Friday, October 14, 2011

Copper J-Pole Antennas

As soon as you get your Amateur Radio license, you start thinking about a good 2-meter antenna for your home or garage.  Commercially built antennas can be expensive but there is a great solution to the problem.   Build a Copper J-Pole antenna.

Construction of the J-Poles is simple.  Careful measurements and soldering skill condenses the soldering construction time into a few brief hours.  If you haven’t soldered copper pipe fittings before, buy a few extra and practice a few times before you start construction on your antenna.  There are numerous videos on YouTube that show the best method for soldering copper pipes. 

You’ll find that you will rapidly become proficient with the skill.  Undoubtedly, you’ll be using it again before long to build more antennas either for yourself, scout troops and as an Elmer helping new Hams set up their own shacks.

I built both Super J-Poles (2-Meter, 70 CM) and single frequency 2-Meter J-Poles for use at home and for other applications.  

N7QVC has posted the plans for both of these antennas on his site.  The comments by his blog readers are equally helpful in choosing the design features of your J-Pole.  Download them and print a copy for the wall above your work bench.

I used 3/4” copper for the Super J-Poles and 1/2” copper for the J-Poles.  There is a substantial weight difference between the two antennas due to their respective lengths and the added pipe mass.

Even though I’d drilled and tapped holes for the stainless screws to attach the copper 3/8” tubing in the Super J-Poles, they didn’t offer the solid connections I’d hoped to achieve.  The connections won’t ever loosen with the stainless hose clamps I put around the copper and over the screws.  The clamps had no measurable affect to the SWR of the antennas.   Just remember to paint the schedule 40 union or threaded PVC splice that you use to separate the sections of the antenna before tightening the screws lest you strip the threading out of the tubes with multiple tightenings.

Tuning the antennas was a snap.  I used sections of split tubing combined with stainless steel hose clamps to attach the SO-239 connector and feed point.  The idea came from comments I read on several blogs and it works very well.   The sections are each 1 1/2” in length.  It was easy to bend back a 1/4” lip at the split and drill holes through it to match the mounting holes on the SO-239.  A Dremel tool quickly carved a curved notch so it could seat deeply enough to align the mounting holes with the copper sections.

Once the split sections were snapped in place about 3” above the base of the “J”, it was simple to slide them up or down slightly to find the best SWR / Antenna Analyzer reading.  Clamping them in place was accomplished using more stainless hose clamps followed by solder between the section and the antenna elements.

The SWR for the Super J-Poles is 1:1.1 and the J-Poles have a 1:1 SWR.  I used 146.500 as the ‘sweet spot’ tuning frequency because it is close to the mid-point of the frequencies used by the repeaters in my area.

I played with the radiation patterns before final mounting but found that it didn’t really matter which way I pointed the antennas because we live in a valley with mountains all around us.  The repeaters I use are all line-of-sight shots on nearby mountain ridges, so hitting them is easy. 

The Super J-Pole is mounted at 25 ft and operates flawlessly. I mounted the J-Pole at 11 ft on a post that supports a side fence by my shop.  I expected some impact due to the reduced height, but it is negligible. 

The simplex tests I ran were all over 30-miles in distance and even at 5-watts, I couldn’t conclusively prove one orientation to be better than another.  Broadcast radiation and reception worked very well with these copper hot-shots.   I suspect that the mountains here almost duct the radio waves along the valleys between them regardless of the twists and turns that I tested.   

You’ll want to look up J-Pole radiation patterns and do some testing at your location before making the final tightening turns on the antenna mount.

The whole tuning process per antenna only took 10-minutes at most.  It literally took more time dialing them in by checking readings between the radios and meters on my work bench and the antennas than it did to solder the connections in place.

In an effort to bring the younger generation into the Ham Radio world, I used Google video chat on a laptop so a grandson could engage in the construction from afar.  He will undoubtedly build ‘better’ antennas in the coming months to show his grandfather that the digital generation can build things out of copper as well as being a master of digital bits, which is just what I hoped for.

My wife likes the look of shiny copper, so rather than painting the antennas that will be used at our home a sky gray color, I polished them with steel wool and covered them with four coats of clear lacquer.  So far, none of the neighbors have spotted them through the Ponderosa pine trees at hour home.  The G5RV antennas between the same pines have never been spotted, so I have little fear of complaints about a couple of copper shafts in the air in different locations.

I used LMR-400 coax wire to the entrance box with its lightning arrestors and multiple 12” bulkheads that feed through the wall into my ham shack.  I really like LMR-400 cable due to very good experiences with it in my HF and other installations. 

If you are considering building a J-Pole but aren’t sure of your skill level, quit worrying and start building.  Even if you haven’t built anything like this before, you’ll quickly learn the minimal required skills and you’ll have fun in the process. 

There are many variations on the J-Pole and recommendations for their construction on the web.  Take a little time, read through them, focus on the folks who have actually built antennas from the plans and choose your design. 

You’ll be on the air with your new antenna sooner than you’d think possible and if you are lucky, the only part you’ll have to buy that isn’t already in bins in your garage or shop will be the SO-239 connector.  If you’ve been to any Hamfests or gatherings, you probably have a handful of them laying around too.

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