[TowerTalk] coax and leaks

[Jim Brown]

On 5/2/2022 9:12 AM, Grant Saviers wrote:

Lots of wisdom here.

> Braid coverage matters.  

Yes.

Commercially double braid or hardline (solid Cu
> shield) is used for jumpers.  For hams, the RG8 size "foil plus braid" 
> is the economical choice if from a reputable brand/supplier.  

In the second edition of his landmark book on stubs, W2VJN noted 
measured degradation of foil/braid LMR400 HF stubs left in his garage 
for 10 years.

> DXEng also 
> sells double braid TFE mini coax jumpers which, while expensive, are 
> very handy for rig to amp to filter jumpers.  Or buy the bulk RG400 
> (stranded center) or RG142 (solid center) TFE silver plated double braid 
> and make your own.

There's a reason why coax like this is used in serious installations. A 
measure of shielding effectiveness is Transfer Impedance, the ratio of 
differential voltage induced inside the cable by shield current on the 
outside of the shield. The components are the resistance of the shield 
at the frequency of interest and the quality (density, uniformity) of 
the shield. DC resistance of the shield, which is multiplied by skin 
effect, establishes the lower limit.

The ultimate, of course, is hard line.

Another fact often lost is that unless it's damaged by intrusion of 
moisture, loss in coax below about 500 MHz is all due to resistance of 
center conductor and shield.

73, Jim K9YC