Desensing of FM band

[Brian Gaff]

Somewhere around here is a very strong carrier that has the effect on most
FM radios of making the radio less sensitive. Its switching on and off every
few seconds and has been doing so for at least a couple of years. Its most
obvious on weaker stations and worse on the low end of the band, but here is
the thing. I cannot find this carrier on my scanner, though there do appear
to be some cross modulated birdies switching on and off at the same rate.
You can tell these are overload noises as an attenuator removes them.
Its very strange and seems to be mainly very local.
Seems to me it must either be a massive signal to have such a broadband
effect, yet I cannot find it. I was wondering if it was some industrial
process, as there are a lot of small businesses about. When the whether is
warmer I'll have to take a portable fm radio around to see where its coming
from.


Brian

[Jim Lesurf]

Could it be getting in via the IF rather than at VHF input? The result
would then look quite 'broadband' but not show up on scanning the VHf
spectrum.

Most radios may have little protection against that.

Jim

Post by Brian Gaff
Somewhere around here is a very strong carrier that has the effect on
most FM radios of making the radio less sensitive. Its switching on and
off every few seconds and has been doing so for at least a couple of
years. Its most obvious on weaker stations and worse on the low end of
the band, but here is the thing. I cannot find this carrier on my
scanner, though there do appear to be some cross modulated birdies
switching on and off at the same rate. You can tell these are overload
noises as an attenuator removes them. Its very strange and seems to be
mainly very local. Seems to me it must either be a massive signal to
have such a broadband effect, yet I cannot find it. I was wondering if
it was some industrial process, as there are a lot of small businesses
about. When the whether is warmer I'll have to take a portable fm radio
around to see where its coming from.
Brian

[Brian Gaff]

No the Icom is pretty well screened from this sort of thing. Its a multi
conversion radio in any case. I'm no thinking of a nearby rf heating
machine that works on a cycle, but the thing is, why is it there on a
Sunday.
.
Brian
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Post by Jim Lesurf
Could it be getting in via the IF rather than at VHF input? The result
would then look quite 'broadband' but not show up on scanning the VHf
spectrum.
Most radios may have little protection against that.
Jim

Post by Brian Gaff
Somewhere around here is a very strong carrier that has the effect on
most FM radios of making the radio less sensitive. Its switching on and
off every few seconds and has been doing so for at least a couple of
years. Its most obvious on weaker stations and worse on the low end of
the band, but here is the thing. I cannot find this carrier on my
scanner, though there do appear to be some cross modulated birdies
switching on and off at the same rate. You can tell these are overload
noises as an attenuator removes them. Its very strange and seems to be
mainly very local. Seems to me it must either be a massive signal to
have such a broadband effect, yet I cannot find it. I was wondering if
it was some industrial process, as there are a lot of small businesses
about. When the whether is warmer I'll have to take a portable fm radio
around to see where its coming from.
Brian

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Electronics
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biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html

[Brian Gaff]

The effect is much reduced this week and sounds like the carriers are
either in different locations or maybe the direction of a beam is different
as it pulses. I still cannot find a fundamental, but it has to be there as
you can hear the effect on a portable at the low end of the fm band.
Brian
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Post by Brian Gaff
No the Icom is pretty well screened from this sort of thing. Its a multi
conversion radio in any case. I'm no thinking of a nearby rf heating
machine that works on a cycle, but the thing is, why is it there on a
Sunday.
.
Brian
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The Sofa of Brian Gaff...
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Note this Signature is meaningless.!

Post by Jim Lesurf
Could it be getting in via the IF rather than at VHF input? The result
would then look quite 'broadband' but not show up on scanning the VHf
spectrum.
Most radios may have little protection against that.
Jim

Post by Brian Gaff
Somewhere around here is a very strong carrier that has the effect on
most FM radios of making the radio less sensitive. Its switching on and
off every few seconds and has been doing so for at least a couple of
years. Its most obvious on weaker stations and worse on the low end of
the band, but here is the thing. I cannot find this carrier on my
scanner, though there do appear to be some cross modulated birdies
switching on and off at the same rate. You can tell these are overload
noises as an attenuator removes them. Its very strange and seems to be
mainly very local. Seems to me it must either be a massive signal to
have such a broadband effect, yet I cannot find it. I was wondering if
it was some industrial process, as there are a lot of small businesses
about. When the whether is warmer I'll have to take a portable fm radio
around to see where its coming from.
Brian

--
Please use the address on the audiomisc page if you wish to email me.
Electronics
https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html

[Jim Lesurf]

This would perhaps be a case where a specan and/or sdr would be handy for
investigations...

