> I've seen coax isolation numbers >60 dB at 100 MHz (I'm
> not sure what
> the measurement condition is).. So, even if the leakage
> quadrupled from a mismatch, it would still be 54 dB down.
We have to be careful here.
The leakage is >60dB but that is in a special standard test
fixture at virtually zero spacing to the cable in a closed
It doesn't mean if we have 1000 watts applied the leakage
power radiated away from the cable is -60dB kW. It's that
field intensity in the test fixture at the spacing they use,
and it is a nearfield data point.
I'm trying to think of a better way to put this into
perspective. Consider two conductors in a well balanced open
wire line at 3" spacing with a kilowatt. One inch away to
the side we would measure an extreme signal level indicating
very high leakage, we could light a 100 watt light bulb. One
might conclude the line is useless.
Three feet away there would be very little measurable power.
We couldn't light a flashlight lamp.
One mile away we couldn't detect a thing.
It's pretty well understood SWR does not generally increase
feedline radiation. We also tend to greatly overestimate the
importance of a closed shield, or multiple shield layers.
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