Dan, here is a preliminary run on a 12 ft monopole model structured as
follows:
base at 6 ft, 10 x 6ft radials. All #14 AWG. Ground - perfect, frequency
3.8 MHz.

Zin = 0.968 - j1847.55 ohms;
Efficiency = 87.4 % (structure copper loss);
Gain = 4.15 dBi;
Take-off angle = 0 deg;
Gain at 27 deg elevation (expected TOA with real ground) = +3.09 dBi.

I will try successive modifications to approach a practical model. The code
I used, modified so it should run in 4nec2, is shown below.

73,

Frank

CM 75 m Vertical 12 ft high
CM base 6 ft up, 10 X 6 ft radials
CM copper conductivity
CE
GW 1 24 0 0 18 0 0 6 0.0026706
GW 2 12 0 0 6 6 0 6 0.0026706
GM 1 9 0 0 36 0 0 0 2
GS 0 0 .3048
GE 1
GN 1
EX 0 1 24 0 1.00000 0.00000
LD 5 1 1 144 5.8001E7
FR 0 11 0 0 3.5 0.05
RP 0 181 1 1000 -90 0 1.00000 1.00000
EN


"Frank's Basement 2" wrote in message
news:dhmKf.6088$_62.3050@edtnps90...
Dan,

The lumped inductance of 4 +j1750 comes from your previous comment about
the
inductance range from 60 - 90 uH. I just chose the mid range value of 75
uH
at 3.8 MHz. To be exact 2*PI*f*L = 1791 ohms. The real part of 4 ohms is
based on an approximate Q of 400.

Incidentaly I am working at another location this morning. The computer
is
an old 600 MHz machine, with 384 MB of RAM, and Windows ME OS. The NEC
code
here takes 17 seconds to run.

73,

Frank
"dansawyeror" wrote in message
...
I see the length is set to 1.8 meters already. A 2 meter elevation
minimum
is
needed to lower ground effects.

How is the lumped inductance set of 4 Ohms and 1750 Z? What impedance
does
that
translate to? How did you calculate this value? Dan

Frank's Basement 2 wrote:
Hi Dan, thanks for the interesting info. You did not specify
dimensions,
but from your comments it appears you are using a vertical about 23 ft
high.
Such a monopole would have a 3.5 ohm input impedance when placed above
a
perfectly conducting ground, and gain about +4.5 dBi. Adding a center
loading coil raises the input impedance to 11.5 ohms, and gain +2.6
dBi.
Base loading provides an input impedance of 5.5 ohms with almost the
same
gain as center loading (Q = 400). Adding ten, 6ft radials, at 3"
above
an
average ground, the input impedance increases to 40 ohms, and
gain -6.3
dBi.

Adding lumped element loading coils, (75 uH, Q = 400) in each radial
(antenna base end) drops the input impedance to 37 ohms, and gain -6.4
dBi.
Don't know why this does not agree with Reg's program. Probably I
made
some
fundamental error with the NEC model. Included the code below, so you
may
see an error I missed.

73,

Frank

CM 75 m Vertical 23 ft high
CE
GW 1 64 0 0 23 0 0 0.25 0.0026706
GW 2 12 0 0 0.25 6 0 0.25 0.0026706
GM 1 9 0 0 36 0 0 0 002.002
GS 0 0 .3048
GE 1
GN 2 0 0 0 13.0000 0.0050
EX 0 1 64 0 1.00000 0.00000
LD 5 1 1 184 5.8001E7
LD 4 1 33 33 4 1600
LD 4 2 1 1 4 1750
LD 4 3 1 1 4 1750
LD 4 4 1 1 4 1750
LD 4 5 1 1 4 1750
LD 4 6 1 1 4 1750
LD 4 7 1 1 4 1750
LD 4 8 1 1 4 1750
LD 4 9 1 1 4 1750
LD 4 10 1 1 4 1750
LD 4 11 1 1 4 1750
FR 0 11 0 0 3.5 0.05
RP 0 181 1 1000 -90 0 1.00000 1.00000
EN





Frank,

Good morning. Let me start at the beginning. I have a loaded vertical
on

75

meters. The combination of the antenna and ground measure about 40
Ohms
at

the

antenna. The models all show such an antenna over a perfect ground
should

have a

radiation resistance of between 3 and 4 Ohms. That says the antenna
system

is

less the 10% efficient.

This then is a journey to reduce ground resistance. Attempts to add

radials and

wire mesh to the ground have had very little if no effect. This leads
to

Reg's

c_poise model. It predicts a coil in the range of 60 uH to 90 uH tuned
to

a 2

meter by 18 mm 'wire' will have a total resistance in the 2 to 4 Ohms

range.

Together this should result is a 8 Ohm system. The ratio can be
directly
inferred as an performance improvement of 5 to 1 or 7 db. This is
worth

some

effort.

To answer your question the first step will be one coil and one
radial.

The

objective is the get the antenna system close to 10 Ohms. From there I

will

experiment with adding radials and coils. I am not sure what to
expect.

Thanks - Dan





Frank wrote:

Not sure I understand what is going on Dan. Are you planning on
loading
each radial element?

Frank


"dansawyeror" wrote in message
...


These results were from Reg's c_poise program. The band is 75 meters
and
the coils were about 70 uH. The coils were a relatively large
diameter,

on

the order of a meter. The wire lengths were about 20 meters. By
varying
the length the coil, the coil wire may be varies from 1mm to 12mm.

Richard Clark wrote:


On Sat, 18 Feb 2006 08:20:38 -0800, dansawyeror
wrote:




The devil is in the details. Modeling shows large coils with 1 mm
wire
have a Q in the range of a few hundred. On the other hand a coil
with

12

mm tubing has a Q of about 2000. The R of the 1 mm coil is about 6

Ohms

while the 12 mm coil is on the order of 1 Ohm.

Given these model results it says there is a significant
difference
between 1 mm and 12 mm coils.


Hi Dan,

In the details, indeed. What is the LENGTH of wire in this 6 Ohm
resistor? What is the LENGTH
of wire in this 1 Ohm resistor? How many turns are in these "large
coils?" What is their diameter? What is their solenoid length?

Without these details, there is nothing said that is significant.

73's
Richard Clark, KB7QHC