Frequency-selective attenuation of VHF radio in car

[NY]

I know that poor aerial connections and signal propagation can attenuate
some frequencies more than others, but is it plausible that this could
cause everything below about 100 MHz (ie BBC R 2, 3, 4) to be virtually
unusable (hiss/buzzing with very faint modulation by the signal, no RDS
channel ID displayed) whereas everything above 100 MHz (Classic FM and
various commercial local stations) to be perfect (ie stereo, no birdies
or hiss)?

Short of taking the headlining out of my car to follow the cable from
the shark-fin aerial on the roof as far as the radio itself, I'm at a
bit of a loss as to how to cure it. It's been like it for several years,
in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio the
other year to fit a mobile phone hands-free kit the other year, I got
good reception below 100 MHz while the radio was hanging loose and for a
few weeks after I'd refitted it, but then it gradually got worse again.
The problem pre-dates fitting the hands-free kit by several years.

The fact that the fault didn't return *immediately* that I refitted the
radio suggests that it's not as simple as a kink in the coax cable or a
badly-fitting aerial plug.

The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and had
similar problems.

It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below the
threshold.


Unfortunately I don't have a portable FM radio that I can use to check
for stray radiation from something in the car. It happens even when the
ignition is off, so if it's car-related, it's something that happens
without the ignition on and/or the engine running.

[charles]

Post by NY
I know that poor aerial connections and signal propagation can attenuate
some frequencies more than others, but is it plausible that this could
cause everything below about 100 MHz (ie BBC R 2, 3, 4) to be virtually
unusable (hiss/buzzing with very faint modulation by the signal, no RDS
channel ID displayed) whereas everything above 100 MHz (Classic FM and
various commercial local stations) to be perfect (ie stereo, no birdies
or hiss)?
Short of taking the headlining out of my car to follow the cable from
the shark-fin aerial on the roof as far as the radio itself, I'm at a
bit of a loss as to how to cure it. It's been like it for several years,
in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio the
other year to fit a mobile phone hands-free kit the other year, I got
good reception below 100 MHz while the radio was hanging loose and for a
few weeks after I'd refitted it, but then it gradually got worse again.
The problem pre-dates fitting the hands-free kit by several years.
The fact that the fault didn't return *immediately* that I refitted the
radio suggests that it's not as simple as a kink in the coax cable or a
badly-fitting aerial plug.
The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and had
similar problems.
It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below the
threshold.
Unfortunately I don't have a portable FM radio that I can use to check
for stray radiation from something in the car. It happens even when the
ignition is off, so if it's car-related, it's something that happens
without the ignition on and/or the engine running.

It's a good many years ago, but I remember someone's burglar alarm blotting
out part of Band II. Even further back there was a home computer which
wiped out Band II for a whole block of housing in a town. This was before
CE certification came into force.

[Woody]

Post by NY
I know that poor aerial connections and signal propagation can attenuate
some frequencies more than others, but is it plausible that this could
cause everything below about 100 MHz (ie BBC R 2, 3, 4) to be virtually
unusable (hiss/buzzing with very faint modulation by the signal, no RDS
channel ID displayed) whereas everything above 100 MHz (Classic FM and
various commercial local stations) to be perfect (ie stereo, no birdies
or hiss)?
Short of taking the headlining out of my car to follow the cable from
the shark-fin aerial on the roof as far as the radio itself, I'm at a
bit of a loss as to how to cure it. It's been like it for several years,
in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio the
other year to fit a mobile phone hands-free kit the other year, I got
good reception below 100 MHz while the radio was hanging loose and for a
few weeks after I'd refitted it, but then it gradually got worse again.
The problem pre-dates fitting the hands-free kit by several years.
The fact that the fault didn't return *immediately* that I refitted the
radio suggests that it's not as simple as a kink in the coax cable or a
badly-fitting aerial plug.
The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and had
similar problems.
It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below the
threshold.
Unfortunately I don't have a portable FM radio that I can use to check
for stray radiation from something in the car. It happens even when the
ignition is off, so if it's car-related, it's something that happens
without the ignition on and/or the engine running.

