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Sherlock Ohms

Freight Train Kills Cellphone Signal

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Ann R. Thryft
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Blogger
Re: Other kinds of traffic?
Ann R. Thryft   5/10/2012 12:56:26 PM
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Thanks, dsbrantjr, for the info. I know about airplane multipath, but didn't know about the automobile effects. Makes sense.

dsbrantjr
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Silver
Re: Alternate solution?
dsbrantjr   5/10/2012 1:16:49 PM
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A higher-gain antenna would be helpful; it would not only increase the signal strength of the desired, direct signal but would also help to discriminate against the multipath signal(s).  If a pattern null could be directed to the direction of the train so much the better.  A small change in height of the antenna might also be useful.  Care would need to be taken in using a shield lest diffraction effects actually increase the undesired signal.

Ockham
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Gold
Re: Hard to believe…
Ockham   5/17/2012 4:29:12 PM
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Wonder about the origin of the 1hz vs 2hz AM rate difference between the passing passenger train vs. the freight train? I note that a standard passenger rail car is 85 feet long, while a standard freight car is 44 feet long. Perhaps there is some connection there, as the breaks in the amplitude modulation may be tied to the differing time constant of the gaps between trains with longer cars vs. the shorter cars. Perhaps they were making and breaking the multipath at each space between the cars?


Dunno - but seems related.

 

 

 

 

Ockham
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Gold
Re: Reflections, reflections...
Ockham   5/17/2012 4:31:28 PM
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Nice detective work there!

akili
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Iron
Re: Reflections, reflections...
akili   6/4/2012 3:58:19 PM
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Many years ago we were trying to plot the polar diagram of a 90 foot satellite dish by firing a tiny signal from a convenient tower some 20 miles away and panning up/down and left/right.  However, we could never get stable signals - always a slow up and down drift of the signal level by a fraction of a dB over several minutes.  Then we realised we were looking across Penzance Bay and it was the tide rising and falling - classic interference, even though the water level was theoretically well below the Fresnel zone.  Curiously we also realised that with such a huge dish, even 20 miles was technically not far from "near field" at 4GHz.  The best way of checking the dish G/T was to do a spot of radio astronomy!

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