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Re: Geosynchronous Satellites


your statements about PAs that have 7% efficiency is plain not true. Another
half truth you stated is the one about that FM occupies INHERENTLY more
bandwidth than SSB or AM. Just not true.

Why do you think the aviation world uses AM  and NOT FM?  They use 15 KHz
occupied bandwith just like VHF FM..  UHF FM uses 25 KHZ bandwith. And they
talk just as far. (That means line of sight.)
It's because AM (as well as SSB) allows you to hear everybody at the same
time. And it has the same efficiency as FM. Watt for watt, KHz of bandwith
for KHz of bandwith, they'll talk just as far. The main difference between
them is the "Capture effect of  FM." This means that the FM receiver will
lock onto the strongest signal and ignore the weak ones. Another difference
is distortion. FM is basically distortion-free. That is why it is used in
Satellite Television Distribution and as well as regular broadcast  FM
Stations, where as in ham radio SSB if you are off zero beat you sound like
Donald duck. A third difference is that FM is allways on full blast weather
you talk or not, where as SSB is on  only when you talk. Almost the same for
AM. So for power budget considerations AM and SSB are a lot less power
hungry than FM.
I gues you know all these facts anyway, and about PA's efficiencies they
reach tipically 50% and if you optimize them, they can reach close to ~ 70%.
I know, I used to design them at Motorola.

About cabbies interference, even if we had PL codes, say a Ham opens the
sat. then the cabbie would capture the sat. Receiver because he has more
power than the HAM. It would'n work.

I personally would like  to see Ham sat. that would be not micro-sat  but
mini-sat that would ride piggy-back on a  Ariane rocket and end up in a geo-
Transfer orbit that had precession, this means that it would vary the
ground-track much like LEO sats. On one pass they would pass overhead Miami,
next pass over Houston, and so on...but the pass would last a few hours and
the coverage would be worl-wide at apogee. I like 2.4GHz uplink, 1.2GHz
downlink, SSB or even AM 1 or two MHZ wide..That way even the little gut
would be heard and no one person would capture the bird.

Food for thought.......

ARRL member

----- Original Message -----
From: Jon Ogden <na9d@mindspring.com>
To: <karn@ka9q.net>
Cc: <amsat-bb@AMSAT.Org>
Sent: Sunday, December 24, 2000 8:57 AM
Subject: Re: [amsat-bb] Geosynchronous Satellites

> on 12/24/00 1:18 AM, Phil Karn at karn@ka9q.net wrote:
> > So it really seems to me that SSB's claimed power efficiency is
> > largely illusory in practice. We might as well switch to nonlinear
> > methods such as FM (or better yet, digital voice) that can give us the
> > high audio SNRs we really want while using much less power than SSB.
> Phil,
> If you think my recent posting was full of errors, that's fine.  I wasn't
> trying to be as mathematically precise as you.  And while I am at it,
> not begin the digital vs. analog debate again.  When I am talking about
> I mean STRICT FM as we know it that you get from your mobile or HT.  FM
> voice, not some specially encoded digital signal.
> If STANDARD FM voice is so much better than SSB, why don't EME operators
> it?  Why is it not used then on VHF weak signal work?  Why don't Aurora
> scatter operators use it?  Or meteor scatter ops?  You have failed to
> that question as it shoots down your statements above.  No one modulation
> form is best for all circumstances.  But ham radio IS a different beast
> commercial communications and yes, hams will tolerate a lower SNR.  In
> most of the time we have to.  That's part of the fun of it too.
> Also, Phil, there are some very valid forms of digital modulation that do
> use AM.  They work quite well, IN THE PROPER application.  FM is used in
> subscriber units, etc. because of the efficiencies involved in the
> modulation (and for some of the reasons you state in your post).  But the
> same cannot be said for that base stations that use amplifiers with
> efficiencies of 7% (and do NOT argue with me on that one as cellular
> amplifiers is my background).
> All in all, this discussion is meaningless because we are coming at it
> different sides.
> Enough.
> Jon
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
> http://www.qsl.net/ke9na
> "A life lived in fear is a life half lived."
> ----
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