Common matching networks for Industrial, Scientific and Medical applications

Why use a matching network?

The Radio Frequency Generator (RFG) is designed to drive a specific characteristic impedance, 50Ω. This is the characteristic impedance of the output cable and allows the generator to deliver full power at maximum efficiency. If the output impedance varies from 50Ω the protection circuits in the RFG reduce the output power in order to protect the RFG from overload.

With the exception of dummy loads and some types of antenna most real world loads differ substantially from 50Ω and a device is needed to transform the output of the RFG to the actual load impedance of the system being driven.

It is sometimes possible to use a conventional wound transformer, but most often the matching device is made up of a network of series and parallel impedances that combine to perform the transformation, i.e. a 'Matching Network'. There are several configurations of these in regular use and the selection of the right one is important.

Representing RF Impedances:

The impedance of an application is rarely pure resistance, often it consists of a combination of resistance and capacitance. This can be considered as an 'equivalent circuit' consisting of a resistor and capacitor. Two forms of this circuit exist, series and parallel. It is important to realize that neither form is correct or incorrect, both have validity and it is possible to represent the output in whichever form is most convenient.

The Parallel form represents a load as a resistor and capacitor in parallel. The resistor would typically be high in value. This form is convenient when considering the 'PI' network as the tune capacitor just adds to the parallel equivalent capacitance.

The Series form represents a load as a resistor and capacitor in series. The resistor would typically be low in value. This form is convenient when considering the 'L'  network as the tune capacitor just appears in series with the series equivalent capacitance. In the case of the 'T' network the series capacitance should be made to resonate with the inductor, leaving just enough inductance to allow it to tune in.

The reactive part of the impedance could just as easily be inductive, I described the capacitive case as this accounts for a large proportion of RF work.

Common Matching networks

This list describes networks used for delivering power into capacitance loads, therefore it does not include the more elaborate matching stages used inside RF amplifiers circuits.

Also see 'Inductive output networks'

• The 'L' Network.
• The 'T' Network.
• The 'PI' Network.