Diathermy inteference on 2M
 

Diathermy was mentioned in the longwire exposure thread and made me think of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M repeater
(145.41) which sounds just like the diathermy I used to hear on 10M long
ago. I could tell that it was a crude 60/120Hz. modulated signal which
swept through the repeater input frequency.
It is easily readable on the input over an area at least 3x3 miles. No one
has had the time to spend and I (with some pretty good equip) only went out
a few times (can't spend a lot of time on the way to work... & when I'm
off, it isn't very active), but have limited time as well.

.. It is more active in the morning rush hour and rarely later in the day
and probably never on weekends.


It has to be some kind of RF heating machine, right? Plastic sealing?


Here is a full symptom account to show that I know what characteristics it
has.
I took a spec analyzer out and could see the signal sweeping down at least
five MHz. (where I had it set at the time). I can hear it sweep through
sometimes slowly and sometimes quickly. It so happens that it conveniently
slows down and comes to a stop right around the repeater input - sometimes
ON the input, but spends no more than 2-3 seconds if it happens to stop
there AND stay on long enough. It has a variable time "on"and doesn't
always make it to the input freq.
Now that the repeater is using CTSS full time (last Sept or Oct another
fulltime input spur appeared!) , it doesn't bring up the repeater. However,
you can hear it as it comes into the input across the freq -- if it is
strong enough it will eventually capture and the station talking will go
away, PL detect dies, the beep goes, then this reverses as it continues
through the input freq.

--
Steve N, K,9;d, c. i My email has no u's.

"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M repeater
(145.41) which sounds just like the diathermy I used to hear on 10M long
ago. I could tell that it was a crude 60/120Hz. modulated signal which
swept through the repeater input frequency.
It is easily readable on the input over an area at least 3x3 miles. No
one
has had the time to spend and I (with some pretty good equip) only went
out
a few times (can't spend a lot of time on the way to work... & when I'm
off, it isn't very active), but have limited time as well.

. It is more active in the morning rush hour and rarely later in the day
and probably never on weekends.


It has to be some kind of RF heating machine, right? Plastic sealing?


Here is a full symptom account to show that I know what characteristics it
has.
I took a spec analyzer out and could see the signal sweeping down at least
five MHz. (where I had it set at the time). I can hear it sweep through
sometimes slowly and sometimes quickly. It so happens that it
conveniently
slows down and comes to a stop right around the repeater input - sometimes
ON the input, but spends no more than 2-3 seconds if it happens to stop
there AND stay on long enough. It has a variable time "on"and doesn't
always make it to the input freq.
Now that the repeater is using CTSS full time (last Sept or Oct another
fulltime input spur appeared!) , it doesn't bring up the repeater.
However,
you can hear it as it comes into the input across the freq -- if it is
strong enough it will eventually capture and the station talking will go
away, PL detect dies, the beep goes, then this reverses as it continues
through the input freq.

--
Steve N, K,9;d, c. i My email has no u's.



Plastic sealing, using dielectric heating, is often done with the ISM at
27.125 MHz. Loading of the machine might (but isn't supposed to) pull the
frequency. The 5th harmonic is a long way from the 2-meter band, and so is
the 6th. If the RF source is quite close to the repeater, maybe you are
seeing an image response to the 5th harmonic.

You don't need to have a heavy-industrial setting; I once tracked some RF
unhappiness in Milwaukee to a neighborhood shop making vinyl auto tops.

Ed
wb6wsn

"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M repeater
(145.41) which sounds just like the diathermy I used to hear on 10M long
ago. I could tell that it was a crude 60/120Hz. modulated signal which
swept through the repeater input frequency.


It could also be a parasitic oscillation in a VHF transmitter as well.
Pager,
police, taxi... Use the spectrum analyzer to see if any nearby activity
jives with the buzzzzzzz.

Pete

Time wise it could be an RF drier for resin.
Lexan for one has to be thoughly dried before molding which would be around
start of the shift. If the top is not fully closed or the electrician has
left the rear panel off then the R.F. would get out. The bottle used has
about 6Kv on it at about .4A so even without an antenna the interference can
be widespread in the area that you mentioned.
Art
"Ed Price" wrote in message
news:jaoXb.72863$fD.59039@fed1read02...

