In message , FranK Turner-Smith G3VKI
writes
"Spike" wrote in message
...
On 06/02/15 12:39, Jeff wrote:
FM has always been a legal mode for 80m in the UK, it's just that nobody
normally uses it intentionally.
Provided you're using no more bandwidth than normal AM I can't see a
problem.

Well perhaps not 'always'. FM was not allowed prior to about 1952.

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.

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?

IIRC, most WW2 gear used simple 'Lo-Fi' forms of AM modulation (grid or
screen-grid), and not hi-level plate and screen (which requires more
valves, more current drain, a heavy mod transformer etc). As such, the
component count would be similar to FM equipment.

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.


--
Ian

On 07/02/15 13:07, Ian Jackson wrote:
In message , FranK Turner-Smith G3VKI
writes
"Spike" wrote in message
...
On 06/02/15 12:39, Jeff wrote:
FM has always been a legal mode for 80m in the UK, it's just that
nobody
normally uses it intentionally.
Provided you're using no more bandwidth than normal AM I can't see a
problem.

Well perhaps not 'always'. FM was not allowed prior to about 1952.

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.

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?


The various different services, Police, Ambulance, Fire, Coast Guard,
were on different frequencies so AM/FM was really of little consequence.

One of the, supposed, ideas behind 'Airwave' is that they can all be
linked but I am not sure how far this has actually been achieved, if at
all. Certainly the functionality that Kent Police were expecting is
still a pipe dream, based on some casual chats with end users who are
most unimpressed.


Compare that to some of the systems in place in the USA, where (even in
small towns) officers can access key information from a vehicle mounted
computer. (No, I've not been 'checked', I know someone who worked on the
systems.)

I have seriously wondered if the considerable delay in 'pushing' the
take up of the 'old' UHF emergency service frequencies wasn't, in part,
caused by concern that it may be necessary to 'rethink' the reliance on
Airwave. After all, I attended a presentation by OFCOM when it was still
not sure if it was the RA or OFCOM ;-) when they were, supposedly, about
to 'fill' the old frequencies with waiting users. Here we are, over a
decade on, and they are still 'talking'. Quite a feat, even for OFCOM ;-)


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.

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

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 07/02/15 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.

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

On 2/7/2015 8:07 AM, Ian Jackson wrote:
In message , FranK Turner-Smith G3VKI
writes
"Spike" wrote in message
...
On 06/02/15 12:39, Jeff wrote:
FM has always been a legal mode for 80m in the UK, it's just that
nobody
normally uses it intentionally.
Provided you're using no more bandwidth than normal AM I can't see a
problem.

Well perhaps not 'always'. FM was not allowed prior to about 1952.

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.

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?

IIRC, most WW2 gear used simple 'Lo-Fi' forms of AM modulation (grid or
screen-grid), and not hi-level plate and screen (which requires more
valves, more current drain, a heavy mod transformer etc). As such, the
component count would be similar to FM equipment.

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.



Actually not. Aircraft worldwide were using AM radios before FM became
popular. To change would require every radio-equipped airplane in the
world, from Cessna 150's to Antonov An-225's, change at the same time,
as well as all land-based stations including ATC, Flight Service
Stations, Unicoms, and even handhelds on the tarmac.

Plus, with the 8.33Khz channel spacing, deviation would be limited to
about +/- about 3Khz. Even with the old 25Khz channel spacing (allowing
about +/- 10Khz deviation), there is no clear advantage to FM over AM.
Fidelity is not a concern for the aircraft band.

It isn't going to happen It's going to take least 13 years just to
get new NextGen navigation system installed in aircraft in the United
States.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

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