[RFI] Link-coupled loop - more

[Roger Parsons]

A couple of points:

The sense circuit does not need a goniometer in its simple form. All you need is a whip antenna physically pretty much in the same place as the loop. A vertical whip is omnidirectional. That needs to be coupled into the loop - a single turn similar to the coupling to the receiver works well. The phase relationship is then correct, although you need to determine by experiment which direction is maximum and which minimum.(The sense direction is at right angles to the loop null directions.) The fly in the ointment is that the signals from the loop (broadside) and the whip should ideally have identical amplitudes. They will not, so you will need an amplifier (a few dB) in the feed from the whip.

I was bemused by the attack on me by Larry, W0QE, who cannot conceivably know how much experience I have in direction finding. I don't think it's remotely relevant, and I don't like blowing my own trumpet, but I don't like being attacked so I feel I have to tell you - or actually W0QE. 


I was originally licensed in 1962 in England and for most of the next 20 years I took part in about 10 DF contests (mostly 160m but some 80m) every year with gaps of several years when I was overseas. Each of those contests had 2 or 3 hidden stations which could each be up to 25 miles from the start. The stations were very, very well hidden, used fiendish antennas, short random transmissions, and some of the contests were at night. I won quite a lot of those competitions and was national champion twice.

I have DFd various nasty noises and spurii on many occasions in many locations in several countries. As my primary interest in amateur radio is 160m DX I have usually started on that band in trying to find the problem, but have switched to VHF as soon as practical as I got closer. I must admit that I have failed to find the nasty on a few occasions - most particularly when it was not strong enough to be heard on my DF receiver, but was still a nuisance on the main antennas.

I have also been professionally involved in underground (coal and hard rock mine) communications for much of my career. Amongst other things, I learnt quite a lot about coupling and extraneous conductors.

So I do have a teeny weeny bit of experience.

W0QE said that my "Comments made about noise in a home getting on the power lines are speculation by folks who have probably never actually done much DFing". Actually, they are not speculation. You may have noticed, Larry, that when two conductors are placed in parallel, a single wire mode signal on one conductor is transfered to the other one. Theory and practice says this must be so. You may have also noticed that most services in North America are carried on parallel overhead conductors - typically power, telephone, cable TV and an elevated 'ground' cable. There is a continual transfer of signals between each of those conductors so the fact that there may be a transformer in one of the legs is irrelevant.


In my experience it is best to triangulate bearings on a map rather than try to go straight towards the target. Others may find the latter technique works best for them.

73 Roger
VE3ZI