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CONVERTING A 2 METER J POLE INTO A SLIM JIM ANTENNA
By Don Butler N4UJW
Having recently posted the2 Meter Slim Jim antennaproject to Hamuniverse.com, I took a long hard look at the old standard J Pole antenna that had been up for some time. I had the brainstorm that maybe it could be converted into a Slim Jim antenna with hopefully added performance over the "J"..
The old J Pole looked like it was in need of a face lift and some minor surgery so rather than fix'er up, it appeared to me that the major difference between it and a Slim Jim was that it lacked the other half wave section that parallels the main element on the Slim Jim and the J Pole needed to be isolated from it's mount after the experimental modification.This looked like a simple surgical procedure to me.
I looked at the Slim Jim project on the site much closer this time as an antenna builder and not as a "webmaster" and went into the operating room, (the hot Texas outdoors), with rubber gloves, face mask, and some ice water.
(It's hot in Texas in August!!!)
The Surgical Project!
After much consultation and great expense, an antenna surgeon was called in!
NOTE: [What follows was a temporary antenna surgical experiment and no great pains were taken for neatness in the antenna construction! The Slim Jim project calls for 1/2 inch copper pipe of which I had none. The existing J Pole had been constructed of tapered aluminum tubing about 1/2 to 3/4 inches in diameter so I decided to not change this side of the antenna. I did not measure the diameters but these measurements are very close.]
FROM THE JUNK PILE TO THE OPERATING ROOM
The pile of scrap aluminum pieces and tubing, (junk according to the XYL), that I had laying around was worth it's weight in gold.
I found and old piece of aluminum channel stock that appeared to be just about long enough for the half wave section addition. Looking at it on end, it looks like a letter U, only squared off more. Like this |_|. It is about 1/4 inch across. (I did not want to cut up my other "junk" aluminum tubing.)
After measuring a couple of them, I found one that was about 40 inches long.
(Length changes later)
I re-measured the longest element of the J Pole and slightly re-adjusted it's length to 58 inches per the project on the site.
I then had to figure out a way to attach a short section of conductor across the top of the J Pole to the half wave addition that would be parallel to the original longest section of the J Pole.
I found a short length of aluminum strap (about 3 inches long and a half inch wide) and attached it across the top of the two elements.
It was attached with an old rusty tubing clamp, ( I cleaned the contact points ) and a screw drilled into the aluminum channel element. The distance between the old J Pole element and the new element was about 2 inches. The idea here is to electrically connect the old J Pole radiator to the new added "1/2 wave" section.
Now all that was left to the surgical modification was to cut or adjust, (I used telescoping tubing for this section), the new added section to allow for an adjustable air space (gap) of about 2 inches between it and the matching section at the bottom of the J Pole antenna and to figure out some way to keep the new added element from touching the other element on the old J Pole. (The aluminum strap at the top was a bit unstable.) I came up with a small piece of plastic (the green object in picture below) from the â€œjunkâ€ box and attached it between the two using electrical tape to keep them parallel.
The surgical construction (destruction) of the antenna was almost complete.
The attachment point for the 50 ohm coax, (according to the Slim Jim project), was about 4 inches from the bottom of the matching section which was about 19 inches long.
I anticipated Swr adjustment for the final installation, so I carefully drilled several small holes in the matching section above and below the 4 inch point on the 19 inch section channel stock and attached a self tapping screw into one of the holes and attached a hose clamp to the other, longest side.
During SWR adjustment and final installation, the center conductor is placed between the clamp and the longest element and the shield to the screw on the other side on the short element and tightened for the test. (Looking at the picture below you will see how to attach the coax center and shield. Center conductor goes to long side, shield to short side.)
These two points will be where the coax attaches to the antenna after final testing and lowest SWR is achieved. The feed points are moved up and down equally for SWR adjustment.
NOTE:The original J Pole antenna was made from an old Ringo bottom section with it's black insulator still attached to a short piece of tubing that extended from the bottom of the old antenna onto a metal mast. The tuning ring was removed.
