Hi,
As I had recently chosen to give uhf a go I decided to assemble an ldmos
amp kit I bought many years ago. It sat in my drawer for 5 years
I found this useful website http://g4fev.atspace.com/mrf18670cm.htm with
some details and I did put it together. Now I have some practical
experience with vacuum tubes, but not much with transistors so I'm a bit
stuck on matching the input and output. I'm well equipped with test and
measurement equipment , power supplies and such so I'm hoping to be able to
tune it eventually.
The values of components in the kit are obviously just a "loose
suggestion". The first time I run my nanovna over the input it was pretty
bad. The s11 dip was at the right frequency but the input swr was pretty
bad. From 2.5 to 3.5. The impedance was 30ohms and 12nH.
I tried adding some series capacitance and some shunt capacitancre but it
shifted the peak around more than it affected the depth.
I'm thinking currently the input is not that bad. But the output is another
story. For example I always get the same power of about 20W out regardless
if I feed 2.5w or 5W.
If this was a vacuum tube I'd fit a resistor on the output of the same
value as its anode impedance and I'd find the cofrect pi network by
sweeping from outside with a nanovna. But it's a non trivial task with the
complex impedance. Perhaps anyone can point me towards some articles how
to match output (and input) with a vna in an ldmos amp8.
73, Lukasz
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