all H23 models dont require BBCs.. they are internal built. all H20, H21, H22 still require it, all H23s are built in which is pain bc if the BBC inside fails to work. you gotta replace the whole reciever... unlike 20, 21, and 22s you can swap out the BBCs.
Unfortunately, sddtvtech didn't really answer the question. The BBCs are used to offset the frequencies from the LNB so that the receiver (non H23 or HR23) can use them. Those frequencies will interfere with the OTA frequencies. So, you can't use diplexers.
actually there are ways to do it, but its alot of headache to explain on here.
easist way to do OTA is i usually have the customer turn off the DTV reciever and use TV channels to watch OTA channels.
HDMI goes into the tv for recieving DTV and then plug the OTA cable directly into the TV's turner to get OTA channels.
diplexing can work.. but ive heard its distorted pictures and not great. i have never carried diplexers and never will.
however im in the HD market where D1Xs are obsolete here so every install, upgrade and even movers connect get HD recievers with free upgrade no matter what the call is. we're even suppose to replace all Dxs model out on a service call so they are up to date on the dish market in our area. if a customer has D12s... we're to swap it out with a H2x reciever. so all HD here.
1. You can't use diplexing at all with the H23 and a conventional DirecTV system.
2. Although it's possible to use diplexing with the H20/21 using the "BBC relocation" approach, that does not work for everyone and is not recommended.
3. It is possible to use diplexing with the H23 if you have an SWM system because SWM frequencies do not overlap into the off-air frequency band. However new DirecTV features will start to use the off-air frequencies so diplexing is not a good idea.