Monday, 2 March 2015

FM Loop Antenna Design for DX

Simple FM Loop Antenna

Not everybody can afford elaborate aerials.  Nor does everyone has the space for large aerial arrays.  So how about an aerial which is small, inexpensive and, more importantly, actually performs well?

For years I have been singing the praises of a simple aerial called an "FM Loop".  I often recommend this little antenna to newcomers to the hobby since they are very simple to construct, cost next to nothing and, more importantly, perform very well considering their simplicity.  How well?  A single 33 inch square (84cm) turn of coax typically has around 5.5 to 6dB gain on its own, which will compare very favourably to many 5 element yagis!  33 inches (84cm) is a quarter wave for the FM broadcast band but you could easily adapt the sizes to suit other bands.  The loop is wideband and therefore suitable for the whole of the FM broadcast band and you can use it horizontally (coax feed point at the bottom) or vertically (coax feed point at the side).

I used to use an FM loop in my bedroom years ago for the purpose of obtaining RDS from meteor scatter signals on the FM broadcast band.  That's right - it would often produce PI codes and PS names on my receiver's display, but the loop was just fixed to the wall!  So why aren't more of us using these aerials?  I don't know!  But I decided to put one to the test while out hilltopping one night.  The resulting log was impressive to say conditions were as good as "flat" and the loop itself was only mounted around 8 feet above the ground, resting on a tree branch!

The reason this article has been published is because the aforementioned log attracted a lot of interest and members wanted to know how to construct the FM loop to try out for themselves.

The history bit
The FM loop is simply a single element cubical quad antenna.  I have seen it described in other articles as "an excellent antenna".  Perhaps it would be simpler to compare its properties with those of the medium wave loop.  It is of a slightly smaller size at band 2 frequencies but has the same figure-of-eight polar response and so receives both "front and back" (180 degrees apart) with side nulls 90 degrees in between.  The side nulls are actually very deep.  You can make this loop more directional by adding appropriately sized and positioned "reflector" and "director" loops as with a yagi though am only covering details of a single element loop for this article.  As with all aerials, the best results will be achieved if it is mounted clear of objects, though this little gem actually still performs pretty well in confined spaces, indoors, and even leaning against a wall or stood up on the ground.  Because of it's characteristics it is not easily detuned by being close to nearby objects.

The construction bit
This aerial uses Bodgitt & Scarper technology.  It is so simple even I knocked one up in a matter of minutes.  You don't need to rely on my design, however, you may wish to make the construction a lot more sturdy, but the purpose of this article is to show how you can achieve good results with the simplest of constructional effort.  If you are going to use the loop externally for an extended length of time you would need to give it more strength.  I constructed the loop quickly to use in mobile situations where I just throw the loop up into a tree or hang it on a branch.  I will get round to constructing something more solid one day - perhaps!  This is what I used ...

*  Square wooden dowelling around 2cm thick (around half an inch). Enough for four x
    84cm lengths = 336cm total (or 4 x 33 inch lengths = 132 inches total).
    half an inch square).
*  At least 15 metres (50 feet) of good quality 75 ohm coax cable
*  Screw terminal blocks
*  Sticky tape - duct / gaffer variety
Alternatively, you could construct the loop out of plumbers copper pipe instead of coax and some 90 degree corner joints.  Make sure you insulate the copper though with thick tape though if you intend to hang the loop on tree branches.  Any physical contact *may* degrade performance.


The receiving element itself is a 33 inch 84cm square loop of the coax, so you will need to construct a suitably sized wooden frame, say 10mm (half an inch) less each side of the wooden dowelling around which you will wrap the coax - this is to make allowances for the coax which will then be the correct size.  The coax needs to have a feed point half way along the bottom of the frame so cut coax to a 336cm (11 feet) length and wrap this around the frame ensuring the inner core and outer (braid) are connected together, each end going into separate connections on a small electrical screw terminal block.  The remainder of the coax will be used for the downlead - you can vary the length accordingly.  Connect the two ends of the loop to either side of the coax downlead via the terminal block, either way around, it doesn't matter.

The impedance of the FM Loop is 75 ohms.  There has been much debate about this but this is stated in ARRL and RSGB information.  Also, recent computer modelling has given the impedance as 75 ohms at resonance too.  The loop should be positioned broadside to the transmitter for best pick-up, while the nulls can be found endways on.  The loop also seems to have other quirky attributes, exhibiting a slightly unidirectional pattern at times, but these quirks can be of great use.

So, in conclusion, this is aerial is very easy to construct.  It is very efficient and seriously outperforms my FM whip with several extra dB of signal.  As I have said, this loop has a big advantage in that it works well indoors and doesn't seem to mind too much where it is located, making it an ideal aerial for beginners or those with no room or availability for roof mounted antennas.  It doesn't look like much af a DX aerial but I have been very surprised by its performance and it has become an essential part of my mobile DX setup.

Used indoors at home I have been able to eliminate Lincs FM on 102.2 and leave Galaxy from Birmingham at noise-free listenable strength - something none of my other yagis or rooftop beams have ever been able to achieve.
Above:  Simple FM loop antenna.
Above:  The coax connection to the aerial.

John Faulkner

Source: Source of Information

Archieved Skywaves Website 
Archieved Working Skywaves Website

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