[TowerTalk] Why the inconsistent line impedance

[Jim Brown]

On 9/3/2012 11:28 PM, K8RI wrote:
> Can anyone tell me why the impedance is a sloping or irregular line 
> where all the single band antennas have a very straight trace with the 
> only blips and dips being connectors, the amp bypass, and the tuner 
> bypass? 

Several thoughts, Roger.  First, TDR is usually done with a linear 
sweep, typically from 1 MHz to 500 MHz, or whatever the range of the 
analyzer.  Because the sweep is linear, half of the energy in the sweep 
is in the top half of the sweep range, where small discontinuities in a 
line are more likely to be detected, but which do NOT relate very well 
to the impedance of a connected antenna in the operating range of the 
antenna.

Second, the impedance we measure from one end includes reflections from 
the other end of the line. If you compare your red and green curves, 
their shape is nearly identical -- the same peaks and valleys, with the 
major difference between them being that the distance scale is more 
compressed.  SO -- those peaks and dips are simply a reflection from the 
other end of the line.

Third, the TDR function in many analyzers (including mine) is an inverse 
FFT of the sweep data, and the shape of the trace will depend on the 
sweep rate, the excitation signal, how the data is filtered, and how it 
is windowed. My VNA, the DG8SAQ VNWA, allows me to change all of those 
parameters to maximize the sensitivity to defects.

73, Jim K9YC