Many PBS stations offer "multicasting" of their OTA broadcast, which means they may offer subchannels such as PBS Create, mHz Worldview, or Documentaries. Comcast sometimes retransmits these OTA "subchannels" on cable, sometimes they don't. In my area, Comcast carries two PBS affiliates (KCTS and KBTC). Comcast offers three feeds of KCTS, but only one of KBTC's four OTA feeds.
If you receive good antenna reception of the desired PBS stations, you could pick up a digital converter box and connect it to your TV so you could switch back-and-forth between the Comcast feed and the OTA feed. The key here, though, is "if you receive good antenna reception". I've experimented extensively with OTA converter boxes because I like some of the subchannels not currently transmitted by Comcast, but digital TV antenna reception is more difficult than analog antenna reception.
