From: SFPhysics@aol.com
Date: Mon Nov 04 2002 - 00:24:45 PST
From: SFPhysics@aol.com Message-ID: <11f.19472978.2af788cd@aol.com> Date: Mon, 4 Nov 2002 03:24:45 EST Subject: Caldecott Tunnel Radio Reception
> Anyone in the SF area might be able to answer this Physics question that
was in the Contra Costa Times today...page A34...
"I commute every day through the Caldecott Tunnel. Why is there radio
reception during the morning commute, but not during the evening commute? It
drives me crazy..."
> Marti Andreski, andreskim@dlshs.org
> De La Salle HS, Concord
>>
Greetings Marti:
Actually, if CalTrans were on the ball you would have reception in both
directions. Each tunnel bore has a repeater antenna throughout its length.
An outside antenna system gathers the broadcast band information (really one
for low frequency AM and one for the higher frequency FM) then a broadband
amplifier fills the tunnel with the radio signal. This is needed because the
steel reinforcing rod in the cement blocks the AM through the tunnel walls
and the FM is directional so that the waves cannot enter the tunnel openings
in a line of sight from the station transmitters. The amplifiers on some
tunnels are old vacuum tube based systems so when the vacuum tube burns out
the amplifier quits and the reception in the tunnel dies. If you look up in
most tunnels, like the Waldo Tunnel going south to the Golden Gate Bridge,
you will see a wire near the roof and it leads outside to a green shed where
the amplifiers are with their exterior antennae. What has happened to the
Caldecott is that the eastbound antenna amp is probably out and no one has
complained to have it fixed. Often vandals find the wires outside, pull them
loose, or cut the power in some way. Give a call to CalTrans and tell them
which bore is out so they can schedule a repair. I have to do this with the
Waldo because no one else seems to complain. My late dad was one of the
designers of the state system in the 1960s.
Now there is one other factor, the standard car "whip" antenna is a vertical
wire that receives waves that are both polarized and *directional* because it
sits above the metal of the car and acts as a detector perpendicular to a
metal plane. A weak signal may work the FM receiver when the car is pointing
with the highest lobe of sensitivity in the direction of propagation; but,
turn the car around where the weakest lobe of antenna sensitivity is located
and there just may not be enough signal to have reception.
The directional sensitivity of receiving antennae was one of the contributing
factors to Emelia Erhart's loss at sea during her attempt to fly across the
Pacific. She took off without a medium frequency antenna on the tail of the
aircraft that could have kept her in contact over a much longer range. The
early loss of a guidance signal left her flying by "dead-reakoning" alone.
It only took less than a degree of error for her to navigate past the island
landing destination.
Hope my usual clear as mud explanation helps, and best wishes to all,
Al Sefl
Who had fun at the NCNAAPT Fall Physics Meeting this last Saturday...
And is looking forward to going to the spring meeting at Sonoma State U...
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