Re: Mirror as ground plane

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.


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~ Liz Tuddenham ~
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