I think Manfred has a point - where the harmonics aren't going
into a load and are being 'reflected' by the harmonic filter, I
think the phase between fundamental and harmonics matters. The
right relationship can enhance Po and linearity, the wrong one can
hurt badly. It's not as simple as looking at what the filter is
presenting, there's also the effects of matching circuits going
back to the drains.
With the hf bands being mostly narrow frequency segments, it
should be possible to tweak the filters for optimum results. In
broadband systems such as commercial or mil use, that's difficult
and it's quite usual to have to find fixes for frequencies where
an amp struggles when presented with harmonic filters.
A real convenience of diplexer harmonic filters for the
experimenter is that you can develop a broadband circuit running
into a dummy load and then get very similar results when you hook
up the filters and antenna.
Steve
Gentlemen,
The real issue here in respect of this excellent discussion about diplexer
termination of harmonic energy is the profound impact on IMD and linearity.
These MOSFET and LDMOS RF power devices have non-linear voltage dependent
parametric capacitances that engender intermodulation distortion generation
and reflected harmonic energy control plays a key role; efficiency is just
one parameter consideration in the overall design objective that must be
judiciously balanced.
Leigh
VK5KLT
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