I have added a further update to the AIM4170 Presentation, showing a TDR
scan on an open-wire feeder.

Go to: http://homepage.ntlworld.com/wadei/aim4170.htm

then click on the link "AIM4170 PowerPoint Presentation".

New slides 72, 73 and 74 show clearly that although Zo = the calculated
free space value at the antenna feedpoint, it is otherwise considerably
affected by the feeder’s proximity to ground and other objects.

Zo actually falls as the feeder descends from the shack window to the
garage, remains more-or-less constant as it passes over the garage roof,
and only begins to increase again as it traverses the hedge and passes
through the tree. Note also that the impedance decreases slightly where
the feeder narrows at the antenna feedpoint.

Yet another example of the incredible versatility of the AIM4170.

--
73
Ian, G3NRW

The AIM4170 Antenna Analyzer:
http://homepage.ntlworld.com/wadei/aim4170.htm