In article ,
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
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