The document describes the design and testing of an anechoic chamber built by Raytheon UMass-Lowell Research Institute to characterize electronic devices between 8-12 GHz. Ray tracing was used to simulate the expected quiet zone performance and attenuation of reflections within the chamber. Measurements found the amplitude taper was less than 1 dB and phase taper less than 22.5 degrees across the quiet zone, matching expectations. Attenuation of reflected signals was measured to be below -45 dB, indicating minimal effect on device measurements.

1. EXPECTED RESULTS
In order to simulate the expected performance of the chamber,
ray tracing techniques were used to perform various calculations.
INTRODUCTION
The Raytheon UMass-Lowell Research Institute
(RURI) is a joint research facility focused on the
advancement of innovative technologies, including
flexible and printed electronics. RURI expects to
expedite the development of these technologies by
providing a single space for designing, prototyping
and testing. Several projects at RURI involve the
creation of electronic electromagnetic (EM)
components using 3-D printing. Before this project,
RURI lacked the appropriate test facilities for
characterizing a device-under-test (or DUT),
specifically within X-band frequencies (8-12 GHz).
CONCLUSIONS
MEASURED RESULTS
Anechoic Chamber Design & Construction
Brian Morrison, Brittany Decker
Raytheon UMass-Lowell Research Institute (RURI)
DESIGN CONSIDERATIONS
Quiet Zone
The primary measure of QZ performance is seen in the amplitude
and phase tapers. The amplitude should taper less than 1 dB on
either side of the QZ. The phase difference between the edges and
center of QZ should be no more than 22.5 degrees. Figure 2
simulates the expected amplitude and phase tapers across one
dimension at 10 GHz.
Comparing expected and measured results has
demonstrated the anechoic chamber built is suitable for
testing devices. The amplitude taper was shown to be less
than 1 dB, and the phase taper less than 22.5 degrees.
Additionally the attenuation of reflected signals was shown
to have minimal effect on the measurement of DUTs.
Future work for the group will likely include validating
chamber at frequency between 1-18 GHz, adding a source
antenna positioner, expanding the software capabilities of
the laboratory for testing, and adding a parabolic dish to
increase the overall path length.
Quiet Zone
The amplitude and phase of the quiet zone
were measured across a 17.25 in x 17.25
in plane located 75 in. from the SA. Using
the XY positioner shown in Figure 5..
measurements were taken in 0.49 in.
increments across the plane. The results in
Figure 6 demonstrate the measured
amplitude and phase tapers across the XY
plane, with the quiet zone marked in white.
ACKNOWLEDGEMENTS
This project was made possible
by a generous donation of
equipment and materials by
Raytheon Corporation.
The authors would like to thank
Tom Sikina of the Raytheon
Corporation for his guidance.
*MATLab simulations provided
by Tom Sikina.
Several constraints dictate the
parameters of the quiet zone:
Path Length (R): the distance
between the SA and DUT. A
longer path length results in a
larger quiet zone. In order to
maximize the path length, a
corner-to-corner configuration
was selected, shown in Figure 2.
Specular Zones: where first-order reflections occur,
and are the primary source of noise in the quiet
zone.
Back Wall: where first or second-order reflections
occur, which result in false measurement of rear
lobes on a DUT turned ~180 degrees from SA.
CHAMBER PARAMETERS
Frequency range: 8.0 to 12.0 GHz
Min. internal attenuation: > -45.0 dB
Power (S21) @ 10 GHz: ~-20 dBm
Path Length (R): 75 inches
Max Device-Under-Test Diameters:
@ 8 GHz = 7.36 inches
@ 10 GHz = 6.65 inches
@ 12 GHz = 6.07 inches
Using the corner-to-corner configuration, the chamber
was designed with three priorities:
1.Provide the largest quiet zone possible.
2.Maximize the range of frequencies for DUTs.
3.Maximize the attenuation of noise.
Figure 3 Simulated amplitude and phase tapers @ 10 GHz.*
Figure 2 Chamber
configuration.
Attenuation
The RAM inside the chamber is specifically
designed to minimize the reflection of
signals within an anechoic chamber. Using
ray tracing, the expected attenuation of
first and second order reflections were
calculated. All second order reflections
were calculated to have negligible effect on
the measurement of DUTs within the quiet
zone. Additionally their expected
magnitudes are beyond the measurements
capabilities of lab equipment.
Attenuation
To measure the attenuation of
the most impactful reflections,
the positioner and antenna
probe were set at an angle
outside the direct path of the
source antenna.
Figure 6 shows a measured amplitude taper of less than 1 dB
across the quiet zone. Additionally, the phase taper is shown to be
less than 22.5 degrees on either side of the quiet zone. Both sets of
data match the expected results shown in Figure 3.
Figure 5 XY Positioner.
Figure 6 Measured amplitude and phase tapers @ 10 GHz.
In order to adequately char-
acterize a device indoors, an
anechoic chamber is used to
isolate a DUT from radiation
during testing. An anechoic
chamber isolates internal and
external EM signals by
creating a Faraday cage.
Additionally, the magnitude of reflections (noise)
within the chamber are reduced (attenuated) using
radiation absorbing materials (RAM). A “quiet zone”
(QZ) is a volume within an anechoic chamber ideal
for testing a device, and is the primary design
parameter in creating an anechoic chamber.
Figure 1 RAM in
Anechoic Chamber.
Figure 4 Specular and
back wall reflections.
Figure 5 Expected attenuation
across X-axis.*
The expected attenuation of
specular (first-order) reflections was
calculated to be less than -45 dB
across the quiet zone. A simulated
ray tracing shown in Figure 5
demonstrates the expected
attenuation across one dimension.
This simulation verifies an
attenuation less than -45 dB.
Figure 7 shows the attenuation of reflections across the XY plane
centered in the QZ. The QZ, denoted in white, shows the attenuation
of signals in excess -45 dB. At -40 dB, the magnitude of reflections
will be 1/1000th
the magnitude of direct signals; and have negligible
impact on the measurement of devices under test.
Figure 7 Measured attenuation at 10 GHz.
Acceptable
taper