Sec.  15.211   Tunnel radio systems.

   An intentional radiator utilized as part of a tunnel radio system may
   operate on any frequency provided it meets all of the following conditions:

   (a) Operation of a tunnel radio system (intentional radiator and all
   connecting wires) shall be contained solely within a tunnel, mine or other
   structure that provides attenuation to the radiated signal due to the
   presence of naturally surrounding earth and/or water.

   (b) Any intentional or unintentional radiator external to the tunnel, mine
   or other structure, as described in paragraph (a) of this section, shall be
   subject to the other applicable regulations contained within this part.

   (c) The total electromagnetic field from a tunnel radio system on any
   frequency or frequencies appearing outside of the tunnel, mine or other
   structure described in paragraph (a) of this section, shall not exceed the
   limits shown in  Sec. 15.209 when measured at the specified distance from the
   surrounding structure, including openings. Particular attention shall be
   paid to the emissions from any opening in the structure to the outside
   environment. When measurements are made from the openings, the distances
   shown in  Sec. 15.209 refer to the distance from the plane of reference which
   fits the entire perimeter of each above ground opening.

   (d) The conducted limits in  Sec. 15.207 apply to the radiofrequency voltage on
   the public utility power lines outside of the tunnel.