Are you talking here only about LW/MW/FM, or are you including DAB? When
you talk of Classic at 101.1 (if in W Yorks) and show poor signal
<below> that frequency, what about other frequencies above 101.1 - such
as Sheffield on 104.1 or York on 103.7?

What make/model/year of car would also help. You mention a shark fin
aerial? Is that one of the small modern ones or one of the older larger
type. Most modern installs use the shark-fin for GPS and the e-SIM
cellular if you car has such, the radio reception is all by wires in the
rear window glass and/or side windows if you have a SUV or estate.

The first guess would be lack of supply to the aerial amp - but lets see
what we can suggest when we know more about the vehicle?

[NY]

Post by Woody

Post by NY
I know that poor aerial connections and signal propagation can
attenuate some frequencies more than others, but is it plausible that
this could cause everything below about 100 MHz (ie BBC R 2, 3, 4) to
be virtually unusable (hiss/buzzing with very faint modulation by the
signal, no RDS channel ID displayed) whereas everything above 100 MHz
(Classic FM and various commercial local stations) to be perfect (ie
stereo, no birdies or hiss)?
Short of taking the headlining out of my car to follow the cable from
the shark-fin aerial on the roof as far as the radio itself, I'm at a
bit of a loss as to how to cure it. It's been like it for several
years, in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio the
other year to fit a mobile phone hands-free kit the other year, I got
good reception below 100 MHz while the radio was hanging loose and for
a few weeks after I'd refitted it, but then it gradually got worse
again. The problem pre-dates fitting the hands-free kit by several years.
The fact that the fault didn't return *immediately* that I refitted
the radio suggests that it's not as simple as a kink in the coax cable
or a badly-fitting aerial plug.
The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and
had similar problems.
It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below the
threshold.
Unfortunately I don't have a portable FM radio that I can use to check
for stray radiation from something in the car. It happens even when
the ignition is off, so if it's car-related, it's something that
happens without the ignition on and/or the engine running.

Are you talking here only about LW/MW/FM, or are you including DAB? When
you talk of Classic at 101.1 (if in W Yorks) and show poor signal
<below> that frequency, what about other frequencies above 101.1 - such
as Sheffield on 104.1 or York on 103.7?

The radio can only receive LW/MW/FM, it's not new enough to have DAB.

I'll check what stations above 100 MHz I can receive tomorrow when it's
light, but I remember at least one and maybe two local stations.

Everything below 100 is mush - both Radio York and the national stations
such as R2/3/4 - and possibly R1 which is close to 100 but just
fractionally below it.

Post by Woody
What make/model/year of car would also help.

The car is a 2008 Peugeot 308 with the factory-fitted radio.


I live near Bridlington, so I'm not sure which would be nearest/best
national FM transmitters in the area.

I also experienced the problem when I lived just off the A64 between
Whitwell and Malton, and when I lived just north of Leyburn.

Post by Woody
You mention a shark fin
aerial? Is that one of the small modern ones or one of the older larger
type. Most modern installs use the shark-fin for GPS and the e-SIM
cellular if you car has such, the radio reception is all by wires in the
rear window glass and/or side windows if you have a SUV or estate.

I was wrong. I've just shone a torch on the car from the house (!) and
it's not a shark-fin, it's a short whippy aerial mounted on the roof
above the rear seats. I mis-remembered.

Do cars with FM radios tend to have a "mast-head" amplifier close to the
aerial, or is all the amplification done at the radio? I wonder if an
aerial amp would be listed in the car's list of fuses, to help check
which fuse might have blown.

Post by Woody
The first guess would be lack of supply to the aerial amp - but lets see
what we can suggest when we know more about the vehicle?

