On 8 Sep 2006 12:58:47 -0700, wrote:

Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio?

FM offers better quality that AM when signal is relatively strong (or
signal to noise ratio is high). As the signal strength decreases, there

For voice communications you want the frequency response narrow.
Probably from a low of 200 or 300 HZ to a high or 2500 or 3,000 Hz.
This is where the majority of the intelligence is located. So,
communications is definitely not hi-fi. Keeping the signal narrow also
improves the signal to noise ratio.

is a point when the quality is identical in both cases and then the
quality of FM deteriorates _rapidly_, while AM is still usable.
So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used. Just a theory
;-)

There is also "capture ratio" or the ratio of signal strengths where
both of two stations transmitting can be heard. Not understood, but
heard. With AM this is about 12:1 and with FM about 2:1. (Jim, did I
get those ratios correct?) The means the weaker station is completely
gone in FM where in AM the tower would know the other station was
stepped on. With FM, if the stronger station starts first and talks
as long or slightly longer than the weaker station there will be no
indication the weaker one even transmitted.



Bartek
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com