In article ,
Buck wrote:

The SlimJim resembles the J-Pole. The 1/4 wave section is a matching
network and it is probably not critical that it be vertical so the
height may be reduced by about 1/3. I am not sure what the folded
section does for it. but without the 1/2 wave section coming back
down from the top, the antenna is just a J-Pole.

The Slim Jim appears to be identical to the "top wire double radiator"
J-pole which is described in Cebik's multi-part discussion of J-pole
designs: see http://www.cebik.com/vhf/jp2.html

His analysis suggests that the performance and behavior of this type
of J-pole are essentially identical to those of a single-radiator
J-pole. There are some slight changes needed in the antenna
dimensions to get it resonated properly (the coax attachment point is
a fraction of an inch higher up and the total length is a bit
different) but once those are taken into account there doesn't seem to
be any real difference in performance between this variant, a
version with two wires which are left unconnected at the top, and a
version with one wire.

At their best, they appear to be simple 1/2-wave radiators. The hype
on the HamUniverse page about this antenna outperforming "1/2wave over
1/2wave over 1/2wave colinear!" seems a bit overdone - if it's ever
true, it would be in comparison to a rather badly-done colinear!

My expectation is that one could create a helically-wound version of
any of these J-pole versions. The matching arrangement would no doubt
need some adjustment. I'd expect the gain to drop off, with more
high-angle radiation appearing, in the usual fashion as the total
length of the radiating section is shortened.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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