Topband: Improving efficiency of a short vertical antenna?

[Chip Owens]

I'm looking for suggestions for improving the efficiency of a short 
vertical antenna on 160M.

Due to local restrictions an antenna height of 35 feet is what I need to 
work with. At present I  have top loaded this 35 ft. vertical with four 
25ft. long wires. They run down at a 45 degree angle. The ground system 
consists of 65 radials ranging anywhere from 40ft. long to about 100ft. 
long. The radial length is limited by the size of the backyard.

The antenna is self-resonant at 2.88MHz, where it looks like 12 ohms. It 
is matched at 1825kHz using a tapped inductor. The antenna connects to 
the top of the inductor and the bottom end is grounded. The 50 ohm feed 
point connects a few turns above ground. I'm using a big edge-wound 
inductor from an old AM broadcast transmitter. I had to short several 
turns at the top of the inductor to bring the antenna to resonance.

At 1825kHz the measured impedance is 5.89-j198 ohms.  The impedance 
doesn't change radically over the 1805 to 1835kHz frequency range. Right 
now the 1.5 to 1  VSWR bandwidth is 14kHz and the 2.0 to 1 VSWR 
bandwidth is 26kHz.

Since the antenna is short electrically I believe the efficiency is 
pretty low. I'd appreciate any suggestions that would help improve the 
efficiency of this antenna system, keeping in mind the 35ft. height 
limitation.

Thanks!
Chip, NW0O