Slim Jim v J Pole
 

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?

--
Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Slim Jim v J Pole
 

In message , Peter
writes
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?

Some websites indicate that the only significant difference is that the
Slim Jim has a somewhat wider bandwidth. Unfortunately, I can't
instantly find a good reference. Here's one, but not too good.
--
Ian

Slim Jim v J Pole
 

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
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!

Slim Jim v J Pole
 

Peter Inscribed thus:

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?

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. There is or was a commercial
design that attempted to reduce that.

73's
--
Best Regards:
Baron.

Slim Jim v J Pole
 

Baron wrote:

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. There is or was a commercial
design that attempted to reduce that.

73's

Modeling of J-poles and ground plane verticals shows considerable mutual
coupling to the feedline, and the Slim Jim almost certainly behaves the
same in this regard. The amount of common mode current this induces and
consequently the amount of pattern distortion it causes depends on the
feedline length and its path to ground. When the current is significant,
you'll probably need two current baluns (common mode chokes), one at the
feed point and the other about a quarter wavelength down, to reduce it
to a low value.

I theorize that the dependence of common mode current on feed line
length is a possible cause for the widely varying reports on how well a
J-pole radiates -- some people get lucky, some don't.

Roy Lewallen, W7EL

Slim Jim v J Pole
 

On Jul 15, 12:33*pm, Baron
wrote:
Peter Inscribed thus:

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?

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. *There is or was a commercial
design that attempted to reduce that.

73's
--
Best Regards:
* * * * * * * * * * *Baron.

I only did it once but the j-pole design papers often call for the
feedline to be coiled tightly -- a few turns -- just below the feed
point. I have made and used several of these "choke baluns" for HF
because failure to use a balun seems to screw up my VSWR readings.
Does this strictly apply to 2m j-poles, too? I don't know.

"Sal"
(KD6VKW)

Slim Jim v J Pole
 

On 7/15/2010 9:52 PM, Sal M. Onella wrote:
On Jul 15, 12:33 pm,
wrote:
Peter Inscribed thus:

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?

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. There is or was a commercial
design that attempted to reduce that.

73's
--
Best Regards:
Baron.

I only did it once but the j-pole design papers often call for the
feedline to be coiled tightly -- a few turns -- just below the feed
point. I have made and used several of these "choke baluns" for HF
because failure to use a balun seems to screw up my VSWR readings.
Does this strictly apply to 2m j-poles, too? I don't know.

"Sal"
(KD6VKW)

I have used 7 turns on 1 inch PVC about an inch below the feedpoint on
the Arrow style J Poles I have built. I see no significant change in
the SWR when coupling my hand to the feedline over the first 2 meters
below the feedpoint on the 2 meter band. Per my version of "significant".

For best results, follow Roy's advice.

tom
K0TAR

Slim Jim v J Pole
 

Sal M. Onella Inscribed thus:

On Jul 15, 12:33Â*pm, Baron
wrote:
Peter Inscribed thus:

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?

The difference is negligible. They both suffer from unbalanced feed
currents and thus feeder radiation. Â*There is or was a commercial
design that attempted to reduce that.

73's
--
Best Regards:
Baron.

I only did it once but the j-pole design papers often call for the
feedline to be coiled tightly -- a few turns -- just below the feed
point. I have made and used several of these "choke baluns" for HF
because failure to use a balun seems to screw up my VSWR readings.
Does this strictly apply to 2m j-poles, too? I don't know.

"Sal"
(KD6VKW)

A few (2 - 4) ferrite beads, the sort used on the glass computer monitor
video lead, works very well slipped over the co-ax just below the
bottom of the J. Best of all you can get them for free...

--
Best Regards:
Baron.

Slim Jim v J Pole
 

I've seen a couple of postings so far recommending a single balun at the
feedpoint. While this takes care of conducted common mode current, it
doesn't eliminate current induced on the feedline from mutual coupling.
All the single balun does is to insure that the current is near zero at
the single point where the balun is inserted. Current is still induced
on the feedline, and you end up with a current maximum a quarter
wavelength down the line -- the distribution looks just like it does on
an antenna (which the feedline has actually become), with the balun
location being the end of the "antenna". The balun can even make the
induced current worse if the effective common mode open circuit at the
balun insertion point results in a more nearly resonant feedline length.
The only way to really effectively reduce the induced current to a low
level is to break up the resonance of the feedline by inserting a second
balun about a quarter wavelength below the feedpoint as I recommended
earlier.

