|20 meter Vertical w/ Balun
Author: Frederick R. Vobbe, W8HDU
April 10, 2009
|My old Cushcraft A3S was in a world of hurt!
I put up my A3S antenna just after being licensed in 1987. It's a pretty decent antenna for the price. At the time they were about $259. I think they are now up to around $550. But for what I paid back in 1987, it's still working and doing the job.
But since 1987, it never received an attention. Even after severe wind storms, lighting, and several freak ice storms that hit our community. Although the performance of the antenna is still good, it was time to remove the antenna from my tower and recondition it, hopefully for another 20 years!
As you can see from the picture above, (click on a picture to enlarge), I installed a 1:1 balun at the feed point. This smoothed out the response of the antenna, and the transciever's VSWR reading came down as I tuned away from the normal resonant portion of the antenna.
However, note the mounting hardware that is almost rusted off! When this bracket for the balun was installed, it was silver and shiney. Now look at it. It's rusted and pitted badly. The balun was also bent because of the weight of the cable that came loose. I had to come up with a better mount.
I always collect materials that I think I can use to make things. Two of those materials were some aluminum bar stock, (6 foot long, 1 inch wide, 1/8" thick), and some teflon cutting board. The cutting board material is made here in town at a place called U.S. Plastics. Sometimes they have some end pieces or damaged sections that can be purchased for a few dollars.
Since it was raining today I needed an inside project. I found a strip of aluminum, some teflon, and some U bolts. The teflon was cut into a 2" x 2" square (50.8 mm). The teflon is .500" thick. All totaled, what you see here cost me about $7.56. The stainless hardware was new from Bolt Depot, but everything else was bargain bin material.
Since this will be on my tower in "wet" Ohio, I used some stainless steel U bolts, washers and nuts. This should keep the assembly from rusting in the future.
The aluminum bar and the U bolts will be used to hold the balun support to the antenna, while the teflon will insulate the balun from the antenna. The reason for this is to eliminate any magnetic fields the balun might encounter while strapped to the center support.
The large holes are for attaching the U bolts.
The two smaller holes allow a screw to hole the aluminum bracket to the teflon.
The two smaller holes are centered in the bracket, and .25" off the edge of the bracket. The large holes are 1" either side of the center, and centered between the edges. I should mention that the aluminum stock I used was .1250" thick. You don't need much as the balun is rather light.
I ended up having to clamp the teflon to an iron block so I could steady it in the drill press. Also, I had to apply some marker to the area where I was drilling just to see my punch marks.
The holes are drilled .500" apart, starting .2500" from the edge. I used a 0.09375" (3/32" or 2.3813mm) drill bit. Don't go to small as trying to tap hard teflon can be a real chore!
The holes were drilled about 1.00" long, and the screws I used allowed the heads to be counter sunk into the aluminum.
First I drilled two holes about .500" from the edges with a .3750" (3/8 or 9.525mm) bit.
Then taking a .46875" (15/32 or 11.906mm) bit, I tapered the hole slightly. I feel that even though it's teflon against plastic, the taper takes away some of the possible fracturing of the ty-wrap.
As I mentioned before, select the screw diameter and the bit for cutting the holes in the teflon carefully. When you thread these into the holes, tapping them for the first time with the threads, it can be difficult. Don't snap off the screw in the teflon. On the other hand, once in, they won't come out!
The counter sink must be deep enough to allow the head to be flush with the plate. When mounting on the antenna, the screw heads will be aligned with the boom of the antenna.
The balun is mounted so the terminals will be horizontal with the antenna elements. I should also mention that the balun was purchased at a hamfest for $2.00, so now we're just below $10.00 on this project.
When I mount this on the antenna boom, the terminals of the balun will be just a few inches below the feed point of the antenna. On my old installation I used a 16g stranded wire by Alpha, which I always noticed would move in high winds. By positioning the balun under the feed point, and using a stiffer wire, I can prevent this problem from happening again.
For the feed wires I made the choice to use Belden 8000, a 14AWG stranded antenna wire. This is a stranded, all copper wire. I have used Belden 8000 for the majority of my wire antennas, and it's a great product.
The wire is crimped into the balun connection, then I soldered it with 60/40. After it cooled, a small amount of silicone was placed over the solder, and carefully I placed a 1.500" piece of heat shrink over the connection, and started to seal it from the end to the center. This compacts the silicone around the connection.
When this is installed on the antenna, I'll heat up the wire and apply some wire grease to keep them from corroding, and trim them slightly to make the shortest run to the driven element of the A3S. Here is the manual for the Cushcraft A3S.
After the antenna is up and in operation I'll have to remember to offer some notes on the difference, (if any), of the balun mounting on the boom of the antenna (the old way), and this new method of mounting the balun.
After putting this all together, I'm seriously thinking of doing something like this for all my antennas. I really like the idea of having the balun mounted firmly in place, yet at a fixed distance from the antenna.
I removed all the old rusted hardware, and installed new stainless steel screws, washers, and wingnuts. I then attached a ring terminal to each wire. Then I soldered the connection after crimping with 60/40 solder.
Before putting heat shrink over the connector, a generous coating of grease was placed over the wire and connections, MINUS the round terminal area that attaches to the antenna. This will keep water from corroding the wire, and ultimatly the connections to the balun and antenna.
Depending on the location, it may be necessary to regrease the wires to keep them shiney and clean. If you live near a salt water area, check the wires often. Once they start to corrode, it's hard to return them to normal.
Till next time, 73.
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