Hi Paul

More for interest rather than on topic.

I use to work for the company that did a tunnel rebroadcast systems for
a series of motorway tunnels in Sydney (Australia) back about 5 years ago.

It had to cover a number of operating freqs, mainly AM, FM, cell phones
and certain VHF/UHF comms. Most were active rebroadcast and effectively
were same frequency linear repeaters. The tunnel helped not letting
radiation out as well. Reradiating was done with two cables the length
of the tunnel. AM was a single wire and the rest was via leaky coax. The
single wire was resistive terminated at one end and coupled with some
large toroidal transformer at the source end. I dont remember all the
details but think that the toried had multiple inputs.

We also had a class A amplifier system on a UHF freqency. (about 401MHz
from memory) It had two yagis spaced maybe 20ft apart facing opposite
directions in the tunnel. Enough isolation was achieved to not cause
feedback and of course we had a second pair of yagis stacked above that
for the other direction.

You even find "mine comms" systems that work in a similar vein.

Interesting story about the Portland & Seatle systems, thanks!

Cheers Bob VK2YQA

KD7HB wrote:


I see the same wire trick in other tunnels. The I-90 tunnels East of
Seattle have the wire.

Paul, KD7HB