[1000mp] What is Wrong with SWR Meter?

Fri, 7 Jun 2002 05:36:25 -0500

Thanks for putting these things in perspective. When I just think about my
own station it becomes clear that a lot of clunky stuff works fine on hf.

73
Steve  wd0ct

----- Original Message -----
From: "Tom Rauch" <[email protected]>
To: <[email protected]>
Sent: Friday, June 07, 2002 5:26 AM
Subject: Re: [1000mp] What is Wrong with SWR Meter?

> > 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
> [email protected]
>
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