Description: Antenna tuners match a radio's impedance to a load. Most ham radio transceivers use a 50 ohm impedance to the transmission line (for example, coaxial cable). However, the impedance presented to the radio by the load (transmission line and antenna itself) can vary. The mismatch in impedance is often caused by operating at a frequency that is not resonant for the load. For example, if trying to operate at 7.1 MHz with an antenna which has a 8.0 MHz resonant frequency. The antenna tuner (or ATU) allows ham radio operators to use an antenna across a broader range of frequencies by matching the source (transceiver) to the load.
Antenna tuners vary in design, complexity and features - from very simple circuits which add capacitance and inductance to create a 50 ohm resistive impedance match to more complicated matching networks that are automated and can find a 1.0:1 SWR match within seconds.
PI networks, SPC (series parallel capacitor) and autotransformer ATUs are common in ham radio.
If you have a balanced line (ladder line) transmission line coming into your ham radio shack (through a tuner) to your rig, does it make sense to add a balun at the tuner? If so, should it be placed on the tuner output side or on the radio side? This article by W9CF explores that issue in depth.
Servo Control Circuit - Used to remote control a lightly modified commercial antenna tuner.
Have a ham radio antenna that needs to be remotely tuned? With a few modifications to an indoor commercial antenna tuner you can manipulate the capacitance and inductance via servo motors as N3OX has done in this project.
