The other John Smith wrote:
"They tell me that shortening the antenna (folded monopole) below
resonance will lower the resistance and introduce capacitance."

I believe that is only half right. A too-short (less than
1/4-wavelength) antenna worked against a ground plane will have a lower
resistance than a 1/4-wave antenna. But, unlike the open-circuit less
than 1/4-wave whip, which has a series capacitive reactance, folding the
too-short element not only transforms its resistance to a higher value,
it reverses the sign of the input reactance. The too-short folded
monopole has a series inductive reactance.

Just like the shunt-fed grounded tower antenna, the inductance can be
tuned out with only a simple series variable capacitor, not an inductor
as is required with a too-short whip. See Fig 19, page 6-10 of the ARRL
Antenna Book, 19th edition for an example of a shunt-fed tower capacitor
tuning arrangement. The shunt-feed arrangement makes a too-small loop
just as a short folded monopole does.

Q of the folded monopole is lower than a whip because the folded antenna
is fatter. This gives more bandwidth.

Best regards, Richard Harrison, KB5WZI