[1000mp] What is Wrong with SWR Meter?

[Tom Rauch]

> bumps in coax came up. I guess these bumps are caused by the coax roll
> being in contact with the floor, a heavy object stacked on top of the
> roll, etc.
>             Seems one fellow worked for a cable company and this
>             "impedance
> bumpy" coax gave them some grief. I believe he said it was almost
> impossible to find coax without these impedance bumps.

"Bumps" in cables have never been a problem for me, except in 
critical applications like video. Most of those problems disappeared 
in the 70's and 80's, with the improvements in cable manufacturing. 

It's important to keep the context of this thread in perspective. SWR 
reading varying in an HF system is much different that critical 
applications like UHF systems carrying video information. We 
sometimes have a tendency to get paranoid about non-problems in our 
non-critical HF systems.

I had temporary cables lying on the ground and driven over them many 
times until they were ellipticity shaped, but I'd never notice a 
change in SWR at HF. It's not only the amount of the impedance bump 
that is important, the LENGTH of the impedance error matters even 
more.

What we will commonly find is because of varying amounts of gas in 
poorly manufactured foamed cables, or because of the limited 
available combinations of center conductor to outer conductor 
diameters, most 50 ohm cables aren't exactly 50 ohms.

Worse yet, most bridges are either not nulled at the cable impedance 
and vary with frequency.

That, and common mode currents on feedlines, are the real culprits in 
this problem.73, Tom W8JI
W8JI@contesting.com