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Re: Geosynchronous Satellites

on 12/24/00 4:52 PM, Phil Karn at karn@ka9q.net wrote:

>> However, most base stations in the cellular industry now use multi-carrier
>> linear amplifiers even though they are amplifying FM signals.  They have to
>> be linear because you are amplifying more than one signal at a time and
>> intermods would result from a non-linear design.  Even your precious CDMA
>> needs highly linear amplifiers.
> Correct. DC power is cheap on the ground, so the low efficiency is not
> a big deal. Not true on a satellite. This is precisely why I think we
> should consider saturating transponders and TDMA for amateur satellite
> use. Techniques like HELAPS can make a linear transponder with
> improved efficiency, but it's complex and is inherently limited in
> bandwidth.

I agree with you here Phil if you are talking about a digital bird.  TDMA or
GSM type modulation would be much better.  And as you have pointed out, a
digital signal can be demodulated much better under very weak signal
conditions (that doesn't sound right, but I am tongue tied as to how to say
it correctly.).

This whole thread started over the idea of a geosynchronous FM bird.
Traditional Voice FM certainly would not work for reasons already pointed
out.  And even a multi-channel digital bird wouldn't work well on VHF/UHF
because of bandwidth limitations, but it could work on the microwave bands.

We don't need phone quality fidelity also (although the fidelity of most
digital phones sucks!).  In fact, we could probably go with a pretty high
number of time slots per carrier.  Motorola's iDEN system uses 6 time slots
per carrier when in the two way radio mode.  Voice quality is still
excellent.  We could probably do a similar thing.  If you have 10 FM
channels, you could end up having 60 QSOs all at once.  In the microwave
bands, you have the bandwidth for such a thing also.

Creating an FM oscillator for transmitting is very easy as well since you
can just apply a modulating signal to a varactor diode.  At Motorola in our
18 and 23 GHz radios we used a 140 MHz IF.  Gee, we could use a two meter
rig as the IF!



Jon Ogden
NA9D (ex: KE9NA)



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

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