components. The resulting impedance match is made over
a wider frequency range, too, requiring fewer adjustments
as frequency changes.
Pi networks with series inductors are very common in
HF (high frequency, 3-30 MHz) transmitter outputs
because the series inductor acts as a filter to reduce
higher frequency harmonics from the transmitter. An L-C
network with inductors in the series signal path between
input and output is called “low pass” because of the
additional attenuation by the inductors at higher signal
frequencies. This is useful in meeting FCC (Federal
Communications Commission) specifications for
transmitter output harmonics and other spurious
T networks are the most common type of circuit used
for adjustable impedance matching units. These are high
pass networks because of the series capacitance in the
signal path. Even though these circuits don’t attenuate
harmonics, the series capacitors make them less expensive
to build and so they are popular in amateur equipment.
Note that both the pi and T networks can be built with
the opposite configuration of L and C, but those in the
figure are the most common in use today for a variety of
Transmission Lines as
Transmission lines can be used to create reflections
that cancel the unwanted reflections. Called synchronous
transformers, the two best-known designs are shown in
Figure 7. The quarter-wave transformer (or Q section)
works by inserting a length of transmission line between
Z1 and Z2 that is one-quarter of a wavelength (λ) long at
the frequency of the impedance match. The characteristic
impedance of the matching section, ZQ, should be the
geometric mean of Z1 and Z2 as shown in the figure.
For example, a one wavelength loop has a feed point
impedance of around 120Ω. The geometric mean of
120Ω and 50Ω is 77.5Ω — quite close to the 75Ω
impedance of RG- 6, RG- 59, or RG- 11. There are many
other types of coax with useful impedances, as well.
The Q section is found in many more places than just
coaxial cable! If you wear glare-reducing coated glasses,
the coating is a stack of transparent films. Each film is
1/4λ thick over a range of light wavelengths, and has a
characteristic impedance such that the result is a set of
canceling reflections created for a variety of wavelengths
of visible light.
A similar trick is employed in the 12th wave section.
Surplus CATV hardline is free or very cheap, but is usually
75Ω cable which would create a 1.5:1 SWR in a 50Ω
system. By inserting two back-to-back 1/12th wavelength
sections of the 75Ω and 50Ω cable as shown in the
figure, reflections from the 75Ω cable are cancelled,
making the system “look like” 50Ω — usually across an
entire band of frequencies.
Are We Matched Yet?
I’ve presented four common techniques for
impedance matching in this column: resistive,
transformers, reactances, and transmission lines. You might
recognize these in circuits you’ve seen on your
workbench. Or, you might find one of these techniques to
be just the thing you need to fix a pesky SWR problem or
clean up some digital signals on a PCB. What’s important
is to remember that once the frequencies of signals start
exceeding a few hundred kHz, everything starts behaving
like an RF signal — because it is! NV
March 2016 61
More Matching Material
We've barely scratched the surface of impedance matching
techniques — a topic widely discussed in amateur radio. The ARRL
has published many articles over the years but one of the best,
"Another Look At Reflections," was written by Walt Maxwell W2DU.
This multi-part article is available online at www.arrl.org/
transmission-lines along with numerous other interesting and