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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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Horizontal Dipole - zero degrees elevation
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#3
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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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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 |
#5
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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 |
#6
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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. |
#7
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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. |
#8
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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 |
#9
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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. |
#10
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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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