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Re: July 2001 President's Letter

Hi Phil,

I am pleased that AMSAT is considering flying a new mode. The bulletin board
already sounds like old QSTs from the 1950's when SSB was new. Here are some
comments and a question.

> I'd like to know more about the 23cm
> power amplifiers that are currently available. How much peak power can
> they produce if the duty cycle is kept low, say to a few percent?

I have only seen two types of amplifiers in general use by hams in the U.S.
on 1296 MHz and I happen to have both.

1) 23 cm transceivers and many transverters use the Mitsubishi M57762 hybrid
module ($90 new). These produce 10 watts with about -25 dB IMD or 16 watts
saturated. There are products using 4 of these to get  64 watts out. These
are peak power limited devices and 16-20 watts seems to be the limit.

2) High-power amplifiers are all based on the 3CX100A/2C39 vacuum tube. I
don't know if they are still being manufactured but they are widely
available on the surplus market for $30-50 each. There are three small
manufacturers of cavities for amateur use that take 1 to 6 tubes and one of
them may still make complete amplifiers.  Although specified for 25 watts CW
output each tube can easily generate 250 watts peak with 1350 VDC plate
voltage. N6CA and others are using these to generate 250W average power by
modifying the tube for water cooling. This tube would work well for TDM with
a duty cycle of 10% or less as the power supply and cooling requirements
could be shrunk by the same factor as long as the repetition rate is above
120 Hz.

> On 23 and 13cm we can assume a power-limited channel (as opposed
> to a bandwidth-limited one) so that dictates a power-efficient
> modulation method like BPSK or QPSK plus forward error correction with
> a rate low enough to get good coding gain -- say rate 0.4-0.5.

Why use QPSK? This is essentially two orthogonal PSK channels. The power
required per bit in QPSK is the same as for PSK but amplification must be
more linear and decoding is harder. Just double the PSK bit rate and use twi
ce the bandwidth.



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