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Test Volume Measurements in Fully Anechoic
Rooms
Fatih Kartal

parameters and conditions during the studies.
Abstract—Radiated emission measurements may be applied in
fully anechoic rooms. When the Fully Anechoic Room (FAR)
method is used, the limits should be defined in relevant
standards. Test volume should be defined by the method which is
described in CISPR 16-4-1. The results must fulfill the
requirements of the standard. The site reference test method is
used during the measurements, there are lots of parameters that
affect the uncertainty of reference measurement. Hence, these
parameters should be well defined and controlled during the
measurements.
Index Terms— CISPR 16-1-4, fully anechoic chamber,
Radiated emission, site reference, test volume, uncertainty
I. INTRODUCTION
An anechoic chamber ("an-echoic" meaning non-reflective,
non-echoing or echo-free) is a room designed to completely
absorb reflections of either sound or electromagnetic waves.
They are also insulated from exterior sources of noise. The
combination of both aspects means they simulate a quiet openspace of infinite dimension, which is useful when exterior
influences would otherwise give false results.[1]
Anechoic chambers, a term coined by American acoustics
expert Leo Beranek, were originally used in the context
of acoustics (sound waves) to minimize the reflections of a
room. More recently, rooms designed to reduce reflection and
external noise in radio frequencies have been used to test
antennas, radars, or electromagnetic interference.[1]
Anechoic chambers range from small compartments the size
of household microwave ovens to ones as large as aircraft
hangars. The size of the chamber depends on the size of the
objects to be tested and the frequency range of the signals
used, although scale models can sometimes be used by testing
at shorter wavelengths (higher frequencies). [1]
In this paper, we describe the method of our measurements
for the test volume measurements in fully anechoic room.
There are lots of critical points that can affect the
measurement uncertainty such as the height of the masts,
direction and position of the transmitting, receiving antennas,
the position of the cables, the distance between the antennas
and absorbers in the room… We aimed to control all these
II. METHOD OF MEASUREMENT
CISPR 16-1-4 is our reference for the method of
measurement. A fully absorber lined shielded enclosure, also
known as a fully anechoic chamber (FAC), or a fully anechoic
room (FAR), may be used for radiated emission
measurements. When the FAR method is used, appropriate
radiated emission limits shall be defined in relevant standards
(generic, product or product family standards). Compliance
with the radio services protection requirements (limits) shall
be established for FARs in a similar way as for tests on an
OATS.[2]
A FAR is intended to simulate a free space environment
such that only the direct ray from the transmitting antenna or
EUT reaches the receiving antenna. All indirect and reflected
waves shall be minimized with the use of appropriate
absorbing material on all walls, the ceiling and the floor of the
FAR.[2]
Site performance may be validated by two methods– the site
reference method and the NSA method. We used the site
reference method during the measurements.[2]
A single position SA (site attenuation) measurement may
not be sufficient to pick up possible reflections from the room
construction and/or absorbing material lining the walls, floor,
ceiling and turntable of the FAR.[2]
The fully anechoic room SA measurements and validation
shall therefore be performed at 15 measurement positions for
both horizontal and vertical antenna polarizations of the
transmit antenna in the test volume ( see Figure 1).
– at three heights of the test volume: bottom, middle and top;
– at five positions in all three horizontal planes: centre, left,
right, front and rear positions in each horizontal plane. The
rear position may be omitted if the distance between rear
position and absorbers is more than 0,5 m. During EUT
testing, the rear position on the turntable is also turned to the
front, and the contribution of the back reflection will then not
affect the maximum signal.[2]
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reference site is to choose the height (h) of the antennas as
follows
h ≥ d × 8/3
where d is the antenna separation. [2]
A height of h = d × 8/3 is recommended to suppress the
influence of the ground, or substantial absorbers which work
down to 30 MHz, need to be placed on the ground.[2]
NOTE: At 3 m separation at 30 MHz there is a significant
near field term (1/d 2) that alone contributes an error of 0,8 dB
for a height of 5/3. This was verified by the national
laboratories of both the UK and Austria. For a site reference
with an uncertainty of less than ±0,5 dB, a height of 8/3 is
recommended if no absorber is placed at the ground.[2]
Figure 1- Measurement positions for the site validations
procedure
For all positions of the transmitting antenna in the test
volume, in both horizontal and vertical polarizations, the
transmitting and receiving antennas shall be aligned on the
measurement axis. [2]
The distance shall be equal to the actual distance dnominal
between the antennas used in the FAR. The antennas are
polarized vertically (horizontal polarization shall not be used
because of stronger interference with the ground-reflected
signal). It also provides a good approximation of free-space.
The clearance from buildings, trees, etc. shall be greater than d
× 8/3 because there may be an influence for vertically
polarized antennas. [2]
Tilting the antennas is necessary to meet this requirement at
certain positions ( see Figure 2 ).
Figure 3- Typical free-space site reference measurement
set-up
III. CONCLUSION
Figure 2- Example of one measurement position and antenna
tilt for the site validation procedure
For accurate site validations at distances less than 5 m, it is
recommended that dedicated pairs of antennas be used to
determine the site reference (transmit and receive antenna). A
quasifree space test site is required. It consists of 2 nonmetallic antenna masts (wood or plastic with εr ≤ 2,5, low
loss, diameter as small as possible retaining mechanical
strength), which allow the placement of antennas at a certain
height above the ground level (see Figure 3). One possible
method of realization of the ±1 dB performance of the
A measurement site shall comply with the following
requirements:
– deviations of the SA or the NSA shall be less than ±4 dB for
both horizontal and vertical polarization and for each
measurement position and measurement frequency;
– the uncertainty budget of the site evaluation according to
CISPR 16-4-2 recommendations must be reported and shall
have the same components as required for field strength
measurements on alternative test sites with ground plane.
There can be some spot frequencies which do not fulfill the
±4 dB criterion. The wavelength should be calculated for the
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related frequency. The distance between antenna and
absorbers should be greater than this wavelength value to
avoid the reflection affect. There is such kind of clues during
the measurements. All these clues will be discussed in the
conference.
REFERENCES
[1]
[2]
https://en.wikipedia.org/wiki/Anechoic_chamber
CISPR 16-1-4 Ed.2: Specification for radio disturbance and immunity
measuring apparatus and methods- Part 1-4: Radio disturbance and
immunity measuring apparatus- Ancillary equipment- Radiated
disturbances