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Mark makes a good point. Those with EZNEC or other modeling program
might try this -- build a model of a ground plane antenna, and do an elevation plot. I'll describe the process with EZNEC, but you can do the same with other programs. With EZNEC, start with the example description VHFGP.EZ, and change the Plot Type to Elevation. Click FF Pat to run a pattern. In the 2D window, select Save Trace As, enter a name, and click Save. Then, from the junction of the ground plane and vertical wire, extend a wire downward for 3/4 wavelength, by adding a wire (in the Wires Window) with end coordinates 0,0,5 and 0,0,4.25 wavelengths. Give the new wire 15 or so segments. Change the Units to Inches or Millimeters, and make the diameter about equal to the outside diameter of your feedline coax. Do a pattern calculation on the new antenna. In the 2D window, select Add Trace, and enter the name of the trace you just saved to show the original pattern for comparison. The distortion of the pattern is due to current induced on the outside of the feedline by the antenna -- the radiating feedline is as much a part of the antenna as the intended radiator. (Those of you enamored with with CFA, EH, or other small antenna take note!) In fact, feedline radiation can actually contribute more to the overall field than radiation from the supposed "antenna". You can see this current in the View Antenna display after doing a calculation. You'll find that very small changes in the length of the feedline wire make profound differences in the pattern. (The phenomenon is similar to what happens when you change the length of a Yagi element.) This open ended feedline of course doesn't do a good job of representing most real feedlines, and the length was chosen to be nearly resonant. The real feedline is likely to be longer, and take a circuitous path, perhaps through the mains wiring, eventually ending at ground, and might or might not resonate. The pattern you'll actually get depends on this path and length, which are often hard to predict or quantify. But the program shows that pretty severe pattern distortion is possible for some feedline lengths, so you might or might not experience it. It also illustrates that the ground radials do an ineffective job of isolating the outside of the feedline from the antenna. I believe it's very possibly a reason people have such varying opinions about ground planes of various lengths and of J-poles -- and why people seem pretty uniformly satisfied with the AEA Isopole. It's not spectacular, but it's consistent, and independent of the feedline. To further your education, try different ground radial angles and lengths in the model antenna, and different radiator lengths. Another fun experiment is to put a "balun" -- a resistive or reactive load of about 1000 ohms -- at the "coax" feedpoint connection (wire 6, 0% of the way from end 1), and see what that does to the current and pattern. Try moving it, and adding a second one at various spacings. Interesting, isn't it? The conclusion you should reach is that a description of the vertical and ground plane wires alone isn't an adequate description of the actual radiating structure -- the feedline is an integral part of the system and has to be included if you're to have any idea of how the antenna will really work. Hopefully, this exercise will help you understand why, when you ask a question about how well a "simple" antenna will work, some of us hem and haw and answer "It depends". Roy Lewallen, W7EL |
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