Hi Bob, I agree, but most of the time, you can use good design practices to keep the currents flowing through the outside of the shield to a minimum... avoiding ground loops, stuff like that.
Simple coax is used for shielding very high gain circuits from 60Hz noise all the time in PA systems. -Chuck Harris Bob Camp wrote:
Hi Yup It is impractical to make coax that has a shield thickness of 1/3”. Even if you do, it’s not going to be very flexible. For a real world system that needs good isolation, coax is not the way to go below 100 KHz. There are a few other issues that come up, but skin depth is a big part of the problem. Another part of the equation is (as Bert points out in another thread) “how good do you need?”. Skin depth simply the point that you have knocked out 2/3 of the current. That probably isn’t what you are after when you ask for “good isolation”. The “inside” of the coax should be below 170 dbm/ Hz to be “quiet” when terminated. If you have -70 dbm / Hz noise signals running around here and there, you need quite a bit of isolation. You might have a spur spec rather than a noise floor spec to meet and that would give you different numbers to go after. In most cases you will need multiple skin depths (like 10 or more) to get the job done in a noisy environment. Bob
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