Using the PC adapter, I’m finding I have about a 6 foot effective range. Does that seem about right?...
I don't think that would be typical. I don't know if the handheld does have a more powerful radio or not (I haven't opened up a CI-1 to see nor used a signal strength meter on them) but I've used the CI-1 at a distance at least twice that through a wall, but that was just playing around with an LM hooked up to a decoder tester board.
Make sure there are no power supplies directly in between (line of sight) the CI-1 and the LM's. RFI can interfere with the signal.
I'm not sure how wide the beam is from the transmitter on different RP products (whether truly omni-directional or not). If the CI-1 is not orientated generally in the direction of the LM's (eg, pointing in completely the opposite direction) try to orientate the CI-1 to be pointing towards the LM's and see if you get better range.
Which way up do your have the LM's installed? The transmitter is on the side with the logo and product name etc. If you've got this side facing down into the frame of the locomotive you could be loosing a lot of signal strength from the LM's.
- Tim