Feedlines for Beginners
Corrosion takes place if the connections are not weather-protected by more than a single layer of tape. Also, there are many products on the market that can be used to protect antenna connections. It is a good idea to pick materials specifically manufactured for use on antennas and coax because some products have components in them that are corrosive or otherwise harmful to coaxial cable or antennas in general. So, check the ingredients carefully and ask other hams or someone who can help you if you are not sure.
I have used several layers of tape and then covered the tape with hot glue. I use a torch with a low flame to allow the hot glue to melt and cover the connection completely. But before you use this method, be VERY sure the connection is not shorted or open or has other problems. If you don’t, you will spend a long, long time removing the hot glue and the tape. I am speaking from experience. But it makes a good waterproof covering and the cable does not get damaged by the hot glue or torch usage, as long as you are careful. I use low temperature glue which can make it a lot easier on the cable. Some of the hot glues require more heat than others. This is one method for protecting your cables.
Some of the newer silicon compounds such as silastics will do fine as long as they do not have acetic acid or other compounds in them that can eat up your connections or antenna components. No matter what method you choose, seal up your feedline at the antenna end. Try to locate your feedline in places where you will minimize the possibility of damage to it. And keep it away from animals. I had a squirrel chew a hole in my coax one time and the coax filled up with water. I didn’t know that this had happened and turned up the amplifier one day and was rewarded with screeches of terror from the wife. Seems the coax went up in flames after it had arced internally and since this was in the winter, it set fire to the dead grass in the back yard. After putting out the grass fire and replacing 65 feet of RG-8/U, I was back on the air. But I am not fond of squirrels.
If you live in a corrosive atmosphere, such as along an ocean or other body of salt water, some method of protecting the antenna would be a blessing as salt water in the air can cause an antenna to develop high resistance joints where it is put together. This in turn causes loss of signal on both receive and transmit. Couple this with a lossy coax connection and you can lose a lot in both the antenna and feedline. Since it occurs gradually, it will slip up on you and you may not notice it until things have progressed beyond simple cleaning of joints and connections. So careful attention to your feedlines will help ensure minimum signal loss in your system.
But what is a feedline? In its most basic form, a feedline is a single wire that goes from your rig to your antenna. Then there is the open line or ladder line that consists of two wires separated by insulators spaced every few inches (centimeters). Next, is the familiar TV twinlead, with variations on the open wire to be found in this family of feedlines. Some of the twinlead has had the insulating material cut out of the center, approximating the open wire feeders. But what we see mostly is the plain old ribbon line as it is called in some places. At my station, I use the common TV twinlead to feed my quadpole antenna and it is tuned by my elderly Johnson 275 watt Matchbox. (No I won’t sell it.) In the past I have used the open line feedline and had no problems with it even when it was covered with ice and I was running over 700 watts into it.
However all of these twinlead type of feedlines are affected by weather in some fashion. They also have the lowest loss of all the feedlines that we can afford. I have thought that if I could take the open line type and put it inside lengths of PVC, I would have the lowest loss line possible and use baluns at each end to match the transmitter and antenna. At VHF and UHF, feedline losses become important, particularly when you have long runs of cable from the shack to the antenna.
As a point of interest, some time ago I got curious and decided to see how much loss 50 feet (15.2 meters) of RG-58 A/U had at 150 MHz had. So, I took a Bird Wattmeter, a 50 watt transmitter and a dummy load and connected it all up to a new 50 ft. roll of coax. With an even 50 watts into the cable, I got 25 watts into the dummy load. Rather shocking, to say the least. Half of the power was being lost in the cable before it even got to the dummy load. I repeated the test on 450 MHz and with 20 watts in, got less than 5 watts into the dummy load. Over 75% of the power was lost in the coax. So you can see that the losses at these frequencies can be terrible if you are trying to do any weak signal work, like moon bounce or other such DX.
Why does the coax have so much loss? If you look at Figure 1, this is the circuit equivalent of a length of coaxial cable. Since it is made up of wire and insulation, you have the equivalent of inductors and resistors in series. Then, since the outer shield is separated from the center conductor by insulation, you have a capacitor and this capacitor extends from one end of the cable to the other. The wire that makes up the center conductor has a certain amount of resistance per foot, so there is loss there. There is also a certain amount of inductance per foot, which adds more loss per foot. There is even more loss per foot due to capacitance per foot. So unless you can afford the expensive stuff, it is very hard to get the losses down on the VHF and UHF bands.
The losses at below 30 MHz are not as bad, but they are still there. So, coax is found in most of the ham shacks in the world. But for the most efficient feed line, open ladder line is the best. It also has the lowest capacity per foot. Plain cheap TV twinlead is the next lowest as far as loss is concerned. There are several different makes of twinlead on the market. The foam filled twinlead seems to be the best for our purposes, and there are many TV accessories that will allow you to run the twinlead in through a window or wall.
I am not advocating a change over from coax to twinlead, but merely pointing out some interesting comparisons between the two types of feedlines. Each has its place in the amateur radio world and I have antennas fed with coax and antennas fed with twinlead and they all work fine. My below 30 MHz antennas are fed with twinlead and the VHF/UHF antennas are all fed with coax, short runs to be sure, and the 10 meter antenna is fed with a short length of coax. They all work well as intended, and the antenna ends are all shielded from the weather as they should be. In all honesty, I have never noticed any affects of weather on any of the twinlead antennas, despite what I have read. Weather may cause problems with TV signals, but none noticed on the ham bands where I operate.
It is an advantage to have a catalog from a manufacturer like Belden or one of the others, as they usually list the characteristics of their feedlines and this is nice to have on hand at times. So keep your connections protected from the weather and your feedlines, if possible, protect the antenna joints and minimize your losses due to corrosion.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 9th March 2024