LF: Re: Balun for tuned loop antenna

["James Moritz" <james.moritz@btopenworld.com>]

Dear Daniele, LF Group,

The high impedance input of your portable should be a good match to the 
loop. It may be an advantage to have 2 or 3 turns coupling winding instead 
of a single turn, but this is best found by experiment.
It is probably the high impedance tuned winding that could benefit most by 
being balanced, rather than the receiver input. The best way to do this is 
to make a centre tap in the main loop winding, and connect this to ground. 
The tuning capacitor must not be grounded, of course. If you prefer, you 
could make a balun with a single centre-tapped winding. The centre tap would 
connect to ground, while the two ends of the balun winding connect to the 
two ends of the loop winding. The balun should have much higher inductance 
than the loop (at least 5 times, say), or it will significantly increase the 
minimum tuning frequency of the loop. The stray capacitance of the balun 
will also reduce the maximum frequency of the loop.
Personally , I have found it makes little difference to pick-up of 
interference, whether the loop is balanced or not. It is probably just as 
effective to leave the tuned winding floating. It is usually more effective 
to move the loop to a different position (often only a few metres will make 
a big difference) if QRM is a problem. Again, experimentation is the best 
way to find out.
Good luck with LF,

Cheers, Jim Moritz
73 de M0BMU