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Re: Geosynchronous Satellites

on 12/23/00 8:32 PM, Phil Karn at karn@ka9q.net wrote:

>> 3.) FM is not efficient as a mode in terms of propagation.  FM is inherently
>> more wide band than SSB or CW.  You may transmit 25 watts on SSB and 25
>> watts on FM but the SSB signal will get you farther.  CW will be even
>> better.  This is because as you increase the bandwidth your power is spread
>> out over a wide range of frequencies.  Hence your power density decreases.
>> This is the reason why weak signal and EME types use SSB/CW and not FM.
> Gee, then I wonder why all the analog C and Ku-band TV satellites use
> FM rather than AM.

Well, Phil, probably because of several reasons:

1.) FM is pretty much the defacto television standard
2.) A videa signal is inherently wide band.  FM produces better fidelity at
least on the audio side.  So why not use it.
3.) An FM transmitter can use a saturated class C amplifier.  In terms of
efficiency on a bird, the power budget for an FM transmitter is certainly
less than that for a linear amplifier used for amplifying AM.
4.) Television is not designed to be "weak signal" work.  The public
wouldn't put up with it.  So very large TVRO antennas are used on the ground
and the transponder power on the satellites is pretty hefty.  Given the
power budget, FM is certainly a better form of modulation to use.  However,
ham satellites don't have the power budget of a big commercial bird.
Therefore, to maximize your link budget for an inherently narrow band signal
(voice) it behooves a person to use a modulation form that uses the minimum
possible amount of bandwidth, thereby resulting in greater power density.

Again, Phil, if FM is a better "weak signal" mode or if it's easier to
propagate FM over a longer distance than SSB or CW, then why do EME
operators not use it?



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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