Roy Lewallen, W7EL

Slim Jim v J Pole
 

On Jul 16, 4:09*pm, Roy Lewallen wrote:
I've seen a couple of postings so far recommending a single balun at the
feedpoint. While this takes care of conducted common mode current, it
doesn't eliminate current induced on the feedline from mutual coupling.
All the single balun does is to insure that the current is near zero at
the single point where the balun is inserted. Current is still induced
on the feedline, and you end up with a current maximum a quarter
wavelength down the line -- the distribution looks just like it does on
an antenna (which the feedline has actually become), with the balun
location being the end of the "antenna". The balun can even make the
induced current worse if the effective common mode open circuit at the
balun insertion point results in a more nearly resonant feedline length.
The only way to really effectively reduce the induced current to a low
level is to break up the resonance of the feedline by inserting a second
balun about a quarter wavelength below the feedpoint as I recommended
earlier.

Roy Lewallen, W7EL

Does the velocity factor of the coax come into play when locating the
second
choke or does the outer shield behave like an open wire, VF ~ 1.0?

Perhaps I am slicing the baloney too thin for an average sandwich ;-)

"Sal"

Slim Jim v J Pole
 

"Sal M. Onella" wrote in news:cc6de1f9-fa1d-4a1c-bf7f-
:


Does the velocity factor of the coax come into play when locating the
second
choke or does the outer shield behave like an open wire, VF ~ 1.0?

Perhaps I am slicing the baloney too thin for an average sandwich ;-)

You will often see emphatic advice as to which side of the thing to
attach the shield, and which the centre conductor.

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the shield
and bond it to the tube, and take the inner horizontally across to the
other tube and connect it.

If physical symmetry is very good, the balun will be very good.

Nevertheless, as Roy observes, the feedline and supporting structure
below the U section is coupled and there is still potential for some
level of common mode current.

(To preempt Roy, the velocity factor of the outside surface of the shield
clad with a thin layer of vinyl will be close to 1, close enough for the
purpose at hand.)

Owen

Slim Jim v J Pole
 

In article ,
Owen Duffy wrote:

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the shield
and bond it to the tube, and take the inner horizontally across to the
other tube and connect it.

.... and take *real* care to waterproof the point at which the cable
exits from the tube and the shield is bonded! Buy a tube of
non-acid-containing (metal-safe, "neutral cure") silicone sealant, and
apply it very thoroughly here... squeeze and press plenty of it into
the braid and then form a layer which completely seals the exit point.

If you don't do this, and leave even a small amount of braid exposed
to the air, it will tend to "wick up" moisture into the braid every
time it rains.. The water can even wick its way down to the bottom of
the cable and you can find water dripping out of (or filling up) the
connector at the bottom! Trust me, it really does happen... I found
my lower N connector full of water, and what I swear appeared to be
algae!

--
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!

Slim Jim v J Pole
 

Dave Platt Inscribed thus:

In article ,
Owen Duffy wrote:

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the
shield and bond it to the tube, and take the inner horizontally across
to the other tube and connect it.

... and take *real* care to waterproof the point at which the cable
exits from the tube and the shield is bonded! Buy a tube of
non-acid-containing (metal-safe, "neutral cure") silicone sealant, and
apply it very thoroughly here... squeeze and press plenty of it into
the braid and then form a layer which completely seals the exit point.

If you don't do this, and leave even a small amount of braid exposed
to the air, it will tend to "wick up" moisture into the braid every
time it rains.. The water can even wick its way down to the bottom of
the cable and you can find water dripping out of (or filling up) the
connector at the bottom! Trust me, it really does happen... I found
my lower N connector full of water, and what I swear appeared to be
algae!

I'll second that ! Having seen TV's and set top boxes damaged by
ingress of water.

--
Best Regards:
Baron.

Slim Jim v J Pole
 

Owen Duffy wrote:
. . .
(To preempt Roy, the velocity factor of the outside surface of the shield
clad with a thin layer of vinyl will be close to 1, close enough for the
purpose at hand.)

I'll just add the reason -- the common mode current, which is what
causes feedline radiation and what we're trying to suppress, is on the
outside of the coax. The commonly specified velocity factor (around 0.66
for solid dielectric coax, a bit higher for foamed dielectric) applies
to the field inside the coax where the differential mode current flows,
not to the outside where the common mode current is. So you use a value
near one as Owen says. And it's not at all critical for this purpose.

A standing wave is present on an antenna or radiating feedline -- every
half wavelength there's a current null, and offset a quarter wavelength
from these are current maxima. For example, there are current nulls at
the ends of a half wavelength dipole (or, an even better example, a Yagi
parasitic element) and a maximum at the middle. When you insert a balun
in a transmission line, it causes a current null at that location, so
there'll be a maximum a quarter wavelength down and another minimum a
quarter wavelength below that if current is induced by coupling and the
length to ground supports that distribution. We want to make that
distribution impossible, and inserting the second balun does that.

This is easily observed by modeling, but you have to keep in mind that
the actual path the current takes to the Earth along the outside of the
coax can be considerably more complicated than most simple models
represent. So models can tell you what *can* happen although maybe not
necessarily what *is* happening in a given installation.

Roy Lewallen, W7EL