Ralf Ballis, DL2MRB wrote:
"The common (method) to provide radio communication in a tunnel is to
use a leakage coaxial cable which isn`t 100% shielded and has slots
along in the shielding."

Yes. Andrew Corp. in the U.S.A. used to get a high price for such a
special cable. 300-ohm TV twinlead is cheap and works very well too when
used between a radio and a load to communicate with other 2-way radios
along the twinlead route in tunnels and elsewhere.

Best regards, Richard Harrison, KB5WZI