In message , RST Engineering
writes
I tried simulating a Slim Jim in EZNEC and did NOT get the expected "aim at
the horizon" low-angle radiation simulated pattern. I got instead the
standard "hole in the donut" that you would expect from a vertical dipole
and a rather fat true donut shape rather than the expected pancake pattern.

I'm not at all sure how to connect a "voltage source" driver at the tap
point without being able to simulate the ground of that voltage source
tapped up from the bottom end as well.

You should have a wire across the bottom of the antenna at the tap
point. The voltage source is in the centre of this wire. A real Slim Jim
would be fed with a balun so both halves of the matching section are fed
symmetrically.

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.

73 Brian GM4DIJ
--
Brian Howie

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.




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.

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

Jim

On Mon, 22 Jan 2007 14:47:21 -0800, "RST Engineering"
wrote:

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

Hi Jim,

Most folks, and many more sophisticated folks lack the skill to really
confirm this. It takes 10% tools and 90% common sense. Most can
afford the tools. The rarer common sense (which includes experience)
again informs us that height above ground makes a considerable
difference in results which are often mis-ascribed to a
"revolutionary" design.

One need only examine your statement about comparing "towards the
horizon" to "up and down." I dare say very few can accomplish FS
measurements of any accuracy in the far field above 50 or 60 degrees
(and not simultaneously create a situation that abounds in reflections
off the platform they are using to achieve that elevation).

Successfully challenging these claims of superior performance is easy,
unfortunately making these claims is far easier. Worse yet are these
claims satisfy the dreams of wish fulfillment. We are regularly
submitted to glowing reports of such dreamers who refuse to consider
the aridity of fact and testing. One such is a designer of 9 foot
radiators for 160 Meters. His antenna design, too, has excellent
performance towards the horizon. One need only look at the list of
testimonials any one of which negates science.

73's
Richard Clark, KB7QHC

My FCC approved antenna pattern range is the overrun area of the local
airport. It is largely comprised of short weeds, rocks, and the occasional
dirt patch {;-)

If it wasn't freakin' winter, I'd put the 16' wooden mast with the up-n-down
mechanism for nulling out ground effects at VHF and see what the pattern of
a "real" SJ is. I think I'll wait for spring/summer for something that
really isn't time critical.

I hate to admit it, but the HP 608 is still the best thing I've found for
accurate attenuator calibration on the range. The Systron-Donner spectrum
analyzer with phase lock down to a few Hz. is the other end of the range; we
have large sheets of black conductive foam to keep the reflections from the
equipment to a minimum and use battery/inverter power to keep power line
interference down.

As for the choke balun referred to in another post, of course we use
decoupling toroids (3 of them are generally sufficient) where the coax
passes by the bottom of the antenna. Choke balun, decoupling ferrites, call
it what you will.

Jim



"Richard Clark" wrote in message
...
On Mon, 22 Jan 2007 14:47:21 -0800, "RST Engineering"
wrote:

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

Hi Jim,

Most folks, and many more sophisticated folks lack the skill to really
confirm this. It takes 10% tools and 90% common sense.

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