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#1
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![]() "W5DXP" wrote in message ... On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote: As built, both the pattern and the impedance already seem to be what I want. And it already has a counterpoise. The counterpoise is where the current flowing on the inside of the coax shield goes when it leaves the coax. The J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL doesn't radiate (much) and is part of the matching section and counterpoise. Quite often, the outside of the coax braid is also part of the counterpoise. -- 73, Cecil, w5dxp.com Thanks, that's what I thought, too. The nature of current flow on the outside of the coax is the reason so many construction articles mention winding a few turns of the coax into a choke, not far* from the antenna. I don't always do it but I probably should. * But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? My head spins just a little. "Sal" |
#2
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On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer. -- 73, Cecil, w5dxp.com |
#3
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![]() "W5DXP" wrote in message ... On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote: * But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer. -- 73, Cecil, w5dxp.com Thankew! "Sal" |
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