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![]() "John S" wrote in message ... On 11/22/2014 10:19 AM, John S wrote: A special purpose antenna for maybe just below the 6M band. For railroad cars (a large metal ground plane). The antenna must be short enough to pass through tunnels and have a 50 ohm feed impedance. I saw this antenna in a book but I can no longer remember which book and, although I've searched, I can't seem to find a reference. It was probably from the 1950's. Anyway... Imagine a folded unipole over a large sheet of metal. It will probably have a high feed resistance of 100 or so ohms. But, if it is bent over 90 degrees starting a short distance above the ground plane, it can be adjusted to match a 50 ohm feed and with no imaginary component. This will satisfy not only the feed impedance but also the short height requirement. Other than the really nice ground plane of a railroad car's roof and using a frequency proportional to the plane, there is no obvious reason this cannot be use in other situations. Is that not really cool? Comments welcome, of course. # No, Guys, nothing that I have read so far is the thing I have in mind. # Picture this... # A folded unipole on a large sheet of conductive material. Let one of the # vertical elements be connected to the sheet. The other is insulated from # the sheet and has the generator attached. # Now, this configuration has a high feed impedance, even when resonant. # However, if you now bend the antenna at an appropriate point vertically # so that the top portion leans horizontally to the sheet, the antenna # gets lower to the sheet and the impedance lowers. # I have an EZNEC file that I can share if anyone is interested. I will # also continue to search my books for the example. Ahhh....the "folded" part didn't sink in when I responded. I suspect my original comments still apply. With two conductors, the current is half in each conductor, thus a doubling of radiation resistance at the feed point in one conductor. So by carefully selecting the height at which the vertical part is bent, there might be a good match for coax. I'll play with that configuration a bit on EZNEC. |
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