[1000mp] What is Wrong with SWR Meter?

Thu, 6 Jun 2002 06:37:56 -0500

Thanks for the information. I've always wondered about why meters read
differently.
         On a related note, on another reflector the subject of impedance
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.

73
Steve  wd0ct
----- Original Message -----
From: "Tom Rauch" <[email protected]>
To: <[email protected]>
Sent: Wednesday, June 05, 2002 4:08 PM
Subject: Re: [1000mp] What is Wrong with SWR Meter?


> > When there is some swr on the line the impedance will change with line
> > length. Meters at different points in the line may read diferent  even
> > though swr doesn't change with line length. Many meters will say that
> > it does change with length.
>
> The reason that happens is almost exclusively because the meter's
> impedance is not the same as the surge impedance of the transmission
> line. Both meters and transmissions lines are not perfect.
>
> Most 50-ohm transmission lines are really between the 48-ohm and 56
> ohms. I have a heck of a time finding true 50-ohm cables for test
> bench work. manufacturers offer precision 75-ohm video cables, but
> very few offer 50-ohm tight impedance tolerance cables.
>
> A second cause is common mode currents changing the SWR of the
> antenna as you disturb the line, if you move the ground length. But
> that has nothing to do with two meters reading differently. That is
> virtually always a meter/line Z0 problem.
>
> Different readings almost always go back to the meter not being
> nulled for the line's characteristic impedance...and the meter
> disagreement has nothing directly to do with actual operating
> mismatch (SWR) of the load since SWR is virtually constant along a
> mismatched line (unless it has significant loss).73, Tom W8JI
> [email protected]
>
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