On Thu, 14 May 1998 22:11:44 +0100 Steve Thompson <amps@txrx.demon.co.uk>
writes:
>I had only a few moments before finishing today, but while I had the
>network analyser on, I looked at a 100pF variable such as might be
>used
>as an anode tuning cap - semi-circular vanes about 1.5" diameter,
>spacing around .1"
>
>I measured between the rear centre bearing and one of the corner bolts
>of the stator rail, frequency range 10-300MHz.
>
>Fully meshed it looks like 110pF at 10MHz, short circuit at about
>100MHz, then goes inductive to an 'open circuit' at 270MHz. Actually,
>it's lossy and looks like 300 ohms or so resistive.
>
>I added a few inches of thick wire to simulate the anode to capacitor
>lead and then, looking at it from the anode's point of view, both
>series
>and parallel resonances ('short' and 'open' circuit respectively) went
>down in frequency a bit.
>
>At half mesh, it doesn't get as far as an 'open' circuit resonance.
>
>The loading cap out of a MLA2500 (fully meshed) goes through 3 low-Z
>resonances and 2 high-Z resonances in the frequency range (2 and 1 at
>half mesh).
>
>More info. next week when I get a chance, got to go pack for a contest
>now.
>
>Cheers, Steve
Steve, next time run the test with the cap in-circuit. Several years ago
I ran the SB-230 on the analyzer and found multiple high impedences
100-300MHz.
If you look at Heath construction you will find the Tune cap has brass
end plates, which sit on cad plated steel washers which then are in
contact with the aluminum chassis. All held together with a pair of 6-32
screws and no shaft to panel contact. Talk about inviting problems.
Then you throw in a "VHF" choke in series with the plate choke that is
inches away and in parallel with the 80-20M tank coil AND has a series
resonance around the reported parasitic frequency and you wonder why
almost a dirty look will cause a zap.
The Heath is somewhat affected but really look at the tuning differences
in a Dentron or Amp Supply product with the covers on and off.
73 Carl KM1H
>
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>
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