Q Bill, W0LPQ, asks, “Is it true that an antenna tuner does not really ‘tune’ the antenna? And if an antenna is cut to resonance already, what good is a tuner?”
A Yes, it’s true—an antenna tuner doesn’t really tune your antenna in the strict sense of the word. It does not, for example, adjust the lengths of your antenna elements, their heights above ground and so on. What an antenna tuner does do, however, is transform the impedance at the feed line input to a value that your transceiver can handle (typically 50 Ω—see Figure 1). Think of an antenna tuner as an adjustable impedance transformer and you’ll better understand its function.
If the antenna is cut to resonance and is designed to match the impedance of the transceiver and feed line, an antenna tuner is not required. The transceiver is presented with a 50-Ω impedance (or something close to it) into which it can deliver its full output. However, the SWR bandwidths of many antenna designs are usually limited to only 200 or 300 kHz. If you cut a dipole, for example, to be resonant and provide a 1:1 SWR at 7100 kHz, you might find that the SWR climbs well above 2:1 when you attempt to use this antenna at 7250 kHz. Most modern transceivers begin reducing output, or may shut down completely, at SWRs greater than 2:1. With an antenna tuner in the line you can transform the impedance to 50 Ω, reducing the SWR to 1:1. The transceiver delivers its full output once again and you’ll be able to operate on 7250 kHz—minus some feed line attenuation between the tuner and the antenna. The attenuation is caused by the fact that the SWR on the feed line between the tuner and the antenna is not affected by the matching function of the tuner; it remains unchanged.
Figure 1—Think of an antenna tuner as an adjustable impedance transformer. Depending on the operational range of the tuner, it can match the antenna system impedance to 50 Ω for your transceiver.
From QST February 2000
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
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?”
A 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
