On Tue, 05 Sep 2006 06:34:33 GMT, RK Henry
wrote:

On Tue, 05 Sep 2006 00:49:23 +0100, wrote:

I cannot remember the exact figures but I seem to remember
communication quality AM is about 8db better than the equivalent FM.
SSB is about 13db better than FM. Remember you need also to specify
power.

I think you have identified precisely the reason why aircraft use AM,
or in some cases SSB, instead of FM. We're going for distance, not
quality. Voice's narrow range of frequencies doesn't require broadband
to useful information. Under many circumstances, punching through with
a noisy, barely readable message is preferable to no message at all.
Flying an ordinary instrument approach might exceed the limits of
narrowband FM, especially if the aircraft happens to be encased in
ice. Over mountains, it's easy to exceed the limits of VHF
communications with ATC, getting a weak, "radar service terminated..."
until you get closer to the airport or to a RCO. I don't think we're
willing to go to the extreme of SSB, so AM offers a reasonable
compromise for most operations.

On the issue of intelligibility, I've always found that a more
important factor is equipment, not modulation. Old, decrepit
radios in need of maintenance send and receive poorly. I've flown
airplanes where one radio was loud and clear and the other was
unreadable. Bad microphones may not give the transmitter much to work
with either. There are still airplanes flying with cheap (relatively)
carbon hand mikes. I was flying with such a mike, from a major name in
aircraft communications, when ATC refused to handle my flight because
of the poor communications. Some headset mikes can be really bad too.
Some aren't even noise canceling, and sometimes even noise canceling
isn't enough, as evidenced by some transmissions I've heard from
helicopters. It should also be noted that communication mikes have a
different frequency response from the broadcast mikes they use at the
FM broadcast station. The frequency response helps to emphasize those
frequencies that will punch the signal out over a distance.

A good AM radio with a good mike ought to give quite satisfactory
results. Unfortunately, sometimes FAA's radios aren't that good
either. I once reported poor transmissions to a controller. He thanked
me, made a switch, and I was then able to report 5x5 to him. Unicom
operators' radios are sometimes virtually useless.

RK Henry

Thank you RKH, I wasn't going return to this thread but you have
detailed exactly what I was trying to put across.

SSB is by far the best for long range communication but requires very
accurate tuning, unless you leave a small amount of carrier and allow
the receiver to do it for you. At least with AM the carrier is
transmitted along with the signal so tuning is relatively unimportant.
You only have to listen to two stations transmitting at the same time
and you can hear the inaccuracy in the form of the heterodyne. If they
were both on frequency you would not hear the hetrodine.

David