PA3CNJ Magnetic loop
A loop antenna is called 'Magnetic Loop' if the circumference of the loop is smaller
then 0.35 x the wavelength. It consists of a loop radiator made of copper or aluminium
tubing and a tuning capacitor. Note that these are the materials of choice for loop
construction. It is also possible to ie. use a bicycle rim, coaxial cable etc. The diameter
of a loop is usually in the range 1/10 to 1/100 of the wavelength. Even for the small
size such a loop constitutes, it can be an extremely efficient antenna.
The bandwidth is always very small and covers only a few KHz. The high Q of the
antenna allows a selective receiption and suppresses effectively QRM of nearby BC
stations, as well as other QRM.
The schematics of a loop antenna are shown here. The mainloop is an LC circuit
where L consists of just a single turn (it is possible to construct multiple turn
loop antennas for the lower frequencies). The current distribution is equal in the
whole loop. The coupling shown in the schematic is an inductive one, using a smaller
loop which is 5 times smaller then the main loop. There are more ways though : I myself
use a gamma match (see images) for easy of construction.
The radiation resistance of the antenna is very low. It is therefore important to keep
the DC losses as low as possible, preferably in the milliOhm range. Hence my choice for
copper pipe where all connections can be thoroughly soldered.
Feeding the loop
There are a couple of different ways to feed a magnetic loop. One of the most popular
methods is using what is called a Faraday loop. Such a loop can easily be constructed from
coax. A Faraday loop basically has a diameter that is 1/5 of the diameter of the loop. So
say you build yourself a 1m loop, use a 20cm Faraday loop. The picture below shows how
to construct one out of coax :
People say that this kind of feed seems to pickup less QRM as it is a shielded loop. I have tried
a Faraday loop myself and in comparisson with the currently used gamma match, I could find
a noticable difference. In order to get the SWR down to a minimum, you would have to
play with the shape of the Faraday loop. The Faraday loop has no electrical connection with
the main loop, it just inductively couples.
The Faraday- or shielded loop should be placed opposite the tuning capacitor.
The gamma match I am using is easier to construct, I think the pictures are self explanatory.
To tune for lowest SWR, I can just adjust the clamp and slide it around on the main loop. Once
the right point has been established, this will remain the same for all frequencies the loop
is used for.
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