[Elecraft] SWR, Transmission lines and Antenna Matching

[k6se@juno.com]

As Bob, AA4PB stated, changing the length of the feedline can drastically
change the measured R and X on the end of the line if the SWR is high,
resulting in inaccurate SWR readings on a fixed-impedance 50-ohm SWR
meter.

A good example is where you have an antenna whose feedpoint is about 100
ohms unbalanced.  If you feed it with a 3/4-wavelength of 75-ohm coax,
your 50-ohm SWR meter will read a 2:1 SWR at the feedpoint, but it will
read 1:1 SWR at the other end of the coax.  This is due to the ability of
a 1/4-wave length of coax to transform  the impedance and the ability of
a 1/2-wave of coax to repeat the impedance seen at its other end.

For Larry, W1HUE/7, who told me in a personal e-mail to "read the book",
I resent that snide remark.  The above paragraph describes exactly what I
meant in my original post (that reliable measurements are only made at
multiples of a half-wave from the feedpoint).  I suggest that YOU re-read
the book, Larry!

The handiest instrument I've found for accurate measurement of SWR is the
Autek model VA-1 Vector RX Antenna Analyst.  It has a feature where you
can enter the length of the feedline and the instrument compensates for
this length and calculates the antenna R and X as (and therefore the SWR)
as it would actually be if measured at the feedpoint of the antenna. 
(Why would they have that feature if the instrument measured the same SWR
anywhere on the line?)

The meter also has a menu whereby you can select virtually any
characteristic line impedance from 25 to 450 ohms, so it's not restricted
to 50 ohms like most meters. 

73, de Earl, K6SE