Before they get to your transceiver

Another trick from the "old timer's" handbook - using a wave trap to send interfering signals to ground before they have a chance to reach your radio's front end. Wave traps were fairly common in the days when ham receivers had wide open "front ends." Today most modern transceivers have tightly configured first stages so interference from broadcast stations isn't the problem it once was. However, I have a nearly-new IC-7000 and when I put it in AM mode I can hear "trash" from a 50kw AM station that is just a few miles away, and many KHz below the 75 meter band.

Not too long ago I had a fine transceiver with a first stage that was SO sensitive I HAD to do something to get rid of the broadcast interference. I knew it was front-end overload, because one click of attenuation would get rid of it but attenuating a receiver goes against my ham instincts. The result was this wave trap which did the trick. It could stay inline at all power levels and made the big bad AM broadcast interference go away.



The wave trap is simple to construct and uses easily acquired parts. You might even have them in your junk box if you have been a ham for any length of time. Only one precaution - build it in a metal box and make sure it is well sealed on all edges. If not, those pesky BC signals will sneak thorough any cracks and defeat the purpose of the trap.

Mine is built in a Bud aluminum box and, as the schematic shows a number 12 wire runs from the input SO-239 directly to the output SO-239. This allows the unit to stay in line between your amp and the antenna tuner at all times. It causes no loss of signal strength or output power. It only attenuates the interfering signal.

Since we are trying to trap broadcast signals and send them to ground we need components that will tune the broadcast band. The capacitor and the coil were purchased from Antique Electronic Supply but, as I said you may have such parts in your junk box. The capacitor is 400pf and could be half of a tuning cap out of an old BC set. The coil is the antenna/mixer coil also as used in old BC radios. Use the SMALLER of the two coils for your trap and leave the other open (It was used to feed the mixer stage in broadcast radios and we don't need that for our trap to work.)

Using the trap is simple. Insert it between your antenna tuner and whatever follows, either your amp or your transceiver. Since all Ham RF goes straight through it can stay hooked up all the time and legal high power will not affect it. Just turn on your radio, and tune the capacitor until you hear the interference drop out. The tuning is critical so you may have to try it a few times to determine the best spot for the tuning knob. But it will send all that AM broadcast hash to ground before it has a chance to enter your radio. If you are hearing strange clicks, pops, and mumbles that you suspect are not QRM from other hams, give the wave trap a try. You can build it in an afternoon and have it working before dinner!