Dan,

Will be interested to know how you get on with the antenna. I suspect that
adding loading coils to radials is about the same as adding loading coils to
any part of an antenna system. They do nothing to effect the radiation
efficiency, only modify the input impedance.

73,

Frank

The effect of the radials is a surprise. I would not expect that short
radials
would work well at all? The system predicts about 9 Ohms. That is closer
to what
is expected and over 6 db better then the 50 Ohm reading.

I will try experimenting and let you know.

Thanks - Dan

Frank wrote:
That's correct Dan. I just wanted to systematically build up the
antenna,
adding a component at a time, to note where the major losses are. This
was
the first trial with no loading -- except for copper conductivity.

From the other model you sent me it seems that any other attempts are
redundant. The major losses are due to ground loss, as expected.
Unfortunately this can only be overcome by increasing the length, and
number, of radials -- something that is pretty well known. Also
inductive
loading of the radials does not seem to have any effect, except for
marginally decreasing the antenna efficiency.

I have been interested in installing a short monopole for 160m, so am
very
interested in your results. I have a fairly large lot (visible on
"Google
Earth), so am not so restricted in radial length.

73,

Frank


"dansawyeror" wrote in message
...

Frank,

I tried the nec below. The result was resonant at 21.9 and about 34
Ohms.
I am not competent at reading nec cards yet, however the model editor
does
not show any coil loads. That could explain the frequency?

Dan

Thanks - Dan

Frank's Basement 2 wrote:

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