> However, the load impedance of most PAs
> will vary
> significantly with the drive level,
I think you meant source impedance.
> and the input impedance
> of a duplexer
> cavity is always reactive
Not necessarily. You can tune a duplexer very close to 50+j0 at the pass
frequency. It's at frequencies off the center frequency that the selective
nature of the duplexer makes it appear as a Z other than 50+j0.
> However, a
> simpler approach is to install an impedance matching device, sometimes
> called a "Z-Matcher", at the output of the PA and adjusting
> it for maximum
> forward power.
I disagree with this and feel it is bad advice. The point where the
transmitter makes maximum power may occur at some load Z other than what the
transmitter was designed for. Just because you can eek out a few extra
watts by futzing with the Z-matcher doesn't mean you've done anything to
improve the stability of the amplifier, nor is it guaranteed to be operating
at maximum efficiency, nor do you know if the increase in power you're
seeing is due to new spurious/oscillation products being generated due to a
bad match.
If there is a means of adjusting the loading on the PA via a Z-matcher, be
it internal or external to the PA, it should first be set "flat" at 50+j0 on
a VNA or return loss bridge, then hooked up to the PA, and while monitoring
current draw, power output, AND spectral purity, make minor adjustments if
necessary to find the best balance.
The Z-matcher itself performs as if it were a narrow-band device in the
sense that it will only correctly match at a single frequency. The load Z
of the duplexer, which varies as you get away from its tuned pass frequency,
is going to present a varying load, which the Z-matcher is going to
transform again, maybe for the better, maybe for the worse. So, at best,
the Z-matcher is a band aid in my book.
I would much rather see an isolator with a very good input return loss on
the output of the PA if there are problems that can defintively be proven to
be caused by the load Z of the duplexer and antenna system. I've always
maintained that if you have amplifier stability problems or problems
achieving rated specs for the PA (power out, current draw, spurious, etc.),
then you've got fundamental problems that need to be fixed either in the
antenna system or the amplifier itself.
--- Jeff
