Thursday, June 24, 2010

I’d like to feed a dipole above my roof using my antenna tuner and a balanced feed line ...

Q Larry Amann, K5TQN, asks, “I’d like to feed a dipole above my roof using my antenna tuner and a balanced feed line. However, my roof is metal and the feed line would have to rest on it for a considerable distance. I know that you need to keep open-wire feeders away from metal, but this is not possible in my case. Could I make my own shielded balanced line by using two lengths of coax in parallel?”

The idea of using two parallel lengths of coax to create a kind of “shielded” balanced feed line has a long history. A number of hams have used this approach in situations where they needed to pass a balanced feed line over an expanse of metal, such as a metal roof. The technique involves placing two coaxial cables in parallel (using tape or cable ties to keep them firmly together along the entire length), shorting the shield braids of both coaxial cables at both ends, and grounding the “shack side” of the braids at your antenna tuner (see Figure 1). The inner conductors of the coax then become your “shielded” parallel feed lines.

The primary drawback with this technique is that you will not enjoy the same degree of low-loss performance as you would with standard twin-lead or ladder line. On the other hand, it should allow you to maintain a balanced line all the way to your antenna, despite your metal roof.

Figure 1—You can make a shielded balanced feed line out of two pieces of coaxial cable. It won’t have the same low-loss characteristics of open-wire or ladder line, but it is a workable alternative when the feed line must pass over a large metal surface.

From QST May 1999