I recall a case perhaps similar to this from some years ago. Taxidrivers in
town kept finding interference when in a given small area of town. Turned
out to be a self-service machine in one of the shops had started radiating
RF! Took ages to track down as it only did it when used in a particular way
by customers!

Jim

Post by Brian Gaff
The effect is much reduced this week and sounds like the carriers are
either in different locations or maybe the direction of a beam is
different as it pulses. I still cannot find a fundamental, but it has to
be there as you can hear the effect on a portable at the low end of the
fm band. Brian

[NY]

Post by Jim Lesurf
This would perhaps be a case where a specan and/or sdr would be handy for
investigations...
I recall a case perhaps similar to this from some years ago. Taxidrivers in
town kept finding interference when in a given small area of town. Turned
out to be a self-service machine in one of the shops had started radiating
RF! Took ages to track down as it only did it when used in a particular way
by customers!
Jim

Post by Brian Gaff
The effect is much reduced this week and sounds like the carriers are
either in different locations or maybe the direction of a beam is
different as it pulses. I still cannot find a fundamental, but it has to
be there as you can hear the effect on a portable at the low end of the
fm band. Brian

Reminds me of a problem that I had with my video recorder which had a
lot of corruption on channels that were on the PSB1 mux, but was fine on
all others.

It took me an embarrassingly long time to work out that it only happened
at night, and even after that, to work out that it wasn't an after-dark
reception problem but something much more obvious - at night you tend to
switch lights on.

My study, which is lit by a row of GU10 LED bulbs pointing in various
directions, is directly below the TV aerial mast. After a long time, I
worked out that reception got much worse as soon as I turned the study
lights on and got better as soon as I turned them off.

From there it was a process of elimination to remove the bulbs one by
one and find the rogue ones - plural, because two of the same make
seemed to splat all over 482 MHz, Belmont's PSB1, but leave other muxes
near by (490 COM5 and 506 PSB2) untouched.

Replacing those rogue bulbs with ones of a different make solved the
problem. The rogue ones are fine elsewhere in the house, so I could
still use them as long as it was in a different room.

[JMB99]

Post by NY
It took me an embarrassingly long time to work out that it only happened
at night, and even after that, to work out that it wasn't an after-dark
reception problem but something much more obvious - at night you tend to
switch lights on.

There is a story of a WWII radar station in North Wales (CHL?) that kept
getting problems with interference.

It was eventually found to be the bulb in the light in the WAAF's latrine.

They changed the bull and all was OK.

[Davey]

On Thu, 7 Mar 2024 15:21:06 +0000

Post by JMB99

Post by NY
It took me an embarrassingly long time to work out that it only
happened at night, and even after that, to work out that it wasn't
an after-dark reception problem but something much more obvious -
at night you tend to switch lights on.

There is a story of a WWII radar station in North Wales (CHL?) that
kept getting problems with interference.
It was eventually found to be the bulb in the light in the WAAF's latrine.
They changed the bull and all was OK.

Back in my student days, we had one cheap 2nd hand TV set for our digs.
I often took it back home for the holidays.
One evening, it let all the smoke out, along with some spectacular
sparks, and it was later reported that at that time, all TV sets in the
street had suffered unexplained interference for several minutes.
I said nothing.

We also had one that stank of curry, until it had burned off, but that
one lasted a while.

[Ian Jackson]

Post by JMB99

Post by NY
It took me an embarrassingly long time to work out that it only
happened at night, and even after that, to work out that it wasn't an
after-dark reception problem but something much more obvious - at
night you tend to switch lights on.

There is a story of a WWII radar station in North Wales (CHL?) that
kept getting problems with interference.
It was eventually found to be the bulb in the light in the WAAF's latrine.
They changed the bull and all was OK.

In the early 60s our evening TV viewing (BBC 405-line, channel 5, around
65MHz) was regularly ruined by a faulty light bulb in a neighbour's
house, upstairs on the landing next to their young child's bedroom. The
interference took the form of a searing horizontal line that drifted
slowly up or down the picture.

