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Myths and Legends of Antennae
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#23
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Myths and Legends of Antennae
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#24
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Myths and Legends of Antennae
Lostgallifreyan wrote:
wrote in : You do understand that a log periodic is a series of 1/2 wavelength dipoles from the lowest to highest frequency of interest? I do now. Point taken, this is not a portable thing... I looked very briefly last night before sleep, my (wrong) assumption was that the elements were basically directors using a log scale to compress the effective result into a smaller space... I didn't realise that each was a separate normal sized 1.2 wave dipole. Well, almost... The longest element is 1/2 wave at a frequency slightly below the lowest frequency and the shortest element is 1/2 wave at a frequency slightly above the highest frequency. The taper in the element lengths is described by a log function. The total number of elements and the boom length effect both the gain and "flatness" of the SWR over the frequency range. -- Jim Pennino |
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Myths and Legends of Antennae
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Myths and Legends of Antennae
Lostgallifreyan wrote:
wrote in news The total number of elements and the boom length effect both the gain and "flatness" of the SWR over the frequency range. I can sort of see why the military would like it. Given their resources, it could be used to rapidly find direction and frequency for many transmissions, one after another, with no immediate need to co-ordinate a triangulation effort. Though I suspect that locating sources isn't their primary reason for using one. Correct; they are used because the military is not constrained to discreet bands like hams and it is much simpler to erect one antenna and tower to cover a wide frequency range, though HF global comm is less and less used by the military these days as everything is going encrypted space based. Short range tactical comm still uses a lot of HF, but the antennas there are usually whips with autotuners at the base. -- Jim Pennino |
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Myths and Legends of Antennae
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Myths and Legends of Antennae
Lostgallifreyan wrote:
wrote in : HF global comm is less and less used by the military these days as everything is going encrypted space based. And, I suspect, by policemen. No fun stuff on the edge of the FM broadcast band these days... Public service is mostly going to VHF/UHF trunked systems. http://en.wikipedia.org/wiki/Trunked_radio_system -- Jim Pennino |
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Myths and Legends of Antennae
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