Some viewers could also lose signals because of what’s known as the digital “cliff effect.” Unlike analog signals, digital broadcasts either come in clearly or not at all, meaning that those on the fringes of analog coverage areas will lose that reception entirely after the transition. Currently, they can still get fuzzy analog signals.
Some viewers may therefore need more powerful antennas — in addition to converter boxes — to continue receiving certain channels. [Emphasis mine]
When means you should bookmark this section of Josh’s blog, Blob Waukesha, where he seems to have made one out of PVC pipe.