Re: Protection from Tempest attacks

From: Allan Herriman (allan.herriman.hates.spam_at_ctam.com.au.invalid)
Date: 06/07/04

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    Date: Tue, 08 Jun 2004 00:55:48 +1000

    On Mon, 07 Jun 2004 14:21:37 GMT, Michael Amling <nospam@nospam.com>
    wrote:

    >Louis Scheffer wrote:
    >> Mok-Kong Shen <mok-kong.shen@t-online.de> writes:
    >>
    >>>Andrew Swallow wrote:
    >>>
    >>>>Faraday cages are oversized bird cages. [...]
    >>
    >>>I know that and had also seen an experiment in which it
    >>>protected a person from a lightening created in lab with
    >>>high voltage. However, as layman in physics I 'conjecture'
    >>>that the mesh size plays a certain essential role. Anyway,
    >>>if the mesh were of, say, 1m*1m size, I couldn't imagine
    >>>that it would prevent e.g. ordinary radio waves going through.
    >>
    >> Your intuition is exactly right. To a first approximation,
    >> waves shorter than the hole size pass though, waves larger
    >> are reflected and cannot pass. This is exactly why your AM
    >> radio fades out when going under an overpass (AM uses about
    >> 200 meter waves) and FM does not (FM uses 3 meter waves)
    >
    > That's most of it. With AM the volume goes down as the signal
    >strength goes does so it's more obvious when you're under that overpass.
    >With FM, the volume stays the same as the signal strength goes down (but
    >the signal to noise ratio gets worse).

    AM radios have an AGC (automatic gain control) to partially counteract
    that effect over the expected range of signal strength.
    Decreasing the signal strength will cause the volume to remain roughly
    constant until the AGC is at maximum gain. Only then will the volume
    decrease (e.g. under the overpass).

    Regards,
    Allan.

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