An antenna tuner, transmatch, antenna tuning unit, or ATU matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match. This mismatch is usually caused when using a non-resonant antenna, one that is not the correct electrical length as compared to the wavelength of the signal. This impedance change is akin to changing the electrical length of the load. An ATU allows the use of one antenna for a broad range of frequencies. A tuned antenna is never as good as a naturally resonant antenna due to additional SWR induced losses on the feed line, and losses in the ATU itself.
Connecting an ATU
transceiver -> SWR bridge -> feed line -> antenna
Using an ATU (a typical three knob version, radio, band, antenna)
- Adjust the transceiver to an empty frequency near the frequency you intend to use.
- Set the radio and antenna knob to a center location.
- Move the band knob until the loudest noise is heard.
- Fine tune both the radio and antenna knobs alternately for louder noise. You are done if this is a receiver only.
- Start transmitting at the lowest power that you can detect on your SWR bridge
- Fine tune both the radio and antenna knobs alternately for lowest SWR (You may need to move the band knob for lowest SWR)
Sources
- ARRL website
- The ARRL Antenna Book
- Reflections Transmission Lines and Antennas by M. Walter Maxwell, W2DU
- many many webpages
- personal experience