Length & number of radials again
 

Radials Continued.

Perhaps some kind person who has been able to afford the latest issue
of NEC4 could calculate the radiating efficiency of a typical vertical
antenna of height 9 metres (29.5 feet) and diameter 50mm (2 inches) -

- when fed against a ground system of 50 uniformly distributed radial
wires, each 1.64mm in diameter (14 AWG) buried to a depth of 25mm (1
inch), of length 10 metres -

- in soil of typical resistivity = 150 ohm-metres and permittivity =
16.

But first I should like to ask, can NEC4 complete such a calculation
without human intervention or assistance? If yes then please
continue, perhaps keeping a record of the time involved.

Using program RADIAL_3 the answer is - Radiating Efficiency = 86.0
percent.

If several of you participate, perhaps using different tools, it would
be interesting to compare results. By all means, join in!

Thank you for your assistance.
----
Reg, G4FGQ.

Length & number of radials again
 

"Reg Edwards" wrote in message
...
Radials Continued.

Perhaps some kind person who has been able to afford the latest issue
of NEC4 could calculate the radiating efficiency of a typical vertical
antenna of height 9 metres (29.5 feet) and diameter 50mm (2 inches) -

- when fed against a ground system of 50 uniformly distributed radial
wires, each 1.64mm in diameter (14 AWG) buried to a depth of 25mm (1
inch), of length 10 metres -

- in soil of typical resistivity = 150 ohm-metres and permittivity =
16.

But first I should like to ask, can NEC4 complete such a calculation
without human intervention or assistance? If yes then please
continue, perhaps keeping a record of the time involved.

Using program RADIAL_3 the answer is - Radiating Efficiency = 86.0
percent.

If several of you participate, perhaps using different tools, it would
be interesting to compare results. By all means, join in!

Thank you for your assistance.
----
Reg, G4FGQ.

Reg, I made some changes to the antenna, but should not effect
the result too much. The maximum number of junctions without
a workaround is 36, so I reduced the number of radials to 36.
Ok, I know that give 37 junctions, but doubt it will effect the
result. I changed the vertical diameter to #14, since I
had a warning with the 25 mm diameter. Again there are
workarounds, but I did not want to spend all day figuring
out segmentation and length tapering.

You did not specify the frequency, but assume from the
dimensions it is 7 MHz. I used 7.000 MHz. The input impedance is
27.33 - j 109 ohms. Since I am only learning how to use the program
I don't know if NEC can provide the total radiated power. I
computed the total radiated power by summing power density over a
hemispherical region. For 100 W input I get a total radiated power
of 30.5 W. It took me 90 minutes.

Regards,

Frank

Length & number of radials again
 

Reg Edwards wrote:
Perhaps some kind person who has been able to afford the latest issue
of NEC4 could calculate the radiating efficiency of a typical vertical
antenna of height 9 metres (29.5 feet) and diameter 50mm (2 inches) -
- when fed against a ground system of 50 uniformly distributed radial
wires, each 1.64mm in diameter (14 AWG) buried to a depth of 25mm (1
inch), of length 10 metres -

Would it help to model this in EZNEC with the radials 1/1000
of a wavelength above ground?

Just heard a funny line on Stargate SG-1 on TV:
"This planet is as dead as a Texas salad bar."
--
73, Cecil http://www.qsl.net/w5dxp

Length & number of radials again
 

"Cecil Moore" wrote in message
y.net...
Reg Edwards wrote:
Perhaps some kind person who has been able to afford the latest issue
of NEC4 could calculate the radiating efficiency of a typical vertical
antenna of height 9 metres (29.5 feet) and diameter 50mm (2 inches) -
- when fed against a ground system of 50 uniformly distributed radial
wires, each 1.64mm in diameter (14 AWG) buried to a depth of 25mm (1
inch), of length 10 metres -

Would it help to model this in EZNEC with the radials 1/1000
of a wavelength above ground?

Just heard a funny line on Stargate SG-1 on TV:
"This planet is as dead as a Texas salad bar."
--
73, Cecil http://www.qsl.net/w5dxp

Ground planes above ground can approximate the results
from buried radials. The wires should be several wire diameters
above the ground, and not 10^(-6) wavelengths -- providing
that a finite ground, Sommerfeld/Norton method, is used.
The reflection coefficient approximation will produce large errors.

73,

Frank

Length & number of radials again
 

Frank,

Just to confirm we are both working on the same system, I have -

Number of radials = 36
Length of radials = 10 m
Diameter of radials = 2 mm
Frequency = 7 MHz
Antenna height = 9 m
Antenna diameter = 1.64 mm = 14 AWG
Ground resistivity = 150 ohm-metres
Ground permittivity = 16

IMPORTANT:

If NEC4 gives you the input impedance of the radial system I should be
very pleased to know what it is.

Otherwise we shall have no idea where the discrepancy arises - in the
radial system or in the antenna efficiency calculation.

Radiating efficiency is estimated by my program by the well-known
formula -

Efficiency = Rrad / ( Rrad + Rradials )

provided antenna and radials reactance are tuned out.

Whereas NEC4 calculates efficiency by integrating power flow over a
hemisphere WITHOUT tuning out antenna and radials reactance.
Altogether different.
----
Reg, G4FGQ

Length & number of radials again
 

Frank wrote:
Ground planes above ground can approximate the results
from buried radials. The wires should be several wire diameters
above the ground, and not 10^(-6) wavelengths -- providing
that a finite ground, Sommerfeld/Norton method, is used.
The reflection coefficient approximation will produce large errors.

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."
--
73, Cecil http://www.qsl.net/w5dxp

Length & number of radials again
 

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.

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.

Since I do not have GNEC I usually test my NEC 4 models
with NEC-Win Pro. Interestingly NEC-Win Pro actually
runs, with no errors, on below ground wires. The results
are usually pretty weird though.

73,

Frank

Length & number of radials again
 

Just to confirm we are both working on the same system, I have -

Number of radials = 36
Length of radials = 10 m
Diameter of radials = 2 mm
Frequency = 7 MHz
Antenna height = 9 m
Antenna diameter = 1.64 mm = 14 AWG
Ground resistivity = 150 ohm-metres
Ground permittivity = 16

IMPORTANT:

If NEC4 gives you the input impedance of the radial system I should be
very pleased to know what it is.

Otherwise we shall have no idea where the discrepancy arises - in the
radial system or in the antenna efficiency calculation.

Radiating efficiency is estimated by my program by the well-known
formula -

Efficiency = Rrad / ( Rrad + Rradials )

provided antenna and radials reactance are tuned out.

Whereas NEC4 calculates efficiency by integrating power flow over a
hemisphere WITHOUT tuning out antenna and radials reactance.
Altogether different.

Correct Reg, Those are the parameters I used, with the
exception that the radials were also # 14 AWG (1.64 mm).

You raise some interesting points -- How do I measure
the radial impedance? I have to think; given a vector
network analyzer, how would I measure a radial system
under laboratory conditions? this is what I need
to replicate with NEC. Since I have never made
such a measurement, I am not sure where to begin.
Would it be valid to consider one radial wire as
an "End fed zepp", and feed one end with an
ideal transmission line? As long as I know the current,
and voltage at the measurement point, I can determine
the input impedance -- problem is; voltage input
with reference to what?

As for the reactive input; this is of little concern to
NEC since it drives the load from a complex
conjugate source.

So far as I have been able to determine NEC does
not provide the total radiated power, only the
normalized far field in peak "Volts" -- i.e. V/m at
1 meter, at every angular increment. Usually
every degree. I take these data to determine the
power density at each increment, and sum
over a hemispherical region; where I take the
elemental area to be:
(r^2)*sin(theta)*d(theta)*d(phi). Since the
pattern is symmetrical I only need 91 points.

Frank

Length & number of radials again
 

Frank,

So NEC4 cannot calculate input impedance of the radial system and we
have almost reached a dead end.

Would it be possible to insert a loading coil ( 2.48 uH ) at the
bottom of the antenna to tune out its input reactance ( which is what
my program does.)

Then repeat the efficiency calculation and tell me what you get.
----
Reg.

Length & number of radials again
 

Frank,

So NEC4 cannot calculate input impedance of the radial system and we
have almost reached a dead end.

Would it be possible to insert a loading coil ( 2.48 uH ) at the
bottom of the antenna to tune out its input reactance ( which is
what
my program does.)

Then repeat the efficiency calculation and tell me what you get.
----
Reg.

===================================
Frank,

Alternatively, or in addition to, you could shift frequency nearer to
8.3 MHz where the antenna is resonant and its input reactance is zero.

And again do the efficiency calculation. Tell me what the efficiency
is and the frequency. Also the antenna input resistance.

You will see I am desperately trying to localise the discrepancy in
efficiency. It is either in the radials or in the antenna. You should
also learn something about NEC4.
----
Reg.