[Elecraft] SWR, Transmission lines and Antenna Matching

George, W5YR [email protected]
Tue Oct 15 18:52:00 2002


That is correct, Bob . . . this is another instance of confusing SWR
readings with impedance readings. This admonition is frequently found in
the CB literature . . .

In a practical situation, if there is common-mode current on the coax outer
braid, then the "indicated" SWR reading from the typical reflectometer type
of SWR meter will in fact vary with position in the line.

But, in the absence of CM current, the only effect on SWR readings from
proper instrumentation would be assigned to loss in the transmission line.

A contrived situation can be created in which the SWR reading *does* vary
with position but it requires use of a very lossy transmission line having
a complex Zo and an SWR instrument whose reference impedance does not match
that of the line. This is not a combination very likely to be found in an
amateur station installation.

73/72, George    
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489      Icom IC-765 #2349     Icom IC-756 PRO  #2121


"Bob Lewis (AA4PB)" wrote:
> 
> > the minimum SWR reading is correct only if the meter is placed in
> the feedline
> > at a multiple of 1/2 waves from the antenna.
> 
> Given no significant loss in the transmission line, what accounts for
> the SWR changing as you insert the meter at different points in the
> line? It seems to me that the power reflected by the mismatched load
> would be the same all along the line, except for the amount dissipated
> by the loss in the line.