Join Date: Jun 2009
Location: King George, VA
Technically, stations are not sharing frequencies. Station A multiplexes station B's data to its stream and sends it out. There are several reasons why this happens in US markets.
First, there are markets, especially small ones, where there are not enough transmitters for all the networks. In the analog days stations would have secondary affiliations, and the programming from the secondary network would be shown at a different time. Now, a station can put their secondary network on a subchannel.
Second, co-owned stations sometimes back each other up with SD subchannels. This happened in LA and occurs in other markets, especially where one of the two stations doesn't quite cover the whole market.
OTA, it's a beauty way to go!