[BC] "VSWR" Metering

[Cowboy]

On Saturday 15 October 2005 09:06 am, Richard Fry wrote:

>Poster 2:
>>On a matched line, the voltage, current, and VSWR is the same
>>at any point along its entire length.  This is not the case on a
>>mismatched line.

 /snip/

>To clarify with an example: if the load impedance at the far end of a 
>perfect 50 ohm transmission line provides a 1.5:1 mismatch to the line, the 
>SWR at every point along that transmission line also will be 1.5:1, and 
>that is what a properly calibrated "Bird" meter should read if installed at 
>the near end, and at any point along the line.

 Ignoring the load presented by the generator to the reflected components,
  and both the forward and reflected losses, I would tend to agree.
 Since we rather deliberately mismatch transmitters to the line in order
 to obtain power transfer efficiencies above 50%, the transmitter makes
 a poor load for reflected components. As a result, a mismatched line in
 the real world has even more at play. As such, I find it quite conceivable
 that a Bird might show differing SWR depending on what point it's
 installed in the line.
 Once upon a time, just to prove to myself that metering could not be
 relied on by itself to provide accurate absolute readings on a mismatched
 line, I did insert a meter and 1/4 wave jumper between transmitter and
 line, into a load known to be mismatched, but within the transmitters
 tolerance.
 Read the numbers, both forward and reflected, and note the ratio.
 Reverse the jumper and meter, so as to maintain the same total length,
 but move the meter 1/4 wave up the line, and read again.
 All of the numbers came up different, leading to my conclusion that
 the meter can not be relied on to provide absolute numbers in a
 mismatched condition.
 ( not greatly different, but different )

 Further, looking strictly at losses in a long line, I've also seen a 3:1
 mismatch at the antenna look like a 1.12:1 match at the transmitter.
 ( Alan's experience being an extreme example )

 So, in a lossless line where reflected components are dissipated ( in a
 reject load ) and not re-reflected, we agree, but in many real world
 situations more is at play, and the simple theory does not necessarily
 indicate all that might be seen.

 So in final conclusion, a meter that indicates all is well could be wrong
 under certain conditions, while a meter that indicates a problem is
 much more likely telling the truth that a problem exists, and any
 change in the indication is also telling the truth in that a change
 of some sort has occured.
 No ?

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