In message ,
W5DXP writes
On Friday, June 7, 2013 4:42:13 PM UTC-5, Ian Jackson wrote:
... one side is fed from the TX, and the other side is open circuit.

The 1/4WL folded monopole is resonant with a feedpoint impedance around
three times a 1/4WL single-wire monopole. Those two antennas have
approximately the same gain.

If one disconnects the folded element's ground connection leaving an
open-circuit, the feedpoint reactance becomes extremely high and the
antenna loses about 6dB of gain compared to the standard 1/4WL
monopole. It looks more like a 1/4WL shorted stub than an antenna.

I thought something like that would happen. However, because of the
close coupling between them, you might instinctively think that the two
conductors would still simply act as a single conductor, and the feed
impedance would be low (more like a non-folded monopole).

Can't you make a trapped dipole (or whatever) using 300 twin for each
leg, but with sections of one conductor cut out so you form a succession
of 1/4WL shorted stubs (of various lengths) in series, acting as traps
on the various HF bands?
--
Ian