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:

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    (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.