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Antenna article
"Jeff Liebermann" wrote in message ... On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food poisoning.org wrote: I can agree with the need for preventing feedline radiation but one thing you and Terry say may be erroneous. I believe the radiating element of a J-pole to be a half wavelength long, not a quarter-wave. You're correct. See my follow up posting. I've been building J-poles driving the short 1/4 wave element instead of the long 1/2 wave element. I looked at Terry's EZNEC wires list and observed the long side (the radiator or driven element) of the J is 57 inches and the short side (the stub) is 19 inches. (These dimensions agree with my idea of a 2m J-pole. I've made a few.) As I understand the action of the J-pole, net radiation is low or nil from currents in the lower third of the antenna (bottom third of the radiator and the adjacent stub). Correct. The original Zepp antenna was designed to be lowered from a hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by high voltage from the antenna to the dirigible would be considered a really bad idea. So, the antenna was designed to have zero voltage and probably zero current at the closest point of the antenna to the dirigible. The desired radiation comes from the top two-thirds (38 inches) of the radiator, which is very nearly a half wave at 2m. Allowing for so-called "end effect," it's almost exactly cut to 146 MHz. Well, here we disagree slightly. I once made a similar comment in this group about the radiation from an end fed collinear antenna came mostly from the bottom of the antenna. I was corrected by Roy Lewallen (W7EL) who pointed out that the current through the length of such an antenna is the same and therefore the radiation from all parts of the antenna are similarly identical. Unless I'm missing something (a real possibility), the situation is the same with a J-pole. https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ This is interesting: http://www.qikzepp.com/QikZepp_technical_information.html It shows a 1909 German patent for a Zepp antenna. However, the accompanying description once again makes the mistake of feeding the 1/4 WL section and not the longer 1/2 WL section: Early fixed installation Zepp Antennas were a half-wavelength long(or multiple) and fed with a 1/4 WL (or multiple) open wire feed line which uses only one of the wires. The feed line provided a matching section for the transmitter. Please check my reasoning and math Your comments are welcome. Thanks. I hate being wrong, but do I seem to be getting used to it. I'm used to it, too, so there's no chance you will monopolize it. I saw your follow up posting. Several comments. Hams who write construction articles sometimes support the notion of a consensus about feeding a J-pole from the stub side. Consensus doesn't carry any scientific weight. Some construction articles acknowledge such a consensus but claim it doesn't really matter. I've been happy soldering a small mounting tab to the short (stub) side, then bolting on a panel-mount SO-239 and extending the center conductor across to the long side. I'm not sure I care enough to experiment, A versus B. It would involve many hours' work to get it right. Maybe you weren't actually clashing with Roy (whom I admire). If he were regarding the upper two-thirds of the structure as the antenna (and regarding the lower two thirds only as a feed/matching-section/transformer), it might only have been a failure to communicate about what is the "antenna." I risk being wrong again by commenting on brief snatches of somebody else's conversation. With an end-fed half-wave antenna, I believe the current maximum is at the midpoint of the radiating section, 19 inches from the tip of the J-pole's long section, for example. The current minima would then be at the ends, most notably the high voltage end feed point. Cecil's 40m Zepp diagram indicates that the common mode currents are comparatively small, against the differential currents that do the work. Some comfort, there. I think we mostly have it nailed. Please, let any additional nails NOT involve my getting crucified. Again. "Sal" |
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