The parents used to leave the light on until the child fell asleep, and
it was purely by luck that I noticed that the interference coincided
with the light being on, and disappeared immediately the light was
switched off.

Almost certainly the interference was being caused by Barkhausen-Kurz
oscillations in the bulb, which I believe can occur if there is a minute
arcing break in the filament.

The up-till-then friendly neighbour (who, at the time, didn't have TV)
got very huffy when approached with a view to getting the light bulb
changed - and absolutely refused to do anything about it. Fortunately,
after a few weeks the interference suddenly disappeared - presumably
because the bulb had failed and been replaced. [Interestingly, a few
weeks later, the family finally got a TV.]

https://www.ofcom.org.uk/news-centre/2020/interference-issue

[Brian Gaff]

I'm thinking along the lines of a mixing product due to overload of the
front end. That means the field of frequencies to look on is very much
wider. Add some harmonics as well, and all sorts of odd things occur. Most
FM tuner aerial stages are quite broadband, even though they are tuned, that
does not preclude mixing products from getting in.
Brian
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Post by Ian Jackson

Post by JMB99

Post by NY
It took me an embarrassingly long time to work out that it only happened
at night, and even after that, to work out that it wasn't an after-dark
reception problem but something much more obvious - at night you tend to
switch lights on.

There is a story of a WWII radar station in North Wales (CHL?) that kept
getting problems with interference.
It was eventually found to be the bulb in the light in the WAAF's latrine.
They changed the bull and all was OK.

In the early 60s our evening TV viewing (BBC 405-line, channel 5, around
65MHz) was regularly ruined by a faulty light bulb in a neighbour's house,
upstairs on the landing next to their young child's bedroom. The
interference took the form of a searing horizontal line that drifted
slowly up or down the picture.
The parents used to leave the light on until the child fell asleep, and it
was purely by luck that I noticed that the interference coincided with the
light being on, and disappeared immediately the light was switched off.
Almost certainly the interference was being caused by Barkhausen-Kurz
oscillations in the bulb, which I believe can occur if there is a minute
arcing break in the filament.
The up-till-then friendly neighbour (who, at the time, didn't have TV) got
very huffy when approached with a view to getting the light bulb changed -
and absolutely refused to do anything about it. Fortunately, after a few
weeks the interference suddenly disappeared - presumably because the bulb
had failed and been replaced. [Interestingly, a few weeks later, the
family finally got a TV.]
https://www.ofcom.org.uk/news-centre/2020/interference-issue
--
Ian
Aims and ambitions are neither attainments nor achievements

[JMB99]

Post by Brian Gaff
I'm thinking along the lines of a mixing product due to overload of the
front end. That means the field of frequencies to look on is very much
wider. Add some harmonics as well, and all sorts of odd things occur. Most
FM tuner aerial stages are quite broadband, even though they are tuned, that
does not preclude mixing products from getting in.

Gets very complicated, I was told that on transmitter sites with
typically the six main services and other lower power services they can
mix in the RF stages of other PMR or utility services on the site. So
they something have to put filters in their outputs - to stop other
services mixing in those output stages.

The rule apparently is the last person to install equipment is
responsible for eliminating any interference even if their equipment is
OK with no nasties.

[Brian Gaff]

Its obviously cross modulation of some kind as an attenuator stops it at
about 10dB of reduction.
Some portables are too sensitive for their own good I fancy. I am old
enough to remember local and distant switches on FM radios and TVs. I guess
anything that generates non linearity will cause all sorts of strang
intermodulation components to be heard.
Brian
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Post by Brian Gaff
I'm thinking along the lines of a mixing product due to overload of the
front end. That means the field of frequencies to look on is very much
wider. Add some harmonics as well, and all sorts of odd things occur. Most
FM tuner aerial stages are quite broadband, even though they are tuned, that
does not preclude mixing products from getting in.

Gets very complicated, I was told that on transmitter sites with typically
the six main services and other lower power services they can mix in the
RF stages of other PMR or utility services on the site. So they something
have to put filters in their outputs - to stop other services mixing in
those output stages.
The rule apparently is the last person to install equipment is responsible
for eliminating any interference even if their equipment is OK with no
nasties.