Bottom post:
"Ed Price" wrote in message
news
N0Zb.3320$C21.1209@fed1read07...
"Steve Nosko" wrote in message
...
"Gary Schafer" wrote in message
...
On Wed, 18 Feb 2004 03:17:37 -0800, "Ed Price"
wrote:
I agree with Steve's observations (although I prefer that he
bottom-post),
but I would still try to keep the process simple.
First, you know the very limited frequency range of the offending
signal
(you said it slowly drifts across your repeater input at 145.41 MHz).
So
that means you know which way it drifts, about how fast the frequency
slew
is, and about how far it shifts. Further, you know the characteristic
modulation on the signal. And, to make it even easier, the signal
isn't
playing tricks on you to spoof your search. Finally, you must have
some
idea
by now as to when the signal is most likely to be present.
Time to get a directional antenna (Yagi, log periodic, a loop
resonant
to
145 MHz, even an inefficient horn) and look for the signal. I would
use
a
spectrum analyzer (those HP boxes are too big and heavy, so look for
a
Leader or Anritsu. Why not turn this into a real learning experience?
Why
not build a 2 meter version of the Poor Man's Spectrum Analyzer into
a
briefcase, maybe updating the display by using a small laptop?
As a first assumption, I suppose we can assume that the offending
signal
isn't in a vehicle. So get out and grab a bearing when you can.
Obviously,
you have to be prepared to get into the field quickly and efficiently
to
make your observations. After a couple of bearings, at least you will
guess
which side of town to look first.
Hey, if this was easy, then T-hunts would be no fun, and clandestine
radio
would be a short path to a bullet.
Ed
wb6wsn
It sounds a lot easier than it is. Sometimes the interference that you
hear is the result of a spur from one transmitter getting into another
and the IM product of the second is what you hear. Try and track that
one down!
The most successful way I have seen is to first find the general
direction the problem is coming from (spectrum analyzer and
directional antenna) and then spending many hours watching the
spectrum analyzer to see what transmitters come on the air when the
interference is present.
Problem is that they are not always there. Temperature difference at a
particular site can cause the spur to change or not be there. Amount
of use of the transmitter can make a difference. These things can
drift across the whole VHF band or more.
73
Gary K4FMX
OK. Got it. Remember to not eliminate Tx spur / IM. We had one spur
occurrence last year and the two repeaters were 20 miles apart...but it
was
a pretty bit Tx spur. Interesting how this PA spur just landed on the
other
repeater input so nicely.
Steve
If it had landed on some other frequency, you would never have noticed it.
The odds of it affecting you were really quite low, despite Murphy's Law.
And logically, there's nearly an infinite number of other problems that
could happen, given just the right tweak or failure someplace.
Ed
wb6wsn
Yea. sorta' except that it sweeps through repeater outputs and other
inputs, just not coming to rest on them, ever.
Weird.
P.S. Did you know that Murphy's law was discovered by a *DIFFERENT* Murhy?