"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me
think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M repeater
(145.41) which sounds just like the diathermy I used to hear on 10M long
ago. I could tell that it was a crude 60/120Hz. modulated signal which
swept through the repeater input frequency.
It is easily readable on the input over an area at least 3x3 miles. No
one
has had the time to spend and I (with some pretty good equip) only went
out
a few times (can't spend a lot of time on the way to work... & when I'm
off, it isn't very active), but have limited time as well.

. It is more active in the morning rush hour and rarely later in the
day
and probably never on weekends.


It has to be some kind of RF heating machine, right? Plastic sealing?


Here is a full symptom account to show that I know what characteristics
it
has.
I took a spec analyzer out and could see the signal sweeping down at
least
five MHz. (where I had it set at the time). I can hear it sweep through
sometimes slowly and sometimes quickly. It so happens that it
conveniently
slows down and comes to a stop right around the repeater input -
sometimes
ON the input, but spends no more than 2-3 seconds if it happens to stop
there AND stay on long enough. It has a variable time "on"and doesn't
always make it to the input freq.
Now that the repeater is using CTSS full time (last Sept or Oct another
fulltime input spur appeared!) , it doesn't bring up the repeater.
However,
you can hear it as it comes into the input across the freq -- if it is
strong enough it will eventually capture and the station talking will go
away, PL detect dies, the beep goes, then this reverses as it continues
through the input freq.

--
Steve N, K,9;d, c. i My email has no u's.



Plastic sealing, using dielectric heating, is often done with the ISM at
27.125 MHz. Loading of the machine might (but isn't supposed to) pull the
frequency. The 5th harmonic is a long way from the 2-meter band, and so is
the 6th. If the RF source is quite close to the repeater, maybe you are
seeing an image response to the 5th harmonic.

You don't need to have a heavy-industrial setting; I once tracked some RF
unhappiness in Milwaukee to a neighborhood shop making vinyl auto tops.

Ed
wb6wsn

I can almost echo that experience. Some years ago, while working on an
automated frequency usage project, I noticed a fast drifting signal. It
would key up several repeaters as it drifted through their input
frequencies (in those days of too little use of "PL"). It also caused
havoc with the automated system as it made it appear that frequencies
were being used that were not being used.
I tracked it down to a dispatch transmitter used by a newspaper in
the 170 MHz range. The Motorola people would fix the radio only to have
it start up some time latter. Finally had a scheme where I would call
the radio shop technicians, many of whom were radio amateurs, and say
"guess who is back?"
Because the transmitter's frequency was above most of the Federal
frequencies, guess who it mostly interfered with.
A study a few years latter in a metropolitan area suggested that
oscillating (at an HF rate) of VHF transmitters was much too common.
Since the signals were always drifting, they just presented temporary
interference to any one user who was inclined to pass it off as
just-one-of-those-things.
Be vigilant. The oscillating transmitters are out there. 73 Mac
N8TT

--
J. Mc Laughlin - Michigan USA

"Crazy George" wrote in message
...
Steve:

Sorry to speak of your employer like this, but this is unfortunately a
true
story. I can provide names, dates and times. Doesn't have to be on a
nearby frequency. When Micor series radios were first delivered,
Motorola
told shops not to tune the finals of the base transmitters after
installation (!!!!). There was a brand new Micor high band pager here
which
had a spur on 440 when cold, but not later in the day (sound familiar
yet?).
Drifted through the 440 machine(s) input(s) all morning, buzzing away,
with
also some commercial FM broadcast content also. Two spectrum
analyzers and
3 months were necessary to nail the problem, and a quiet promise to
make
their 400 foot tower about 398 feet shorter one night got them to tune
the
$&*%* thing and fix the problem. They quoted the Motorola directive
to me
when I first contacted them. However, given my reputation, they
believed
the promise and went against Motorola's advice.

--
Crazy George
Remove N O and S P A M imbedded in return address

Mac:

I'm sad to hear that others had to go through the same experience, IIRC,
back in the early 70s. Few labs had spectrum analyzers, and almost no hams
had them, and it was very difficult to find these drifting signals
otherwise. I studied enough of the spectrum to get the idea that the type
acceptance of certain designs should never have been issued. Hopefully all
those transmitters are now defunct.

