Re: Mirror as ground plane

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.

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

Yes, if they are the only wires you are interested in; across open
ground, for instance.  In a house, when trying to trace a specific wire,
you need some sort of identifying signal such as a bleeper.  I also have
an exceptionally noisy electric drill that can be used to mark a circuit
(it was used in the construction of the Brabazon airliner and pre-dates
effective suppression methods).

It is easiest to track the magnetic field if you can get to both ends of
the wire and make a complete circuit, but if the wire has been cut and
you don't know where it goes, the electrostatic field is more traceable
as long as there isn't a lot of earthed metal or damp material in the
way.  Sometimes there is enough capacitance to earth at the cut end of
the cable that some highish frequency current can be driven down it and
make electromagnetic detection possible.


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