[AMPS] Resonances

John Lyles jtml@lanl.gov
Thu, 14 May 1998 19:10:21 -0600


Steve Thompson sez:
>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"

Are you using the smith chart or what? and is it calibrated (open, short,
load) before all this. Otherwise all bets are off.

>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.

The short circuit appears to be series resonance. When it went open
circuit, or 300 Ohms, you have to trust that your open cal was done right,
as you are on the edge of the smith chart.

Inductance in this measurement could also be your leads or whatever you
attached the end of the network analyzer test cable (where the calibration
standards were applied) to the actual frame of the capacitor.
It's a tough measurement. If it is series resonant with the network
analyzer, you might want to try different leads to see if it changes. You
are basically connecting a 50 Ohm source across the cap, with wires or
straps.

On a different subject, was looking at the book "Fixed and Variable
Capacitors" by Dummer and Nordenberg (1960, McGraw Hill) and they listed
some interesting trivia on air variable capacitors. For instance, the tests
of Ekstrand in 1940 with sphere gaps at 1.8 MHz and 60 Hz determined that
the best breakdown strength for RF could be made when the the spacing was
about twice the thickness of the electrodes. With well rounded plates, the
thickness of the plates should be 1/3 - 1/2 of the air gap spacing.
Capacitors were tested with thin plates and larger air gaps, and found to
have large differences in 60 Hz versus RF breakdown. One held 25 KV at 60
Hz, 14.3 KV at 700 Hz, and only 11.7 KV at 1.57 MHz.

A chart shows typical breakdown ratings of air capacitors. 1.5 mm = 1 KV, 3
mm = 2 KV, 4.5 mm = 3 KV and so on. I thought inquiring minds would want to
be reminded.

John
K5PRO




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