The fact that LW and MW are poor makes me suspect pretty wideband
interference or attenuation across all bands in the radio. Do car radios
tend to use the roof aerial for LW/MW or do they use an internal aerial?
Odd that for FM it is split so noticeably, rather than being poor for
all frequencies.

[Woody]

Post by NY

Post by Woody

Post by NY
I know that poor aerial connections and signal propagation can
attenuate some frequencies more than others, but is it plausible that
this could cause everything below about 100 MHz (ie BBC R 2, 3, 4) to
be virtually unusable (hiss/buzzing with very faint modulation by the
signal, no RDS channel ID displayed) whereas everything above 100 MHz
(Classic FM and various commercial local stations) to be perfect (ie
stereo, no birdies or hiss)?
Short of taking the headlining out of my car to follow the cable from
the shark-fin aerial on the roof as far as the radio itself, I'm at a
bit of a loss as to how to cure it. It's been like it for several
years, in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio
the other year to fit a mobile phone hands-free kit the other year, I
got good reception below 100 MHz while the radio was hanging loose
and for a few weeks after I'd refitted it, but then it gradually got
worse again. The problem pre-dates fitting the hands-free kit by
several years.
The fact that the fault didn't return *immediately* that I refitted
the radio suggests that it's not as simple as a kink in the coax
cable or a badly-fitting aerial plug.
The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and
had similar problems.
It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below
the threshold.
Unfortunately I don't have a portable FM radio that I can use to
check for stray radiation from something in the car. It happens even
when the ignition is off, so if it's car-related, it's something that
happens without the ignition on and/or the engine running.

Are you talking here only about LW/MW/FM, or are you including DAB?
When you talk of Classic at 101.1 (if in W Yorks) and show poor signal
<below> that frequency, what about other frequencies above 101.1 -
such as Sheffield on 104.1 or York on 103.7?

The radio can only receive LW/MW/FM, it's not new enough to have DAB.
I'll check what stations above 100 MHz I can receive tomorrow when it's
light, but I remember at least one and maybe two local stations.
Everything below 100 is mush - both Radio York and the national stations
such as R2/3/4 - and possibly R1 which is close to 100 but just
fractionally below it.

Post by Woody
What make/model/year of car would also help.

The car is a 2008 Peugeot 308 with the factory-fitted radio.
I live near Bridlington, so I'm not sure which would be nearest/best
national FM transmitters in the area.
I also experienced the problem when I lived just off the A64 between
Whitwell and Malton, and when I lived just north of Leyburn.

Post by Woody
You mention a shark fin aerial? Is that one of the small modern ones
or one of the older larger type. Most modern installs use the
shark-fin for GPS and the e-SIM cellular if you car has such, the
radio reception is all by wires in the rear window glass and/or side
windows if you have a SUV or estate.

I was wrong. I've just shone a torch on the car from the house (!) and
it's not a shark-fin, it's a short whippy aerial mounted on the roof
above the rear seats. I mis-remembered.
Do cars with FM radios tend to have a "mast-head" amplifier close to the
aerial, or is all the amplification done at the radio? I wonder if an
aerial amp would be listed in the car's list of fuses, to help check
which fuse might have blown.

Post by Woody
The first guess would be lack of supply to the aerial amp - but lets
see what we can suggest when we know more about the vehicle?

The fact that LW and MW are poor makes me suspect pretty wideband
interference or attenuation across all bands in the radio. Do car radios
tend to use the roof aerial for LW/MW or do they use an internal aerial?
Odd that for FM it is split so noticeably, rather than being poor for
all frequencies.

Now that makes sense. If it is a bee-sting aerial which is what you
describe then there are a few possibilities.

The easiest is that the bee-sting has a break in it. Many such aerials
are in two parts - a wire-wound section to cover LW/MW and a stiffer
wire/rod section for VHF. It could be that the LW/MW section is broken
and everything is relying upon what is left of the VHF section. Replace
the bee-sting aerial section - it screws in - although you may have to
go to a scrap yard to get one.

