages:
No big frames on your table.
Strong support, so it never falls over.
Gives you extra space.
Easy to move and rotate, it also has 4 wheels!
Easy to clean.
Easy and quick to hide under the desk/table.
You can place your radio on top of the aerial windings, or connect to receiver via cable.
Alberto San Juan's Loop Aerial design
So there you are - LOOP and FRAME AERIALS - cheap and very easy to make! The portable loop certainly looks the part, being very neat and tidy. The larger frame aerials are by their very nature more obtrusive, but can be even more effective at collecting radio wave energy due to their extra size. If construction is kept neat and tidy with the rough edges rounded off and the controls housed in a neat box a big loop need not be a major eye-sore.
Good Luck with YOUR loop aerial and happy LW and MW DX-ing!
Next I'll take a look at Antenna Tuning Units which will help match your long random wire aerial to your radio. ATU's, as they are known, will help with Short Wave reception as well as Long and Medium Wave too. ATU's Aerial Tuning Units
A QUESTION ABOUT AERIALS
Hi Mike, I am in outback Queensland Australia, quite remote, so radio is very important to us out here. My use for my antenna will be to pull in distant AM broadcasts from within Queensland (200 to 500 kilometres away) and just very occasionally from interstate in outback areas of Australia. Mostly the signal may not be reflected, just very weak due to distance. Hope you understand my dilemma.
I have constructed a square loop frame with 750 mm sides (diameter) with two 320pf air gap tuning gangs. It tunes stations relatively well. My question relates to the wire length for the primary winding. I took some suggestions regarding length calculations for aerials and found most way too long for the gangs to be able to tune. I removed wraps one at a time as you suggest until I could get the higher frequencies to ‘peak’. That is all good, but the result is that I now have only six (6) turns of wire in the primary. Only 18 metres. Since a full wave is about 300 metres, I feel there ought to be at least a quarter wave of wire in the primary, 75 metres. Is there any use in increasing the length of the primary coil to 75 metres? If so how can that be done?
Your advice will be enormously appreciated. Yours faithfully, Geoff Douglas. (February 2015)
Thanks for your email.
I found the medium wave loop antenna to be an extremely rewarding project. I am sure that you will find it very useful too.
With regard to your question concerning wire length: The length of the wire does not really have quite the same relationship to wavelength that you might expect if you were dealing with more familiar receiving (and transmitting) aerials.
The aerials that many radio enthusiasts may well be familiar with are the Half Wave Dipole, the Quarter Wave Vertical Monopole or Full Wave Loop, for example.
The Half Wave Dipole is fed at the centre and consists of two quarter wave 'arms' either side of that central feedpoint. Such an antenna is often seen installed horizontally, so its form will look like a T - the vertical feeder cable feeding the two 1/4 wave 'arms'. The total length of the antenna is therefore half a wavelength long. So for a medium wave frequency of (for example) 720 kHz ( 416 metres) the aerial would be (approx*) 208 metres long.
The Quarter Wave Vertical Monopole is simply one half of a Dipole, fed against ground. So it would be (approx*) 104 metres high. The ground may be a network of radial wires and earth rods. Sometimes the vertical section may be 'loaded' to shorten it, thus reducing its efficiency. Alternatively, for reception, it may be installed as an Inverted L form: Made out of wire, the vertical section would be supported on a non metallic vertical pole as high as practicable, with the remainder of the aerial wire run out horizontally to be supported at its far end by another supporting pole, building or object.
The Full Wave Loop is, as its name suggests, is a loop of wire that is one full wavelength long at its design frequency, in this case (approx*) 416 meters long. It might be supported as high as possible above the ground on suitable poles or towers, as a horizontal circle or more often a square. The feedpoint will be a some point on its circumference.
The medium wave frame aerial is slightly different. Certainly, for a given size of former (frame) a longer length of wire will be needed for longer wavelengths. However, the loop forms a tuned circuit with the capacitor. The frequency (wavelength) that the loop will be most sensitive to is determined by the capacitance of the variable capacitor and the inductance of the loop's aerial winding: The more turns of wire on the inductor, the lower the frequency that will be attained. Also, the larger the inductor, the lower the frequency will be covered - so for a given frequency, the larger the frame the fewer number of turns will be needed.
