The transformer of an EFHW? matching network matches the 50 ohm transmitter
to an expected impedance into the wire of between 2500 and 3000 ohms.
Power is Volts*Amps, or Volts*(Volts/Ohms).
At 5 Watts of RF power,? ? 5watts = Volts*Volts/3000ohms,? ?therefore? Volts = sqrt(3000*5) = 122 Volts RMS.
At 100 Watts, it's still around 548 Volts RMS, not yet kilovolts
The polyvaricons are rated for something around 100 Volts:
? ??
Generally, you can press a polyvaricon somewhat beyond 100 Volts.
A polyvaricon works at 5 Watts for an EFHW, though?an air-variable cap would be a better choice.
If an air gap does arc over, it will self-repair once the ionized air circulates out.
And an air gap capacitor has a higher Q, and thus lower losses.
The qrpkits Sota Tuner successfully uses a polyvaricon:in an EFHW tuner:
? ??
They claim a max of 5 Watts CW, 10 Watts PEP.
On 10 Watt peaks, that would be? sqrt(3000*10) = 173 Volts RMS.
Perhaps they are more concerned about heating than they are about arcing?
Anything other than a resonant EFHW will be lower impedance,
and thus lower voltages across a tuning capacitor.
Jerry, KE7ER
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On Mon, Apr 15, 2019 at 12:33 AM, Sandeep Lohia wrote:
End fed 1/2 wave is a voltage fed antenna Sarma ji, there might be more then kilo volts present even with QRP at end of coil...
Therefore thin PVC variable Gang Capacitor won't help...
