I built a paint can 6 meter helical coil duplexer, and also built an 8 stub heliax 6 meter duplexer. The mechanics don't seem all that complicated, but getting the rejection and insertion loss you want can take a lot of messing with it. The heliax was difficult to find. Traded a large tray of donuts to a local electrical contractor for 6 pieces of 8 foot long scrap heliax. I found a partial tray of 3-30 pf trimmer caps at a hamfest. The paint cans didn't hold up to temp changes at all.- Drift all over the place like others said. I cut the heliax cable, drilled and installed BNC connectors, selected stubs resonant at the higher frequency then used ceramic piston trimmers to couple the coax to the stub, then a duplicate trimmer to tune stub down to desired freq.
I have built a 1/4 wave heliax stub filter using same technique with a 1/4 wave phase line to a T connector to make a band pass filter for 2 meter APRS. I suspect heliax will make a more stable and smaller duplexer then oil cans. Happy tinkering! Ed N3SDO
