Wimpie wrote:

With a VSA you can discriminate between the voltage component from the
PA itself and the reflected component (with slightly different
frequency). With a normal spectrum analyser, you can only determine
the magnitude of the PA's reflection coefficient (or VSWR as you
like). Given the dynamic range of today's equipment, you can inject a
very low level signal that may mimic load mismatch well below VSWR =
1.1.



One needs to have an analyzer with a narrow band detector for this, though.

Inexpensive analyzers like the TAPR VNA have untuned detectors, so the
PA's main signal will screw things up.

The N2PK uses a form of direct conversion detector, so your test signal
would have to be far enough away from the main signal so that the LPF in
the detector filters it out. The original N2PK design uses, I think, a
100 Hz filter, and the adc samples at 15.36 kHz with a digital filter.
The overall BW is something like 5 Hz, so putting your test signal 1kHz
away would probably work.