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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