Similarly, the larger the capacitance, the lower the frequency.
It's worth noting that the larger the frame, the larger the area, and hence the greater the signal pick-up - which is a useful consideration when trying to receive weak and distant stations. 1 metre square frames are quite popular.
You'll see, therefore, that it's not quite as simple as a quarter wavelength of wire is needed for a given wavelength!
I wonder what specific stations you are trying to receive?
I hope that helps.
(* Aerials such as a Half Wave Dipole or Quarter Wave would, in practice, be about 5% shorter than the actual calculated length)
Geoff replied: Hi Mike, Thank you for getting back to me so soon, and thank you also for the detailed explanation. It is difficult to find someone with such knowledge nowadays. Yours is a dying art among the general community.
It seems I have it working as well as can be expected. I have added a secondary single turn loop. It has helped the gain considerably. So much so that I often receive signals from Asia which drown out the Australian ones on some stations. Not the antenna’s fault I know.
I now want to add some coax so that I can mount the antenna on a stick above ground outside the building at about 10 metres height. So the receiver will be about 15 metres from the loop antenna. Is that going to work without a booster. If a booster is needed, can you suggest a good one.
Thank you so much. Appreciate your advice. I don’t have a lot of neighbours close about who I could ask for such advice.
Yours faithfully, Geoff Douglas
Hi Geoff, You can use coax to connect the loop to a radio receiver. The effectiveness can depend on the receiver itself - some tuners work quite well with a direct connection, while others may be overloaded and would work better with an inductive coupling coil placed near, or on their own ferrite bar antenna.
A booster should not be required, but it may be possible to use something like a differential amplifier, perhaps, if one was needed. I doubt it will though.
With regard to remote location; bear in mind that once placed away from the radio receiver, the loop will be tuned to just one frequency - one station. To change the frequency, you'd have to go outside every time and re-tune the 320pF capacitor! I would experiment first; the performance should be better outside, but the difference may not be worth the bother. Remember too that the aerials components would have to be fully and effectively weather sealed.
Best wishes, Mike.
Hi Mike, Thank you indeed for sharing your knowledge. It is nice to communicate with someone who is clued on these subjects.
I should have mentioned earlier, the receiver is a Sangean ATS- 909. It has a external antenna jack, as well as the ferrite rod and telescopic. It also has adjustable gain control. The frequencies I am most wanting are between 540 khz and 700 khz. There is the odd commercial station around 1100 - 1400 khz, but they are not critical.
Thank you for the heads up on the external antenna set up. I was planning to make it demountable so that it can be inside in inclement weather and in situations where it is not needed externally. We have pretty dry conditions mostly anyway, and no frosts at all.
Other folk around here are very interested in your thread. So you are helping several other than just me.
Thank you again, Cheers, Geoff.
Hi Geoff, It's worth trying a direct connection to the ATS-909 if it has a variable gain control. The usefulness of the gain control depends on where it is placed in the front end circuit. If is is placed after the first stage of any RF amplification, it may not be entirely effective with strong signals as the first stage may already be overloaded before reaching the subsequent gain control.
Overloading will be noticeable by an increase in background noise and inter-modulation problems, for example. In which case you could add a variable attenuator into the feed-line, or simply use inductive coupling to the radio's internal ferrite bar antenna.
With regard to some stations being swamped by other stations from Asia, then you can try rotating the frame aerial to null out the interfering signal. The the nulls are fairly sharp and can help reduce the unwanted station enough to make the wanted station audible. The long distance interference will no doubt be received at dusk and after dark when the lower D Layer (about 50 miles above the earth) of the ionosphere dissipates and allows signals through to be reflected from the higher F Layers (about 100 to 300 miles above the earth) in the ionosphere, causing longer distance (DX) reception of those stations.
I am glad that others are interested in this idea too!
Thanks for getting in contact.
Best wishes, Mike.
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