[1000mp] What is Wrong with SWR Meter?

Thu, 06 Jun 2002 10:44:52 -0700

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" <[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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