LOOP & FRAME AERIALS
and ANTENNA TUNING UNITS
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.
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 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.
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 acheived.
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
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