HP 8530A, Руководство по эксплуатации и программированию, Страница: 240 / 439

Time Domain RCS Measurements
Gating During RCS Calibration
The HP 8530A RCS Calibration procedure allows you to use gating during calibration. The
gated calibration applies the Gate to the frequency response error term (Er) only , allowing
removal of the eects of shadowing and scattering caused by the reference target.
Gating (during RCS calibration) better characterizes the reference target and the frequency
response error (Er) of the measurement system.
The \Gating during Calibration" feature is located in the T arget Response menu. There are two
selections:
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GATING YES If selected, the HP 8530 will use the current gate settings during the gated
calibration. If Gating is currently turned OFF , the HP 8530 will prompt you to
rst set up and turn on the gate before measuring the reference target. NOTE:
you can re-enter the existing calibration by pressing \RESUME CAL". Once you
have set up the gate , you do not have to start the calibration over again.
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GATING NO
If selected, no gate is used (in the calibration), regardless of whether gating is
turned ON or OFF .
With gating ON during RCS calibration, the receiver applies the current Time Domain gate
settings to the data that results from the raw measured data of the reference target minus
the background. The gate provides better characterization of the reference target, because
it removes the eects of interactions of the reference target with the test chamber . Gating
can remove the eects of reference target scattering, and it can also reduce the eects of
reference target shadowing. Remember that gating only removes responses that lie outside of
the gate .
Setting the Gate for a Gated RCS Calibration
Y ou should select gate Start, Stop , Center , and Span times while viewing the reference target
response in the Time Domain. Make the gate span wide enough to include the full target zone
response . At minimum, the gate span should be at least wide enough to be centered on the
peak response of the reference target and include its full Time Domain response .
If your target is a sphere , the placement of the gate is critical. Spheres produce a secondary
reection caused by RF energy propagating around the back of the sphere , which ultimately
arrives back at the receive antenna. This produces the secondary reection, known as a
\creeping wave ," located in time by (2 + ) 2 radius 2 c (c=velocity of light) after the main
(specular) response from the front of the sphere .
If the creeping wave is clearly separate from the main target response , place the gate around
just the main response . In this case make sure you select the
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OPTICAL SPHERE model when
you dene the RCS target (as explained the section on RCS calibration).
If the main target response and the creeping wave are not clearly distinct and separate ,
position the gate around both of them. In this case make sure you select the
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TARGET: SPHERE
model when you dene the RCS target.
Because the gate start and stop indicators show the 06 dB points of the gate lter , it is
important to locate them beyond the peak of the responses that are to be kept within the gate
to prevent distorting the responses .
Do Not Use Gating When . . .
Y ou are measuring a small frequency span or number of points .
If using a frequency list where the points are not evenly spaced.
Introduction to Time Domain RCS and
Antenna Measurements
13-15