Robbie builds a 2 meter J-pole antenna
Thanks to my ham club and a few members (Thomas, Dale), I have finally decided to undertake the J-pole antenna. I've been using a 2 meter dipole for three months, and I need to see if I can have a better performing antenna.
It is possible that the area that I have my antenna installed is just a bad spot, but this is something that will never be known since I don't have much to work with..I just crimped connectors to it, but I don't believe that they are doing a good job. Then the losses in the coax are the next issue..I'm using cheap RG-58 coax with aluminum wire and shielding, and I cannot get it to take solder. .There are a few items that make my dipole less effective. Not using quality coax is the big one.
To solve these problems I am using quality coax with quality SO-239 connectors purchased from R & L Electronics. I went with Jetstream coax in a length of 50 feet with SO-239 connectors installed. I don't like installing connectors on coax.
I estimate that the JT400 coax has about 1.5dB loss per 100 feet at 150MHz. This gives me about 0.75dB loss with my 50 foot run. I'm sure that I have well over 3dB loss with my current coax. The dimensions for the J-pole can be found on many websites. I found my measurements from Buxcomm. I used their calculator which gives you the same dimensions that you arrive at by doing the math by hand.
These are the sizes that you need to cut from your ten-foot piece of 1/2" copper pipe:
- Long vertical piece = 58"
- Short vertical piece = 19.2"
- Short horizontal piece = 1.8"
- Mast mount piece = about 20" (I don't really think this matters)
You'll also need the following fittings:
- One - 1/2" tee
- One - 1/2" elbow
- Three - 1/2" caps
Finally, you'll need tools and the following parts:
- One - SO-239 connector
- Piece of heavy gauge wire for the SO-239
- Three hose clamps to mount the antenna to your mast (size depends on your mast dims)
- Run of coax to go from antenna to radio (or SWR meter then radio)
The steps to assemble the antenna are fairly simple. The hardest part is soldering the SO-239 to the antenna. The copper wicked the heat away really fast, but getting the connector too hot melted the plastic insulation. Some of the measurements are pretty wacky for a measuring tape. 19.2 inches is really close to 19 and a quarter. 1.8 inches is close to 1 and three-quarters. I used the thickness of the saw blade to get the final length. I'm sure that a little here or there won't be enough to really throw your SWR off. Solder the caps on last. If you need to remove some material, you can. If you need to add material, the cap can be used to add some.
Some people swear by the ugly balun, so that is what I'm going with. I'm using five loops at 5 inches diameter. The main purpose of this "choke" is to keep RF from radiating from the coax.
After pre-fitting the pieces of the antenna, find a place that won't catch fire to solder your antenna pieces together. I chose a nice wooded area inside my shack. I did open the door for ventilation and opened a window for a cross draft. If you've never soldered copper pipe, find a video on YouTube to get a look. It's like electrical soldering, but on a much higher heat setting.
Next, you'll want to connect the SO-239 connector to the antenna. This is the feedpoint on the drawing. The center of the SO-239 should be at 1.92" from the outer edge of the elbow. The center conductor of the SO-239 is also 1.92" from the top edge of the tee. I insulated these connections with electrical tape.
Finally, attach your antenna to your mast. Coil your cable into a choke (I used 5 turns at 5"), and tape it off. I probably should have used cable ties for longevity, but it was too late. I had already raised my antenna, and it's a pain to take it down. Attach the PL-259 connector to the SO-239 and seal it.
Raise your antenna and check you SWR. It should be good if you made it to tight tolerances. I have a 1.2:1 SWR, and I am happy with that.
- Copper pipe and fittings - $17 (Lowes)
- Flux and solder, torch - Already had it, but can be expensive
- 50' Jetstream JT400 coax w/ PL-259 connectors - $58 (w/ shipping from R & L)
- Electrical tape or seal for cables - Already had it
- SO-239 connector - $3
- 3' LMR400 jumper for SWR meter - $14 (MPD Digital)
- MFJ 812B SWR meter - $53 (w/ shipping from HRO)
- Mast - Already had it
- ***Total for my project - $145 of which $25 was shipping, but you can't drive around for that...