ARTICLE FROM SEPTEMBER 1997
Last month we began a discussion on tuning up the antenna and feedline for best performance. Here we go with the rest of the discussion!
We solved the feedline problem in true ham tradition. Just like my early days on six meters, we used what we had for feedline. In this case we used 75 ohm 1/2" cable television (CATV) coax. I quickly consulted the Antenna Handbook and looked up a Smith Chart to compute what the SWR would be with a 50 to 75 ohm mismatch. I was please to discover it would be a low 1.5:1.
Now this was good news because SWR's that low are hard to come by on UHF. No baluns are needed or wanted. We're talking very low loss here. I made up a set of modified "N" connectors for the TV line and it was installed the next week. It has worked perfectly ever since. SWR is exactly 1.5:1 as predicted be the Smith Chart.
The lesson here is that you do not need expensive heliax for your repeater feedline. You don't need to use high loss RG-8 type coax. CATV coax is cheap (as in free) and it is very low loss. We get ours by asking the local cable companies for tail ends off of the big reels they use. Often they have pieces 200-300 foot long for the asking.
You are going to see 75 ohm CATV coax in two sizes and jacketed and unjacketed. The 1/2" size is the most common. There is also a 5/8" size that is also 75 ohm. In the past CATV coax did not have a jacket. Now to protect it from corrosion a version is available with a tough plastic jacket. If you can get it, this is the best type.
CATV coax has a solid aluminum shield. It is about as flexible as you would expect an aluminum tube to be: that is, not very. It is very easy to kink and thus ruin this coax. Be very careful when installing it. Keep the radius of any bends to three foot or more. If you need to zig-zag the feedline around at the ends, use short "pigtails" made from RG-8. Special cable type female "N" connectors are made just for the purpose of making up these jumper cables.
You DO NOT need to have 75 to 50 ohm baluns. I have read a great deal of worrying on this subject on the news groups. Put your mind at ease, if you check the math, you will discover that the loss from the mismatch (at 1.5:1 SWR) is less than the losses caused by the balun itself and its connectors. Also note the these baluns being resonate line section devices they have a narrow band width. In other words, you are better off without them.
If you absolutely have to use matching baluns, you can find plans for them in the old edition of the RSGB VHF handbook. You will need access to a lathe to manufacture them.
Connectors have proven to be only a minor problem for 75 ohm CATV coax. The common 1/2" line is no problem at all for PL-259 connectors. All you need to do is visit your local plumbing supply store and buy some brass copper tubing compression couplings. You want a fitting that is used to couple two pieces of 1/2" flexible copper tubing together.
Slide the coax in one end, and the body of the PL-259 in the other. Trim the center conductor for proper fit with the PL-259 and tighten the coupling with a pair of wrenches. All that is left is to solder the center conductor and your done! If you really want to do a professional job you can silver or gold plate the compression couplings. I would recommend this if the connector is to be used outdoors.
Attaching "N" connectors to 1/2" CATV coax is a little more difficult. There are two methods. The simplest uses a modified "N" connector. A better method uses a connector fabricated from a compression fitting and a "N."
The only drawback of the simple method is that the connector may not be water proof and it is difficult to get them tightly attached to the coax. You start by choosing a Male N connector that has a center pin made for 9913 style coax. The center pin has a slightly larger inner diameter to fit over the larger diameter center conductor used in the new lower loss cables. These will press fit over the center conductor of 1/2" CATV line.
Start by using a tubing cutter to cut off a small piece of the CATV coax's aluminum shield. The length will be determined by the style of "N" connector you are using, so you will have to experiment. Split this ring on the side so it can be slid back over the outside of the shield. This will be used as a compression ferrule inside the connector to tighten the bushing onto.
Next take the bushing and clamp it in a vise. Using a spiral flute reamer or a cone cut drill, bore the hole in the bushing out to 1/2". You are now ready to trim the length of the inner conductor and shield to fit your "N" connectors body. Be very careful to get the center pin correctly positioned and the right length. Examine a cut off connector from the Hamfest or check the assembly drawings in the ARRL Handbook for a guide.
When you are sure of your dimensions, solder the center pin. Be sure you get all the dielectric foam cleaned off the center conductor prior to soldering. Assemble the connector by placing the bushing on the cable first, then the homemade ferrule and if you have it, the rubber "O" ring for sealing. If the ferrule is the right length, you should be able to make the connector a tight fit when the bushing is tightened.
I used this method to make the connectors for the UHF hub repeater installation. It took some adjustment to get them on tight, but they did work and have continued to work well.
A much better way to fabricate connectors is to solder a Male "N" to a copper compression fitting. Mark Atherton, N0RXD, and myself spent some time mystifying the clerk at a local hardware store while we went through almost the entire inventory of fittings. It turned out that a 1/2" compression to 1/2" female NPT adaptor works well. The compression side fits the coax perfect. The NPT side will accept the "N" connector body after its inside is turned out slightly on a lathe. The threads are turned out to the outside diameter of the "N" for a depth of .2" into the adaptor.
The connector is press fitted into the NPT end of the adaptor and soldered. I used ordinary solder with some extra flux (NOT soldering paste or acid!). To keep the rubber seal in the "N" from being damaged I set the assembly upright on a brick and filled the end of the connector with water. I very carefully heated the assembly with a small flame from my propane torch, all the while keeping the water replaced as it boiled out.
OK, it was touchy, but I got the job done. As soon as the solder joint was finished I cooled the assembly off by dropping it in a can of water. The result is a solid water proof connector. After silver plating these homemade connectors are as good as any commercial ones.
One last "tune up" reminder. In the name of safety and sanity always tune your antennas on the ground. If your vertical's radials or decoupling section really works, the antenna will tune on the ground just like it will up on the tower. Just place it on a metal post a few feet high and tune it. Try to keep at least twenty feet from any large metal objects that could affect the pattern and thus the tuning.
Beams can be tuned by simply sitting them on their reflectors and pointing them straight up in the air. The first time I saw this done I had a hard time believing it would work, but it does. As long as the antennas reflector is doing its job, the proximity of the ground seems to have little affect on tuning.
Just to be on the safe side I usually sit the antenna on a small wooden box and support it a vertical position using twine tied to two stakes. It looks odd at first, but it is very easy to work on beams in this position. Plus you can't fall off the tower.
One last hint is if you are using a conventional SWR bridge, install the bridge as close as possible to the feed point of the antenna. Never install the bridge on the transmitter end. You will not get an accurate reading if the feedline is accidentally a harmonic length.
To test your setup, make up a short jumper coax that is not a even or odd 1/4 wave length long. Check the SWR and log the reading. Then insert the jumper into the feedline with a barrel connector. Check the SWR reading again. If it goes up, then your SWR meter was accidentally at a standing wave null point. Retune the antenna for minimum SWR with the jumper in place.
David Metz, WA0AUQ
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