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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 11:29:04 -0800
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On Thu, 28 Nov 2024 15:47:11 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>john larkin <JL@gct.com> wrote:
>
>> 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-cond
>> >> u > 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.
>
>I once built an IR detector into the body of a little hand torch so that
>I could carry it unobtrusively around the site where I worked.  The
>management hadn't told us they were installing surveillance equipment,
>but the IR illuminators for the hidden cameras showed up quite clearly.

It wouldn't be hard to include a circuit that down-samples
ultrasonics.