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[Amps] 2m W6PO input T network tuning.

To: amps@contesting.com, VHFtubeamps@yahoogroups.com
Subject: [Amps] 2m W6PO input T network tuning.
From: Hugh Duff <hduff@cogeco.ca>
Date: Fri, 27 Apr 2007 13:53:29 -0400
List-post: <mailto:amps@contesting.com>
I'm trying to get the L-C-L  "T network" input circuit of my newly built 
W6PO 2m amp to look like 50 ohms at 144 MHz.
The network consists of two series inductors with a variable shunt 
capacitor in the middle to ground. The 8877 tube is installed and the 
input circuit was constructed per the article. Using my MFJ-259B 
analyzer I find that the circuit looks best like 50 ohms with low SWR at 
108 MHz.  Tuning the shunt capacitor and spreading/compressing the coils 
varies the resonant point within a few MHz but I'm still a far way off 
from 144 MHz.

My initial thought was to remove a turn or two from each of the two 
inductors, or wind some new ones however when I spread the windings of 
the coils (decreasing the inductance), the resonant frequency decreases. 
This is backwards to what I expect. I guess I'm thinking in terms of 
parallel resonant circuits where the frequency increases with a decrease 
in the value of the L or C, so the above scenario doesn't make sense to 
me. I'm willing to accept that the T network is a combination of complex 
series and parallel circuits but I'd like to understand why.

As my boss at work likes to say, "do whatever makes it work", so I'll 
wind some larger inductors and hopefully end up near 144 MHz.  I'm just 
bothered and puzzled as to why it isn't working with the coils wound as 
prescribed.

Comments welcome and appreciated with thanks in advance.

73 de Hugh VA3TO
www.va3to.com

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