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June 19th 07, 09:58 AM
posted to rec.radio.shortwave
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J-Pole Antenna Question
On Jun 19, 1:33 am, Ross Archer wrote:
On Jun 17, 11:54 pm, RHF wrote: On Jun 17, 11:54 am, wrote: In article .com, " wrote: On Jun 17, 9:46 am, "Robert11" wrote: Hello, Are vertical J-Pole VHF antennas (essentially) omni directional in coverage ? Thanks - - Yes it is omnidirectional. - - This antenna is merely a 1/2 vertical end-fed - - with a 1/4 wave matching stub. - - It's not a good broadband antenna for SWL. - - -- - Telamon - Ventura, California WHY ? ~ RHF . . . . Because it uses a 1/4 wave matching stub section to transform the low impedance coax input to the high impedance of an end-fed halfwave ("Zepp") at around 5K ohms. Because a 1/4 wave section is only a quarter-wave at a narrow band of frequencies, the impedance match gets poor too far from the design frequency (although I think odd harmonics work find, so 3/4 or (2n+1)/4 wave all work exactly the same. But that's a comparatively small percentage of total frequencies, so it's somewhat of a narrow-band antenna. As far as the radiation pattern, it is omni-directional in the horizontal plane if it's used vertically. The energy is somewhat "squashed donut" so there is some gain toward lower angles of radiation vs. higher angles. It's a fairly OK antenna where moderate gain and a relatively narrow band of frequencies are acceptable. Hope this helps. -- ross- Hide quoted text - - Show quoted text - RA - TYVM for the Explanation ~ RHF |
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