----- Original Message -----
From: Chip Owens<mailto:owensj@atd.ucar.edu>
To: topband@contesting.com<mailto:topband@contesting.com> ;
owensj@ucar.edu<mailto:owensj@ucar.edu>
Sent: Tuesday, August 16, 2005 9:23 AM
Subject: Topband: Improving efficiency of a short vertical antenna?
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
_Chip ... It seems you have done well within your constraints...... The only
thing I might suggest is making the top wires a bit longer and correspondingly
less loading coil.
John K9DX
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