At 12:58 PM 10/1/99 -0400, you wrote:
>
>Thanks to all who took the time to respond.
>
>*****
>
>This doesn't say I thought the SWR varied ALONG the cable. I'll
>rephrase: In a system where (at the far end) the load (antenna) and
>cable have different impedances, the impedance seen looking into the
>near end of the cable will change depending on cable length. Correct?
>
Correct. The only time when the impedance measured at the amp-end is the
same as the impedance measured at the antenna-end is when the line (cable)
is exactly 1/2 wavelength long (at what ever frequency), when the antenna
impedance is something other than the characteristic impedance of the line.
You're right and the interconnecting cable is an impedance transformer.
>Therefore, does that not imply that the SWR at, say, the amp/cable
>interface (50 ohm source, changing load), also change depending on the
>length of said cable?
>
You would think so, but no. The VSWR does no change anywhere along the
cable, but the impedance (Z) does as the resistance and reactance (R +/-
jX) change with line length. Don't be fooled into thinking that VSWR is
also equal to the ratio of the complex impedance at some point on the line
divided by the surge (characteristic) impedance of the line, it isn't.
There are good explanations for all this in the various ARRL Handbooks,
Reflections by W2DU, Antenna Impedance Matching by W. Caron, and a QST
article by Byron Goodman (can't recall the title anymore) that sums it up
nicely.
Phil
>73
>Mike
>n2mg@contesting.com
>
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