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Re: 1.2 Ghz amplifier question


Downeast Microwave makes a 35w amp using two modules...after bad
experiences with kit builders it is only available as an assembled unit.
Steve also makes a four-module 65w unit.  I have a single M57762-18w unit
as driver for my 7289-K9EK water-cooled amp (not QRV at this point).

Ed - AL7EB

At 11:57 AM 2/27/03 +0000, Grant Hodgson wrote:
>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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