Another time proven performer is the old quarter wave stub. The idea here is to insert a tee connector in line with the antenna cable to the scanner. Next, cut a piece of rg-58 cable 20 inches long and install a connector on one end while leaving the other end open. Connect this stub onto the open port of the tee connector. Now comes the tricky part. You need to tune the scanner to a weak signal either on or very near to the frequencies in question. Any ham repeater on the 145 mHz segment would probably suffice. Next, while listening to the weak signal, take a diagonal cutter and begin clipping very short pieces from the end of the cable, no more than 1/8" max. After clipping a few pieces, you should notice the signal beginning to get weaker with each piece you clip off. You should find a spot where the signal doesn't get any weaker and will start to strengthen again if you continue cutting. Stop here!
This stub filter will have about 20-25dB of signal rejection at the tuned frequency. It will have minimal loss up in the 154-170 mHz band. NOTE...due to the impedance repeating characteristics of coaxial cable, this filter will also have similar loss at 3X the tuned frequency where it acts as a 3/4 wavelength stub. That should not have a serious effect at the UHF public safety channels.
You can easily make this stub filter tunable by cutting an additional 1/4 inch or so off beyond the above mentioned "stop point" and then connecting a small trimmer capacitor across the open end of the cable to tune the notch frequency. Something in the neighborhood of 5-15 pF should do nicely.
John
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