For the Traditional Cs/Cp Omega Match, Cp the shunt capacitor is
between the antenna and ground so it has to withstand substantially
the same voltage as the series capacitor, which is many KV.
In my shunt fed tower setup the shunt capacitor is around 75pF and
for that I use a short piece of hardline. This is obviously a fixed value.
My series capacitor is a gigantic air variable with about 3/8"
spacing between plates.
Paul - KW7Y
At 05:08 AM 7/29/2009, k8gg@voyager.net wrote:
>For the Shunt
>Capacitor:
>The RMS voltage V = I x R, so Vrms = 5.5 x 50 = 275
>volts.
>The peak voltage (assuming a sine wave) is about 1.42 x Vrms =
>390.5 volts peak.
>This has to be adjusted upwards for VSWR, but with
>a VSWR of 2.5 or less a 1000 volt capacitor should be sufficient for the
>shunt capacitor between the feed point and ground.
>
>For the
>series capacitor from the feed point to the bottom of the omega match wire
>or tube,
>The voltage across the series capacitor depends on the
>reactance (Xc) times the current.
>Using 200 pfd as the value of the
>series capacitor,
>Xc = -435j ohms. from the standard formula,
>Xc = -1/(2 x Pi x F x C)
>where F is frequency in cycles or Hertz and
>C = capacitance in farads
>
>The voltage across the series
>capacitor is then Vcs = 5.5 amps x 435 ohms =
>2391 volts rms and 3395
>peak volts, at a VSWR of 1.0
>Multiply the peak value by
>the VSWR to get a recommended minimum voltage rating
>for the series
>capacitor. In this case I would use a 10 KV vacuum capacitor, since
>a 2:1 VSWR gives almost 6800 volts across the capacitor.
>
>I hope
>this helps.
>
>73 to all
>
>George K8GG
>
>
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