Sunday, July 4, 2010

My transceiver has a built-in antenna tuner, but it can ...

Q Mike, K4JIF, asks, “My transceiver has a built-in antenna tuner, but it can provide a 1:1 SWR only in the middle of the 80-meter band using my G5RV antenna. I want to operate phone on the high end of the band and CW on the low end of the band, but my tuner can’t seem to deal with the mismatch at the band edges. Should I use an external tuner?”

A The internal antenna tuners provided in modern rigs are quite limited in range. They will not normally tune mismatches that result in SWRs of more than 2.5 or 3 to 1.

An external antenna tuner should do the job. Although they are more expensive, the roller-inductor models are particularly versatile. Make sure to switch your internal tuner off, of course, before attempting to use an external tuner. Another alternative would be to try different antenna designs. A broad-banded design such as a cage dipole might provide a sufficiently broad SWR bandwidth that your internal tuner could handle.

If you are going to invest in an external tuner, I’d suggest that you try it with a simple dipole fed with 450-Ω ladder line. Just cut the legs to equal lengths, as long as possible. If the dipole is a total of 62 feet in length or longer (a quarter wavelength at 3.750 MHz), I’m willing to bet that your external tuner will allow you to operate wherever you want on the band.

From QST November 1999