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#13
February 4th 07, 04:12 PM
posted to rec.radio.amateur.antenna
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Best Books/Websites for Antenna Theory and Building Recommendations
"Richard Fry" wrote in message ... "Cecil Moore" : An antenna's abilty to "load" is proportional to its electrical length. An antenna's ability to radiate seems to be proportional to the physical length of the antenna that is carrying the highest current. __________ Perhaps unexpectedly, the intrinsic pattern and directivity of a physically/electrically short, unloaded monopole radiator are not greatly different than those of a self-resonant 1/4-wave monopole. The big problem with an unloaded, short radiator is the reactance at its feedpoint, which means that very little current will flow into the short radiator from any practical r-f source. But for the current that DOES flow in it, its radiation performance will not be much different than that of a self-resonant 1/4-wave monopole, at that same current flow (as NEC will show). Using a "loading" reactance to resonate the radiator allows maximum power transfer from the r-f source into the feedpoint. But the remaining issue is the low radiation resistance of the short radiator even when it is resonant, which is a small fraction of the other series resistances in the antenna system (ground and coil loss, mostly). As a result, much of the available transmitter power produces heat rather than EM radiation. RF I had an electronics instructor (not sure what he knew about antennas) say thet there was a 10/90 rule about antennas. That an antenna 10 % as long as a 1/4 wl will radiate 90%as well, wile he didnt say it I assume this means with all other sources of loss minimized. Is this anywhere near true? /Jimmie |
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