October 1960 Popular Electronics
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Here is a short tutorial on how to construct a 1/4-wave stub
"trap," or filter to attenuate even-order harmonics from transmission
lines, be they antennas or sections of track running on a microwave
substrate. The article appeared in a 1960 edition of Popular
Kill Those Harmonics
By Kent A. Mitchell, W3WTO
Inexpensive, easy-to-make tuned stubs will eliminate harmonics
from your CB or ham rig
Whether you're a Citizens Bander or a ham operator, harmonics
from your transmitter can ruin your neighbor's TV pleasures
and bring him pounding on your door. Likewise, the FCC takes
a dim view of anyone who clutters up the bands with spurious
Fig. 1. - Quarter-wave stub filters can be
of either the "shorted end" (A) or "open end" (B) types. Both
types eliminate even-order harmonics.
One sure way to help clean up your signal is to connect a
stub filter to your antenna transmission line. Although relatively
simple and inexpensive, quarter-wave stub filters are very effective
in eliminating even-order harmonics (2nd, 4th, 6th, etc.) from
the output of a transmitter feeding a single-band antenna.
There are two types of quarter-wave stub filters. In one
case, a quarter-wave stub with a shorted end is connected in
parallel with the transmission line; in the other, a quarter-wave
stub with an open end is hooked up in series with either leg
of the transmission line. Let's see how these two types of stubs
work and how they are used.
Figure 1 (A) shows a shorted quarter-wave stub connected
in parallel with the transmission line from transmitter to antenna.
Since the stub is a quarter wavelength of the signal frequency,
it presents a very high impedance to the transmitted signal,
and the signal passes on to the antenna with little or no loss
in power. Even-order harmonics, however, are confronted with
a virtual short circuit, since the stub offers a very low impedance
at these frequencies. The parallel shorted stub is easily connected
to coaxial transmission lines as well as twin-lead and open-wire
Figure 1(B) shows an open-end quarter-wave stub hooked up
in series with the transmission line. The stub offers little
or no resistance to the fundamental frequency, allowing it
to pass to the antenna. Even harmonics, on the other hand, "see"
some multiple of one-half wavelength - a near-infinite impedance
for these frequencies-which prevents them from reaching the
antenna. Open-end series stubs are not suitable for coaxial
transmission lines since they are difficult to connect to this
type of line. However, connection to either a twin-lead or open
line is simple.
Fig. 2. - T-connector inserted in coaxial transmission line
makes convenient jack for connecting shorted-stub filter.
To make a stub filter for your ham or Citizens Band transmitter,
use a piece of transmission line of the same type and impedance
you presently use. To determine the length of the stub, substitute
the fundamental frequency of your transmitter in the following
Incidentally, coaxial cables such as RG-8/U, RG-58/U, RG-11/U,
and RG-59/U have a velocity factor of 0.66; flat 300-ohm TV
twin-lead has a velocity factor of 0.82; tubular 300-ohm line
is rated at 0.84; and the popular 450-ohm open-wire transmission
line at 0.90.
As an example, let's say we are going to cut a shorted stub
filter for the 6-meter amateur band on 50.1 mc., using coaxial
cable. Applying the formula, we find:
= 3.24' or 39"
A quarter-wave shorted stub for the popular CB channel 11
(27.085 mc.) would be determined as follows:
Hook up the stub to your coax transmission line using aT-connector
(Amphenol 82-36 or equivalent) as in Fig. 2. Then
Hook up the stub to your coax transmission line using a T-connector
(Amphenol 82-36 or equivalent) as in Fig. 2. Then just attach
a male coax connector (Amphenol 83-851 or equivalent) to the
stub so that it can be easily connected to the T-connector.
For twin-lead or open-wire stubs, solder the stub directly to
the line as shown in Fig. 3.
Fig. 3. - Open-stub filter can be soldered
directly into transmission line using open-wire or twin-lead
Keep in mind that a stub filter is not intended to replace
a low-pass filter but rather to supplement one. A stub filter
is more efficient in attenuating troublesome even harmonics
which can be a cause of TVI, while a low-pass filter attenuates
all harmonics very effectively.
Posted May 12, 2014