More 9m Monopole
 

Reg, I think I now have a more accurate working program.
I am computing the E and H fields at only 20m, which
produces dramatically different results. Using
the Poynting Vector, S = 1/2*Re(E X H*), where
"X" is the vector cross product (H* is the complex conjugate
of H). Integrating |S| over a hemispherical region, of radius
20 m, shows a radiation efficiency of 80.3%, and a radiation
resistance of 27.7 ohms.

I have moved the test frequency to 8.1 MHz where the input
impedance of the antenna (99 x 10 m radials) is:
34.523 +j0.18. This implies a radial input impedance of
6.823 + j 0.18 ohms. These results appear to be much closer
to your program. Of course, now I am starting to wonder
if I should not redo the computation at 10 m to see how
significant the ground wave losses are. Closer than 10 m
is probably not practical since I do not think NEC can
compute the near fields in cylindrical coordinates.

The results also verify your comments concerning the
contribution of the ground wave to the total radiated
power. The "Sky wave" radiated power represents
only about 35% of the total input power.

My new Excel spread sheet contains over 4,000 active
cells, but is much easier to use than the previous method.
Using rotational symmetry, and other methods, the
NEC run time has been dramatically reduced.
I have also used almost lossless wire of conductivity
1E12 S/m. Perfect wire crashes NEC when using
the "Numerical Green's Function" -- which helps speed
up calculations.

To verify program accuracy I have computed the radiation
efficiency of an ideal 9m monopole over a perfectly conducting
ground. The results are accurate to within less than 0.25%.

Obviously my previous results are no longer valid, So will
have to re-calculate all the test antennas.

Frank

More 9m Monopole
 

"Frank's" wrote in message
news:mokIg.17654$365.9204@edtnps89...
Reg, I think I now have a more accurate working program.
I am computing the E and H fields at only 20m, which
produces dramatically different results. Using
the Poynting Vector, S = 1/2*Re(E X H*), where
"X" is the vector cross product (H* is the complex conjugate
of H). Integrating |S| over a hemispherical region, of radius
20 m, shows a radiation efficiency of 80.3%, and a radiation
resistance of 27.7 ohms.

I have moved the test frequency to 8.1 MHz where the input
impedance of the antenna (99 x 10 m radials) is:
34.523 +j0.18. This implies a radial input impedance of
6.823 + j 0.18 ohms. These results appear to be much closer
to your program.

===============================

I'm not surprised.

===============================
Of course, now I am starting to wonder
if I should not redo the computation at 10 m to see how
significant the ground wave losses are. Closer than 10 m
is probably not practical since I do not think NEC can
compute the near fields in cylindrical coordinates.

The results also verify your comments concerning the
contribution of the ground wave to the total radiated
power.

The "Sky wave" radiated power represents
only about 35% of the total input power.

==============================

This should have been obvious from the radiation pattern of a vertical
antenna above ground. For all antennas less than 5/5 waves in height,
maximum radiation always occurs along the ground, ie., at an elevation
angle of zero degrees, ie., the ground wave. The take-off-angle
computed by Eznec-type programs is misleading in this respect.

==============================

My new Excel spread sheet contains over 4,000 active
cells, but is much easier to use than the previous method.
Using rotational symmetry, and other methods, the
NEC run time has been dramatically reduced.
I have also used almost lossless wire of conductivity
1E12 S/m. Perfect wire crashes NEC when using
the "Numerical Green's Function" -- which helps speed
up calculations.

To verify program accuracy I have computed the radiation
efficiency of an ideal 9m monopole over a perfectly conducting
ground. The results are accurate to within less than 0.25%.

Obviously my previous results are no longer valid, So will
have to re-calculate all the test antennas.

Frank
==============================

To repeat - I'm primarily interested in the difference between the
sum of (antenna + radials impedance), and the antenna impedance, at
frequencies around 8.1 MHz and 24.3 MHz, corresponding to an antenna
height of 3 metres.

Thanks for keeping me posted.
----
Reg.