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Horizontal Dipole - zero degrees elevation
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 |
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