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

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.

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

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.

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

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.

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.

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.

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."

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."

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.

73's
Richard Clark, KB7QHC

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

On Sat, 20 Jun 2009 20:50:59 -0400, wrote:

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.

Hi Pat,

You can use the "near field" table to then build a second antenna
(representing your own at the proper distance) with a load, and then
look at the power developed to that load to see how things perform.
This goes beyond the normal use, but for those who will slog through
it, there's a way.

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.

A combination of both, naturally.

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.

This is the contribution of ground reflections (which occurs for the
horizontal as well).

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)

It takes discipline to look at the situation without the filter of
expectation driving you.

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).

You now enter into the realm of the "Brewster Angle." This is a
phenomenon specific to vertical polarization (originally from the
field of optics) when an incident ray of radiation meets an interface
and is wholly absorbed by the second media (earth in our case) without
any reflection possible, but only for a "critical angle" (another name
for this) or less. If you were to change the characteristic of the
earth's conductivity (accessible in EZNEC), then you would find you
can shift that angle. Selecting a "perfect" ground will present you
with a total absence of the Brewster effect, and as high a zero angle
response as you might ever want to see.

73's
Richard Clark, KB7QHC

On Sat, 20 Jun 2009 23:19:34 -0700, Richard Clark
wrote:

On Sat, 20 Jun 2009 20:50:59 -0400, wrote:

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.

Hi Pat,

You can use the "near field" table to then build a second antenna
(representing your own at the proper distance) with a load, and then
look at the power developed to that load to see how things perform.
This goes beyond the normal use, but for those who will slog through
it, there's a way.

snip

73's
Richard Clark, KB7QHC

Thanks again, Richard. I just purchased an EZNEC license so I will
try that to further my learning. (I had been using the demo version).

73,
Pat