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October 3rd 06, 03:38 PM
posted to rec.radio.amateur.antenna
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Rhombics
"Ian White GM3SEK" wrote in message ... Alan Peake wrote: Dave Oldridge wrote: Properly designed, they have a good reputation for doing what the theory says they will do. Just remember, though, that you're going to have to sewer almost half your transmitted power into the terminating resistor. But that's the half that would be going the wrong way, basically. But it would get there - eventually  Long or short path. But I don'tknow if both paths are ever open at the same time to the same extent. If not, then it may not be a problem. What about running a transmission line from where the terminating R would be, back to the feed point? Assuming you can match it all that is. No problem. Pipe the signal back from the far end into the shack, feed it into a circulator, and add it to the outgoing signal. Cecil will explain what happens to the power :-) Replying to Yuri's point: from personal experience of using a rhombic 100 wavelengths long for 2m moonbounce, it had only about the same maximum gain as a box of 4 mid-size yagis - and that is only while the moon is passing through the very narrow main beam, which only happens for a magic 20 minutes on certain days of the month. In other words, the rhombic did work, but the performance was nowhere near as spectacular as we had expected from its huge electrical length. What is undeniably true is that it *looked* spectacular! I've used many kinds of antennas since then, up to an 85ft dish, but not one of them has given me the same buzz as that rhombic. And there is the trap: buzz isn't the same thing as performance. We need to be very careful about applying dual standards. An unavoidable feature of all very long rhombics is that the main beam is very narrow, because the edges of the main lobe are sliced away by large numbers of sidelobes that are not many dB down. If we saw that kind of E-plane pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so what's "good" about the same feature in a rhombic? -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
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