I am reminded of when I (actually my employer and you taxpayers) bought a
shielded room to enclose "my" RF laboratory. This was modular, 16' x 20' x 10'
and composed of 2" thick particle board, clad on both sides with tin-plated
sheet steel. Because I needed to be able to move a rack mounted test position
with a table top in and out, I specified a four-foot wide door.
The door was constructed similarly with a brass frame around the door and was
prehung in a brass jam and massive hinges. It was installed using a forklift to
position it while the fasteners were tightened. The jam had a continuous
channel around the periphery that housed a double row of beryllium-copper finger
stock backed up by conductive foam. The door frame had a brass "knife-edge"
around the periphery that drove into and was pinched by the finger stock in the
jam.
On of the specs that I remember was 120 dB of isolation at 10 Ghz. We measured
this by setting up a source with a horn antenna on a tripod outside the room
with another horn and LNA into a spectrum analyzer inside the room. We
normalized the path with the door open and the horns aligned and then closed the
door and measured the difference. It made 120 dB, but just barely; this is not
a trivial achievement.
To drive home the necessity of maintenance, the vendor engineer said, "Watch
this." He took a dollar bill out of his wallet and laid it across the finger
stock and then closed the door. I don't remember the exact number but it was
yuge; the attenuation went down 30 or 40 dB just from breaking the connection of
the knife edge with the finger stock over the width of a dollar bill. (For what
this room cost, he should have been using a c-note.)
Wes N7WS
On 4/14/2017 1:33 PM, Jim Brown wrote:
On Fri,4/14/2017 1:13 PM, KENT TRIMBLE wrote:
The assumption has always been that the driver and passengers are riding
inside a Faraday Cage
There are HUGE holes in that assumption in the form of non-metallic parts and
paint preventing contact between metal parts. An enclosure forms a Faraday
cage ONLY if it is continuous. and ONLY if all conductors penetrating it have
either a feed-through to the enclosure or, if a shielded conductor, the shield
is bonded to the enclosure at the point of entry.
Modern vehicles have lots of paint between metal parts. When I used Hamsticks
on my Volvo S80, I had to bond the mount to the trunk, and I had to bond
around the trunk hinges.
73, Jim K9YC
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