About the user reactions. Only hams, it seemed, were concerned with
actually finding and getting those problems fixed. As you observe,
commercial users just passed it off, and let the dispatchers suffer. More
ham repeaters were 'open' than commercial, but the interference was there,
nevertheless. The commercial shops found the hams a convenient whipping
boy, blaming them for the various problems. Here, the records I kept
indicated about a 50 to 1 ratio of commercial caused problems to ham caused
problems.

--
Crazy George
Remove N O and S P A M imbedded in return address


"J. McLaughlin" wrote in message
...
I can almost echo that experience. Some years ago, while working on an
automated frequency usage project, I noticed a fast drifting signal. It
would key up several repeaters as it drifted through their input
frequencies (in those days of too little use of "PL"). It also caused
havoc with the automated system as it made it appear that frequencies
were being used that were not being used.
I tracked it down to a dispatch transmitter used by a newspaper in
the 170 MHz range. The Motorola people would fix the radio only to have
it start up some time latter. Finally had a scheme where I would call
the radio shop technicians, many of whom were radio amateurs, and say
"guess who is back?"
Because the transmitter's frequency was above most of the Federal
frequencies, guess who it mostly interfered with.
A study a few years latter in a metropolitan area suggested that
oscillating (at an HF rate) of VHF transmitters was much too common.
Since the signals were always drifting, they just presented temporary
interference to any one user who was inclined to pass it off as
just-one-of-those-things.
Be vigilant. The oscillating transmitters are out there. 73 Mac
N8TT

--
J. Mc Laughlin - Michigan USA

"Crazy George" wrote in message
...
Steve:

Sorry to speak of your employer like this, but this is unfortunately a
true
story. I can provide names, dates and times. Doesn't have to be on a
nearby frequency. When Micor series radios were first delivered,
Motorola
told shops not to tune the finals of the base transmitters after
installation (!!!!). There was a brand new Micor high band pager here
which
had a spur on 440 when cold, but not later in the day (sound familiar
yet?).
Drifted through the 440 machine(s) input(s) all morning, buzzing away,
with
also some commercial FM broadcast content also. Two spectrum
analyzers and
3 months were necessary to nail the problem, and a quiet promise to
make
their 400 foot tower about 398 feet shorter one night got them to tune
the
$&*%* thing and fix the problem. They quoted the Motorola directive
to me
when I first contacted them. However, given my reputation, they
believed
the promise and went against Motorola's advice.

--
Crazy George
Remove N O and S P A M imbedded in return address

"Ed Price" wrote in message
news:jaoXb.72863$fD.59039@fed1read02...
THAT
"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me
think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M repeater
(145.41) which sounds just like the diathermy I used to hear on 10M long
ago. [snip]
Steve N, K,9;d, c. i My email has no u's.

[snip] If the RF source is quite close to the repeater, maybe you are
seeing an image response to the 5th harmonic.
wb6wsn

Not an image. Spec analizer and many other receivers hear it, as well
as other repeaters.

Steve

"Steve Nosko" wrote in message
...

"Ed Price" wrote in message
news:jaoXb.72863$fD.59039@fed1read02...
THAT
"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me
think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M
repeater
(145.41) which sounds just like the diathermy I used to hear on 10M
long
ago. [snip]
Steve N, K,9;d, c. i My email has no u's.

[snip] If the RF source is quite close to the repeater, maybe you are
seeing an image response to the 5th harmonic.
wb6wsn

Not an image. Spec analizer and many other receivers hear it, as well
as other repeaters.

Steve


Well, if you have that much time to observe the offending signal, start
DF'ing it.

Ed
wb6wsn

Ed:

I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I. I have tried to track one of these spurs
with a receiver while simultaneously attempting to get a bearing, and gave
up. The Doppler DF systems need it in the passband and frequency stable for
at least one "revolution" of the virtual antenna, which doesn't happen, so
let's hear what works.

--
Crazy George
Remove N O and S P A M imbedded in return address

"Ed Price" wrote in message
news:mXnYb.502$C21.147@fed1read07...

"Steve Nosko" wrote in message
...

"Ed Price" wrote in message
news:jaoXb.72863$fD.59039@fed1read02...
THAT
"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made me
think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M
repeater
(145.41) which sounds just like the diathermy I used to hear on 10M
long
ago. [snip]
Steve N, K,9;d, c. i My email has no u's.

