Tech Papers and Guides

Antenna Tuning

Tune an antenna to make sure it presents a 50-ohm load to the circuit and the match is centered for the frequency band of operation, minimizing return loss. This will make sure that the maximum transmit power is absorbed into the antenna instead of reflected, or “lost.”

Power transfer theory can explain the requirement to match the antenna impedance to the characteristic impedance of the circuit (typically 50 ohms). This theory states that the maximum power is transferred when the resistance of the load equals the resistance of the source as viewed from its output terminals.

In terms of impedance matching, it can be shown that maximum power is transferred when the source impedance matches the complex conjugate of the load, or:

Due to the theory of reciprocity, the impedance of the antenna is the same in both transmit and receive states.

Consider this simple RF block circuit diagram:

 RF Circuit block diagram

In the diagram, all three ports of the T/R switch present an impedance of 50 ohms, since the TX and RX matching networks transform TX Zopt (the optimum impedance presented to the TX) and RX Rn (the noise resistance of the RX) to 50 ohms. Note the T/R switch may not necessarily present a purely 50-ohm resistive load to the circuit, just that they’re designed for a nominally 50-ohm system. Finally, the antenna matching network transforms the nominal 50-ohm impedance of the network to the complex conjugate of the antenna.

Important: Antenna impedance at the feed point is influenced by the medium surrounding the antenna. It’s important that the antenna is tuned in an example of the application environment. For example, if developing an application that requires a potted PCB assembly, tune the antenna in a mold of the surrounding potting material.