RST Engineering commented:

A real Slim Jim
would be fed with a balun so both halves of the matching section are fed
symmetrically.

I've never seen that configuration, just a coax feed to a tap on the
antenna.

If the feedline continues vertically downward from the antenna (J-pole
or Slim Jim) then RF current will flow on the outside of the coax, and
the feedline will both radiate and receive. A common-mode choke or
current balun (same thing, different name) at the feedpoint will largely
prevent this.

If you don't deliberately include the feedline in the model, you see
only the pattern of the antenna itself - it's like having a perfect
feedline choke. The difficult part of antenna modeling is to include a
realistic model of the feedline... and most people don't do it.



This however won't change the radiation pattern. There's nothing magic
about a slim jim, it's just a voltage fed folded dipole so the radiation
pattern will look like a dipole.

That's not quite true, for two reasons. An end-fed dipole has a slightly
different pattern from a centre-fed dipole, because the current
distribution is slightly different (one end has the feed current flowing
into it, while the open end has zero current). Secondly, the currents in
the two arms of the feed stub do not completely cancel. This leaves some
common-mode current in the stub, which will radiate and modify the
pattern of the dipole.

These real differences from a dipole tend to tilt the radiation pattern
upward, even in free space. Variations like the Slim Jim are aiming to
bring it back down towards the horizon... but there's very little
evidence that actually happens.


Yet most folks experimental results seem to show a radiation pattern
squirted a lot towards the horizon and very little up and down.

The patterns of simple antennas are extremely difficult to measure - it
needs accurate level measurements and also a very large test area which
is completely free of reflecting objects (including the experimenter
himself). Amateurs can meet the first of these requirements, but not the
second. G2BCX, the original developer of the Slim Jim, did have some
measuring facilities, but he still had to work in his own backyard.

Another major source of errors is that most experimenters fail to use a
choke balun at the feedpoint, so they are also measuring the radiation
from some undetermined amount of feedline.

What "folks' experimental results" most often consist of is changing the
antenna and checking how well it hits some distant repeater. That's fair
enough, but it tells them less about the antenna than they believe. For
a start, the new antenna will be in better physical condition... and
then very few people can resist the temptation to make a few other
improvements at the same time, like replacing the old coax and re-making
all the connections.

All those practical things can make much bigger difference than the
basic antenna design.

The differences between any of the J-pole variant designs are really too
small to be worth arguing about. What makes ALL the difference is the
engineering - how these designs are brought to life.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek