Stronger HD Signals

[Nickolas Noseworthy]

I am an avid listener to HD radio, at home with my roof antenna, as well as in my car. But I do get annoyed from time to time at the lack of power that seems to be pushing these signals out. Driving around town in Merrimack, all the Boston (including stations not transmitting directly from the Pru) HD signals will fade in and out. It takes a ride down Route 3, almost to the Lowell Connector, before the HD signals become reliable. Do stations have the ability to raise the power on their digital transmitters?  
 
On another note, some stations seem to have a better digital signal then analog signal. From my home in Merrimack NH, my roof antenna picks up a barely listenable analog signal of WSNE near Providence. But it always catches a full signal of WSNE HD-1, HD-2, and now HD-3. WBLM out of Portland is masked by WLLO-LP in Londonderry, but the HD signal always kicks in with no interruptions. What makes an HD signal travel farther then the analog wave?