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
