October 1960 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is a short tutorial
on how to construct a ¼−wave stub "trap," or filter to attenuate even-order harmonics
from transmission lines. It applies whether the transmission line is feeding an
antenna or is a section of copper foil running on a microwave substrate. Author
Kent Mitchell (W3WTO) discusses both an open stub and a shorted stub. In case you
are not familiar with how quarter−wave transmission lines stub work, a short at
the far end appears as an open circuit where the stub connects to the main transmission
line, and an open stub line appears as a short circuit. That is because there is
a 180° phase shift at the end of the shorted stub and a 0° phase shift at the
end of the open stub. Therefore, there is a total of 360° (i.e., 90°+180°+90°=360°,
equivalent to 0°) with the shorted ¼−wave stub so it has no effect where
it attaches to the main transmission line. The open stub experiences no phase shift
at the open end, so there is a 180° phase shift (90°+0°+90°=180°) seen at the main
line attachment point. The moral of the story is that if you make the open stub
equal in length to ¼ of the wavelength (in the cable, while accounting for velocity
factor) of the interfering signal, it effectively gets shorted out. This article
appeared in a 1960 edition of Popular Electronics magazine.
Kill Those Harmonics
By Kent A. Mitchell, W3WTO
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.
Fig. 2. - T-connector inserted in coaxial transmission line
makes convenient jack for connecting shorted-stub filter.
Fig. 3. - Open-stub filter can be soldered directly into
transmission line using open-wire or twin-lead construction.
Inexpensive, easy-to-make tuned stubs will eliminate harmonics from your CB or
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 radiations.
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 lines.
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 fre-quency,
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.
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 formula:
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.
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 August 24, 2020
(updated from original post on 5/12/2014)