So....If I happen to insert my Heath, Bird, or japanese SWR/Power meters
into my transimssion line at a high impedance point, ,
say 200 ohms or
so, will the meter measure true SWR at the antenna, (minus the error
created by the losses in the transmisson line) or will it indicate an swr
roughly 200 divided by 50, or approximately 5:1? Can we use transmisson
line transformer theory to match the feed point impedance a the
transmitter to the transmitter output impedance?
73, de Pat AA6EG/N6IJ
aa6eg@tmx.com
On Thu, 18 Dec 1997, L. B. Cebik wrote:
> SWR and Impedance:
>
> Like the reflection coefficient on which it is based, the SWR along a
> length of transmission line (lossless) will be everywhere the same. It
> will slightly decrease in accord with line losses per unit measure and
> total line length.
>
> Impedance will not be the same everywhere along a transmission line
> (unless the line and load happen to be matched precisely). Impedance is
> the vector sum of resistance and reactance, which change continuously
> along the line--and with them, the impedance.
>
> Look in a handbook at the equations for SWR and for impedance and you can
> see the difference.
>
> So, if you measure Impedance at the antenna and at the rig, you can get
> legitimate different numbers. If you measure SWR at the antenna and at
> the rig, then they should be the same--or, for line loss, a little lower
> at the rig (except for feeding UHF rigs with WWII surplus RG-8, in which
> case, SWR at the rig will be lots lower than at the antenna, while power
> at the antenna will be lots lower than at the rig).
>
> You can check the R and X transformations (and calculate Z) along
> transmission lines with any input antenna values with one of the HAMCALC
> programs or use Dean's TL, etc. It is a good thing on a dead-band
> day/night to run lots of values systematically through such programs to
> familiarize yourself with trends. Makes interpreting measured results a
> lot easier with fewer wrong paths.
>
> Hope this is useful. Happy holiday season and a (sun)spotty New Year to
> all.
>
> -73-
>
> LB, W4RNL
>
>
>
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