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On Fri, 20 Jan 2006 06:56:42 GMT, Owen Duffy wrote:
One of the things that intrigues me is the common "expert" advice to cut radials for 7MHz to 33' long and bury them. It seems to me that when buried and considering the wire as a transmission line, the velocity factor will be somewhere between 0.3 and 0.8 depending on the soil type, so that 33' is likely to be closer to a half wave electrically, and present a relatively high and reactive impedance at the antenna base if it were not for the attenuation of the wave on the radial. Hi Owen, Calling them "tuned" radials is an artifact of their length being described in free space wavelength. The proximity of earth negates such illusions. The association with the necessity of being a quarter wave long comes by the field data obtained by Brown, Lewis, and Epstein. This was simply an arbitrary selection born more of the available wire being portioned out in binary increasing counts (2,4,8,16....) such that 119 radials depleted their stock (short of that magic 128). Their work has been offered on the web through the interests of our discussions here, and by one or several correspondent's scanning and posting their report. Google this newsgroup for that link using the authors as a keyword search. This was offered last summer. 73's Richard Clark, KB7QHC |
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