Check your MFJ-259B on a good dummy load. I just got done checking some
loads with mine. I thought I had a bad load but found that the MFJ is only
good to around 60 MHz and then it starts showing swr above that.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Hugh Duff
> Sent: Friday, April 27, 2007 1:53 PM
> To: amps@contesting.com; VHFtubeamps@yahoogroups.com
> Subject: [Amps] 2m W6PO input T network tuning.
>
> 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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