[RFI] Link-coupled loop - more

[Larry Benko]

Roger,

I will make this my last comment on this topic.

I thought about my comments of " a thousand hunts" and I certainly 
didn't do hunts 1 per week for 20 years.  What I did do was to go out in 
the woods with my wife and daughter occasionally who would take 2 walkie 
talkies and talk to each other.  They each had a beacon transmitter 
operating on 2 different frequencies.  I then tracked down beacon #1, 
which was subsequently moved, while I hunted down beacon #2, which then 
was moved etc.  The 2 beacons were about 1/4 mile or so apart in the 
woods and I could easily hunt about 25-30 beacons in a afternoon.  This 
practice was very worthwhile and I could usually find the next beacon in 
less than 5 minutes but occasionally there was one which was difficult 
to find.  Something like just over a ridge line so that knife edge 
refraction to the ridge caused a wide range of bearings to be seen would 
be problematic.  Over time and with enough practice you get very good at 
dealing with a few bad bearings along the way and recognize possible 
pitfalls.  This repertoire of experiences is incredibly valuable when DFing.

You are correct that most RFI signals have a significant vertical 
polarization component.  I have found this to be true when more than 
several hundred feet from the source but not always when close to the 
source which is why I stop trying to use a null for bearings and instead 
rely on maximum signal.  Also when close, the signal strength changes 
much more quickly for a given distance change.  I am familiar with both 
the Adcock and Wullenweber antennas which are not portable.  It would be 
interesting to get a dozen or so people who claim to be proficient at 
tracking down noise and see what equipment they use and how easily they 
find the noise source.  I'm sure not all the equipment is not equal if 
we all had a chance to use the other person's gear that would be very 
instructive.

73,
Larry, W0QE



On 4/3/2014 7:50 AM, Roger Parsons wrote:
> Larry:
>
>
> There is no magic sense circuit design of which I am aware. My first DF receiver used a DF96 1.5V filament valve as the sense amplifier. My next used a germanium transistor which worked fine until I got too close to the hidden station's antenna and all went quiet. I've updated a bit since then.
>
>
> Most locally received signals have a significant vertical polarisation component, and so the loop and sense whip can work well. The problem comes when there is a predominant horizontal component because this can skew both sense and bearing, as whilst a loop will respond to horizontally polarised signals it has no directivity in that case. A small movement will generally allow better results when one is fairly close to the target. In my experience, the real problem arises when trying to DF skywave signals, and even more so where ground wave and skywave are of similar strength.
>
> There is a magic DF antenna, invented during WW1 by Frank Adcock. However, it would be difficult (but perhaps not impossible) to make an 80m one small enough to be carried around. Even better is the Wullenweber antenna, but that is definitely not remotely portable.
>
> A thousand DF hunts is quite a lot - once a week summer and winter for nearly twenty years. I am not, and was not, saying that it is impossible to DF a source to a particular building. I was saying, and am saying, that accidental radiators usually couple into random conductors and that it is very easy indeed to be misled.
>
> 73 Roger
> VE3ZI
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