I think that Steve, G8GSQ, probably has the answer:
I have now convinced myself that it is nothing to do
with the matching network per se, or with flywheel
effects. Many grounded grid amplifiers like/need to
have a capacitor directly across the input. My belief
is that this is sometimes (and only
sometimes)necessary to ensure unconditional stability
over all of the driving cycle, and only marginally to
reduce harmonics.
As Steve rightly points out, it is not possible to
'remote' a capacitor over any length of transmission
line unless the reactance at the end of that
transmission line is optimised for each particular
frequency.
The use of a pi network at the amplifier input allows
a capacitor to be placed directly at the input whilst
preserving (on average) a resistive input.
This could be proved with a bit more effort than I am
prepared to make. A T network could provide exactly
the same impedance transformation and Q as its
equivalent pi counterpart. But it would not place a
capacitor directly across the amplifier input, and I
wager it would not achieve the desired effect....
73 Roger
VE3ZI
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