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

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

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

Hi Dan,

I have run the program to determine the precise resonance. The parameters
are as follows: Inductor 89.3 uH, and resonant at 3.92 MHz. I can send you
a zipped NEC output text file. It is about 190 kB. Also the NEC code I
used. You can plug in the appropriate data into an Excel spread sheet. If
you need any specific graphical output I can it as a JPEG file. I used the
default input source of 1 V peak, which accounts for the low power values in
the output file. The E-field data is far field, normalized to 1 meter.

The inductor is described as a lumped element, complex impedance, of 4.9 +
j2200 ohms. I arrived at this value based on your Q of 450, and just played
around with the imaginary value to achieve resonance within the 75 meter
band.

Let me know if I can send the above information to the address shown in your
posting.

Regards,

Frank

Frank,

My e-mail address is above. Thank you for all your help. I will try this.

Dan

Frank wrote:
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


Hi Dan,

I have run the program to determine the precise resonance. The parameters
are as follows: Inductor 89.3 uH, and resonant at 3.92 MHz. I can send you
a zipped NEC output text file. It is about 190 kB. Also the NEC code I
used. You can plug in the appropriate data into an Excel spread sheet. If
you need any specific graphical output I can it as a JPEG file. I used the
default input source of 1 V peak, which accounts for the low power values in
the output file. The E-field data is far field, normalized to 1 meter.

The inductor is described as a lumped element, complex impedance, of 4.9 +
j2200 ohms. I arrived at this value based on your Q of 450, and just played
around with the imaginary value to achieve resonance within the 75 meter
band.

Let me know if I can send the above information to the address shown in your
posting.

Regards,

Frank