§  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 § 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 § 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 § 15.207 apply to the radiofrequency voltage
   on the public utility power lines outside of the tunnel.