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## [TowerTalk] Antenna Heights and EZNEC

 To: [TowerTalk] Antenna Heights and EZNEC k1ttt@arrl.net (David Robbins) Sun, 3 Feb 2002 16:02:22 -0000
 ```> > Unfortunately in the real world you are not going to find a station > > where you could take those measurements that is not encumbered with > > stuff that would make your measurements questionable. Things like > > other towers, other antennas, power lines, buildings, hills, rivers, > > the ocean, etc, etc, etc. and even if you did find a reasonable place > > where someone had put a single 10m antenna at 200' with nothing around > > it how would you make the measurements? Days of helicopter flights up > > and down and around and around? Then prove the helicopter wasn't > > disrupting the pattern you were trying to measure? > > > > Now, once you have made such a single set of measurements, what do > > they prove? You get a pattern for one antenna on one tower over one > > specific topography. The worst part is that the pattern you measure > > would change when the ground froze or thawed, when another antenna was > > put on the tower, when antennas on the tower rotated relative to each > > other, when the neighbor moved his RV, etc, etc, etc, ad nauseum per > > real world. > > So instead, we assume can use a model and get accurate results > 100% of the time that apply to the same situation? > > Or am I missing something? > 73, Tom W8JI > W8JI@contesting.com If you stay within the limitations of the model's methods, which for nec have been pretty well documented, then you should be able to compare models to each other. The 'accuracy' of the comparison would depend on how accurate your model inputs were... that is, if you model only the antenna and not the tower you would be less 'accurate' than if you included the tower. Now how important that is to the decision you are trying to make versus the added calculation time is something you would have to decide based on your knowledge of the possible interactions and the software. That is one great problem with the proliferation of relatively high powered computers and the free availability of this very complex software. Very few users of this software really understand its limitations, and I fear even fewer understand the complex interactions between antennas and their surroundings well enough to know what is important to model and what isn't. As the developer of a program that models lightning's effects on hv power lines (a task probably similar in complexity to modeling antennas), I know that even if I could make the program provide perfectly accurate results when compared to the real world that once I give it to a user to analyze their power line their results won't be perfectly accurate. Why? Because the user probably won't have enough detailed information about their power line, the ground under it, the tolerances of the installation, modifications from the initial installation, etc... and most likely the user won't understand the importance of certain inputs and will either ignore or just approximate values that he thinks are ok. In building antenna models to try to model a real station there are many of the same problems. Lets say I am trying to model my 160m inverted L's. I have 2 of them 100' high with a 30' top section 1/2 wave apart with 8 approx 1/4 wave radials raised 10' off the ground under each of them. Now, I can make a model of that very quickly from that information and compare patterns for them fed in phase and 180 degrees out of phase and get patterns that look just like are seen in an antenna text book. BUT, my antennas don't perform like that. I know the nulls off the side aren't as deep as predicted... but why? Well, it might be that there is a 120' tower about 50' from one of the vertical sections and a 150' tower about 50' from the other... it might also be that there are other guy wires, 2 or 3 other towers, a house, power lines, a pond, and all sorts of other variables... I can spend weeks building that model, days of calculation time, and find out that the side nulls aren't as deep as the bare bones model predicts. Or I can put it up based on the bare bones model and see that it performs reasonably well and accept the difference from prediction because I know that I didn't model all the possible interactions. If I am real curious I know I can add a couple of simple towers to the model and see how that changes the pattern to verify what my experience tells me, someone not as experienced may not know that a particular tower height or guy wire length might be important. Now someone else may put up the same antenna based on that model and find it works a quite differently because their surroundings are different, or they aren't really 1/2 wave apart, or they couldn't raise the radials off the ground, etc, and if they don't understand the implications of the changes they have made they will say the software doesn't work, where really they have gone outside the bounds of the model as defined. This was also a problem with some of the early software that automatically optimized yagi's (YagiMax was one of the first I tried that did this). If you blindly set up a yagi model and sent it off to optimize it you would get a really SUPER result sometimes... and if you blindly assumed the software was right and went to build it you would find it didn't work. What you had to do was apply some knowledge of the real world and what the limitations of the program were so that you could constrain the problem to come up with a buildable antenna... if you did that it worked rather well... not 100% by any means, but much better than trial and error. I guess what I am trying to say is that the software models are great, but you still have to know something about what you are modeling or you won't know that the results are even reasonable. David Robbins K1TTT e-mail: mailto:k1ttt@arrl.net web: http://www.k1ttt.net AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net ________________________________________________________________________ Where do you get ICE bandpass filters & beverage matching boxes? The same place that pays for the hosting of this list: The eHam Store. Order online at http://store.eham.net. ----------------------------------------------------------------------- ----- FAQ on WWW: http://www.contesting.com/FAQ/towertalk Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
 Current Thread [TowerTalk] Antenna Heights and EZNEC, (continued) [TowerTalk] Antenna Heights and EZNEC, Grant Bright, Ph.D. [TowerTalk] Antenna Heights and EZNEC, David Robbins [TowerTalk] Antenna Heights and EZNEC, Tom Rauch [TowerTalk] Antenna Heights and EZNEC, Pete Smith [TowerTalk] Antenna Heights and EZNEC, Tom Rauch [TowerTalk] Antenna Heights and EZNEC, Guy Olinger, K2AV [TowerTalk] Antenna Heights and EZNEC, Pete Smith [TowerTalk] Antenna Heights and EZNEC, Tom Rauch [TowerTalk] Antenna Heights and EZNEC, Pete Smith [TowerTalk] Antenna Heights and EZNEC, Tom Rauch [TowerTalk] Antenna Heights and EZNEC, David Robbins <= [TowerTalk] Antenna Heights and EZNEC, Greg Gobleman Fw: [TowerTalk] Antenna Heights and EZNEC, Grant Bright, Ph.D.