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.
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|