[cyberham]

I have a similar problem. I have an antenna and also cable service. The cable service provides all channels, but only the antenna delivers my 3 local channels is good quality HD. The cable service delivers those channels in SD. My TV has only one RF input and the TV internally selects either Antenna or Cable channels. So I want to diplex the antenna signal with the cable signal into this single TV RF input. Even if I succeed at this, I will still need to switch the TV between antenna mode and cable mode, but at least I could do this via remote control without physically changing connections to the TV.

The 3 antenna frequencies for the HD locals fall on cable channels 8, 91 and 104 which would only conflict with cable channel 8 (program guide channel) since no cable channels are available on cable channels 91 and 104. Diplexers I see are always designed to combine satellite signals with lower frequency signals. But I need a diplexer that will combine the antenna signal with the cable signal. How do I solve this?