So when I said that the J-pole was soldered together, I neglected to mention that the coax - the thing that I connect my radio to - had not been connected to the antenna.
To begin with, I used a borrowed antenna analyzer (a MFJ-259) to find the point on the antenna where I got the lowest SWR. I then marked those points (there's a point on the 3/4 wave section, and a point on the 1/4 wave stub, and in theory they should be at equal height) on the antenna with a Sharpie. Then I screwed a chassis mount SO-239 connector (photo) into the 1/4 wave matching stub, and added some copper wire wrapped around the base of the connector. This was intended to improve the electrical connection between the 1/4 wave stub and the side of the connector that connects to the shield of the coax. I'm not sure it was strictly necessary, though. :)
I had previously soldered a piece of solid core copper wire that I had laying around to the center conductor of the SO-239. After I got the connector screwed into the 1/4 wave stub, I tried soldering the end of the wire to the 3/4 wave section of the antenna at the low SWR point I had previously marked. My soldering iron wouldn't get the copper pipe hot enough for the solder to stick - not surprising, since copper is an excellent conductor of heat - so I ended up using the blowtorch I had used to solder the antenna together to begin with. This worked much better, and when all was said and done, I had an SWR of between 1.1 and 1.2 at 146 MHz.
Today I covered the wire and everything but the threads on the SO-239 with electrical tape to protect them from the elements, and to make sure that nothing could short the matching stub and the 3/4 wave section (apart from the bottom of the "J"). I then (with my upstairs neighbor's permisssion) attached a couple of U-bolts to the railing of the back staircase. They will hold the mast that the j-pole is mounted on to the railing. I might fabricate another mast out of something or other later to get some additional height - I haven't decided yet. I didn't connect any feed line to it, as I don't have a UHF -> F adapter.
Soon, I will have a working outdoor antenna!
Showing posts with label j-pole. Show all posts
Showing posts with label j-pole. Show all posts
Saturday, February 5, 2011
Monday, January 31, 2011
The J-pole is soldered together!
Tonight I soldered together the J-pole I mentioned in my last post.
Conditions were less than ideal for this. Not having a garage, and it being awfully cold outside, I had to work in a basement. On the plus side, the basement was relatively warm. On the minus side, it was awfully dirty down there. I kept things as clean as I could, though.
Another less than ideal thing was the flux I used. I purchased a kit from the hardware store that came with a small roll of solder and a small tube of flux. The tube of flux had a thick plastic lid, and I didn't have anything to puncture it with, so I ended up heating up a piece of scrap copper wire and using that to puncture the tube. I only managed this once, however, so the flux came out in a thin ribbon.
But, all problems and less than ideal conditions aside, the j-pole is together! Now all I need to do is find the 75 Ohm match point and I'll be good to go. Now, experienced hams might have noticed that I said 75 Ohms in my last sentence and be a bit confused. After all, most ham equipment is designed for 50 Ohm loads, and you generally want the best impedance match you can get, so as to maximize your power output. But as it happens, I have a bunch of leftover 75 Ohm RG6 that various cable installers have left with me, and I don't want it to go to waste. So I'm going to try and use that as feedline, and try and set something up to match that impedance as best I can. It was suggested in this thread on QRZ that if I make sure my 75 Ohm line is a multiple of 1/2 a wavelength, I can then connect a bit of 50 Ohm line at the radio end. We'll see how that goes. :)
Conditions were less than ideal for this. Not having a garage, and it being awfully cold outside, I had to work in a basement. On the plus side, the basement was relatively warm. On the minus side, it was awfully dirty down there. I kept things as clean as I could, though.
Another less than ideal thing was the flux I used. I purchased a kit from the hardware store that came with a small roll of solder and a small tube of flux. The tube of flux had a thick plastic lid, and I didn't have anything to puncture it with, so I ended up heating up a piece of scrap copper wire and using that to puncture the tube. I only managed this once, however, so the flux came out in a thin ribbon.
But, all problems and less than ideal conditions aside, the j-pole is together! Now all I need to do is find the 75 Ohm match point and I'll be good to go. Now, experienced hams might have noticed that I said 75 Ohms in my last sentence and be a bit confused. After all, most ham equipment is designed for 50 Ohm loads, and you generally want the best impedance match you can get, so as to maximize your power output. But as it happens, I have a bunch of leftover 75 Ohm RG6 that various cable installers have left with me, and I don't want it to go to waste. So I'm going to try and use that as feedline, and try and set something up to match that impedance as best I can. It was suggested in this thread on QRZ that if I make sure my 75 Ohm line is a multiple of 1/2 a wavelength, I can then connect a bit of 50 Ohm line at the radio end. We'll see how that goes. :)
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