[snip] If the RF source is quite close to the repeater, maybe you are
seeing an image response to the 5th harmonic.
wb6wsn

Not an image. Spec analizer and many other receivers hear it, as
well
as other repeaters.

Steve


Well, if you have that much time to observe the offending signal, start
DF'ing it.

Ed
wb6wsn

Crazy George wrote:
"I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I."

Terman says on page 1050 of his 1955 edition:
"The errors in bearing caused by downcoming horizontally polarized sky
waves can be eliminated by replacing the loop antenna with an Adcock
antenna, which in its simplest form consists of two spaced vertical
antennas, connected as shown in Fig. 26-28."

The ARRL Antenna book also gives information and says construction is
not critical.

Best regards, Richard Harrison, KB5WZI

And how does that help track a non - (frequency) - stationary signal? And
150 MHz two-way has nit to do with downcoming horizontally polarized sky
waves, I theenk.

By the way, Terman and Pettit got themselves into a heap o' grief with that
simplistic approach. I've sent them on to the archives now, but there is a
large volume of correspondence in Proc. IRE back about '47 about how that
doesn't work.
--
Crazy George
Remove N O and S P A M imbedded in return address
"Richard Harrison" wrote in message
...
Crazy George wrote:
"I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I."

Terman says on page 1050 of his 1955 edition:
"The errors in bearing caused by downcoming horizontally polarized sky
waves can be eliminated by replacing the loop antenna with an Adcock
antenna, which in its simplest form consists of two spaced vertical
antennas, connected as shown in Fig. 26-28."

The ARRL Antenna book also gives information and says construction is
not critical.

Best regards, Richard Harrison, KB5WZI

Actually, if there is an *easy* way, I'd sure like to know!

I started my DF'ing in the Navy in the late 60's, so I'm no slouch.
This is a very sticky problem; few ways to do this. All phase systems
(Doppler, TDOA) require the signal to be well centered in the IF and a well
behaved IF to boot. The amplitude systems (beam, single / dual cardioid)
are better for this, but the sweep-width of this signal makes the beam less
effective due to the change in gain as you get off the design freq.

If I had the time, I'd modify a wide band FM receiver (like a broadcast
receiver 88-108) with a *really* aggressive AFC so it would follow the bogie
along. Then a TDOA should work. Actually any would be better with this
receiver, but I would have to do a little characterization across the
expected freq range first.

I think *any antenna* and a spectrum analyzer watching signal strength is
the best bet...already done some of that. The biggest problem is that it
is most active when we have the least time to hunt.
Steve





"Crazy George" wrote in message
...
Ed:

I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I. I have tried to track one of these
spurs
with a receiver while simultaneously attempting to get a bearing, and gave
up. The Doppler DF systems need it in the passband and frequency stable
for
at least one "revolution" of the virtual antenna, which doesn't happen, so
let's hear what works.

--
Crazy George
Remove N O and S P A M imbedded in return address

"Ed Price" wrote in message
news:mXnYb.502$C21.147@fed1read07...

"Steve Nosko" wrote in message
...

"Ed Price" wrote in message
news:jaoXb.72863$fD.59039@fed1read02...
THAT
"Steve Nosko" wrote in message
...
Diathermy was mentioned in the longwire exposure thread and made
me
think
of
this long standing problem here in northern IL..

For some years now, there has been interference on the local 2M
repeater
(145.41) which sounds just like the diathermy I used to hear on
10M
long
ago. [snip]
Steve N, K,9;d, c. i My email has no u's.

[snip] If the RF source is quite close to the repeater, maybe you
are
seeing an image response to the 5th harmonic.
wb6wsn

Not an image. Spec analizer and many other receivers hear it, as
well
as other repeaters.

Steve


Well, if you have that much time to observe the offending signal, start
DF'ing it.

Ed
wb6wsn

Not sure how this applies. It is non-stationary in FREQUENCY that is the
problem.


"Richard Harrison" wrote in message
...
Crazy George wrote:
"I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I."

Terman says on page 1050 of his 1955 edition:
"The errors in bearing caused by downcoming horizontally polarized sky
waves can be eliminated by replacing the loop antenna with an Adcock
antenna, which in its simplest form consists of two spaced vertical
antennas, connected as shown in Fig. 26-28."

