Length & number of radials again
 

Fellow Experimenters, Frank and Hasan.

I havn't the foggiest idea what you are doing with NEC4 but you should
be aware that, according to Radial_3, there are 3 resonant frequencies
with a single radial at lengths shorter than 10 metres and at a
frequency of 7 MHz.

The propagation velocity is very low. VF = 0.225

Funny unexpected things happen on multi-resonant lines especially when
Zo has a relatively large positive angle. Before you draw any
conclusions about deducing attenuation from your output data you
should take into account the line is -

1/4-wave resonant at 2.4 metres.
1/2-wave resonant at 4.8 metres.
3/4-wave resonant at 7.4 metres.

and at 10 metres it is very near to full-wave resonance. It can be
assumed the far end is open-circuit. Actually it isn't. It behaves
as if it is slightly longer.

It is significant that at 10 metres and 7 MHz, you have concluded that
the radial is about 20dB long. Which approximately agrees with my
program as being the length beyond which there is not much point in
extending it.

But the best way of determining attenuation is to do what I have
suggested - increase radial length in short increments and observe
what happens to radial input impedance. Eventually, Zin will converge
on Zo if it hasn't already done so. I should very much like to know
what Zo is and at what length it occurs. I have to assume NEC4 knows
what it's doing! ;o)
----
Reg.

Reg, Note that I am making all my calculations at 8.07 MHz, where
the structure is very close to resonance. NEC does indicate
the resonant lengths of the radials as follows:

1/4 wave = 2 m;
1/2 wave = 4 m......etc., to 1.25 wavelengths at 10 m.

I will try your suggestion of gradually increasing radial length
until I see a convergence trend at the complex Zo.

Frank

Length & number of radials again
 

Frank's wrote:
Here's what the EZNEC manual says: "Horizontal wires should not
be placed exactly on the ground, but should be at least 1/1000
wavelength above (and in the case of EZNEC/4, also below) the
ground."

Cecil, Probably the 1/1000 WL limit contains a safety margin.
This does not appear to be addressed by either the NEC 2,
or NEC 4 user manual.

From the NEC-2 User's Guide, p. 11: ". . .for a horizontal wire with
radius a, and height h, to the wire axis, [h^2 + a^2]^1/2 should be
greater than about 10^-6 wavelenths. Furthermore, the height should be
at least several times the radius for the thin-wire approximation to be
valid." All I can find in the NEC-4 manual is the restriction in terms
of wire radius.

Cebik's book "Intermediate Antenna Modeling", p 1-12,
states: "The minimum height for wires above
a Sommerfeld-Norton ground has two dimensions. The first
relates the height above ground limit to the wire radius.
The wire height (h) should be several times the wire radius
(a), that is, h~a. As well, the minimum height is related to
the wavelength for the frequency in use:
(h^2 + a^2)^(1/2)10^(-6)Lambda. If a is very small
compared to h, the wires may approach 10^(-6) Lambda
toward ground. ......reflection Coefficient approximation....
... the general recommendation is that ......
horizontal wires should be () 0.4 Lambda above ground".

Obviously, from the manual quote, EZNEC can invoke a
Sommerfeld-Norton ground.

Yes. EZNEC's "Real, High-Accuracy" ground is the NEC Sommerfeld-Norton
ground.

. . .

Roy Lewallen, W7EL

Length & number of radials again
 

Frank,

After correcting the resonant lengths for the change in frequency from
7 to 8.07 MHz there is almost exact agreement between NEC4 and
Radial_3.

Keep a record of length, Rin, jXin for each incremental increase in
length. They could be useful.

When using Radial_3 set the number of radials to 1. The input
impedance of the radial system will then be same as the input to the
single radial and will be displayed with a greater number of
significant figures.

When you compare results between the two programs set the frequency on
Radial_3 also to 8.07 MHz. The resonant frequency of the Radial_3
antenna is slightly higher - it's something to do with the end-effect
and the fact that a vertical antenna needs pruning by a few percent to
make it resonate at the theoretical value of 75/Height MHz. Since at
present we are concerned only with the radials it is better to use the
same frequency for both programs.

I think that will complete all I have in mind. It may involve you
with a tedious amount of work. If you find it interesting you could
do something similar at 28 MHz. The 20dB limit may be reached with an
even shorter length of radial. Radial resonances ( which BL&E never
dreamed of ) will be much more pronounced especially with high ground
resistivity.

Has anybody ever generated an input table for 120 radials?

It's all in the cause of Science. Thanking you in advance.
----
Reg.

Length & number of radials again
 

Reg Edwards wrote:
Frank,

After correcting the resonant lengths for the change in frequency from
7 to 8.07 MHz there is almost exact agreement between NEC4 and
Radial_3.

Keep a record of length, Rin, jXin for each incremental increase in
length. They could be useful.

When using Radial_3 set the number of radials to 1. The input
impedance of the radial system will then be same as the input to the
single radial and will be displayed with a greater number of
significant figures.
(snip)

Does Radial_3 assume that each radial is independent of its neighbors,
regardless of how close or far?

Length & number of radials again
 

From the NEC-2 User's Guide, p. 11: ". . .for a horizontal wire with
radius a, and height h, to the wire axis, [h^2 + a^2]^1/2 should be
greater than about 10^-6 wavelenths. Furthermore, the height should be at
least several times the radius for the thin-wire approximation to be
valid." All I can find in the NEC-4 manual is the restriction in terms of
wire radius.

