Peter:
      "Both or none. I meant the cover joints
       normally used in RF proof equipment
       boxes, with a channel and an o-ring clad
       with wire (and sometimes flashing), as
       compared to oven doors. The simple box
       joints get it down by 40-55 dB in VHF/UHF
       in my experience, and they are more
       complex than the oven door joint."

I've had extensive experience sweeping and tuning both
commercial and amatuer band duplexers at between
50 MHz and 1300 MHz. I've also designed, modeled
and built duplexers for use in this same range of
frequencies. (A 'duplexer' allows the simultaneous use of
an antenna on two different frequencies - most normally
one frequency is receive and the other is transmit. The
duplexer supplies the much needed islolation between
the two thereby allowing simultaneous, concurrent
operation as required in a "repeater".)

These duplexers exhibit attentuation values from 40 dB
(the simpler designs) to greater than 95 dB attenuation
(the better-built commerical designs by the likes of
dB Products and Collins). It requires a great deal of
skill and the proper equipment using  double-shielded
(or "Super-flex" Heliax) at times to assure one is not
simply seeing coaxial cable leakage (at 150 MHz and
up) as opposed to DUT (device under test) 'leakage'.

Some of this product uses construction techniques *far*
simpler than you describe (i.e. an aluminum cylinder with
a tight-fitting cast end-cap fitted with coupling loops and
pop riveted in place,  *no* gaskets, no finger stock, etc.)
- with much better isolation and shielding results than you
describe.

It makes me wonder - what it is you were using to
make these measurements? Were the contacting
'metal' surfaces painted? Were you using double-
shielded coaxial cable on your test/measurement
instruments?

Jim

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