[RFI] Link-coupled loop - more.

[Larry Benko]

Ken

You are continuing to over think the DFing .  I'm 100% sure I could find 
your noise sources relatively  quickly.  Why not try the following?

1.) Build a small noise generator (I posted one last week).

2.) Have your son plug it into an outlet in your house and you try to 
find it.  No maps with vectors drawn.  Just go find it from a few blocks 
away.  Do it several times coming from different directions. Then get 
him to hide it at a nearby friends house.  Again find it and do it 
several times.

3.) Attach a 6ft. wire to each end of the noise source.  Have your son 
hang it vertically from a tree in some park or wooded area. Again find 
it and notice how easy it is to find.  Then get your son to mount it 
horizontally (tie the 2 ends to some trees).  Now notice how it is easy 
to find from the end and the bearings are off 90 degrees from the 
broadside.  This is because the loop has the nulls 90 degrees apart for 
vertical and horizontal polarizations.  All horizontal dipoles are 
vertically polarized from the ends.  This is a fact of life and no loop 
shielding or any thing else is going to change this.  The horizontally 
polarized signal is much harder to find.  Fortunately this is generally 
only a problem in close unless the horizontal antenna is high off the 
ground.

4.) Comments made about noise in a home getting on the power lines are 
speculation by folks who have probably never actually done much DFing.  
The transformers that feed houses attenuate the noise considerably.  
Again do your own test with a controlled noise source and draw your own 
conclusions.

Other comments:

1.) The loop is hard to adjust!  Duh, you wanted to hear the signal from 
the start and the only way to achieve that is to make the loop large or 
to resonate the loop and increase the Q.  High Q is your friend and the 
adjustment should only take a second and it doesn't have to be perfect.  
Also the ability to de-tune the loop acts like having an attenuator 
which is needed when you get close to the source.  Again a DPDT toggle 
switch with a couple of padding capacitors can give you 20-380pF, 
290-650pF, and 570-930pF (20-380pF var. cap. and either one or two 270pF 
silver mica caps.  Again rebuilding the loop is just a waste of time IMO 
until you have actually used it to successfully track down numerous  
sources.

2.) Often I will use the nulls from the antenna when far away but when 
getting close use the peaks.  This means that you need to be comfortable 
with changing gears mentally and need a good signal strength indicator 
on your radio.  The downside of using the nulls is the 180 degree 
ambiguity and that you can go past the source.

3.) Your ability to become good at tracking down noise is NOT going to 
be enhanced by getting advice but by actually hunting stuff down and 
that means all the way to the source.  This is why you need your OWN 
CONTROLLABLE NOISE SOURCE.

73,
Larry, W0QE



On 4/1/2014 4:24 PM, Kenneth G. Gordon wrote:
> 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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