[RFI] Link-coupled loop - more.

[EDWARDS, EDDIE J]

Ken, 

It sounds like you are trying to track RFI like it is just ordinary RF.  It may not be acting like an ordinary RF signal.  Also, you will be seeing multiple RFI sources in multiple ways everywhere you look.

RFI on power lines can be either directly radiated from the actual source directly to the receiver, conducted down power lines (or even other types of conductors), and it can also be re-radiated from all the way along a line where it is being conducted.  Add to these facts it can also be induced from one conductor onto another conductor.

Until you get very close to an actual RFI source, you will not be using a bearing system to track the RFI because you are not receiving the direct radiation of the RFI.  You are tracking it along the transmission line that is conducting the RFI noise away from the actual source.  

So the first step is to track it along the conductor or transmission line: this requires either a lot foot work or a directional antenna mounted on the outside of a vehicle.

While tracking RFI along a conducted line, you will see variations of peaks and nulls: where you have peaks on the vertical, you will have a null on the horizontal and vice versa.  This makes using a bearing system impossible and worthless until you are close enough to the source to receive the directly radiated RFI noise.

Once you get close enough to receive the noise radiated directly and receive it at the highest possible frequency, then you can start taking bearings to see what is the actual source of the noise you are tracking.

73, de ed -K0iL

-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Kenneth G. Gordon
Sent: Tuesday, April 01, 2014 5:24 PM
To: Tom Thompson
Cc: Rfi at contesting.com
Subject: Re: [RFI] Link-coupled loop - more.

OK. My son, Brendan KB7QEU and I went out after lunch and took 8 more 
bearings with one of Tom Thompson's loops which I had constructed over 
the past few days. 

Tom's loop tunes VERY sharply. I intend to add a bandswitch, a large scale, 
and a vernier dial to it to make it both easier to peak and easier to take down 
a data point.

We carefully plotted the bearings we took on a map of our area of the city.

I connected the loop to the FT-890 in AM mode (BW 6 KHz), tuned to 3573.5 
KHz where "my" noise peaks, and used the FT-890's "S" meter to take 
readings.

First of all, the nulls are not quite as sharp, nor as deep, as those I get with a 
shielded loop, but they suffice. (The shielded loop I was using would 
commonly show a difference from null to peak of over 30 db, while the 
unshielded loop most often provides a difference of only about 10 db).

The results we plotted are somewhat confusing to me as three of the 
bearings did not result in anything common to the other five.

4 of the bearings definitely converge on an area that appears to be around 
an area of about 1 square block which is 4 blocks east of us.

A 5th is close, but outside the convergence zone of the other 4.

Signal strengths became greater as we got up on a hill to the east of my 
station which is also nearer to that 1 block area than some other bearings.

Those three bearings that gave confusing results were the weakest ones, 
and the differences between the null and a peak were only about 1 "S" unit.

However, one odd thing is that two of those three bearings, if I take a 90 
degree normal bearing to those, both show up in the same area as the 4 
"good" bearings. One, in fact, is exactly coincident at its end with two of the 
other "good" bearings.

I don't know how to read those odd ones, but I am not in the habit of 
throwing out data points just because they don't "fit" either. So I don't know 
what to do about them right now.

I suppose I should drive over to that spot 4 blocks east of us and take a look 
around, then take some more bearings.

Probably tomorrow...

Ken W7EKB.
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