Thursday, July 15, 2010

I’m uncertain about how I should attach a 450-Ω ladder transmission line to my radio ...

Q George, KD4KYM, asks, “I’m uncertain about how I should attach a 450-Ω ladder transmission line to my radio. I have SO-239 coax connectors on my Kenwood TS-520 transceiver. Can I assume that it is simply a matter of soldering one side of the ladder line to the center conductor of a PL-259 and the other to the outer conductor? I want to use the ladder line to feed a multiband 80-10 meter dipole antenna.”

A Your TS-520 was designed for 50-Ω unbalanced coaxial cable. Your ladder line is 450-Ω balanced, so there is more to deal with than just the physical differences. If you were going to use the ladder line on just a couple of bands, you might be able to get away with feeding the ladder line to a balun (a balanced-tounbalance feed line transformer) with an appropriate impedance ratio (4:1, for instance), then using coax between the balun and your radio. But since you’re aiming for broad, multiband coverage, I’d recommend an antenna tuner.

Most antenna tuners are designed with built-in baluns to handle the impedance transformation. There are usually three posts in the back of a tuner—two for ladder-line and one for random wire antennas. There may be a jumper between the random wire post and one of the ladder-line posts. Make sure the jumper is removed it is only used for a single random-length wire.

If your tuner does not have provisions for ladder-line, you can connect it as you say, by soldering the ladder line to a PL-259. It may work, but not as well as using a balun. Of course, you could always install an external balun in the line between the ladderline and the antenna tuner (see Figure 1), although we don’t recommend that you run high power with such a setup.

Figure 1—If you own an antenna tuner that does not include an internal balun, you can use an external balun. Just keep the coaxial connection between the balun and the tuner as short as possible.

from QST May 2000