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1.2 Ghz amplifier question


There is no fundamental problem with designing and building a 1.3GHz PA
using two Mitsubishi M57762 PA modules.  The only issue is with
cost/perfomance; you start to get into the realms of diminishing returns.

To use 2 PA modules properly, networks are required to perform the input
splitting and output combing functions.  These networks should offer a high
degree of isolation such that the two modules cannot 'see' each other, even
under worst case fault conditions whereby if one module was to fail, the
other module should still work properly and see good 50 ohm impedances at
the input and output.

Such networks exist in a number of forms, including the Wilkinson splitter,
90 degree (or branch line) hybrid coupler,  Gysel combiner and 1/4 wave Sage
wire-line amongst others.  All of these work well, but all of them require
some form of terminating resistor, which is 50 ohms in all cases apart from
the Wilkinson which requires a 100 ohm balance resistor.  Other networks
exist that do not require a terminating or balance resistor, but as far as I
know none of these networks provides any isolation between the ports.  (A
2-way antenna splitter is such a network where isolation is not required.)
Under fault conditions, the resistor has to be able to dissipate a large
portion of the output signal from the remaining module.

There are also losses in the networks, although with care these can be
reduced to small levels.

The PCB for a dual-module PA is therefore much greater than twice the size
of a PCB for a single module, and you have to add the cost of a high-power
resistor. Of course the heatsink has to be twice the size to dissipate the
heat.  With all these factors added together, a dual-module PA costs more
than double the cost of a single module PA, and due to the losses you will
get slightly less than 3dB extra output signal - which is less than 1 S
point on most meters.

There are various designs for 2-, 4-, 8 and even 16-module PAs, an internet
search for M5772 amplifier will yield a number of results. I beleive that
there is a JA station who uses 32 modules for EME - I'd like to see his 140
amp PSU....


Grant Hodgson  G8UBN
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