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Re: [TowerTalk] Why the inconsistent line impedance

Subject: Re: [TowerTalk] Why the inconsistent line impedance
From: Jim Brown <>
Date: Tue, 04 Sep 2012 08:57:09 -0700
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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

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