Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: J Pole ant lenght ?  (Read 1077 times)
SWMAN
Member

Posts: 549




Ignore
« on: November 05, 2012, 06:59:32 AM »

 I was just real curious about something and hopefully someone would know why ??
On a 2 meter J Pole antenna, the full length of it is usually 58 to 68 inches long. But the half wave length for example a dipole is only 38 inches with 19 inches as a quarter wave lenght. My question is, why when you buy a J Pole is it so long for a single band 2 meter antenna ??  I bet someone has a good answer.
 Thanks and 73  Jim  W5JJG
Logged
K9YLI
Member

Posts: 853




Ignore
« Reply #1 on: November 05, 2012, 07:02:16 AM »

From  you measurments  I would guess that the long sectin is
a full wave long and the  J section  matching stub is about a 1/4 wave..

do the math...
Logged
G0VKT
Member

Posts: 64




Ignore
« Reply #2 on: November 05, 2012, 08:17:30 AM »

It is an end fed 1/2 wave fed with a 1/4 wave matching stub. 39+19=58.
Any extra length is at the bottom for mounting.
Logged
W5FYI
Member

Posts: 1044




Ignore
« Reply #3 on: November 05, 2012, 11:13:45 AM »

A J-pole is a half-wave antenna matched to the coax with a ¼λ wave section. A half wave at 146MHz is 38.5" and a quarter-wave is 19.25". Throw in velocity factor for the quarter-wave section, and you could have up to 29" for the quarter-wave section. 38" plus 29" equals 67"; and there's usually an inch or two at the base for mounting to a mast or mount.

I suggest that you read more about J-poles on the Internet--there's a lot of good information on how to build them to obtain optimum performance, for single band use, or on multiple bands (VHF/UFH). GL
Logged
SWMAN
Member

Posts: 549




Ignore
« Reply #4 on: November 05, 2012, 01:16:51 PM »

 Thanks for the good info as usual, I see why now, I need to do the math for shure. Jim
Logged
W6EM
Member

Posts: 736




Ignore
« Reply #5 on: November 05, 2012, 07:03:45 PM »

....Throw in velocity factor for the quarter-wave section, and you could have up to 29" for the quarter-wave section. .....

In the case of the J, the velocity of propagation in the stub is very close to 1 since it's in air.  Or, at most a few percent under.  This results in a slightly shorter length, not a longer one.  It's basically like a half wave section sitting atop one side of a 1/4 wave stub.  The feed point is adjusted on the stub to the correct impedance of the feedline.  Could be 50, 75, or even 300 ohms.  The feed should be balanced.

The Super J that is also quite popular is yet another 1/4 wave stub (sticking out to the side) and another 1/2 wave section on top.
Logged
SWMAN
Member

Posts: 549




Ignore
« Reply #6 on: November 06, 2012, 08:06:34 AM »

 Do you think that a regular half wave dipole cut for 2 meters hung vertically would be just as good as a J Pole at the same height even though the J Pole would be cut for a full wave ? I know that I have alot of questions but thanks again. Jim W5JJG
Logged
W5FYI
Member

Posts: 1044




Ignore
« Reply #7 on: November 06, 2012, 09:17:38 AM »

Quote
In the case of the J, the velocity of propagation in the stub is very close to 1 since it's in air.

Not all stubs are open-air; roll-up J-Poles, where the stub is often TV twin lead, and encased J-Poles come to mind. In these instances, velocity factor should be a design consideration.
Logged
WB6BYU
Member

Posts: 13042




Ignore
« Reply #8 on: November 06, 2012, 10:02:10 AM »

Quote from: SWMAN
Do you think that a regular half wave dipole cut for 2 meters hung vertically would be just as good as a J Pole at the same height even though the J Pole would be cut for a full wave ?


A J-pole is a half wave radiator with a quarter wave matching stub.
In theory, the radiation pattern is exactly the same as that from a half
wave dipole, quarter wave ground plane with sloping radials, or any other
antenna that is about the same length.

In practice things are somewhat different due to the effect of the feedline.
A vertical half wave dipole works well if the feedline can be kept from
affecting the pattern, for example, by side-mounting the antenna on a mast
using a stand-off with the feedline running away at right angles to the antenna.
If you just stick the dipole on top of the mast with the feedline running down
beside the lower element, that doesn't work as well.  (One common commercial
solution is to bring the feedline down inside the bottom half of the dipole.)

Similarly, a J-pole is prone to common mode currents on the feedline and/or
mast, because it isn't really a balanced antenna.  These can affect the
radiation pattern:  we put up one J-pole that couldn't hit the local repeater
until we insulated it from the support mast.

So both antennas are expected to have identical performance when properly
installed and decoupled.  Both antennas may have degraded performance
when antenna currents flow anywhere other than the desired antenna
elements.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!