[TowerTalk] Balun question shield leakage

[Tom Rauch]

> 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 
> were
> 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 
box.

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.

73 Tom