[RFI] Link-coupled loop - and DFing...progress

[Kenneth G. Gordon]

On 1 Apr 2014 at 17:57, Roger Parsons wrote:

> Your noise is
> almost certainly being re-radiated from a power line or other overhead conductor
> even if it originates within a building.

I agree.

> When you are DFing your noise you should make sure that you are as far away from
> any overhead conductors as possible. If you get ambiguous bearings go further
> out from your suspected site to a clear area - this is really important and does
> help a lot.

I agree with this also. Trouble is, there are overhead power lines literally 
everywhere in this area.
 
> It is often helpful to have a sense antenna as well as the loop. When an
> omnidirectional antenna, such as a whip, is combined with the loop and with the
> correct relative amplitudes then a cardioid pattern results. 

Yes. I know about those, and have used them, but I believe they require a 
variable phasing device, like a goniometer, in order to make them effective.

I had a DAG-1 once. With its sense antenna, the 180 degree ambiguity was 
non-existent.

> I think that the major benefit of a shielded loop (and a balun) is that the two
> nulls will be closer to 180 degrees apart. With an unshielded loop it is harder
> to maintain balance and so the nulls may have several degrees of difference
> between the two directions.

OK. Thanks. I have been using only one "side" of my loop since I thought 
that would at least eliminate one extra variable. 

It wouldn't be all that difficult to build a shielded variety of this loop: all it 
would take is a piece of RG-8 about 6' long, a small box, and some RF 
connectors. Coupling to it would be a problem though.

> It has often been pointed out on this forum that it is highly desirable to use
> the highest frequency possible to track down noise sources - VHF is best if the
> signal can still be detected there.

It cannot, yet. I am beginning on that frequency, 3573.5 KHz, which is most 
annoying.

> Relatively low frequencies such as 80m are
> horrible in built up areas just because of the number of conductors around.

Yes. Tons of them! On every street corner.

> Sorry if this has all been mentioned before, and good luck in your hunting.

Not a problem. I am happy to be reminded, and thanks.

Tonight, I decided to go to the area that my bearings seemed to converge 
on, and take a closer listen.

I did find a very strong RFI source near a Church, but a bit south of it, right in 
the area, about 4 blocks away, where my bearings converged.

I also found standing waves of the noise.

Signal strength on the loop alone is now about S-9 in that area, while it is S-7 
at max in my back yard.

I also discovered that one of my "odd" bearings had been misread by our 
son, and it too points in the same general vicinity of the other 4 "good" 
bearings. So that is now 6 out of 8 bearings that all end up in the same 1/2 
block area.

I'll go back tomorrow when it is lighter out, to do some more searching.

I'll try to post another report tomorrow.

Ken W7EKB