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#1
April 2nd 14, 09:21 PM
posted to rec.radio.amateur.antenna
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Radials
Ian Jackson wrote:
In message , writes Ian Jackson wrote: In message , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. Yebbut ........ You've just said "the ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequency". I assumed that "ideal" = "best". Poor choice of words on my part. Most of the literature I've seen on the subject recommends radials about 5% longer than the driven element. I don't recall anyone saying why. All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. True - but what's the angle of the radials got to do with their length? Not a whole lot but they do interact. I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. I had presumed you had already do this (or something similar) in order to say that slightly longer than a 1/4 wavelength was ideal. However, I have always assumed that the steeper the angle of the radials, the more the groundplane becomes like a vertical halfwave dipole - and the lower becomes the angle of radiation. The angle of radiation in free space for a vertical dipole and a GP no matter what the radial droop are both 0 degrees. The thing that changes the angle of radiation is the antenna distance from ground in wavelengths. -- Jim Pennino |
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