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LOOP AERIALS
And ATUs

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LOOP & FRAME AERIALS
and ANTENNA TUNING UNITS

LOOPS
A Loop or Frame aerial is a wonderful tool to assist longwave and mediumwave 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. 

ATUs
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.


LOOP AERIALS

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 instaling 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.

Construction

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

Loop Diagram
Constructional details

LW Frame
A Long Wave Loop Aerial
The illustration opposite was taken from a very old listening guide and shows the basic method of constructing a traditional style large frame aerial.  It is 40 inches (100cm) square and made of wood with the loop windings wound over the four plywood 'paddles.  I have tried this method and it works very well. Certainly the increased surface area really improves signal pick-up and is ideally suited to 'communications' receivers.

I have also experimented with different shapes, since 40 inches (100cm) can be a bit too wide for some small rooms.  My favourite is taller than it is long and is hexagonal in shape being 150cm tall and 70cm wide.

For Medium Wave reception 9 turns are required for the main winding.  A swiched capacitor to extend the tuning range could also be included if the tuning range is found not to cover the whole of the band.

Coupling or Connecting To The Receiver:

When using a portable radio with a built in ferrite rod ("Loopstick") aerial, it could be quite easy to place the radio inside the loop and therefore use 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.

However when using a communications receiver that has a dedicated aerial socket or antenna terminals then the frame aerial must be connected to the radio by a cable. Since a communications receiver will not, in most cases, have a ferrite rod / loopstick antenna, the frame aerial cannot be inductively coupled to the radio in the same way as for an ordinary portable radio:

Therefore a second loop of wire - the inductive coupling loop - is wound over the main winding: This is just one single turn of wire, the ends of which can form a 'fly-lead' that can be connected to the antenna terminals on the receiver. This single turn of wire is shown in blue in the diagrams a little further down this page.

Long Wave

For use for Long Wave reception approximately 30 turns will be needed. The exact number will be established by a little experimentation and adjustment. The photo on the left shows a Long Wave loop aerial.  The windings are wound over the 'paddles' described above and consist of 31 turns plus the one coupling turn.

The frame is made from broom handles which are joined together using halving joints and a screw.  The base is used for this aerial and a Medium Wave loop aerial and is made from an offcut of kitchen worktop which is dense and heavy.  A block is screwed to the base with a hole bored in it to suit the diameter of the 'broom handle' frames.

The tuning capacitor, switch and sockets are neatly housed in a plasic enclosure of the same type as the one used for the portable loop described above.
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
Basic Wiring of a Medium Wave Loop Aerial


Medium Wave Loop Aerial - Using a dual gang tuning capacitor to tune to lower frequencies
Using a dual gang tuning capacitor to tune to lower frequencies


Medium Wave Loop Aerial - Increasing the low frequency tuning ability by adding a switchable 470pF capacitor
Increasing the low frequency tuning ability by adding
a switchable fixed 470pF capacitor


Medium Wave Loop Aerial - Increasing the low frequency tuning ability by adding an additional switchable tuning capacitor
Increasing the low frequency tuning ability by
adding an additional switchable tuning capacitor




LW Loop
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.


LW Loop
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 acheived.


Band Coverage

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.


The portable loop in use, the portable radio is simply placed into the centre of the loop and the signals collected are inductively coupled to the internal ferrite rod antenna of the radio

The Portable MW Loop Aerial, shown to the left, is much smaller than many traditional frame aerials at 40cm (17") diameter. It is designed for use with a portable radio.  The radio is simply placed in the middle of the loop and the signals collected are transferred to the radio via its internal ferrite rod aerial.

The circuit for a loop aerial could not be simpler:  A continuous length of  wire, such as 7/0.2mm 'hook-up' wire, is wound around the 40cm former to form a loop of approximately 10 turns*. The  tuning capacitor is connected across the loop so that it can be adjusted to resonate at different frequencies. 

(*about 40 to 50 turns for Long Wave).

If it is required to connect the loop to a radio via its aerial input terminal then a second winding of just 1 turn of wire is wound over the main 10 turn winding.  This secondary winding acts as an inductive coupling coil - this is then connected to a suitable socket so that a cable  ('fly lead') can be run from the loop aerial to the radio receiver's antenna jack or terminals.


The circuit diagram of the loop showing the 10 turn main winding (100uH) and the tuning capacitor, together with a second capacitor that can be switched into circuit to provide tuning of the lower frequencies of the medium wave band.  The second 1 turn coupling winding allows direct connection to the aerial terminals of a receiver.

For Long Wave reception about 40 to 50 turns may be required.

The circuit diagram on the left shows the main loop winding of ten turns (100uH) and the variable capacitor which tunes the loop aerial to the required frequency.  Ideally the tuning capacitor should have a value of 700pF to cover the whole of the medium wave band. However standard 500pF tuning capacitors seem to be more widely available and will generally tune the medium wave band from around 700 kHz to 1600 kHz with a 10 turn winding. 

To Tune the lower portion of the band a second capacitor can be switched into the circuit to provide the increased capacity required.  The second capacitor can be in the form of a variable trimmer that can be pre-set to the required value, usually around 200pF.  The second capacitor could be another tuning capacitor (as shown in the diagram), but that could be rather expensive.  Alternatively a fixed capacitor could be used, the best value determined after a little experimentation. 

