Concepts and Features
R&S
®
ZNB/ZNBT
208
User Manual 1173.9163.02 ─ 55
EMC Test Site Validation
EMC test sites for radiated emission measurements rely on free-space conditions to
minimize the influence of reflections on the received signal. Practically, near free-space
conditions are achieved by shielded enclosures fully lined with RF absorbing material.
Site validation
determines deviations from free-space conditions that must meet the
acceptance criterion for making EMC compliance measurements in a FAR (= fully
anechoic room). It is performed by measuring the
site voltage standing-wave ratio
(S
VSWR
), which is the ratio of maximum received signal to minimum received signal,
caused by interference between direct (intended) and reflected signals.
blue arrow
= direct signal
red and green arrows = reflected signal
TD S
VSWR
Calculation
The ANSI time-domain method relies on a complex transmission measurement (S
21
)
using a vector network analyzer (VNA).
A time-domain transformation of the frequency domain data shows the impulse
response between the two antennas. Since the direct antenna-to-antenna response is
related to the shortest distance, the earliest impulse is the direct antenna response.
Reflections from the test site are from farther distance away, thus come at later time. It
is therefore possible to separate the direct antenna response S
21, direct
from reflections
S
21, reflected
by using time gating and to calculate
TD
S
VSWR
= (1+r) / (1-r)
,
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