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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