[AMPS] Pi-Net math

[Richard W. Ehrhorn]

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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