The aerial also has an amp which is in fact two sections, one for LW/MW
and one for VHF, but they are both fed down the same cable. Your
description sounds like the LW/MW section has died. The load of this
dead amp may induce the frequency selective effects on the VHF section.

On modern cars the aerial amp is often fed with power up the aerial
cable - certainly for all VAG for instance - but with a car of the age
of yours it will likely have a separate supply cable. You need to check
if 12V is arriving at the aerial and also that the aerial still has a
good earth to the car body through its mounting. If the local earth has
failed then the amp could be working by earthing through the braid of
the aerial cable from the radio, which could at least theoretically form
part of a frequency selective circuit causing serious signal
attenuation. You should be able to get at the aerial by removing the
runner seal strip along the top of the tailgate aperture and gently
lowering the roof lining.

It always used to be said that the French make lovely motors that are
well designed, well built, comfortable, and mechanically reliable - but
their electrics were and still are cr*p! I found that when my temporary
Co car was a Renault Laguna 1.8i Sport estate which blew the fuse every
time the footbrake was applied.....

For the record, around Brid you should be getting VHF from Belmont near
Horncastle in Lincs, or Holme Moss near Glossop. Belmont R2/R3/R4 is
88.8/90.9/93.1, HM is 89.3/91.5/93.7. Note that most transmitters have
2.2MHz spacing between R2-3-4; Belmont should be on 88.7 but was changed
to avoid interference wit BBC Sheffield on 88.6.
There is also a VHF radio transmitter on Oliver's Mount at Scarborough
but I don't have the frequencies to hand.

Good luck.

[NY]

Post by Woody
For the record, around Brid you should be getting VHF from Belmont near
Horncastle in Lincs, or Holme Moss near Glossop. Belmont R2/R3/R4 is
88.8/90.9/93.1, HM is 89.3/91.5/93.7. Note that most transmitters have
2.2MHz spacing between R2-3-4; Belmont should be on 88.7 but was changed
to avoid interference with BBC Sheffield on 88.6.
There is also a VHF radio transmitter on Oliver's Mount at Scarborough
but I don't have the frequencies to hand.

Ah, I wasn't sure whether there were transmitters closer than than Holme
Moss and Belmont. I didn't know that Oliver's Mount did FM radio as well
as TV.

I'll have a look at the aerial and see if there is obvious sign of
damage anywhere - if I can work out how to remove (and more importantly
*replace*) the headlining.

It came as a surprise to learn only last year that there is normally a
fixed spacing between R2/3/4 frequencies. Is there an equivalent pattern
for the UHF frequencies used for TV from a given transmitter? Adjacent
channels (eg 482 MHz for PSB1 and 490 MHz for COM5) would have been
unthinkable in analogue days! I'll be interested to see which set of
frequencies I've stored in my radio's memory and therefore which
transmitter I've selected as being strongest.

I wonder if the radio does do phantom powering of an aerial amplifier. I
don't remember seeing anything about aerial amplifier listed in the list
of fuses, so the amp may well be powered from the radio.

If the amp has lost its power, it's intriguing that only the lower
frequencies in the VHF band have suffered, and that higher ones are not
only passable but actually good (no obvious hiss or flipping in/out of
stereo).

I know that radio and TV reception can be very frequency-dependant: I
get flawless reception (*) of all the terrestrial TV multiplexes from
Belmont, apart from COM4 which drops out sometimes and which is the
highest frequency - either it is attenuated most by the hill 1/2 mile
from our house (flawless line of sight for 60 miles, and there's a hill
in the last 1/2 mile!) or else the aerial (which probably dates from
analogue days) was narrow-band grouped and not designed to go as high as
COM4.


