Randy This is 10m RF getting into your 15m yagi. A filter on the 10m amp will achieve nothing. You need a coaxial stub on the amplifier end of the feeder to your 15m mono-bander. RG213 or similar with VF 0.66 will require a 23ft shorted stub. This will null both 20 & 10m RF. A lot less expensive than an high power BPF. 73 Bob, 5B4AGN