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jimbo wrote:
Dan Richardson wrote: I'm wondering if you are using a common mode choke? Also what type of mast are you using to support the antenna? I have tried with and without a coax loop, 5 turns about 6 inches in diameter. I couldn't see any difference. I use a 1/8 inch diameter rope to hang the J-Pole from a joist in the basement or from a roof rafter in the attic. A single choke blocks conducted common mode current. But a J pole (and ground plane, for that matter) can also have considerable common mode current due to coupling between the antenna and feedline, which can distort the pattern and reduce the gain at the horizon. This can actually be made worse by a choke in some cases. Two chokes, about a quarter wave apart, are necessary to reliably reduce the coupled current. The amount of conducted and coupled current you have without chokes depend on the feedline length and orientation, so it can be small in some installations and large in others. I suspect that's one reason why people report such widely varying degrees of success with this antenna. Roy Lewallen, W7EL |