(*) Unless there is "lift", and Crystal Palace right down in London
interferes - because it shares the same UHF channels with Belmont on the
basis of them being far enough apart in normal conditions. Hopefully
Belmont stomps on Londoners' reception as much as CP does on ours ;-)

[Mark Carver]

Post by Woody
Belmont R2/R3/R4 is
88.8/90.9/93.1, Note that most transmitters have
2.2MHz spacing between R2-3-4; Belmont should be on 88.7 but was changed
to avoid interference wit BBC Sheffield on 88.6.

No, R2 Belmont has always been 88.8, since day one (Nov 1966). The non
conformity to the 2.2 MHz pattern is down to continental frequency
planning reasons.

Radio Sheffield's use of 88.6 (a year later) is not (directly) related

[Ian Jackson]

Post by NY

Post by Woody

Post by NY
I know that poor aerial connections and signal propagation can
attenuate some frequencies more than others, but is it plausible that
this could cause everything below about 100 MHz (ie BBC R 2, 3, 4) to
be virtually unusable (hiss/buzzing with very faint modulation by the
signal, no RDS channel ID displayed) whereas everything above 100 MHz
(Classic FM and various commercial local stations) to be perfect (ie
stereo, no birdies or hiss)?
Short of taking the headlining out of my car to follow the cable
from the shark-fin aerial on the roof as far as the radio itself,
I'm at a bit of a loss as to how to cure it. It's been like it for
years, in various parts of Yorkshire (so for a variety of different
transmitters). The annoying thing is that when I removed the radio
the other year to fit a mobile phone hands-free kit the other year,
I got good reception below 100 MHz while the radio was hanging loose
and for a few weeks after I'd refitted it, but then it gradually got
worse again. The problem pre-dates fitting the hands-free kit by several years.
The fact that the fault didn't return *immediately* that I refitted
the radio suggests that it's not as simple as a kink in the coax
cable or a badly-fitting aerial plug.
The odd thing is that LF and MF radio reception is also very poor -
sometimes when I've not been able to get R4 FM I've tried R4 LW and
had similar problems.
It's not a blanket below-100 MHz problem because there are locations
where I get good R2/3/4 reception - presumably where the signal is
particularly strong - but the radio is selectively more deaf below
the threshold.
Unfortunately I don't have a portable FM radio that I can use to
check for stray radiation from something in the car. It happens even
when the ignition is off, so if it's car-related, it's something
that happens without the ignition on and/or the engine running.

Are you talking here only about LW/MW/FM, or are you including DAB?
When you talk of Classic at 101.1 (if in W Yorks) and show poor
signal <below> that frequency, what about other frequencies above
101.1 - such as Sheffield on 104.1 or York on 103.7?

The radio can only receive LW/MW/FM, it's not new enough to have DAB.
I'll check what stations above 100 MHz I can receive tomorrow when it's
light, but I remember at least one and maybe two local stations.
Everything below 100 is mush - both Radio York and the national
stations such as R2/3/4 - and possibly R1 which is close to 100 but
just fractionally below it.

Post by Woody
What make/model/year of car would also help.

The car is a 2008 Peugeot 308 with the factory-fitted radio.
I live near Bridlington, so I'm not sure which would be nearest/best
national FM transmitters in the area.
I also experienced the problem when I lived just off the A64 between
Whitwell and Malton, and when I lived just north of Leyburn.

Post by Woody
You mention a shark fin aerial? Is that one of the small modern ones
or one of the older larger type. Most modern installs use the
shark-fin for GPS and the e-SIM cellular if you car has such, the
radio reception is all by wires in the rear window glass and/or side
windows if you have a SUV or estate.

I was wrong. I've just shone a torch on the car from the house (!) and
it's not a shark-fin, it's a short whippy aerial mounted on the roof
above the rear seats. I mis-remembered.
Do cars with FM radios tend to have a "mast-head" amplifier close to
the aerial, or is all the amplification done at the radio? I wonder if
an aerial amp would be listed in the car's list of fuses, to help check
which fuse might have blown.

