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...
On Sat, 20 Jun 2009 21:13:51 GMT, "Dave" wrote:


what you are missing is the 'real world'. eznec is probably modeling over
a
perfectly flat infinite surface. In the far field in a perfect world the
signal along the surface is a combination of ground wave and sky wave, the
ground wave decays rapidly with distance leaving the sky wave which will
always be very small along the surface. now remember, the frame of
reference is at ground level, not the antenna height, so zero degrees is
along the infinite flat surface. And there is nothing in there that
models
where the other antenna is... it just creates a picture of how the
strength
of the fields are at a given elevation/azimuth angle from the reference
point.

Dave,
Your paragraph above helped. For VHF and above, in the real world, am
I better off using EZNEC's "free space" setting instead of real
ground? I know at HF frequencies, where antennas are often close to
the ground, it makes a big difference, but could free space be a
better approximation of VHF antenna many wavelengths off the ground?

Pat

Hi Pat

I recently re-entered the Ham community after being away from electronics
for 40 years. I was extreemely resistant to accepting the accuracy of
computer modeling. As I have become more familiar with computer modeling
data, I now respect its value very much.
Before acceptance of the accuracy of computer modeling, I recently made my
own complex impedance measurement equipment for 2 meters. I also made
actual radiation pattern measurements using polar orbiting satellites at the
"range illuminator".
Your question about how well the data from EZNEC matches "actual"
radiation pattern is probably related to our not being able to feed the
appropriate information into the computer modeling program.

You are probably interested in 'just learning' how accurate EZNEC is.for
predicting antenna sensitivity toward the horizon when the signal is
horizontally polarized. But, if you want to get actual radiation pattern
data from any given antenna at VHF where polar orbiting satellites are
sending a beacon signal, you can simply record the receiver RSSI while the
satellite passes overhead.
I have some EZNEC and actual patterns measured using the 137 MHz signals
from NOAA satellites. My data wont convince you that EZNEC is quite
valuable for predicting the performance of YOUR dipole. But, if you have
interest in knowing more about what I have done for recording antenna
pattern data using polar orbiting satellites, contact me. I suspect you
already know more about what you need than anything I can add.

Jerry KD6JDJ