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Re: Beginner birds

I have to disagree that an linear (SSB) transponder is simpler than FM.

In an FM transponder the high-power stages operate class C which is
inherently very efficient and 80-90% of the DC power is converted to RF. A
traditional earth-bound design can be copied and work well. Integrated
circuits are available to reduce the weight to a few ounces.

In a linear (SSB) transponder up to 50 independent signals must be
multiplexed onto one power amplifier. A class AB amplifier as used for
terrestrial HF SSB communication is useless in this application as the
distortion levels are too high. Earth-bound equipment that handles multiple
signals, such cable TV and cell phone base stations, use class A amplifiers
with about 10% efficiency. The HELAPS power amplifiers used on AO-11, AO-13
and AO-40 acheive higher efficiency by using two class A amplifiers in a
bridge arrangment plus presistortion to increase efficientcy. They are much
more complex than the amplifiers that are used for terrestrial communication
and more susceptable to failure. If I remember correcly, the maximum
published  power efficiency for the AO-13 HELAPS PA was 50%.

A third method that could combine the simplicity and efficienvy of FM and
the multi-channel capability of SSB transponders would be a digital TDM
transponder but we won't see it for a few years.



> Add to that the fact that FM transponders are necessarily more complex,
> and consequently more failure prone, than SSB transponders (which can
> be as simple as
> antenna->bandpass->preamp->oscillator/mixer->filter->output
> amp->antenna!), and SSB's tendency to sound natural even with multiple
> stations talking at once -- just a lot of people talking at the same
> time -- and it becomes evident that SSB is a superior and far more
> versatile mode.  Add to that the fact that it's SIGNIFICANTLY narrower
> and a whole lot cleaner in most cases, and lends itself to a whole
> range of other modes like PSK31 that take advantage of its linear
> nature, and you begin to see what a dead end FM is, technically
> speaking.

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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