I know a capacitive hat (or capacitance hat) lowers the resonant frequency
of a whip by electrically lengthening it, but by how much? When I was in
the service, one of our techs loaded an AFRTS BCB transmitter (1520 kHz, I
think) into a 35-foot whip that had a capacitive hat. The hat was a five
foot diameter ring of #10 wire with four spokes off the top of the whip. I
don't recall (or never knew) what other matching he did. 50 watts went
about ten miles, so it worked OK, not great.

The ARRL antenna book makes it look like the physical length is about 2/3 of
the electrical length (60 degrees physical length plus 30 degrees from the
hat).