Update: I can get a signal between 155.9975 and 156.0025, but only if I remove the antenna and install a terminator on the antenna port. If I do that, I get a faint signal (about -111dBm) that's just enough to break squelch. So there is a little something there, but atmospheric noise picked up by a Diamond RH77CA (-104dBm) drowns it out thoroughly.