Hi Bob...
With all due respect; your statement below appears to me to be incorrect. The
addition of an "L" network to the output side of the plate pi adds another pole
and typically increases harmonic attenuation by about 12-15 dB at the second
harmonic, somewhat more for higher orders. Achieving that much improvement by
increasing loaded "Q" of the pi network, if doable at all in practice,
typically would result in excessive circulating rf current, place exorbitant
requirements on tank components to avoid overheating, and increase tank losses
significantly.
Trying to implement FCC harmonic-suppression standards for a typical 1.5 kW
peak output ham amplifier using only a pi output tank is marginal design. The
extra harmonic attenuation which can be realized relatively easily by adding an
"L" network of loaded "Q" ~2 to 3 -- a relatively small "L" net inductor and a
bandswitch section -- generally is the most practical way to end up with an
acceptable design margin... at least in commercially manufactured amps, which
are most likely to be subjected to FCC scrutiny for conformance to rules.
Further, a pi-L yielding a given level of harmonic suppression will have
significantly wider bandwidth than a simple pi of equivalent suppression. IMHO
it's also/therefore more tolerant of frequency excursions, as well as of
antenna impedance variations. This would be expected intuitively, because the
pi-L's tapered-transmission-line equivalent at a given frequency has one more
step of impedance taper than does the pi's equivalent.
Have I missed something? It happens all the time...Peter? Ian? John? Tom?
Anybody?
73, Dick W0ID
-----Original Message-----
From: bob alexander [SMTP:realex@flash.net]
> Sent: Thursday, August 12, 1999 8:48 PM
To: Ian White, G3SEK
Cc: amps@contesting.com
Subject: Re: [AMPS] Pi-Net math
>Hi Ian,
> It is the Q of the input l-network that establishes
the bandwidth and harmonic rejection of the filter.
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