<MISC> Electrical Interference

["Curtis A. Ingraham" <ingraham@xxxxxxxxxxxxxxxx>]

Earlier Bob asked:

> My new '00 528iA sport-premium came with a dire warning (GLUED! to the
> inside of the windshield, took me a half hour to get off with Goo Gone!)
> about using non-BMW-approved cell phones inside the car. Claims that a
> cell phone with an antenna inside the passenger compartment may cause
> damage to the poor baby car's sensitive electronics.
> True? Or just a ploy to get me to spring for the expensive BMW StarTAC
> phone?

Not a marketing ploy.  More likely just lawyers working overtime in 
a holy war to eliminate risk for their client (employer).

And Barrie K. Davis wrote:

> The Lexus had a problem with cell phones:
>
> Quoted from http://www.motortrend.com/jan00/lt98lexus/lt98lexus_f.html
>
> "The GS Series was the subject of ... technical service bulletins or 
> recalls ... and to change a yaw-rate sensor in the stability control 
> system.  The latter would interpret electromagnetic waves from cell 
> phones or, in our case, a two-way radio, as a command to aggressively 
> apply the brakes. "

Engineers normally design electronic widgets to be resistant to EM 
fields, and this is not a trivial task; apparently the Lexus folks 
slipped up on the design or fabrication of the original yaw-rate 
sensor or something related to it (wiring, maybe).

And then in v09.n1532 Shapero <dshapero@domain.elided>
wrote:

> However, if the devices "in" the car can pick up waves in the car, 
> and not from an antenna mounted "on" the car, how do the EM-waves 
> not penetrate the car.  My understanding of EM-waves is that they 
> go "everywhere" (unless you need to use your cell phone).

You are mostly correct.  EM waves inside a car from a cell phone
antenna inside the car (i.e. a hand-held cell phone) can be a *lot* 
stronger that those from the same device operating outside the car.

> This sounds a lot like the airline industry claims about using a 
> cell phone in a plane, as I remember the WSJ ran an article that 
> said it was a bogus claim.

What exactly were the claims, and why were they bogus?

There are two reasons not to use a cell phone inside a plane.  The 
first is that it can possibly interfere with the plane's electronic 
systems (as with the Lexus brakes).  I think there are relatively 
few documented cases of electronic devices doing this, but the risk 
is real and the consequences significant.  

The second reason not to use a cell phone from a plane is to protect 
the cellular telephone system from traffic overload.  When on the 
ground a cell phone is within range of only one or two fixed cell
sites; thus it uses only one or two channels in a limited area.  
Once airborne a cell phone might be in range of dozens of cell sites 
and might therefore tie up ten times the normal number of channels 
in the cellular system.  The FCC bans airborne use of cell phones
for this reason.

Curt Ingraham
72 2002tii
Oakland, CA

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End of bmw-digest V9 #1537
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