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Re: Geosynchronous Satellites



on 12/24/00 10:12 AM, Jose Mihotel at jmihotek@gte.net wrote:

> 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.

Excuse me, Jose, but they are.  I used to design PAs for Motorola as well
and now I sell them for another major amplifier company.

Most of the amplifiers produced and used in the cellular industry today are
Feed Forward Multi-Carrier LINEAR amplifiers.  These amplifiers are VERY
linear, have a capability of handling peak to average ratios of 10 or higher
and as a result are VERY inefficient.  If you doubt me, look up the specs of
amplifiers from companies like Powerwave or Spectrian and see how much
current they draw at 27 Volts and see how much power they rate their modules
at.  Then you'll see I AM correct!

And FM DOES have more bandwidth than SSB or AM!  Why don't they then use FM
down in the MW or Shortwave broadcast bands?  After all, the fidelity would
be so much better!

I can listen to an SSB conversation on HF with a 2 KHz filter.  Put two KHz
guard band on either side and you basically have 6 KHz total bandwidth for
an SSB channel while well over 25 KHz for an FM channel (yes, there is an
FCC move to make manufacturers develop FM radios with 6.25 KHz channels).
You can have narrow bandwidth FM, sure.  But I am speaking in the context of
a TYPICAL ham radio setup.

> 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.

Not true.  AM has less efficiency than FM voice in a single channel.  FM
voice can utilize saturated power amplifiers.  AM requires linear
amplification.


> 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.

No kidding!  Gee, I didn't know that you could get a 50% efficient PA.  I
guess the one I built for HF is 50% efficient just by accident!

Oh stupid me.  I guess I don't know jack!  It doesn't matter that I used to
build, test and install linear amplifiers and it doesn't matter that I now
sell them.  I guess I an everyone else in my industry are just plain wrong.

Jon

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

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

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