[TowerTalk] Open wire measurments

[Dan Maguire]

Wes wrote:
>>> I have no doubt that less reflective loads can be adequately measured without common-mode suppression

Many years ago Wes alerted me to a method of determining transmission
line characteristics that uses two load impedances other than open and
short.  For example, when measuring 50-ohm coax the loads might be 25
ohms and 100 ohms.  When measuring open wire line the loads might be
roughly twice and half the estimated Zo of the line.

The math is described in a QEX technical correspondence letter by Bert
Weller, WD8KBW (now SK I believe), that appeared on pg 60 of the
Nov/Dec 2001 issue.  The same technique is described in an article by
James McKay that appeared on pp 136-137 of Electronic Design, November
22, 1976.

Here's a screen grab of the Weller letter:
https://i.postimg.cc/Kz8Mc4dT/image.png

As of Zplots version 2.3 (July 2015) this technique can be used in
conjunction with the "Measure Transmission Line Parameters" feature.
Basically you do a sweep of the two different loads by themselves
followed by sweeps at the input end of the transmission line with
those two different loads at the far end.

Zplots web page:  https://ac6la.com/zplots1.html

Dan, AC6LA