Cebik's book "Intermediate Antenna Modeling", p 1-12,
states: "The minimum height for wires above
a Sommerfeld-Norton ground has two dimensions. The first
relates the height above ground limit to the wire radius.
The wire height (h) should be several times the wire radius
(a), that is, h~a. As well, the minimum height is related to
the wavelength for the frequency in use:
(h^2 + a^2)^(1/2)10^(-6)Lambda. If a is very small
compared to h, the wires may approach 10^(-6) Lambda
toward ground. ......reflection Coefficient approximation....
... the general recommendation is that ......
horizontal wires should be () 0.4 Lambda above ground".

Obviously, from the manual quote, EZNEC can invoke a
Sommerfeld-Norton ground.

Yes. EZNEC's "Real, High-Accuracy" ground is the NEC Sommerfeld-Norton
ground.

. . .

Roy Lewallen, W7EL

Thanks Roy, To be honest I did not really check the NEC 2 manual,
just NEC 4, so understand why I could not find it. My NEC 2
manual is probably a different version -- WDBN version 0.92,
and it appears on page 13.

Regards,

Frank

Length & number of radials again
 

"Reg Edwards" wrote in message
...
Frank,

After correcting the resonant lengths for the change in frequency from
7 to 8.07 MHz there is almost exact agreement between NEC4 and
Radial_3.

Keep a record of length, Rin, jXin for each incremental increase in
length. They could be useful.

When using Radial_3 set the number of radials to 1. The input
impedance of the radial system will then be same as the input to the
single radial and will be displayed with a greater number of
significant figures.

When you compare results between the two programs set the frequency on
Radial_3 also to 8.07 MHz. The resonant frequency of the Radial_3
antenna is slightly higher - it's something to do with the end-effect
and the fact that a vertical antenna needs pruning by a few percent to
make it resonate at the theoretical value of 75/Height MHz. Since at
present we are concerned only with the radials it is better to use the
same frequency for both programs.

I think that will complete all I have in mind. It may involve you
with a tedious amount of work. If you find it interesting you could
do something similar at 28 MHz. The 20dB limit may be reached with an
even shorter length of radial. Radial resonances ( which BL&E never
dreamed of ) will be much more pronounced especially with high ground
resistivity.

Has anybody ever generated an input table for 120 radials?

It's all in the cause of Science. Thanking you in advance.
----
Reg.

I find this very interesting Reg, and learn more about NEC
all the time. I think I could probably do a 120 radial model;
particularly with segment length tapering. With linear
segmentation, and 0.025 m segment length, the 36 radial
model has almost 15,000 segments. It seems that
radial segments can be sloped to their final depth, so
can probably reduce the segmentation requirement.

Frank

Length & number of radials again
 

Reg Edwards wrote:

[snip]

. . . Radial resonances ( which BL&E never
dreamed of ) will be much more pronounced especially with high ground
resistivity.

Reg,

I have no idea what BL&E might have dreamed of, but I did find one
curious item on the fifth page of their paper (page 757 in the original).



Where there are radial ground wires present, the earth current consists
of two components, part of which flows in the earth itself and the
remainder of which flows in the buried wires. As the current flows in
toward the antenna, it is continually added to by more displacement
currents flowing into the earth. It is not necessarily true that the
earth currents will increase because of this additional displacement
current, since all the various components differ in phase.



Whether this is "resonance" I cannot say. However, it is pretty clear
they understood that the radial currents did not monotonically increase
as the distance from the antenna decreased. There was some sort of
variation.

Their figure 42 seems to show significant "resonance", but there does
not appear to be any discussion of that behavior.

73,
Gene
W4SZ

Length & number of radials again
 

"Gene Fuller" wrote

Whether this is "resonance" I cannot say. However, it is pretty
clear
they understood that the radial currents did not monotonically
increase
as the distance from the antenna decreased. There was some sort of
variation.

Their figure 42 seems to show significant "resonance", but there
does
not appear to be any discussion of that behavior.

==========================================
I find it easier to think in terms of ground currents flowing 'away'
from the focal point rather than coming into it.

At 3 MHz, where BL&E made their measurements, in ordinary soils there
are hardly enough resonant effects to be noticed. At MF and below
there are no resonant effects. The equivalent transmission line is
mainly resistive. There is inductance of the radial wire but
propagation is largely independent of the value of soil permittivity
and hence on 'capacitance'.

Resonance effects begin to show at 7 MHz, At 21 MHz permittivity and
inductance predominate - especially with high ground resistivities.

At higher frequencies in very high resistance soils, buried radials
take on the characteristics of the elevated variety. But nobody uses
buried radials with vertical antennas at 30 MHz and above. Everybody
switches to dipoles!
----
Reg.

Length & number of radials again
 

"John Popelish" wrote
Does Radial_3 assume that each radial is independent of its
neighbors,
regardless of how close or far?
=======================================

No John. The input impedance of a set of radials is not the sum of
the individuals all in parallel. Input impedance is a non-linear
function of N, the number of radials.
----
Reg.

Radiating Efficiency
 

Cecil, could you or somebody, please use Eznec or something, to
determine the radiating efficiency of a 9.0 meter long vertical
antenna with a ground-loss connection resistance of 5 ohms, at its
1/4-wave resonant frequency slightly above 8 MHz.

Will discuss the result later.

- and oblige Reg.