Well, one of the side benefits of this list ... lots of smart and
knowledgeable people. A summary and then it can pass into the archive ...
1. The first of "my" tunnels is in Newcastle CA [between Auburn and
Sacramento on the old US40 and Lincoln Hwy route] and was constructed
sometime around the end of the 19th/beginning of the 20th centuries.
It's not quite 1/4 mile long. There is no visible wire or radiax in
it. The hill it runs thru is full of water and you get leaked on when
driving thru it, even in summer.
2. In the early 80's, the company I worked for then had a contract to
rehab the communications for the Bay Area Rapid Transit system. We used
a 300 ohm twinlead with a hollow core about 2 1/2 in in diameter along
the ceiling of the tunnels and underground sections. It was 150 MHz
land mobile stuff and the twinlead was fed from a combiner that put 5 or
6 transmitters into it [train control, fire, security, etc.] It worked
very well. Aligning the combiners [which actually looked a bit like a
still [:-) ] was a bear in the equipment spaces in the tube under SF Bay
but it worked well. They also wanted 800 MHz simulcast throughout the
service area, a requirement probably still waiting for a real solution.
3. Other than under bridges, in canyons, beside heavily forested roads,
and in the garage, where it's obvious the path to the satellite(s) is
blocked, we don't experience any XM drop outs. She's going to drop the
subscription, it's expensive and my new Honda Ridgeline has become our
travel vehicle, but she's had it since 2013.
4. I've wondered if there wasn't some sort of waveguide effect in
tunnels. For BART, one of the many problems we had with simulcast was
that it leaked into the tunnels, even as far as the bottom of the
Transbay tube. I don't know the XM satellite frequencies but I thought
they occupied some spectrum abandoned by the Cellphone industry.
5. [Bonus Factoid]: The pine forests of the Southeast US are opaque to
800 MHz.
Thanks for all the ideas and peripheral info.
73,
Fred ("Skip") K6DGW
Sparks NV USA
Washoe County DM09dn
On 4/15/2017 10:40 AM, Clay Autery wrote:
I would think it would depend almost entirely on the orientation of the
tunnel... (and satellite reception azimuth).
First, the longest of those two tunnels is only 425 feet or so. The
other is significantly shorter (southbound).
The tunnels are oriented generally north/south which is the preferred
direction IF you have to monitor geosynchronous satellite transmissions
from overhead... sort of).
You are right on the edge of a large body of water, which while not
brine, has better conductivity than the soil around the lake.
Significant potential for reflections off the high ground on almost all
sides....
Don't know where that first tunnel is, so I can't comment. But if you
gotta hear in a tunnel.... those two would be great candidates...
73,
______________________
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 4/14/2017 10:47 PM, Fred Jensen wrote:
Semi-related curiosity regarding shielding. My wife's car has
Sirius/XM radio. It usually loses contact with the satellite driving
under Interstate bridges and the like. Likewise in the garage. OTOH,
at our previous home there was a tunnel through a small hill, perhaps
1/4 mi long. The XM worked fine through it. There are a pair of
tunnels at Cave Rock next to Lake Tahoe through a granite mountain.
XM works fine through them too. Anyone know why?
73,
Fred ("Skip") K6DGW
Sparks NV USA
Washoe County DM09dn
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