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Sherlock Ohms

Freight Train Kills Cellphone Signal

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Beth Stackpole
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Commuters wouldn't stand for degraded signal
Beth Stackpole   5/9/2012 6:44:13 AM
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That's a case of deductive reasoning I can follow. But I'm wondering about the zillions of commuters who are glued to their cell phones riding trains every morning and afternoon rush hour. I'm assuming no impact on cell signal or it would be front page news and trending topics on Twitter. Any thoughts as to why this isn't a more regular occurance?

David Zawislak
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Re: Commuters wouldn't stand for degraded signal
David Zawislak   5/9/2012 7:50:55 AM
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I wouldn't expect this to be a problem because when you're on the train, it doesn't change its shadowing. It is always constant, so it would modulate at DC.

Beth Stackpole
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Re: Commuters wouldn't stand for degraded signal
Beth Stackpole   5/9/2012 12:13:56 PM
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Thanks for clarifying, David.

Ann R. Thryft
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Other kinds of traffic?
Ann R. Thryft   5/9/2012 4:55:45 PM
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This raises the question of whether there aren't other kinds of traffic in between that could produce a similar effect, such as planes in the air or automobiles on the ground.


Tim
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Good view
Tim   5/9/2012 6:46:06 PM
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Good job going on the roof to get an actual view of the problem and not just relying in instruments or tests of the interference.  We had a similar issue at our facility where a machine would consistently have a high amount of defects at the start of the shift but would then run great all day and overnight.  On inspection of the cell, the defects were kicked out based on an automated vision inspection.  The rising sun through a plant skylight each morning would change the light profile on the part causing false rejects.  The solution was dark shielding on the whole cell and the problem did not re-occur.

JimT@Future-Product-Innovations
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Hard to believe…
JimT@Future-Product-Innovations   5/9/2012 7:15:52 PM
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This anomaly is just remarkable to me.  Understand the dynamics of traveling signals, and consider this: The mere fact that literally 1,000's of commuters are simultaneously accessing any one particular Cell Tower at the same time;  and then consider that  each of the individual subscriber signals  are dynamically "handed-off"  to next cell tower, (usually about 7-10 miles away) as they zip down the Interstate at 80mph;  and last, to consider that each of these dynamically changing subscriber signals are not interfering with each other, nor any of the other of the 1,000's of cars from which the signals originate (being an avg.  of 4,000 pounds of steel moving at 80mph)  is a marvel of 21st Century technology  that just about everyone takes purely for "granted".  With that degree of RF precision executing routinely in everyday life, I'm struggling to understand exactly how the train on the horizon affected the signal integrity;  but the author's rooftop observations seem to correlate the evidence.  Hard to believe...

dsbrantjr
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Silver
Re: Other kinds of traffic?
dsbrantjr   5/10/2012 9:31:22 AM
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Any type of moving traffic can and does cause similar effects.  "airplane multipath" or "airplane flutter" is a well-known phenomenon which affects FM radio and both analog and digital TV reception.  I am certain that many of us have noticed our auto FM radios fading in and out when another vehicle moves nearby when you are stopped at a traffic light.

wawaus1
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Iron
Alternate solution?
wawaus1   5/10/2012 10:05:23 AM
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What was the angular difference between your direct line of sight to the other antenna and the reflected path via the railway line?

Could you have installed a small shield adjacent to your antenna to block the alternate path via the railway line?

A strip of earthed mesh or similar may have sufficed.

ChasChas
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Platinum
Re: Alternate solution?
ChasChas   5/10/2012 11:17:23 AM
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Do the passengers on the train have a good signal there? Maybe more testing IS needed.

Towerman
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Iron
Reflections, reflections...
Towerman   5/10/2012 12:32:37 PM
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This sounds like a classic case of Fresnel Zone interference. When I was researching microwave paths for TV station studio to transmitter and remote pickup links, I always had to determine what would be in the Fresnel zone or transmission path problems could cause problems down the road. Usually the obstructions in the Fresnel zone were stationary, but sometimes not, as in this case.

I once worked at a TV station where the studio to transmitter microwave link path passed across the Mississippi river. The path was only about five miles long, so signals were usually quite strong. However, occasionally we would get a partial fade out of the signal. Something we would have a complete loss of signal for a few seconds. Naturally this would happen during a dramatic point in the action of a story or worse, during a daytime soap opera! We received many irate calls from our viewers. This went on intermittently for a few years, and I was tasked to investigate the problem.

I determined that the problem only started after a second bridge was built over the river about five years earlier. Further, the problem was the worst during the spring at times of high water in the river.

I eventually figured out that the center of the beam of our microwave link path was only about 80 feet above the high water level in the river. The microwave path passed under both of the bridges, just to the right of a support column for the original bridge, and then just to the left of a support column for the second bridge. We were shooting through a keyhole about 200 feet wide and about 80 feet high that was placed directly in the middle of our signal path!

Whenever the water was high and a large ship crossed the path, it would partially obstruct the signal for a few minutes, sometimes completely obstructing the signal and causing a complete drop out. Because the bridge support columns effectively blocked most of the Fresnel zone signal, there was nothing to prevent a complete loss of signal when the main beam was obscured.

To document what was happening for management, I connected a chart recorder to the AGC line of the microwave receiver. I stationed an observer on the tower with binoculars and a radio to alert me when a large ship passed downstream. As ship passed through the link path, I was amazed to see the pen of the chart recorder clearly delineate the bow of the ship, the wheelhouse, the stern, and the trailing wake of the ship on the chart paper.

We eventually rerouted the microwave path via a dogleg path using a taller building in the downtown area. After that, the signal remained stable and the calls from our viewers ceased.

 

 

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