On 07/02/15 11:14, FranK Turner-Smith G3VKI wrote:
"Spike" wrote in message
...

There was an adaptation of the WS19 (I think it was the WS32) that
used FM instead of AM, to test the use of FM on the battlefield. I
guess it was unsuccessful or other considerations mitigated against
it, because it wasn't adopted in that form, but some manpack sets and
WS19 candidate replacements were FM. I think that only about 100 WS32
were made.

To ensure compatibility it would have been necessary to swap all the
military AM radios to FM at the same time.
The middle of a global war is not the time to make changes on that
scale, especially as the advantages of doing so seem minimal.

I'm not sure that would have been a problem. New units would equip and
train with the new equipment, and then be sent to the front, replacing
those with older gear who would be retrofitted during their rest and
refit time. Units operated tactically, individual tanks talking to each
other and back to their own HQ. Comms between HQs (Company to Battalion
to Division, etc) would have been a simpler affair as far less radio
sets were involved.

And! Don't forget even the 32 set included CW, Signaller/Gunners were
trained in the mode, so there was interoperability built into the
system. Also, the 32 set RT was was AM/FM selectable, and physically and
electrically compatible - a line-replaceable unit.


--
Spike

"Hard cases, it has frequently been observed, are apt to introduce bad
law". Judge Rolfe

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

Jeff

Very true. Weren't there all sorts problems later on when some of our
emergency services got radio, and some areas used and AM, and some used FM?

Even in the areas where the police used FM they were required to have
one AM channel for compatibility (on VHF). Some of the later police
mobiles were capable of both AM or FM on a channel by channel basis.

Jeff

In message , Jeff writes

Very true. Weren't there all sorts problems later on when some of our
emergency services got radio, and some areas used and AM, and some used FM?

Even in the areas where the police used FM they were required to have
one AM channel for compatibility (on VHF). Some of the later police
mobiles were capable of both AM or FM on a channel by channel basis.

As they (don't) say, "Standardisation is next to godliness"!


--
Ian

It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions.

very true .....

On 07/02/2015 17:24, Jeff wrote:

Even in the areas where the police used FM they were required to have
one AM channel for compatibility (on VHF). Some of the later police
mobiles were capable of both AM or FM on a channel by channel basis.

I did that with a Pye Cambridge AM10B in the early 80s.

145.800 AM using the original circuit, plus few simplex and R4 (GB3FF)
for FM using the Garex NBFM IF kit, with audio applied to the screen
grid of the oscillator for transmit.

I could've saved a lot of work if I'd bought a Pye Whitehall to start with.

On 07/02/15 17:20, Jeff wrote:

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

The 'capture' effect of FM is rather limited with NBFM. While you may
not be able to understand if two transmissions are present (just as you
may not on AM) you can often tell if there are.

To really gain (or perhaps not in this application) from the capture
effect, you don't really what NBFM.

The capture effect was mentioned as one of the reasons for UK CB being
FM but it was rather a dubious one, certainly a 'make weight' in the
RA's argument.

On 07/02/15 17:24, Jeff wrote:

Very true. Weren't there all sorts problems later on when some of our
emergency services got radio, and some areas used and AM, and some
used FM?

Even in the areas where the police used FM they were required to have
one AM channel for compatibility (on VHF). Some of the later police
mobiles were capable of both AM or FM on a channel by channel basis.

Really? The only ex-emergency service radio I've seen with AM and FM was
the 'Whitehall' which, if memory serves, was a Low Band unit and useful
for 4m. I repaired on once, quite a beast, with a mass of cables to
connect the boot unit to the control box.

Later radios tended to be pretty standard PMR sets from the likes of
Storno.

The 'Met' police did have a mix of AM and FM radios at one time but they
were different units. I think the AM sets were around 150MHz. FM was
just above 2m and UHF (450 or 460 ish). I think the cars, especially
those on traffic, used AM. Certainly the personal radios were UHF FM. I
knew someone who worked in the Met comms side.

I recall a major fire locally in 1990 or so when the police and fire
couldn't talk to each other at all via radio. In the end, they had a
couple of RAYNET people relaying messages between them, one was with the
senior fire officer the other with the senior police officer. The police
didn't even have enough radios for all of their officers and relied on
RAYNET.

In message , Brian Reay writes
On 07/02/15 17:20, Jeff wrote:

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

The 'capture' effect of FM is rather limited with NBFM. While you may
not be able to understand if two transmissions are present (just as you
may not on AM) you can often tell if there are.

To really gain (or perhaps not in this application) from the capture
effect, you don't really what NBFM.

The capture effect was mentioned as one of the reasons for UK CB being
FM but it was rather a dubious one, certainly a 'make weight' in the
RA's argument.

I thought that the RAs insistence on FM was that the constant signal
envelope level was less likely to interfere with 'things' (apart from a
click at start and end of a transmission).
--
Ian

On Sat, 7 Feb 2015, Ian Jackson wrote:

In message , Brian Reay writes
On 07/02/15 17:20, Jeff wrote:

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

The 'capture' effect of FM is rather limited with NBFM. While you may not
be able to understand if two transmissions are present (just as you may not
on AM) you can often tell if there are.

To really gain (or perhaps not in this application) from the capture
effect, you don't really what NBFM.

The capture effect was mentioned as one of the reasons for UK CB being FM
but it was rather a dubious one, certainly a 'make weight' in the RA's
argument.

I thought that the RAs insistence on FM was that the constant signal envelope
level was less likely to interfere with 'things' (apart from a click at start
and end of a transmission).

As I mentioned earlier, that was certainly one reason narrow band FM was
suggested for the HF bands decades ago. AM would get rectified by first
stages in audio ampliers, and the neighbors would be able to identify the
voice. None of that with FM.

But I remember tuning CB here in Canada in the early seventies, nad much
of the time, at least in the summer, it was a mass of heterodynes. Come
to think of it, since that was with a shortwave receiver, I wonder what it
was like on a channelized CB receiver?

The capture effect has always been attributed to FM, but in reality, it's
the limiters that bring on the capture effect. You can't have limiters
with AM, since that would wipe out the modulation. But if an FM receiver
had no limiters, where does the capture effect come from? The limiter
makes sure that a relatively modest difference between signal levels means
one will be on top. That said, I can remember instances of hearing two FM
signals at the same time, presumably they were pretty much identical
signal strength at the receiver.

On the other hand, maybe CB sets where FM is used don't have good
limiters. I finally found an SSB CB set a year or two ago, and once I
found information about it, discovered that the IF filter is relatively
wide. I was expecting a nice narrow SSB filter (which is why I'd hoped
for years go fined one), but instead it was sort of mediocre bandwidth,
wide enough for AM, and "narrow enough" for SSB. So they saved on the
flter. The odd part is, a good audio filter will make sure the
transmitted signal is narrow (if the actual bandwidth of a voice isn't
good enough), the IF filter only needs to knock off the unwanted sideband.
And I suppose on receiver, the channelized nature of CB means a wider
filter doesn't matter, the next channel up is far enough away so a wider
filter won't let in interference.

Michael