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.