wrote in message
...
On Sat, 20 Jun 2009 12:28:30 -0700, Roy Lewallen
wrote:

Frank wrote:
The horizontally polarized transmission has its E-Field parallel to
earth. Earth is a conductor (albeit a poor one, but in comparison to
free space, it is quite a short circuit). That E-Field's two
potentials are being laid across that conductor during the wave
propagation to that far point where EZNEC then sums up all field
contributions to present you with the lobe characteristic. It stands
to reason that at that great distance, the wave will have attenuated
considerably - hence the low value.

Removing the short circuit (going to free space) removes this
attenuation.

73's
Richard Clark, KB7QHC

NEC will calculate "Space wave plus surface wave" if required.

Frank

Have you tried doing this calculation with a horizontally polarized VHF
antenna? What did you find?

Roy Lewallen, W7EL
I'm not sure what you mean. EZNEC seems to say that a horizontally
polarized dipole seems to have zero gain (-99.99DBi) at zero degrees
elevation regardless of the frequency. So far, I have only tried 14
(the 20 meter example that came with EZNEC) Mhz, 491 Mhz (TV channel
17 center), and 527 MHz (TV channel 23 center). I switched to 527
because I can actually see a channel 23 transmitting antenna from my
window. 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?

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

I'm still confused,
Pat, N8CQV

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.

Now, in the real world... the ground is never level, even on the ocean where
it may look flat it curves down in every direction... so the horizon is not
at zero degrees for any antenna above ground level, over flat ground its
always below horizontal, on a hill or from a tower its even more negative,
and in a valley it can be way above horizontal. both of those cause those
predicted patterns to be changed a bit. Also, if the other antenna is not
at ground level then it is at some positive angle above horizontal... or if
its far enough away maybe a negative angle. You should also note that many
broadcast antennas for TV and FM are designed with a tilt to send the signal
down toward the ground, especially if they are on high hills or big towers,
they would have weak signals near the tower if they didn't tilt it down...
of course they don't care about beyond the horizon stuff anyway.

to confuse things even more on vhf/uhf frequencies signals are easily
reflected from hills, buildings, and other objects... they are also bent by
changes in air temperature and humidity. So in many cases a vhf/uhf signal
sent out toward the horizon may get bent down toward the ground and go
beyond the horizon by quite a distance, see 'tropospheric ducting' for more
info. hf signals of course get refracted back down toward the ground by the
ionosphere, so for very distant stations the arrival angle can still be
quite high, and almost never straight from the horizon.