alhearn wriote:
"That is, when it`s too short for resonance, reactance is capacitive and
is inductive if too long."

True that a too-short open-circuit vertical radiator is impeded by a
capacitive reactance and that a slightly too-long open-circuit radiator
is impeded by inductive reactance. We are aiming for a 1/4-wave antenna.
The switch from leading to lagging power factor or vice versa when
passing through the resonance point is abrupt. See Terman`s diagram. The
folded monopole is a short-circuit 1/4-wave transmission line stub and
it behaves like one.

My 1998 ARRL Handbook displays a group of too-short antennas in Fig
20.44 on page 20.22.
Item (E) of a group of 6 would be 1/4-wave antennas is called a tri-wire
unipole. It`s a vertical tower with a top support to suspend a parallel
wire on either side of the tower. The tower itself is #1 wire of the
tri-wire assembly and it is grounded, not insulated at the earth. Wire
#2 connects the top of the tower to the earth through a variable
capacitor. wire #3 drives the top of the tower and it is insulated from
the earth, and is driven against the earth.

The variable capacitor is used to tune out the too-short folded
monopole`s excess inductance and present 50 ohms to the feedline. The
text says:

"This technique will not be suitable for matching to 50 ohm line unless
the tower is less than an electrical quarter wavelength high."

Why? Over 1/4-wavelength high, the folded unipole is capacitive and
adding more capacitive reactance will detune it even more. The unipole
is synonymous with monopole.

It`s a fact that the sign of the reactance in the folded antenna is
inverted in the too-short folded antenna from that in the too-short
open-circuit antenna. That is a powerful advantage in that low-loss
capacitance can be used to match the too-short folded antenna and your
practical choice may have to be a lossy loading coil to match the
too-short open-circuit antenna.

What`s right is right and isn`t decided by a vote, but on the
probabilities from all the examples I`ve presented they are likely right
by the numbers.

I have a comment on the results of WA4GKQ`s modeling results of the
folded monopole. Models can be wrong for many reasons including garbage
in, garbage out. I`ve measured many folded monopoles in service, at
resonance, accepting measured full power and reflecting negligible
measured power and I`m sure these antennas are working well as evidenced
by expected performance every day over decades at many places here and
abroad. Seeing is believing for me.

Best regards, Richard Harrison, KB5WZI