The German word is "Sperrkreis". Yopu can find them regularly on ebay. But they're not cheap. You can determine up to 4 the frequencies (in this case 3) that will by attenuated by 15 dB. In my place, 13 km from both of the stronger Flensburg transmitters here (3x25 KW, 2x20KW) the wave trap is only neccessarry with the FM+ if I point the horizontal 5 element into that direction +/- 40 degrees. If the antenna was only a little higher the transmitters would be in sight. The wave traps also have a general attenuation but only by maybe 3 dB.
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