"What is "Line A" protecting?"
From the ARRL Repeater Directory: "The band (420-450MHz) is shared by amateurs with government Radio Location Services (RADAR); amateurs must not interfere with these priority government stations."
Here is an example of the type of government radar installation that "Line A" is possibly designed to protect:http://en.wikipedia.org/wiki/Cavalier_Air_Station
Elsewhere on the internet I found the following:
"PARCS is located at Cavalier Air Force Station (AFS), in Cavalier, North Dakota, near the Canadian border. It is a north-facing phased array radar used for ballistic missile tracking and detection, providing warning information on any missiles launched over the polar region directed at the CONUS. In conjunction with its missile warning function, PARCS performs a corollary duty of space warning for the SSN. The radar can detect targets within a volume of 140 degrees azimuth and an elevation scan of approximately 93 degrees. Also known as the AN/FPQ-16, PARCS operates in the 420-450 MHz region of the UHF band at a maximum power of 14.3 megawatts. Average output power is 715 kilowatts.