HP 8530A, Руководство по эксплуатации и программированию, Страница: 248 / 439

Time Domain Antenna Transmission Measurements
signal transmission path. After antenna calibration is turned ON, the main antenna
transmission response in time shifts to zero seconds .
Gating
The HP 8530A Time Domain gating feature can remove the eects of unwanted Time Domain
responses . F or antenna test ranges where there is adequate separation of the line-of-sight and
multipath responses , Time Domain gating can remove the eects of the range multipath, with
signicant results . When converting back to the Frequency Domain, the eects of the Time
Domain responses outside of the gate are removed. See the earlier section on RCS Gating for a
discussion of gate lter shapes .
F or automated antenna systems , the Time Domain gating feature can remove the eects of
multipath from antenna pattern measurements . This procedure involves making broadband
measurements at discrete rotation angles (stepped positioner motion). During the measurement
(or afterwards in post-processing), the Time Domain gating feature can be used to remove the
multipath from each multiple frequency measurement. After the measurement is complete , the
end result is up to 801 dierent antenna patterns (one for each measurement frequency) that
have had the eects of multipath removed. F or more information, obtain the paper \Making
A ccurate Antenna and Radar Cross Section Measurements ," which is available upon request.
Errors Caused by the Gating Process
When using gating to remove the eects of unwanted response paths , it can be dicult to
determine the resulting measurement accuracy in the Frequency Domain response . Usually
the net result is better because of the reduction of the unwanted signal, but it is dicult to
determine just how much better . The following guidelines are suggested.
First, consider the eect of the unwanted signal on the desired response . T able 13-4 lists the
measurement error that can be caused by stray signals at dierent levels relative to that of the
desired response .
T able 13-4. Gating Uncertainty
Unwanted Signal to
Desired Signal
Ratio
6 Amplitude
Uncertainty
Phase Uncertainty
10 dB Completely in error Completely in error
0 dB +6 dB 0 1 dB 645 deg
010 dB +2.4 dB 0 3.3 dB 618 deg
020 dB +0.83 dB 0 0.92 dB 65.7 deg
030 dB +0.27 dB 0 0.28 dB 61.8 deg
040 dB +0.09 dB 0 0.09 dB 60.57 deg
If setting a gate span also reduces some of the main response , then you will incur the an error
proportional to the relative level of the response to the main level. Again, T able 13-4 is a good
guideline in determining the eect of this operation.
Introduction to Time Domain RCS and
Antenna Measurements
13-23