Dual Band J-Pole Project

[KG7SPS]

I planning to the antenna described in this article, http://www.n5dux.com/ham/files/pdf/A 146 and 445 MHz J-Pole Antenna.pdf

Any comments on the build and a couple of modification would be helpful.

1) Adding the mast decoupling stub from the post, Mast Mountable J-Pole Antenna. Is mounting the antenna (as designed) on a grounded Steel Pipe the preferred method or should the antenna be isolated from ground except through the cable.

2) Would shorting the upper element, the two parasitic selection and tunning stub by about 1 1/2 inch and adding a tuning stud described in (http://techdoc.kvindesland.no/radio/vhf_antennas/20061030163006789.pdf) allow for fine tuning. The original article states that element lengths are critical,

3) Would using LMR-240 (Low Loss Flexible LMR-240 Outdoor Rated Coax Cable Double Shielded with Black PE Jacket) coax be okay, in place of the RG-8 (having a problem finding 3/8 copper pipe/tube with an ID of .331 with an OD less .5)? I can find copper pipe with OD of .375 and ID .25 which the LMR-240 will fit.


Sent from my iPad Air

 

[n2nov]

This is the one our group has been using since just after 9/11. Maybe you can get some ideas from the pictures and specs.

Arrow Antenna J Poles 2m 146 70cm 440 OSJ

 

[dsalomon]

Depending on what you're trying to accomplish, consider whether you want a J-pole or a Slim Jim. The two antennas are similar in construction, except the Slim Jim has a lower angle of radiation. This typically equates to better distance than a J-pole. Now, if you happen to be in a valley, then a J-pole might be the better choice. On flat terrain, the Slim Jim might perform better. Do some reading about Slim Jim vs J-pole antennas before you put your effort into building / buying one. Here's an example of a guy who sells a dual band (146/440) Slim Jim made from ladder line. I bought two of his antennas, one for portable use and one for home. It performs SIGNIFICANTLY better than the ground plane I replaced (Comet CX-333) and also better than the dual band home brew J-pole I tested (also a project from a QST article).

73 - David, AG4F

 

[dsalomon]

Oops, forgot the example I referred to in my reply: Slim Jim Info. Reviews: N9TAX Dual band slim jim portable antenna by 2way electronix Product Reviews

73 - David, AG4F

 

[KG7SPS]

Another Opion

Depending on what you're trying to accomplish, consider whether you want a J-pole or a Slim Jim. The two antennas are similar in construction, except the Slim Jim has a lower angle of radiation. This typically equates to better distance than a J-pole. Now, if you happen to be in a valley, then a J-pole might be the better choice. On flat terrain, the Slim Jim might perform better. Do some reading about Slim Jim vs J-pole antennas before you put your effort into building / buying one. Here's an example of a guy who sells a dual band (146/440) Slim Jim made from ladder line. I bought two of his antennas, one for portable use and one for home. It performs SIGNIFICANTLY better than the ground plane I replaced (Comet CX-333) and also better than the dual band home brew J-pole I tested (also a project from a QST article).

73 - David, AG4F

Thank you.

Own a couple Slim-Jim's, including the one you referenced.

Looking answer/comments on my original post.

 

[majoco]

For info - the original "Slim Jim" was published in 1978 by Frank Judd of "Practical Wireless" fame. The original article is here....

Original Slim Jim Antenna Article by F.C. Judd - The 2BCX Slim Jim Antenna - G2BCX

....and I have an original of the booklet "Out of Thin Air" which is full of useful info and antennas to build.

 

[KG7SPS]

Still looking for on point comments.

 

[jshuggins]

Thank you.

Own a couple Slim-Jim's, including the one you referenced.

Looking answer/comments on my original post.

Please note the Slim-Jim operates practically identically to the standard j-pole (assuming neither connect to a mast). Many folks incorrectly apply the Slim-Jim's original comparison to a 5/8 wave test antenna as a comparison to a dipole. This error has, unfortunately, propagated wide and far on the Internet leading many astray.

To address your points:

1) Adding the mast decoupling stub from the post, Mast Mountable J-Pole Antenna. Is mounting the antenna (as designed) on a grounded Steel Pipe the preferred method or should the antenna be isolated from ground except through the cable.

The traditional J suffers from energy transfer to everything it conducts or inducts with, thus essentially making the mast part of the antenna. I believe the decoupling stub offers a good compromise while offering a solid DC connection to the mast. If you desire to use the traditional J, then a means to RF isolate it from the mast is a must.

2) Would shorting the upper element, the two parasitic selection and tunning stub by about 1 1/2 inch and adding a tuning stud described in (http://techdoc.kvindesland.no/radio/...0163006789.pdf) allow for fine tuning. The original article states that element lengths are critical,

We should ultimately be able to construct a copper antenna to exact dimensions and have it work. However, mounting conduction and other local variables can perturb the tuning of the antenna so adjustments are nice to have. I personally don't do this preferring to trim the copper pipe during the tuning process.

3) Would using LMR-240 (Low Loss Flexible LMR-240 Outdoor Rated Coax Cable Double Shielded with Black PE Jacket) coax be okay, in place of the RG-8 (having a problem finding 3/8 copper pipe/tube with an ID of .331 with an OD less .5)? I can find copper pipe with OD of .375 and ID .25 which the LMR-240 will fit.

I have no qualms using LMR-240. I do suggest investing in the direct burial variant even if not burying it. Little nicks here and there will self-seal from the goo inside making for a longer lasting cable. Of course if you place the cable inside a conduit, the advantage of the goo fades. I'm a big fan of 240 for lots of applications short of full duplex use.

John, kx4o