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November 14th 06, 02:56 PM
posted to rec.radio.shortwave
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End-fed dipole
Bart Bailey wrote: In ps.com posted on 14 Nov 2006 04:37:18 -0800, wrote: Begin A vertical HF antenna can be built with, dah, a vertical electrical 1/4 wave over either very conductive, think salt water marsh, or an array of raidals. The more the better up to around 190 or so, Contrary to popular ham myth 16 radials is barely enough. 32 is better with 64 and moer starting to get there. I have a Butternut HF2V with 0 (zero) radials, but a fairly good ground, no salt marsh, just two stakes three feet apart driven into the ground in an inner city neighborhood in San Diego, and I typically propagate on 80m better than the guy who gave it to me that uses the same model, but with an ideal radial pattern. We run similar power levels so am thinking I might be getting some parasitic excitation of very close power lines that he doesn't have, nor the accompanying noise either. Those power lines are a 138kv tie line, a 12kv distribution line and a 4kv distribution line, all less than 100 feet from the antenna. Another possibility is that since everything in the shack is very well bonded and grounded, that at RF maybe I'm getting some counterpoise effect on the entire city wide grid, now that's a long wire! -- Bart When it comes to radiation patterns it is hard to calculate what is really gonig on , and even harder to make meaningful measurements. Ground reflection, amoung many other things, makes a lot difference. Whne I was much yougner, and I expected to be able to make meaningfull measurements, I wasted several weekends by trying to measure the real pattern from a 20M dipole at about 30' elevation and a 20M vertical on a large grounded metal roof. I nver got results that made any sense. Your observation about power lines is correct. "Nearby" wires can produce pattern distortion beyond the abiltiy to calculate. One nice thing about 6M and up is the ease with which you can make readings that knid of relate to reality. My 1/4 has a 6dB lobe toward the front because I drive a hatch back Civic and ther is moer surface metal toward the front. Of course the lobe isn't exactly straight ahead, but canted about 15 degrees to the right for an unknow reason. The NEC modeling program prediction and measurements in an open field agree to a impressive degree. And by using the remote S-meter in a local repeater and having it repeat the level as I drive a tight circle confirmed the theory and my measurements. But I still can't explain the lobe's offset. The antenna is centered to within a few mm. I thoguht maybe wiper blades so I removed them, then I thought BCB radio antenna so I removed it. No change. Sadly at HF reality and theory have almost nothing in common. The nice peaks and nulls of a 10MHz horizontal dipole are seldom as distinct as the graphs shown in text books. If it works then it is good regradless of what theory might say. Some of my first DX HAM contacts were on really bad, not effeceint, random antennas, When I started using "correct" antennas, dipoles for the most part, my ratio of heard and worked versus heard and not worked increased dramaticaly. But I still have a wide range match box with my QRP rig just in case I can't errect a real antenna. I once worked Scotland runing 5W CW on 80M into a random, ~~100', wire wraped along a rail fence. Terry |
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