From PETER CONNORS
Most radio amateurs can answer Simon Gardner’s question (Letters, 8
February) on how a portable CD player could interfere with a plane.
The sad fact is that boxes of digital consumer electronics radiate hefty
radio frequency signals from connecting cables. In the case of the portable
CD player the radiating cable is the headphone lead. The source of the radio
frequency spectrum is the fast risetime of digital waveforms (even at the
CD player’s 44 kHz sampling rate) which generates noise up into the VHF
region.
When such noise is placed within the confines of a metal box (such as
an aircraft fuselage) an unpredictable standing wave pattern will be built
up, with signal strength peaks and nulls all over the place. Site the source
of the spectrum close to the point where, say, the wiring loom to the tailplane
controls happens to pass, and the unpredictability of spurious signal pick-up
becomes of the nail-biting sort. Actual effects are likely to be more subtle
than this crude scenario indicates but subtleties can still produce a devastating
effect in an electronic cockpit at critical times.
Peter Connors Stockport
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