I guess the bottom line is that Jon needs a decent filter at the output - a
single cavity would probably do it. Of course, Jon, you could roll your own
- there have been a number of designs in the older RSGB VHF Manuals, and it
should be quite possible to pick up suitable air variables (the old
Hammarlund MAPC type) at Dayton for a buck a piece or less.
Someone commented on the IMD performance of the bricks and the bias. All the
solid state 'linear' amps I've worked on needed decent bias supplies to make
them linear. Way back when, RCA did an HF linear power transistor with an on
chip diode to get really good thermal coupling; this on chip was the
temperature sensor, and a simple little 3 transistor DC amplifier was used
for the bias. This really worked well. I guess that with an op amp, it's
even easier. (Way back when was when op amps like the 709 were almost a
weeks pay for a technician!). Just run the bias at constant current,
(although it needs a bit of temperature compensation, as beta goes up with
temperature). A shunt diode bias supply is OK for CW and FM!
Interestingly, if you look at some of the ads in CQ and QST, they give
figues for output v input for some of these amps. Something like a 3dB
increase in input gives under a db increase in output - that has to be the
give away. As a result, the '160 watt' brick is about 100 watts PEP or less
in reality.
I take it 'ICS' means ' Intermittent Commercial Service'. That suggests that
the limit is current, not voltage. Incidentally, just because a filter is
designed for a 50 ohm system, it doesn't necessarily mean that it looks like
50 ohms at the input when correctly terminated. The input impedance will
depend on the type of filter and the design parameters used, and will be
related to pass band ripple. In elliptic function filters for example,
passband ripple and thus SWR excursion is a parameter that you choose in the
design.
73
Peter G3RZP
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