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Recycling  Andrew Corporation  900MHz Power Amplifier modules for 23cms

Introduction
Probably everyone knows the Andrew Corporation for their excellent antennas and microwave dishes. In fact the “red flash” logo seems to be on every picture or TV shot of a microwave dish these days.  I was therefore surprised to find some surplus PA modules manufactured by Andrew.

They appear to come from a 900MHz band power amplifier designed for cellular base station use. There are two types, one with a single MRF9045 LDMOS driver device. And one with a push-pull pair of MRF 9060 LDMOS devices.

MRF9060 module

The MRF9060 is rated at a minimum of 60W CW output at 950MHz and the MRF9045 at 40 Watts. Pity we don’t have a band at 900MHz you might say. Well I’ve seen designs for 23cm PAs using both MRF9045 and MRF9060 devices so I felt a challenge coming on!  How can I get these modules working on 23cms?  

Well, I bolted the 9045 stage, to a heatsink and powered it up. LDMOS needs a positive bias on the gate so I rigged up my variable supply bias, stuck 28volts on the drain and set the standing drain current to 350mA as per the data sheet. I then swept the input frequency from 850 upwards, watching the output power, and as expected, it was WAY down when I got to 23cms.

Recycling begins
First of all, there is NO impedance data for these devices above 960MHz, so the design would have to be “cut and try“ Time to get out the scalpel and copper tape!

Looking at some photos and real hardware for other designs for this and similar devices (see DG0VE, G4HJW  and DB6NT) and getting out my vernier callipers, it seemed that to match these devices at 23cms requires a pretty simple microstrip network with low impedance (i.e wide) lines on the input and output. Half a day playing with Appcad and my Smith chart programme  allowed me to get a reasonable handle on the required matching circuit.

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