fast the driven shaft turns). A load with high mechanical
impedance takes a lot of torque to turn.
One mechanical impedance matching device we are
all familiar with is the gearbox (or transmission) in Figure
3. The job of a car’s transmission is to transfer power to
the wheels which are turning at a wide range of speed,
while keeping the engine happy near its optimum speed.
It does so by converting one combination of torque and
speed (impedance) to another, while losing as little power
as possible to friction.
The situation is very similar to an RF transmitter
designed for a load impedance of 50Ω but which can be
connected to an antenna system presenting all sorts of
different impedances. The electrical gearbox used here is
known as an antenna tuner or transmatch.
One combination of voltage and current go in, and
another combination of voltage and current come out,
while heating up the matching unit with losses as little as
possible. There are lots of different electronic components
and circuits that can act as electrical gearboxes. Let’s meet
a few of those.
Resistive matching is not very useful if the goal is to
deliver power efficiently, such as via the power grid.
Instead, we use a transformer to change the high voltages
and low currents of power distribution lines to the low
voltages and higher currents in homes and businesses.
Power is transferred as magnetic energy from the
transformer’s primary to the secondary circuits through
the transformer’s core. This is called a flux-coupled
Figure 4 shows the basic math for how a transformer
converts impedances. Let’s say you have 115 VAC at the
AC outlet and want 12 VAC for your project. Your step-down transformer would need a turns ratio of n = 115 /
12 = 9. 6 from the secondary to the primary. If your
project drew 1A of current, that 12Ω impedance would be
transformed to a load of ( 12 V / 1 A) x n2 = 12 x 92.2 =
1106Ω to the AC line.
You can find impedance matching transformers in
many different types of electronic circuits. A very
common use is to match the output of a transistor audio
amplifier (which may have an output impedance of several
hundred ohms of resistance) into the 4, 8, or 32Ω of
speakers or headphones.
In ham radio, it’s not unusual for antenna systems to
have impedances of 100Ω to 1,000Ω, so transformers are
available with several selectable turns ratios to transform
impedances by ratios of 2:1, 4:1, and 9:1. The result is an
impedance that is close enough to 50Ω, so that either the
transmitter is happy or only requires a little bit of
While power transformers for use at 50 Hz or 60 Hz
have laminated steel or iron cores, transformers for RF use
March 2016 59
FIGURE 4. An electrical transformer transfers power with
one combination of voltage and current (electrical
impedance) through its core to another combination of
voltage and current in the secondary circuit. The amount of
transformation is determined by the ratio of turns on the
transformer core's secondary winding to the number of
turns on the primary winding.
FIGURE 5. Four types of L-C impedance matching
networks. Each type of network is named for the letter it
resembles. A component in the signal path is called a series
component. A component connected from the signal path to
common or ground is called a shunt component. The circuit
at A is a series-L shunt-C network, for example. The pi and T
networks can each be viewed as back-to-back L networks as
described in the text.