HP 8530A, Руководство по эксплуатации и программированию, Страница: 243 / 439

Time Domain Antenna Impedance Measurements
Figure 13-11. Typical Frequency and Time Domain Response of an Antenna
Interpreting the Time Band P ass Reection Response Horizontal Axis .
NOTE: When you read the following paragraph, you need to know the denition of the term
\calibration plane ." A \calibration plane" is the exact place (electrically) where you placed
the calibration standards during a calibration. This is the only location in the system that is
actually calibrated.
In Time Band P ass reection measurements , the horizontal axis represents the amount of time
that it takes for an impulse , launched at the \calibration plane ," to reach the discontinuity in
the antenna and return. Thus , this is the two-way travel time to the discontinuity .
The Marker reads out both the time and the electrical length to the discontinuity . The
electrical length is obtained by multiplying the time by the velocity of light in free space
(2.997925E8 m/sec). T o get the physical length, multiply the displayed electrical length
by the relative velocity of light in the transmission medium of the antenna, or use the
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VELOCITY FACTOR function. Using V elocity F actor along with Electrical Delay can produce
accurate distance measurements in dispersive media such as waveguide . Refer to the
description of the Electrical Delay feature in the Response chapter .
Interpreting the Time Band P ass Reection Response V ertical Axis .
The quantity displayed on the vertical axis depends on the selected display format. Time
Band P ass defaults to the Linear Magnitude format, which displays the response in reection
coecient () units . This can be thought of as an average reection coecient of the
discontinuity over the frequency range of the measurement.
Other useful formats are listed in T able 13-3 . The Time Band P ass response gives the magnitude
of the reection only and has no direct impedance information (R, L, or C).
T able 13-3. Useful Time Band P ass F ormats
F ormat Trace V alue
LINEAR MA G Reection Coecient Units
LOG MA G Return Loss (dB)
SWR SWR Units
13-18 Introduction to Time Domain RCS and
Antenna Measurements