Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old July 16th 10, 03:52 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Oct 2009
Posts: 74
Default 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)
  #2   Report Post  
Old July 16th 10, 04:35 AM posted to rec.radio.amateur.antenna
tom tom is offline
external usenet poster
 
First recorded activity by RadioBanter: May 2009
Posts: 660
Default 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
  #3   Report Post  
Old July 16th 10, 10:38 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2009
Posts: 37
Default 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.
  #4   Report Post  
Old July 17th 10, 12:09 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 1,374
Default 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
  #5   Report Post  
Old July 17th 10, 06:09 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Oct 2009
Posts: 74
Default 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"


  #7   Report Post  
Old July 17th 10, 06:03 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 464
Default 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!
  #8   Report Post  
Old July 17th 10, 07:21 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2009
Posts: 37
Default 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.
  #9   Report Post  
Old July 19th 10, 04:41 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 1,374
Default 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
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with slim jim nec model [email protected] Antenna 23 April 17th 13 05:54 AM
J-Pole vs Slim Jim? Ian Jackson Antenna 4 February 10th 07 02:55 PM
Slim Jim vs. EZNEC RST Engineering Antenna 9 January 24th 07 12:55 AM
Helical wound slim jim? John Smith Antenna 103 April 22nd 05 04:08 PM
Anyone here of a "SLim Jim" antenna? [email protected] Antenna 9 January 19th 05 08:09 PM


All times are GMT +1. The time now is 12:07 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017