Antenna Tuner Circuits

  

 

An antenna tuner, transmatch, or antenna tuning unit (ATU) is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna. An antenna tuner 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. 
 
For systems which need to operate over a wide frequency range such as a power amplifier operating over the range 1 MHz to 30 MHz in some solid state designs a series of wide band transformers wound on ferrite cores can be used. This design has the advantage of not requiring any tuning when the operating frequency is changed.  An ATU allows the use of one antenna for a broad range of frequencies.
 
An antenna plus matcher is never as efficient as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the ATU itself, although issues of pattern and capture area may outweigh this in practice. In solid state RF power amp design these networks are useful because MOSFETs and bipolar transistors are designed to operate with low resistance loads.
 
This type of design can also be used to match an antenna to a transmission line, it has the advantage of not requiring any tuning but it has the disadvantage of not being capable of fine adjustment. These networks can be used to extend the useful range of a conventional narrow band ATU. Valved RF amplifiers are very different because the load resistance which a valve is designed to operate with is normally much greater, hence for power designs the circuit designs are often very different.

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