I then attached a small hose clamp across the bottom of the matching section to give a better mechanical and electrical connection for the bottom end of the antenna.
The coax was attached to both sides of the antenna with the center conductor to the longest side and the shield to the shortest side per the Slim Jim project article.
The antenna was then attached to a 10 foot section of gray outdoor PVC conduit and leaned up against the corner of the house for temporary support and ease of testing.
This gave the needed "free space" distance of at least 20 inches from any metal object and the base of the antenna was 10 feet off the ground.
After a bit of time experimenting with the tuning of the converted J Pole, I discovered that the gap had to be shortened to around an inch for lowest SWR. This was most likely due to the two different sizes of the parallel elements that I used in this experimental antenna.
The final SWR readings were:
144MHz.....1.6 to 1
146MHz.....1.4 to 1
148MHz.....1.5 to 1
I did not bother to get that "perfect" 1 to 1 SWR and was not concerned since the "patient" was doing well but feeling a bit dizzy at this time and after a glass of ice water, I was revived.
These readings above were taken with a random length of Rg58 attached to the antenna not longer than about 20 feet for the tests using low power at the transmitter on 146.000MHz.
The "patient", shown above, after passing out from the HEAT!
THE RESULTS AND SOME OUTPATIENT COMMENTS
Please note the picture above is without the clamp below the bottom of antenna and the air gap distance above the short element is opened up for the picture for clarity.
The actual air gap will vary with your construction but will be somewhere around one to two inches. I used telescoping lengths of tubing in the upper "1/2" wave length, (the air gap side), anticipating having to change it's length to get the right air gap. Also note in the picture above the shorting strap is not visible at the top of the antenna. You must short the top sections together! Don't get mislead by the picture.
It is wise to add an air choke coil at the base feed point area consisting of about 6 turns or so of coax to help prevent rf from getting back down into the shack and also distorting the antenna pattern.
Various on the air tests with different repeaters scattered around East Texas both in daytime and at night yielded very promising results using 15 to 50 watts from an older Radio Shack HTX-212. Keep in mind that the base of the antenna was only 10 feet off the ground and about 2 feet above the eve of the house with the roof directly on the West side very close to the antenna but not within the "free space" distance AND very sloppy construction techniques were used with materials that were on hand, with no expense for the surgical conversion except for the "Doctor" being present. His fee was added to my account.
If you use good construction surgical techniques and materials in your project, you should have very good performer and a nice addition to your antenna farm.
The total length of the antenna is under 6 feet.
The surrounding lay of the land where the house is located is mostly flat out to the horizon. Towards the West, it gradually turns to rolling hills about 15 to 20 miles out.
I have been able to "hit" repeaters out to about 80 to 90 miles using 50 watts.
I did not have another antenna to compare the converted J Pole with Slim Jim, but if memory serves as a comparison between the old J Pole and the new Slim Jim, it was well worth the effort!
I also feel like I lost a few pounds in the surgical process working with the "patient" in the Texas HEAT! HI!
The "patient" was revived and sent to recovery and was discharged from the "hospital" after the testing was completed. It is now resting comfortably atop a long length of PVC pipe! Mild sedatives are given from time to time to keep it calm when distant repeaters call.
For what it is worth....a 300 ohm twin lead Slim Jim antenna was hastily put together in the shack and suspended from an inside door inside the house as a test....I worked several Dallas repeaters about 60 miles away and others at greater distances with the top of the twin lead not higher than a standard door! All on ground level with most of the house between the antenna and Dallas!
This J Pole to Slim Jim conversion project above was just an experiment and used very sloppy and hasty construction techniques using the very highest quality "junk" material on hand and the most skilled surgical techniques from years of practice .....I usually am not this critical when building antennas! Little grin........
If you have experimented with this antenna using your surgical skills and would like to share your results, mods, etc.....
Please email me with details for possible publication on the site. firstname.lastname@example.org
SLIM JIM BY KE5FXU (SK) I will miss you Rich!
73 DON N4UJW
I'll send you my bill in the mail for these surgical instructions!
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