The second coupling winding is of one turn and allows the aerial to be direcly connected to any radio with antenna terminals or and aerial socket.

Scanned in below are my notes for the construction of the portable loop.  The former of the loop was made out of 'Trent' plastic edge strip that was available from my local DIY store.  This edging is about 20mm wide with a 6mm channel, though any similar edging or plastic product such as curtain track, perhaps, could be used.  The strip is bent into a circle of 40 cm in diameter with the channel on the outerside and fastened to a wooden block with some large head screws.  The 10 turns of 7/0.2mm hook up wire are carefully wound side by side around the former and connected to the tuning capacitor.  I used red strip and blue wire to be colourful. 

The single turn coupling winding is wound next to the main winding and connected to the output socket.  I simply used a 3.5mm jack socket as this is the same as on a Sony portable radio, though any coaxial socket could be used such as Belling Lee or PL259 etc.

The loop and wooden block are fixed into a suitable plastic box of about 150mm x 100mm x 60mm, the wooden block and heavy tuning capacitor adding weight to aid stability.  A suitable box would be BOX034 from Bowood Electronics.


Drawing showing the external appearance of the Portable Loop Aerial

Fixings
Drawing showing how the loop is fixed to a wooden block and secured into the enclosure

Internal
Internal photo of loop aerial showing air-spaced Tuning Capacitor (bottom left), Range Switch (top left), Output Socket (top), Wooden Block to which the loop former is attached (centre).

[Note the 3.5mm jack socket on the back panel (top right), this is for a crystal earphone as this loop is also a portable crystal set - see below]


Enclosure
A suitable box with cut-outs to accommodate the entry of the loop into the box and holes for tuning capacitor, switch and output socket.

Tuning capacitor
A solid Dielectric Jackson type tuning capacitor can simply be mounted through the front of the box and held in place with the brass nut.
Tuning capacitor
A traditional air-spaced tuning capacitor can be glued to a wooden block using Araldite which is then screwed to the bottom of the box.

Connections
Connecting a loop to a radio receiver or Hi-Fi tuner



A Crystal Set Loop!

Crystal Set
Crystal Set Loop!

The circuit diagram above is an interesting modification to the loop aerial, and can be made to any loop aerial.  With the addition of a germanium diode (not silicon) such as an OA81, OA91 or OA47, a 47k ohm resistor and a crystal earphone, the loop aerial becomes a portable crystal set which is quite effective given sufficient signal strength at your locality.

If signal strength is low then a long wire aerial could be connected to the crystal set and used in a more standard crystal set configuration. Such a long wire aerial could be coupled to the tuned circuit via a small trimmer capacitor of, say, 100pF. The capacitor would be connected to the top of the tuned circuit ("hot" side) at the junction of the main loop, tuning capacitor and diode. Alternatively the aerial could be connected by inductive coupling and would be acheived by adding a second winding around the loop consisting of, for example, one, two or three turns of wire: One end of this wire would be connected to the long wire aerial and the other end of the loop to the 'earthy' side of the tuned circuit (junction of main loop, tuning capacitor, 47k resistor and earphone in the above diagram.)


In Use

The Loop In Use
The Portable Loop in use!
The photo on the left shows the finished Portable Medium Wave Loop in use, in this case merely placing the radio inside the loop will obtain much improved reception!


The loop is tuned to the frequency of the required radio station with the tuning knob which will really peak up the reception. 


Rotating  the loop will maximise the signal strength &/or minimise co-channel or adjacent channel interference for clearer reception.


Using this loop I can hear distant local stations that would otherwise be completely impossible to receive and it helps improve reception on all other weak stations.  It's a nice little project that produces a really useful listening aid.


Once you've built this little beauty you may want to try something a larger frame aerial design shown near the top of this page.






LOOPS FOR HIGHER FREQUENCIES

I have done a little experimentation with loops above 1.8 Mhz, but not too much.  My observations are thus:

It will take a little experimentation with the number of windings on the loop to get the required frequency coverage.  The actual number depends on the size of the frame that you are using.  To be able to resonate these higher frequencies the loop aerial requires fewer turns than the standard medium wave loop.  Try removing 2 or 3 turns of winding from a medium wave loop and see where that gets you:  Then subtract one turn at a time until the desired frequency range is achieved.  Ideally the starting point of tuning at the lower end of the range should be around 1.8 Mhz with the tuning range going up to about 3 MHz.  This should cover some interesting transmissions including the "Top Band" of the Amateur Radio  ('HAM') bands.

The coupling loop remains at one single turn whatever frequency range you are receiving.

It is important to note, however, that the fewer turns that the loop has (i.e. in order to tune to these higher frequencies) the less signal will actually be picked up - in my experience this reduced signal pick up has been quite noticeable!  I found that for acceptable pick-up the frame needed to be at least 1m square.  Smaller loops just don't present enough signal.  It is for this reason that loops designed for long wave reception will necessarily have many more turns than a medium wave loop and will therefore be much more effective at collecting signals.

A loop for the higher frequencies above the medium wave band could be useful for direction finding and particularly for reducing co-channel interference (assuming the interfering station is at about 90 degrees to the required station), but for chasing weak signals a long random wire, possibly with an ATU, will be much better from my observations.  An interesting experiment none-the-less.


A Reader's Frame Aerial
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.

Inconspicuous aerial
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





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