The ARRL Antenna book also gives information and says construction is
not critical.

Best regards, Richard Harrison, KB5WZI

Crazy George wrote:
"And how does that help track a non-frequency-stationary signal?"

It senses the line between the transmitter and receiver that contains
the path of the signal
by finding a null along that line.

You have a broadband antenna array in the Adcock which produces a null
simultaneously in horizontal and vertical polarizations.

You can have confidence in the null produced by the Adcock array.
Cross-polarized reception causes no error so long as the antenna remains
balanced regardless of the frequency of reception. The Adcock doesn`t
require self-resonance nor a definite spacing between elements. Its
bandwidth means the balance can be good throughout the 2-meter band if
that`s the design frequency.

The longest dimension can be about 40 inches which makes the antenna a
practical size for the 2-meter band.

Best regards, Richard Harrison, KB5WZI

"Steve Nosko" wrote in message
...
Actually, if there is an *easy* way, I'd sure like to know!

I started my DF'ing in the Navy in the late 60's, so I'm no slouch.
This is a very sticky problem; few ways to do this. All phase systems
(Doppler, TDOA) require the signal to be well centered in the IF and a
well
behaved IF to boot. The amplitude systems (beam, single / dual cardioid)
are better for this, but the sweep-width of this signal makes the beam
less
effective due to the change in gain as you get off the design freq.

If I had the time, I'd modify a wide band FM receiver (like a broadcast
receiver 88-108) with a *really* aggressive AFC so it would follow the
bogie
along. Then a TDOA should work. Actually any would be better with this
receiver, but I would have to do a little characterization across the
expected freq range first.

I think *any antenna* and a spectrum analyzer watching signal strength is
the best bet...already done some of that. The biggest problem is that it
is most active when we have the least time to hunt.
Steve





"Crazy George" wrote in message
...
Ed:

I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I. I have tried to track one of these
spurs
with a receiver while simultaneously attempting to get a bearing, and
gave
up. The Doppler DF systems need it in the passband and frequency stable
for
at least one "revolution" of the virtual antenna, which doesn't happen,
so
let's hear what works.

--
Crazy George


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

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

"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

"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

Bottom post:
"Ed Price" wrote in message
news:pN0Zb.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?

I can not for the life of me understand why some of you put responses at
the bottom and make one have to scan way down. Leaving that aside, this
is what I did:

I correlated which stations were, and were not, on the air at the
same time as the spurious signal. Then I was able to use the frequency
of the intended frequency to track the offending station. Our DF
capabilities were only good enough to give a sector.
73 Mac N8TT
--
J. Mc Laughlin - Michigan USA

"Crazy George" wrote in message
...
Ed:

I think Steve would be interested in hearing a methodology for DFing a
non-stationary signal, as would I. I have tried to track one of these
spurs
with a receiver while simultaneously attempting to get a bearing, and
gave
up. The Doppler DF systems need it in the passband and frequency
stable for
at least one "revolution" of the virtual antenna, which doesn't
happen, so
let's hear what works.

--
Crazy George
Remove N O and S P A M imbedded in return address

J. McLaughlin wrote:
I can not for the life of me understand why some of you put responses at
the bottom and make one have to scan way down.

Because anything else is a malicious violation of netnews guidelines?
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
J. McLaughlin wrote:
I can not for the life of me understand why some of you put responses at
the bottom and make one have to scan way down.

Because anything else is a malicious violation of netnews guidelines?
--
73, Cecil http://www.qsl.net/w5dxp

Part of the task of composing a rational response is the ability to edit the
previous post. A good response shouldn't burden you with excessive
scrolling.

Ed
wb6wsn

Ed Price wrote:
"Cecil Moore" wrote:

J. McLaughlin wrote:
I can not for the life of me understand why some of you put responses at
the bottom and make one have to scan way down.

Because anything else is a malicious violation of netnews guidelines?

Part of the task of composing a rational response is the ability to edit the
previous post. A good response shouldn't burden you with excessive
scrolling.

My posting was tongue-in-cheek but it did meet your guidelines. :-) However,
don't be surprised if you are accused of deleting the important stuff.
--
73, Cecil http://www.qsl.net/w5dxp



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