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Time Domain RCS Measurements
RCS Alias-Free Range
Alias-Free Range is the length in time that a measurement can be made without encountering
a repetition of the response (see Figure 13-4). The repetition of the Time Domain response
occurs at regular intervals of time and is a consequence of the Frequency Domain RCS
response being measured at discrete frequencies , rather than over a continuous spectrum. The
separation between Time Domain response repetitions is known as the Alias-Free Range , and
it is exactly equal to 1/1F (the spacing between Frequency Domain data points). Alias-Free
Range is therefore directly proportional to the Number of P oints and inversely proportional to
the frequency Span (Stop 0 Start frequency). Alias-Free Range can be calculated using the
following formula.
Alias-Free Range = 1/1F or (Number of P oints 0 1) 4 Frequency Span
Aliasing
The term aliasing is an undesired condition where Time Domain repetitions overlap . T o prevent
aliasing with RCS measurements , make sure:
Time Domain Alias-Free Range approximately twice the RCS chamber length
As a sample calculation, for a 401 point measurement from 8.2 GHz to 11.2 GHz
(SP AN = 3.0 GHz), the Alias-Free Range is:
(401 0 1) 4 3.0 GHz = 133 nsec (40.0 meters).
Thus , the length of the test range from the range antennas to the chamber back wall must be
20 meters or less (one half of the Alias-Free Range). Otherwise the Time Domain responses will
overlap (aliasing occurs). Remember to multiply by the relative velocity of light to get actual
physical length.
How to Increase Alias-Free Range
T o increase the Alias-Free Range , it is recommended that you rst increase the Number of
P oints measured in the Frequency Domain. If going to the maximum Number of P oints (801)
is not sucient, then you must reduce the Frequency Domain measurement span. Reducing
frequency span also reduces the RCS Time Domain resolution. So reduce the measurement
frequency span only as much as is absolutely necessary .
How to Distinguish an Aliased Response from a Real Response
When aliasing occurs , it can be dicult to tell the real Time Domain response from an aliased
response . Here is one way to determine whether a response is real:
1. Save the Time Domain response into memory .
2. Select Frequency Domain.
3. Decrease the measurement frequency span a small amount.
4. Allow a full frequency sweep .
5. Select Time Domain.
6. Display data and memory .
Since reducing the frequency span increases the Alias-Free Range , any aliased response will
move along the time axis . Any real Time Domain response will remain stationary , although it
will change in appearance slightly .
13-8 Introduction to Time Domain RCS and
Antenna Measurements