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.