A few years ago I put up a temporary 60 foot vertical over
my 230 foot diameter ground screen. It was top loaded with 2
"umbrella" wires sloping down. The bandwidth was MUCH
narrower than your 65 foot vertical. IIRC, the 3:1 VSWR
bandwidth was less than 50 kHz. The feedpoint impedance
was about as predicted by EZNEC over a perfect ground.
IE, very low. It got out really well in contests
(anecdotal "data"). If you are seeing a bandwidth broad
as a barn door, it can probably only be explained by
substantial ground losses. I understand you can't get
a ground like I have, do the best you can and get on
the air.
We have to be very careful. Bandwidth of an antenna system is not just
related to loss resistance. bandwidth is also related to the ratio of
applied energy to energy stored in and around the antenna, or reactance in
the loading system. Bandwidth can go all over the place even without losses
going opposite of what we think.
For example, a 60ft vertical of #14 AWG wire over perfect ground and
virtually no loss can have a 3:1 bandwidth of 100 kHz when hat loaded.
The same antenna coil loaded with a coil, with significant loss, could be
less than 20 kHz wide.
There are countless cases where an antenna with wider BW has better
efficiency, and countless cases where they have less efficiency.
73 Tom
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