Topband: Fw: 25G insulators

[Tom Rauch]

> A shunt feed capacitor value of 120pF on 160m would be needed if the
> electrical height of the structure was between 3/8 wave and 1/2 wave
> (closer to 1/2), and yes, the voltage across the capacitor would be quite
> high.

Actually there is a good case for using a thick gamma conductor.

Assuming the tower is resonant, all the gamma reactance does is cancel
inductive reactance of the shunt wire. Thinner and/or longer shunt wires
require higher voltage capacitors. Bandwidth is also narrower, because the
cap and shunt wire form a series L/C circuit, the Q of which is set by the
ratio of Xc to feedline R.

Voltage across the cap must be the peak voltage of the highest
**instantaneous** power peak plus a safety factor. Some rigs, like the
IC775DSP, can overshoot well over 3X the adjusted power. Also any static
discharge around the tower can add to the voltage and start an arc, and air
variables are prone to reduced voltage holdoff from debris and moisture when
outdoors.

All things considered a person should probably use at least a 2X or 3X
safety factor for PEAK voltage, which is 1.414 times RMS. So a 600pF cap
would be about 1000v peak if you had a perfect rig, times the safety factor
of 2 or 3X.

> If the structure was 1/4 wave electrically, the tuned value of the
> capacitor would be about 650pF (134 ohms X), therefore the voltage with
> 1500 watts would be 5.5*134=737 VRMS.

Which requires at least 1000v breakdown times over 2X or 3X safety, more if
the radio has slow ALC and the amplifer has headroom and handles that radio
overshoot without arcing or clipping.

73 Tom