Ian Wade G3NRW wrote:

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.

I'm surprised that there were no FUs to this (at least in ukra).

This looks like a seriously useful bit of kit for the antenna
experimenter, but whether I could personally justify one for my
string-and-sealing-wax approach to short wire antennas is
questionable.

Although this device means one could fine-tune one's antenna, the one
thing that affects very nearly every Amateur is the ground around
their installation, probably up to 10 lambda before the various fields
and rays begin to sort themselves out. There seems to be no equivalent
piece of kit that would enable such an assessment to be made or its
effect on the station's performance to be estimated. I guess most of
us take the view that one lives with what one has, and can't do much
about it anyway.
--

Spike