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Old June 20th 09, 02:44 PM posted to rec.radio.amateur.antenna
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Default Horizontal Dipole - zero degrees elevation

Hello all you antenna experts,
I have been a ham and an engineer for a long time, but I have never
delved into antenna theory. So, consider me a newbie for this
question.

I recently was helping a friend set up a crude antenna to connect to
his DTV converter box following the June 12th conversion to DTV. I
explained that a dipole was a simple antenna that could be used. He
was very interested in the subject so the conversation soon turned to
theory. I explained the 468/freq formula and eventually mentioned
EZNEC. I have never used EZNEC myself so I downloaded the demo
version and now have all of 24 hours experience with it. I started
with the included backyard 20 meter dipole. I was surprised that
there was no radiation toward the horizon. I figured that I was too
close to the ground so I changed the frequency to 491 MHz (RF TV
channel 17) and shortened the dipole accordingly. Still no zero
degree radiation. I raised the dipole to 100' - then 1000' - then
10000' - still no radiation at zero degree elevation. I then found
this in the help section:

-----------------------
Because the far field sky wave from a horizontally polarized source is
zero at a zero elevation angle for any ground type, and a vertically
polarized source produces zero sky wave for any finite-conductivity
ground, attempts to calculate a 2D pattern without the ground wave
component under these conditions will result in an error message.
-----------------------

Sure enough, changing to free space instead of a real ground changed
the pattern to what I would have expected. I would have thought being
many wavelengths above ground would be just as good as free space, but
EZNEC doesn't think so. Am I missing something? Does a horizontal
dipole really have a problem seeing a broadcast TV transmitter out on
the horizon? Thanks. ...Pat
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Old June 20th 09, 06:03 PM posted to rec.radio.amateur.antenna
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Default Horizontal Dipole - zero degrees elevation

On Sat, 20 Jun 2009 09:44:14 -0400, wrote:

Sure enough, changing to free space instead of a real ground changed
the pattern to what I would have expected. I would have thought being
many wavelengths above ground would be just as good as free space, but
EZNEC doesn't think so. Am I missing something? Does a horizontal
dipole really have a problem seeing a broadcast TV transmitter out on
the horizon? Thanks. ...Pat


Hi Pat,

EZNEC is presenting you with lobe characteristics that are at a very
great distance (and, yet, it has no sense of skip as that would be
tested in the domain of a propagation modeler). Zero degrees up to 10
degrees are minutely examined by those interested in skip, but this is
not a TV phenomenon (not to be confused with ducting which can offer
similar DX from great distances).

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
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Old June 20th 09, 06:27 PM posted to rec.radio.amateur.antenna
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Default Horizontal Dipole - zero degrees elevation

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


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Old June 20th 09, 08:28 PM posted to rec.radio.amateur.antenna
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Default Horizontal Dipole - zero degrees elevation

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
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Old June 20th 09, 09:13 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jun 2009
Posts: 18
Default Horizontal Dipole - zero degrees elevation

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



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Old June 20th 09, 10:13 PM posted to rec.radio.amateur.antenna
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Posts: 797
Default Horizontal Dipole - zero degrees elevation


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.

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Old June 20th 09, 10:15 PM posted to rec.radio.amateur.antenna
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Default Horizontal Dipole - zero degrees elevation


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


p.s. you want to see more real world get one of the terrain analysis
programs that lets you place real antennas over real non-flat ground and see
where the signals really go.

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Old June 20th 09, 10:59 PM posted to rec.radio.amateur.antenna
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Posts: 18
Default Horizontal Dipole - zero degrees elevation

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
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Old June 21st 09, 12:26 AM posted to rec.radio.amateur.antenna
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Posts: 797
Default Horizontal Dipole - zero degrees elevation


wrote in message
...
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


yes, it will probably be a better approximation, especially if the two
antennas are in sight of each other. the ground effect is mostly
appropriate for hf and at very long distance. vhf has many other effects
that cause reflections and ducting that kind of over ride the ground image
model that most modeling programs use.

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Old June 21st 09, 12:41 AM posted to rec.radio.amateur.antenna
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Posts: 133
Default Horizontal Dipole - zero degrees elevation


wrote in message
...
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


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