Roy Lewallen wrote:
"Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893. This can be resonated as Richard Harrison recently pointed out
with a series capacitor. There`s no free lunch though---at 1kW, the
voltage across the capacitor is almost 9000 V RMS (about 12000 volts
peak) and even at 10 watts its almost 900 volts RMS.

I agree that at 1KW input to Roy`s folded dipole the power-correction
capacitor has 8466 volts across it. That`s close enough to 9 KV for me.

No single antenna fits all applications and alterations may adapt an
antenna for more than one application.

Antennas have a voltage to current ratio (Zo) which is a function of
position along along the conductor. Zo is also a function of conductor
length to diameter ratio. Fat wires have lower Zo than do thin wires.
Low Zo means low voltage (relatively).

Also spacing the folded antenna conductors farther apart lowers
impedance and Q. This helps bandwidth.

Raising the current by lowering Zo is no panacea as the volts across the
capacitor are Amps x XC.

The capacitance of 1893 ohms at 14.2 MHz is about 0.000006 pF. If the
plate size is kept significant, the spacing should be good for 12 KV
with no problem.

The Andrew Corporation folded monopoles I am familiar with were usually
working with 500-watt VHF FM transmitters in our land-mobile operations.
Bandwidth required was 2f + 2d, if I recall, and the (f) was maximum
modulation frequency, and the (d) was the peak deviation. Bandwidth was
less than 20 KHz. Half-duplex was the communications mode so we needed
the antenna only to work at one carrier frequency.

It was a cakewalk. Antennas only flashed over on lightning strikes and
the 50-ohm Heliax saw most of the lightning as a common-mode disturbance
and rejected its passage through the coax (via counter-emf from coax
distributed inductance).

The VHF Andrew folded monopole element was similar to the slide pipe on
a trombone only made of stainless steel. It had clamps to hold its
position once set. Andrew set its length for 50-ohms at our frequency, I
suppose, and adjusted the reactance for a net zero. When we set it atop
our tower we always had about 500 watts forward and nearly zero
reflected power. Some of these are surely operating well at this moment
after 50 years or more, though they`ve surely accumulated many small
pits from countless lightning strikes.

Best regards, Richard Harrison, KB5WZI