Some folks are confusing a mismatch between the load and the line
with a mismatch between SWR meter and the line. If the meter's
characteristic Zo is different from the line Zo and the line is
mismatched to the load, then the VSWR "reading" will vary with
the position of the meter, even though the actual VSWR along the
line is constant (by definition assuming a low-loss line). This
happens because the impedance "locus" traced out as you move along
the mismatched line isn't a smith chart circle centered on
characteristic Zo of the VSWR meter, its a smith chart circle centered
on the characteristic Zo of the line. If the Zo of the meter and Zo
of the line are the same, then the impedance locus will fall on
the so-called constant VSWR circle and you will get a constant
VSWR "reading" with the VSWR meter.
73 de Mike, W4EF..........
----------
From: measures[SMTP:measures@vcnet.com]
Sent: Saturday, October 09, 1999 7:32 AM
To: AMPS
Subject: Re: [AMPS] Hold on - SWR meter placement
>
>Rich,
>There are several possible reasons why your measurements "varied all
>over the place."
>
>1. You have a defective SWR meter
>
A halfwave of 93 ohm coax was connected to a 50 ohm termination. The
50-ohm swr meter) was inserted at the end of the 93 ohm section and the
reading was 1:1. At the center of the halfwave section, the swr was
93/50 =1.86:1. . . The bottom-line is that when you have a matched
line, SWR-reading will not vary no matter where the meter is inserted.
However, with a mismatched line, placement of the SWR meter matters.
>2. You have a balanced load at the end of an unbalanced coax resulting
>in currents on the outer surface of the coax shield which are screwing
>up the measurements
>
>3. Your load is radiating and causing the same condition on the line as
>in item 2
>
>4. When you moved the SWR meter you did not make good connections to
>the meter and/or to the splice in the cable...see item 2
>
>5. The coax or connector(s) is bad...see item 2
>
>6. If you are calculating SWR from forward and reverse power
>measurements, you forgot to take the square root
>
>7. You misread the meter
>
>If you get different readings at different points on the line, you have
>a measurement error probably caused by one of the above choices.
>SWR on a mismatched line is the same from one end to the other (with
>slight modification due to cable losses).
>
>------------
>Jon,
>
>Emin = Emax defines a 1:1 VSWR so how can the voltage vary with a 1:1
>VSWR?
>
>--------
>
>Bob, W5AH
>
>--
>FAQ on WWW: http://www.contesting.com/ampsfaq.html
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>
- Rich..., 805.386.3734, www.vcnet.com/measures.
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