Hi Ari

I always wonder whether broadcast at the most common IF frequencies
would be a viable alternative? (eg 455khz for AM) Legality issues aside
of course... This would make frequency selection a little easier to
engineer. You wouldnt have to divide your power budget into many
operating frequencies as well. Problem is that you'd need a lot higher
ERP especially since car mounted radios tend to be better shielded than
portables. (You'll need to experiment some)

On 3 Oct 2005 18:52:41 -0700, wrote:

Wonder no longer, because broadcasing at IF frequencies has been for
year the standard in the US. In spite of some rather silly speculative
posts, this mechanism has proved itself for more than 10-years here.

In a confined space such as a tunnel, a transmitter of 10-watts more
than sufficint to get the job done, and using nothing more
sophisticated than a simple wire radiator running the length of the
tunnel.

In Boston, we also retransmit commercial radio broadcasts into our
tunnels, but that requires equipment dedicated to each radio channel
that we re-broadcast, and that becomes very costly after 10 stations or
so, hence there is a limit.

The real challenge is in maintaining emergency communications to the
outside from within our tunnels. All are serviced by the traditional
leaky coax that runs along the top of the tunnel, but considering that
all of these emergency services operate on their own indepdendent
frequency bands, so servicing them simulteously becomes somwhat
problematic. particularly when hand-held, low power devices enter the
big picture.

Harry C.

Problematic? That would be an understatement.
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