On Sunday, February 24, 2002 11:54 AM Tom Rauch wrote:
> What we see in programs like EZnec are the results at a distance
> of hundreds of wavelengths over flat homogeneous earth, and I
> think that exaggerates what we picture as happening. If it did, we
> would never work anyone except coastal stations.
> What actually happens is the slope of attenuation changes and as
> we view a system at large distances the total attenuation is much
> different along the earth. I can give a good practical example of this.
> If you model a vertical over poor soil you will see nearly ZERO field
> strength at zero degrees. This is in spite of the fact we know we
> can hear BC stations for 25 miles or more on groundwave. We also
> know within a few miles of the antenna there is very little difference
> in FS at 20 degrees or along the ground!
The "standard" versions of EZNEC have the limitation of not including a
ground wave component. However "EZNEC-pro
(http://www.eznec.com/eznecpro.htm), which is a more sophisticated (and more
expensive) version of EZNEC, has an option for calculating ground wave,
which it adds to the sky wave component. The result is more accurate than
sky wave alone. With the ground wave analysis enabled, you will see that
vertically polarized ground wave signals suffer much less attenuation than
horizontally polarization, which, of course, is why commercial AM broadcast
stations use vertical polarization.
73, John W1FV
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