Post by Woody
The first guess would be lack of supply to the aerial amp - but lets
see what we can suggest when we know more about the vehicle?

The fact that LW and MW are poor makes me suspect pretty wideband
interference or attenuation across all bands in the radio. Do car
radios tend to use the roof aerial for LW/MW or do they use an internal
aerial? Odd that for FM it is split so noticeably, rather than being
poor for all frequencies.

As only one end of the FM band is affected, could it be that the local
oscillator is doing something funny? Usually it simply stops, but as the
signal suddenly turns into mush, instead could it be 'squegging'?

[NY]

Post by Ian Jackson
As only one end of the FM band is affected, could it be that the local
oscillator is doing something funny? Usually it simply stops, but as the
signal suddenly turns into mush, instead could it be 'squegging'?

Would it help at all in diagnosis if I recorded (mobile phone recording of
radio's speaker output) the type of mush? From memory, the sound is mostly
roaring hiss with very faint distorted audio in the background, as if it
were a very weak signal. Plus some hum which I suppose could be residual
alternator noise getting through the filtering on the power lines and only
being audible because the RF gain is turned way up.

I may not get round for a while to investigating how to remove (and
replace!) the headlining to find the mast-head amplifier and see if it's
getting power and whether there's any obvious sign of breaks/corrosion in
the aerial connections - until it gets a bit warmer for sitting in the car.

What I'll try in the meantime is manually tuning to R2/3/4 on Holme Moss,
Belmont and Oliver's Mount frequencies, and to seeing which stations
are/aren't receivable. Each time we moved house (Malton to Leyburn to
Bridlington) I've found a good reception spot and seek-tuned to each of
R2/3/4 stations, and then updated the 2/3/4 buttons with the new frequency.
In the past that's fine because as long as I've got a good signal, RDS does
the job of moving me on from one frequency to another if I travel out of one
transmitter's range into another's. But without detectable RDS, that auto
tuning isn't going to work: to my embarrassment I'm not even sure that the
presets are pointing to a sensible transmitter - probably Holme Moss or
Belmont. And seek-tuning finds *nothing* below about 100 MHz. I need to make
sure everything is set up to the best possible settings.

I'll also test with/without the engine running and with/without the ignition
turned on - I think my car allows a few minutes activity with the ignition
turned off before it goes into power-saving mode.

I'd love to know why when I had to remove the radio to fit the mobile
hand-free kit (which mutes the radio output and substitutes the mobile phone
audio while the call is in progress) the radio reception got better - and
remained better for a few weeks before reverting to mush. I can understand a
poor connection between the aerial cable and the radio being improved by
being physically joggled, but I'd expect it either to remain good or else to
go bad as soon as the radio and the cable are back in their original
position.

To what extent, typically, is VHF signal strength affected over the narrow
88-108 MHz range by poor joints in cables or an unpowered head-end
amplifier?

[Mark Carver]

Post by Ian Jackson
As only one end of the FM band is affected, could it be that the local
oscillator is doing something funny? Usually it simply stops, but as the
signal suddenly turns into mush, instead could it be 'squegging'?

My 2001 Peugeot 406's radio had some 'crunchy' noise on stations above
about 100 MHz, which peaked at about 105.0 MHz. It made reception of
anything above 105 difficult, unless there was a lot of signal.

It was only present with the engine running, but was not related to
engine speed, so I assumed some gunk from the ECU ?

[JMB99]

I don't know if relevant but VHF is affected by standing waves (as is
all RF).

It was quite common on 144 MHz that if someone was a weak signal, you
could often get a better signal by driving forward a few feet.

It really confused strangers!

[Mark Carver]

Post by JMB99
I don't know if relevant but VHF is affected by standing waves (as is
all RF).
It was quite common on 144 MHz that if someone was a weak signal, you
could often get a better signal by driving forward a few feet.

I've used that trick for FM radio at traffic lights, to continue
listening to what someone is saying. I reversed once too !