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.