N3CRK builds a 220 MHz Modified Coaxial Antenna


I ran across an article on the internet for a coaxial antenna by John E. Portune, W6NBC.
I find this a lot easier too build than a J-Pole and the author claims his antenna will perform as well as a J-Pole.
I worked out a better way to build one, built it and it seems to work fine. It took me about an hour to build it. Here is how I did it.



The materials: 12" length of 1/2" copper tubing with the ends cleaned for soldering.
A copper sleeve (used for joining two pipes.)
An SO-239 connector.
At least 25" of brazing rod. (I happened to have 3/32" which worked perfectly)
A 1/2" copper end cap. This is a quick and easy way to center the radiator.
A small rubber grommet to fit in the end cap. (Not shown in this photo.)

Step 1: Solder the sleeve to the pipe, make the total length 12 1/2". We just need a good electrical connection.



Step 2: Cut off the four corners of the SO-239. Grind or file away rough edges. You want the connector to fit easily into 3/4" schedule 40 plastic pipe.
If you have to clamp the connector by the threads, use an old PL-259 to protect those threads.
Step 3: Solder the brazing rod to the center pin of the SO-239.



You should now have this much complete.



Step 4: Solder the SO-239 into the copper sleeve. It should be a nice fit. To solder it you must have the pipe vertical.
You will be using enough heat to re-melt the solder between the brazing rod and the SO-239.
You don't want the brazing rod to fall out. (Believe me, I have done it.)
Allow it to cool slowly in the vise.



Step 5: Meanwhile drill a hole in the copper cap for a small rubber grommet. Fit a grommet into the hole.
You should now have this much done.



Step 6: Slide the cap with grommet over the brazing rod onto the end of the pipe.
An electrical connection is not needed but a dab of solder will keep corrosion from causing problems outdoors.
Step 7: Cut the brazing rod to 12 1/2" from the copper cap and you are finished.


A true coaxial antenna is fed in the center and is naturally 75 0hms.
This version is end-fed. That is why I call it a "modified" coaxial antenna. It seems to match 50 ohms reasonably well.

I slid mine into a length of 3/4" PVC pipe to protect it from the weather and make it easier to mount.
I will try to document that in another article.

I will also show a quick and easy way to side mount an antenna on a tower with electrical conduit.