3-16 PN: 10580-00343 Rev. H S820E UG
3-5 Distance-To-Fault (DTF) Cable and Antenna Measurements
Windowing
The theoretical requirement for inverse FFT is for the data to extend from
zero frequency to infinity. Side lobes appear around a discontinuity because
the spectrum is cut off at a finite frequency. Windowing reduces the side lobes
by smoothing out the sharp transitions at the beginning and the end of the
frequency sweep. As the side lobes are reduced, the main lobe widens, thereby
reducing the resolution.
In situations where a small discontinuity may be close to a large one, side
lobe reduction windowing helps to reveal the discrete discontinuities. If
distance resolution is critical, then reduce the windowing for greater signal
resolution.
If two or more signals are very near to each other, then spectral resolution is
important. In this case, use Rectangular Windowing for the sharpest main
lobe (the best resolution).
In summary:
• Rectangular Windowing provides best spatial distance resolution for
revealing closely spaced events, but the side lobes close to any major
event (large reflection) may mask smaller events which are close to the
major event. Excellent choice if you suspect multiple faults of similar
amplitudes close together.
• Nominal Side Lobe Windowing provides very good suppression of
close-in side lobes, but compromises spatial distance resolution
compared to Rectangular windowing. Closely spaced events may
appear as a single event, often non-symmetrical in shape. Excellent
overall choice for most typical antenna system sweeps.
• Low Side Lobe Windowing provides excellent suppression of close-in
side lobes, but spatial distance resolution is worse than Nominal Side
Lobe. The additional suppression of side lobes may be useful in locating
very small reflection events further away from large events. This is not
often used for field measurements.
• Minimum Side Lobe Windowing provides the highest suppression of side
lobes but the worst spatial distance resolution. It can be useful for
finding extremely small events spaced further apart than the distance
resolution. This is not typically used for field measurements.
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