Woody wrote:
>> The ones fitted are properly screened.
>
>
> I assume you mean the aircraft (electronics) has proper screening? Why
> screen a mobile - it is designed to radiate?
>
Aircraft electronics is *very* well shielded at the point of origin of
any potential interference, (Things like ferrite beads on all leads
passing through the casing, properly bonded joints where metal plates
join for access plates & so on)& any antennae are shielded from internal
interference by being on the outside of the metal fuselage, usually as
far away as possible from interference generation on the manufacturer's
fitted equipment.
To get the type approval for the phones currently used in aircraft cost
a fortune, & the test program for the new cellphone units is still in
progress with a budget of millions of pounds so far. Flight testing
using a 747 isn't cheap, & separate approval has to be given for every
individual aircraft design. The new airbus designs are going to be even
harder to certify, as they use a non-conducting fuselage material.
Non type-approved transmitters, transmitting variable frequencies at
varying power from random locations in & around the airframe are a
*very* hard to shield effectively from, & there are limits imposed by
weight considerations. It's easier & cheaper, not to mention a lot safer
just to ban them altogether. There are a number of recorded incidents of
aircraft having to miss a landing & go around, with all the risks & cost
that involves, due to somebody making a cellphone call while the plane
was on final approach.
> Having said that I read recently that mp3 players generate many times
> more RFI than either a mobile phone or a laptop, although I don't own a
> mp3 player so I can't test the validity.
>
It would depend enormously on the design. I've got one that's built in
what amounts to a faraday cage, with a metallic coating on the inside of
the case, I've got another with no shielding, just the plastic case.
Mobile phones & laptops are more expensive units, so I would think less
corners are cut during design & manufacture. MP3 players run their
processor more slowly, so the interference would occur starting lower in
the spectrum than mobile phones & laptops, with closer spacing of the
harmonics. On the other side of the problem is the fact that MP3 players
use the lowest possible power consumption processors available, while
mobiles & laptops use the circuits that give the best results with
reasonable power consumption, so there's more power being switched than
in an MP3 player.
There are tests which they're all supposed to pass regarding RFI
generation to get approval for sale in various territories, but how many
of the cheaper units are even tested, or would pass if they were is a
moot point. The approval mark is very easily reproduced for a casual
buyer/ non-suspicious customs officer, especially if there is a decent
quality forged certificate of compliance with the shipment.
I've not got the gear to check properly for myself, but I note that
neither of the MP3 players or the laptop I'm typing on interfere with
the FM receiver I'm listening to at the moment, but the
GPRS datacard in
the laptop sometimes does, even when it's just changing cells.
Toss a coin, would you, please. ;-)
--
Tciao for Now!
John.
...
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