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Slim Jim v J Pole
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Peter wrote: Hi all I have just modelled both the Slim Jim antenna and J Pole antenna using MMANA-GAL at the same frequency (145MHz) all other parameter equal and was some what surprised that they appear exhibit the same gain and radiation pattern. Based on an article I read in a English magazine many years ago I have always believed the Slim Jim had a slight edge over the J Pole. Wonder if others had a similar view and if my modelling is correct are there any advantages with the Slim Jim design? I recall reading though Cebik's excellent web pages on J-poles and variants thereof, a few years ago, and his modeling results pretty much corresponded to yours. Compared to a simple J-pole with the radiating element consisting of a single wire, the "Slim Jim" variant has two parallel radiators (e.g. the two conductors in a twinlead or window-line). The two radiators may or may not be shorted together at the top. As far as I can tell, electrically, the addition of the second radiator doesn't make much difference in how the antenna behaves. It doesn't change the overall shape of the current distribution curve on the radiator... just divides the current flow between the two wires. Since the current curve shape is the same, and the vertical span of the radiator doesn't change, the gain pattern will be the same. It does seem to have the effect of making the radiator "fatter", which can both lower its resonant frequency a bit, and make it a bit more broad-banded. I believe that for most purposes, the two antennas can be considered equivalent - they're both end-fed half-wave radiators, and will have the gain pattern you'd expect from a half-wave dipole. Both are equally vulnerable to having their patterns disturbed by current flow on the feedline or the mast, if you don't take some precautions to isolate these (e.g. via chokes, a half-wave coax balun, etc.). -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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