[AMSAT-BB] Diplexer advice

Over Labor Day weekend I went out of town and took the FT847 with me hoping to hand out Em15 on a few satellite passes. Instead of taking 2 mobile whips with me, as I have in the past, I decided to try and simply things by running 1 dual band mobile whip and a diplexer for the radio. I bought a Comet CF-416 Diplexer which is supposed to have 60DB of isolation between ports. Results were less than expected, though. I really couldn't hear myself on the satellite downlink at all, where I usually have no problem with separate antennas. Have others used a diplexer with 1 antenna with good results? What might I be doing wrong? Any advice on how to make this setup work?

73 John AF5CC