> I'm most familiar with Maxtrac's with regards to "less than
> full power" operation, i.e. that they can produce spurs &
> other undesirables when run that way.
>
> So other radios can have the same issues?
Many do, but not all, hate to over-generalize...
> I read that it is
> because when running below rated levels (anywhere in the tx
> chain) there exist impedance mismatches between stages, so
> "things" aren't running correctly.
It tends to most often be a class C amplification stage issue. The
multiplier stages in the exciter, if tuned properly, shouldn't cause
problems. As far as I'm concerned, only a poorly-designed transmitter would
rely on adjustments in early exciter stages as a means of varying the final
power output power...
> E.g. bandpass filters
> might be mismatched to active stages so they can't do what
> they're supposed to do. I'm thinking that most (all?) stages
> of an FM tx are running at saturation(?).
I'm not sure if you're talking about early exciter stages (multiplication
stages, buffers, etc.), or the PA, or both. Multiplier stages are, to
greatly simplify, harmonic generators. They produce harmonics (distortion)
which are then filtered/resonated typically by LC circuits to yield the
desired harmonic. So, yeah, I guess you could say that those stages are in
saturation. Buffer stages that act as limiters to remove any amplitude
variations would likewise be in saturation obviously.
Most class C PA's are comprised of cascaded gain stages. If you're lucky,
the inter-stage impedances will be 50 ohms. Many of the two-way equipment
we all know and love is designed this way, either at the inter-stage level
or, in some cases, at the inter-board level for PA's that are comprised of
multiple boards/strips. This makes it easy to get the power output level
you really need if you have a PA that's "too big" for the job at hand. If
you have, for example, a 100 watt amplifier but only need 25 watts, you
would bypass the last stage or two so that you're only running the earlier
stages to get the 25 watts out that you need, running those early stages at
or near their designed power output level.
Reducing collector voltage (power supply voltage) is *usually* a safer way
of reducing power output as compared to under-driving a class C amplifier as
far as stability goes. Efficency will likely degrade, but if you're running
the amplifier below its continuous duty power output rating in doing so,
most likely dissipation isn't going to be a problem. Of course, if you have
cascaded gain stages, this can be a catch-22 situation; if you reduce the
collector voltage to all of the stages, you've also reduced the drive to
each successive stage...
As a last resort, a high-power attenuator on the output can be used for
reduced output.
> I don't
> know if anyone has ever looked at spectrum of output for
> junque. Is there some better way of running at lower power
> w/o making RF crud?
Anyone who puts a repeater transmitter on the air without looking at it on a
spectrum analyzer post-install should be publicly flogged at the next
hamfest with a 4' scrap of 1/2" Heliax ;-)
Again, at the risk of repeating myself, there are well-designed class C amps
that remain extremely stable at reduced power levels. For example, I run a
lot of GE UHF Delta-S's as link radios, and you can turn them down as low as
they will go (about 4 watts) and they remain unconditionally stable, I've
never been able to get one to spur, and I've purposefully tried via load
mis-matching, opens, shorts, you name it. Many others I've found to be a
lot less forgiving. Several of the made-for-the-amateur-market repeaters
have been particularly bad in this regard.
--- Jeff WN3A
