Projects
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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