Frank,

Thank you. Is there any way you can forward the saved parameters. This is a
screwdriver antenna, I will remeasure the coil and double check.

My modeling of the free space antenna showed about 4 Ohms but it was with a much
simpler program. It was that program I used to measure Q.

Thanks,
Dan

Frank wrote:
The coil measures about 60 uH. The antenna is elevated about 3 feet on a
short tripod. The radials angle down the tripod legs and then out.

The coil is about 4 inchs in diameter, number 12, wound on a fiberglass
form. It is centerloaded. I am looking at it accross the yard, it is about
6 inches long. It is would with about a point .5 pitch. Calculations for a
1:1 pitch predict a Q of about 450.

Thanks,
Dan


Dan, I have just run a NEC 2 model of your antenna. I have used the
Sommerfeld/Norton ground, with average parameters of: Er = 13, and Sigma =
5 S/m. The top of the antenna is at 18 ft, and the base at 3 ft. I have
twelve 40 ft radials, spaced at 30 deg, and within the limitations of NEC 2,
placed them at 3" above ground. The first 5 ft of the radials drops from 3
ft to 3" at an angle of 45 deg. The monopole is center loaded with an
inductor of Q = 450. The model has 640 (6") segments and takes 3 minutes to
run (3.5 - 4.0 MHz in 50 kHz increments). What I notice is that I need 92
uH to resonate at 3.9 MHz. The input impedance is 12 ohms. I used a lumped
element model for the inductor. I may try a physical helix later. These
data do not seem to agree with your measured results. NEC 2, with the
Sommerfeld/Norton ground solution, is supposed to give a reasonable result
with wires at 10^(-3) wavelengths above ground (Basic Antenna Modeling,
Cebik p. 15-16 Nittany Scientific).

Gain and take-off angle are excellent, with max gain of -3 dBi at 28 deg.
elevation. The lower 3 dB point (8 deg elevation) gain is -6.6 dBi. The
NEC output file indicates an antenna efficiency of 54%. A free space model
shows an input impedance of 8 ohms, so your ground losses are not
significant (At least with my model).

Apart from adding horizontal wires, in "T" or inverted "L" fashion, I doubt
any antenna you could put up would match its performance at distances over
500 miles.

With 100 ft of LMR 400 the additional loss is about 0.45 dB. I would be very
interested to know if anybody has any ideas why my calculations appear to be
different from the measurements.

Regards,

Frank