[1000mp] What is Wrong with SWR Meter?

[Steve M]

John,

Thanks. Murphy was right. Nothing is as simple as it appears.

73,
Steve

----- Original Message -----
From: "John R. Winward" <k3aot@rcn.com>
To: <1000mp@mailman.qth.net>
Sent: Thursday, June 06, 2002 12:44 PM
Subject: Re: [1000mp] What is Wrong with SWR Meter?


> Steve,
>
> It's not as simple as one might think. There may
> be "Bumpy coax", this would be caused by the
> distance between the inner and outer conductors
> not being the same along a given length of line.
>
> One often overlooked source of "Impedance Bumps"
> is caused by using the PL-259 / SO-239 family of
> plugs and barrel connectors to terminate your coax
> in.
>
> Type N connectors, as well as the BNC connector
> family, are sometimes referred to as "Constant
> impedance" connectors and tend to minimize
> impedance bumps.
>
> Generally, the higher in frequency you go, the
> worse this phenomenon (and other losses) gets.
>
> 73,
> John Winward, K3AOT
>
> Steve M wrote:
>
> > 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" <w8ji@contesting.com>
> > To: <1000mp@mailman.qth.net>
> > 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
> >>W8JI@contesting.com
> >>
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