670 kHz 

The AM frequency of 670 kilohertz is defined by the FCC as a Clear Channel Class A frequency.  However, some class B and D stations are assigned clear channel frequencies, as long as protection is provided to the Class A station.  In Idaho, KBOI is the only station on 670.  Across the US, however, there are 12  stations assigned to this frequency, several in Canada, and over 30 in Mexico and other North American areas.  It is for this reason stations on this frequency either reduce power  or switch to a directional antenna (or both) at night to reduce interference.     (Information updated February 2008)