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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Mirror as ground plane
Date: Thu, 28 Nov 2024 07:26:11 -0800
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On Thu, 28 Nov 2024 11:15:01 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>John R Walliker <jrwalliker@gmail.com> wrote:
>
>> On 27/11/2024 16:41, Joe Gwinn wrote:
>> > On Wed, 27 Nov 2024 14:24:20 -0000 (UTC), Phil Hobbs
>> > <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> > 
>> >> legg <legg@nospam.magma.ca> wrote:
>> >>>
>> >>> Anyone had experience using metalization on glass (mirror)
>> >>> as a ground plane or shield?
>> >>>
>> >>> Any data on conductivity etc?
>> >>>
>> >>> RL
>> >>>
>> >>
>> >> The best quality second-surface mirrors are coated with silver, followed by
>> >> copper plating and a coat of paint.  Those should be pretty good if you can
>> >> get wires on them.
>> > 
>> > A classic way to connect to such things is conductive silver epoxy,
>> > probably to silver-plated copper wire.  Not tin-plated for long-term
>> > use.
>> > 
>> > .<https://www.masterbond.com/properties/silver-filled-electrically-condu
>> > ctive-adhesives>
>> > 
>> > As for shielding effectiveness, the key question is the resistance of
>> > a square of the coating, connected only on opposite parallel sides.
>> > 
>> > 
>> >> Poorer ones have aluminum coatings around 2-3 nm thick and no plating.
>> > 
>> > Probably won't work, between high square resistance and oxide layer
>> > preventing reliable connection.
>> > 
>> > Though people do use metallized Mylar film, with a long bare tinned
>> > copper ground wire in direct contact with the aluminum side, and in a
>> > cable this does work.
>> > 
>> > Joe Gwinn
>> At high enough frequencies there is no need for direct connection.
>> Adhesive copper tape stuck to the varnish film on the back of an
>> ordinary mirror may provide enough capacitive coupling to make a
>> good connection. 
>
>This can even work at highish audio frequencies:  
>
>I was trying to trace some disconnected house wiring by feeding about
>50v of 1 Kc/s audio into the accessible end and following the signal
>capacitively with a high impedance probe connected to a tuned amplifier
>and headphones.  The wires were in the space between the ceiling of the
>downstairs rooms and the floorboards of the upstairs rooms; it was
>easiest to trace them from below because there was a lot of furniture
>and other clutter in the upstairs rooms.
>
>The signal led towards an outer wall of the house which had had a garage
>built onto it.  From inside the house, the wires appeared to be running
>along the wall in the garage , but there were no wires visible in the
>garage  ... and from the garage, the signal appeared to be coming from
>inside the house.
>
>Then I realised that there was a large mirror inside the house on that
>wall and the signal was being capacitively coupled to the top edge of
>the mirror by wires that must have been at least a foot above it and
>separated by a plasterboard [drywall] ceiling.  The whole mirror was
>re-radiating the signal.

It's interesting to walk around and listen to ambient e and h fields,
and light too.

You can trace wires by listening to the 50/60 Hz fields, usually with
lots of harmonics.