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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Mirror as ground plane
Date: Thu, 28 Nov 2024 11:15:01 +0000
Organization: Poppy Records
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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.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk