Winter 2007 Issue


Coaxial Cable

By Kent Britain, WA5VJB

Photo A. Big coax vs. small waveguide.

This time we will covering odds and ends concerning coaxial cable. You might be surprised as you read about one of the most necessary pieces of your amateur radio station. We also feature a couple of Cheap Yagis, one at N7BHC’s QTH, while the other belongs to your columnist.

Coax Maximum Frequency

From Brian we received a question from the last column, where we talked about using TV station antennas. I had said that the coax the TV station was using had a maximum frequency of about 1 GHz. Maximum frequency for coax? Yes, if you look up the loss charts for a sepcific coax, such as 7/8-inch hardline, the loss line will go up to about 6 GHz and just stop. In photo A you see a section of coax and a section of 10-GHz waveguide. The coax is a lot bigger than the waveguide. If you tried to send a 10-GHz signal down the coax, the signal would have trouble deciding if it was going down coax or waveguide. This mixed mode of propagation means that sometimes the waves add, and sometimes they cancel when they get to the other end. In short, the output is a mess. In figure 1 we see a typical plot of coax loss to frequency on the left. If the chart continued, then it would look something like the plot on the right. Like I said: A mess!

At 6 GHz, the 7/8-inch coax shield starts to act like waveguide. Therefore, instead of being coax, it tries to become like waveguide. The connectors are not designed for this use and the coax line gets very lossy. Microwavers make coax of different sizes without the center conductor. This is called elliptical waveguide—just coax without the center wire, squished a bit, and different connectors. It is a nice long tube.

Elliptical waveguide is fairly common on the surplus market, but the connectors are not so common and are usually expensive. Figure 2 shows a mod I first saw used by Eric Ericson, KØKE. Eric drilled a hole for a connector, mounted an SMA connector with a probe on the end, and then closed off the open end of the waveguide. He simply modified the end of the elliptical waveguide into a coax to waveguide transition. You have to be careful not to put too much stress on the coax connector. Even so, this is a simple, low-cost, low-loss way to avoid needing those big, expensive waveguide flanges.

In photo B I show that there is another way to use elliptical waveguide. Find some short sections of brass waveguide that will just fit into the elliptical waveguide. You really want a brass or copper waveguide, since the last step is to solder it in. Cut off a section of waveguide and, with a big file or bench grinder, round off the outside of the corners as much as practical. Now hammer it into the elliptical waveguide as best you can, bend the copper back flush with the rectangular waveguide, and solder. I have used this with 5.7-, 10-, and 24-GHz elliptical waveguide with good results. It has a few tenths of a dB more loss than the proper waveguide flanges, but this way is 20 dB cheaper!

Life Expectancy of Coax

The photo of some contaminated RG-8 took quite a bit of scrounging (photo C). You see, there is only one section of RG-8 actually used in my station. Furthermore, it has terminals crimped on each end, and I use the RG-8 as a ground strap on one of my towers.

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