I am reminded of the time before retirement that I put together an RF lab at
Hughes Missile Systems. All of the equipment was housed in a shielded room,
since in addition to all of the garbage generated in our own factory we were
located at Tucson International Airport with all of their comm and radar stuff
operating. I specified the room to have 120 dB isolation from a few kHz to 10
GHz. It was modular panels 2 inches thick with galvanized steel shielding on
both sides. The door was massive with a brass jam and a knife edge that was
driven into double finger stock backed up by conductive foam.
The vendor performed proof-of-performance by putting a spectrum analyzer fed
with a standard gain X-band horn inside and a TWT amplifier on the outside also
feeding a horn aimed at the one inside. The system was normalized with the door
open and then tested with the door closed. It met the requirement, but with
little margin.
I was given instructions on how to maintain the room, with emphasis on cleaning
the door edge and finger stock. To drive home the importance, the guy took a
dollar bill out of his wallet and placed it across the finger stock channel and
then closed the door. The isolation dropped over 20 dB.
Wes N7WS
On 12/12/2018 10:56 AM, jimlux wrote:
At JPL we regularly do measurements of signals at <-150 dBm into sensitive
deep space receivers, and we're obsessive about phase (since that's how we
measure the distance to spacecraft to a gnat's eyelash). Historically, we
found, for instance that most signal generators leak more through the chassis
than come out the output jack at minimum level. We test the receivers in a
screen room, with the generators outside, so you get about 100dB from the
screen room to help. Waveguide helps too (WG below cutoff is a very effective
filter) and it's not too tough to make waveguide joints that have better than
150dB isolation to the outside (lap them smooth, use pins, wrap in foil tape)
Coax is always a challenge because the shields leak.
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