Roy Lewallen, W7EL wrote:
"By my reckoning, a capacitive reactance of 1893 ohms at 14.2 MHz is 5.9
pF."

Dang me and my sliderule. Neither of us keeps track of decimals very
well. 6 pF can be obtained with wide spacing and high breakdown volts
with small plates. I don`t see much of a hurdle to clear because 6 pF is
a small capacitance.

As a practical matter, Andrew Corporation used another method to tune
its folded monopoles, I believe, because they had d-c continuity. They
supplied these antennas for decades to work at VHF. To move from 10m to
20m brings problems of scale, mechanical and electrical.

The original question said that the open-circuit ground plane has a
35-ohm feedpoint (at some elevation), and a folded ground plane has
about 140 ohms as a feedpoint. Neither ground plane matches the usual
coax at the antenna resonant frequency. Commercial antenna makers
advertise and deliver open-circuit and folded radiator ground plane
antennas which are nearly 50 + j0 ohms feedpoint impedance at a
specified frequency when mounted high and in the clear.

The folded radiator offers more lightning protection than the
open-circuit radiator. The folded radiator contains the ability to
step-up feedpoint impedance in cases where an open-circuit radiator
would have an inconveniently low feedpoint. Most TV yagis, for example,
use a folded dipole as the driven eleement due to the low feedpoint
impedance caused by mutual coupling with the parasitic elements.

Most energy in a lightning strike is at lower frequencies. Tune the
bands during thunderstorm season and notice where the static crashes are
worse, though much of this is due to propagation, some is due to the
shape of the transient. Where the folded antenna loop is small in terms
of wavelength, the loop is nearly a short-circuit and differential
energy is small.

I saw lightning problems solved by replacing open-circuit antennas with
folded-element antennas. As lightning is an interference problem taken
to an extreme, folded elements are also useful in solving some other
interference problems. But there are cautions. A folded dipole has a
resonance where it is only 1/4-wave from tip to tip. Its circumference
is 1/2-wave and resonates. This gives a folded dipole twice as many
resonances as an open-circuit dipole.

I make arithmetic mistakes more frequently when I don`t know for sure
that the number I calculate is reasonable or not. I do know that 20-kV
to 40-kV sparkplug voltage does not ordinarily leap many feet through
the air. I also have a formula for capacitance:

CpF = 0.225 K A / S

CpF = capacitance in pF

K = dielectric constant

A = area of one of the 2-plate capacitor plates
(sq. in.)

S = spacing between the plates in inches

For air, K = 1.0006

For a vacuum, K = 1

6 pF is not much so it should be easy to create.

Best regards, Richard Harrison, KB5WZI