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Figure 38-13. Connecting 146MHz bandstop notch a TV receiver.
Figure 38-13 shows a typical but simple television distribution system. These are
used in large homes and businesses like motels where one TV antenna is
connected to several TV receivers. To do this properly the TV signal needs to be
amplified and distributed with the correct impedances. The television distribution
amplifiers that do this are broadband amplifiers. They can amplify a very wide band
of frequencies including 144-148 meter amateur band. They are easily overloaded.
If overloaded from an amateur VHF signal, then all TV receivers will be affected.
Unfortunately, many of the distribution amplifiers have poor filtering and the solution
is to add rejection on the frequency that is overloading the amplifier. One method is
to add a quarter wave open circuit notch filter via a "T" connector on the antenna
side of the distribution amplifier.

A quarter wave notch for 146MHz will not be long. Make sure you use good quality
coaxial cable when making stubs. Calculate the length: 300/146 x 0.25 x 0.66 =
340mm. It is a good idea to make the notch a bit longer than necessary and prune
its length very slowly while watching the interference. The end of the notch should
be open circuit. The notch can be rolled up into a small loop. This will provide around
20dB of attenuation on 146MHz and a bandwidth of approximately 1MHz. If you
need more attenuation, then you can use two notches spaced one quarter wave
apart. With two notches you can get 30-35dB of attenuation determined primarily by
the quality of the cable used.

Something to try

If you have a radio receiver on any band, one that you can get to the antenna
terminals, tune the receiver to a constant signal and make a notch for that frequency
to eliminate or reduce that signal. If you try this, work out the stub length from the
above equation and cut it a little long. Connect it to your receiver and then trim 2-3
mm off at a time using pliers. Watch the 'S' meter dip as you cut (tune) the stub. This
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is a practical way to tune any transmission line notch filter.
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