Hi, Richard. Thanks for sticking with me throught this. I have added
comment and more questions below. 73, Pat

On Sat, 20 Jun 2009 17:08:25 -0700, Richard Clark
wrote:

On Sat, 20 Jun 2009 16:13:07 -0400, wrote:

I switched to 527
because I can actually see a channel 23 transmitting antenna from my
window.

Hi Pat,

If you think this experience contradicts EZNEC (or conventional
teachings which it concurs with), then the confusion comes from the
sense of "very far away." Truly, if you can see the Channel 23
transmitting antenna, then it is not that far away in the scheme of
things. It merely points out that you have not correctly modeled your
experience.
So,EZNEC models "very far away" and, in my example, the channel 23
tower is 5.1 miles away (about 10,000 wavelengths) so I need a
different model? A different program? I think you are correct in
saying that is my main confusion.


For those who may not missed my original post, I find it hard
to believe a horizontal dipole tuned to the right frequency (near 1:1
SWR with 75 ohm source) would not be able to hear a signal coming from
zero degrees elevation. In the real world, there are all sorts of
reflections off of all sorts of things that will make it work, but is
it true that there should be no signal if everything was ideal?

What is going to be a reflector to a source that is bore-sight with
the horizon?

Richard explained the attenuation of the E-field. That makes sense to
me, but doesn't really explain the other nulls at 6 degrees elevation
and every 6 degrees above that.

You didn't ask about that.
Sorry about that. The null at zero degrees is what surprised me so I
figured that if I understood it, the rest would make sense.

There are strong positive lobes at 3
degrees and every 6 above that. The plot looks like a nice flower :-)
I would think that attentuation of the E-Field would explain zero
degrees, but as elevation increased, the attenuation would decrease.

You have, again, lost sight of the meaning of "very far away."
I don't doubt that!

The EZNEC plot looks more like it is showing additive and subtractive
combining of the signal.

It is. What you see is called the Fresnel Zone if you were line of
sight.

Here, even if you can "see" the Channel 23 transmitting antenna, then
its various reflections could add up to ZERO. This, again, confounds
expectation, but it is the experience of every mobile operator who
encounters "picket fencing."
Good example. I started in this hobby 41 years ago on 6 meter AM.
However, I always thought that picket fencing was caused by
reflections from various objects (power lines, airplanes, metal
fences, water towers, etc,etc) rather than the radiation patterns of
the antennas.


Another reply mentioned a different program
that calculated ground wave in addition to skywave. Maybe that is
what I am missing. I normally think of ground wave as why VLF, LF,
and MF signals travel further than line of sight, though. Does ground
wave have a significant effect at VHF/UHF?

Yes, it is dead within a mile for Channel 23.

I'm still confused,

and so are a number of your respondents.

That may be true, but I appeciate them trying to help.

Since my last post, I changed the polarization of my EZNEC dipole to
virtical. I expected a nice donut shaped pattern, but instead saw
another flower shaped pattern with deep nulls at various elevations
including zero degrees. When I select free space, I get the donut.

I truely believe EZNEC gives valid results when provided with a proper
model. So, either I am not providing a good model to EZNEC (likely)
or a simple virtical dipole radiates very little at zero degrees
(-90dBi) and a lot at one degree (7.33 dBi), etc, etc (which seems
less likely to me).

73's
Richard Clark, KB7QHC