Simulation of Anechoic Chamber. C. Di Giulio LNGS Assergi (AQ). Chicago Sep. 2010. OUTLOOK. Introduction Shielding Absorber Material Theory Absorber Model First simulation test. Introduction to Anechoic ( “ No echo ” ) chambers.
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Simulation of Anechoic Chamber
C. Di Giulio
LNGS Assergi (AQ)
Chicago Sep. 2010
A radio frequency “anechoic chamber” is a shielded room whose walls have been covered with a material that scatters or absorbs so much of the incident energy that it can simulate free space.
aluminum foil which has contact adhesive on one side and simply adhere it to the wall of the enclosure.
Family:
Ferrite Tile .
Magnetic Losses
Preferred technology for Low frequencies (up to 1GHz), it has low profile.
It cannot be used for high frequencies
Wedge and pyramid
Electric Losses
A variant of pyramidal absorber wedge does not show backscattering. Preferred technology for QZ (Quite Zone) treatment and for RCS(Radar Cross Section) chambers.
Flat laminate .
Electric Losses
Preferred technology for laboratory set ups. It is a sandwich of different foams with different amounts of carbon loading.
About 20dB absorption as frequency increases.
Hybrid Absorber .
Electric and Magnetic Losses
Preferred technology for EMC
Applications. foam has to have special formula for good matching with ferrite tile at the bottom. At High frequencies its performance is not as good as MW pyramid of equal size. Flat top causes undesired reflections at MW range.
if the antenna is large compared to the wavelength:
D refers to the maximum physical dimension of the antenna.
R refers to the distance away from the antenna.
D
Else:
This region is only used to guide the absorber layout design.
This material is volumetrically loaded having the same constitutive parameters through the volume of the pyramid
Popular types of absorber have constitutive parameters of:
Non magnetic material
Low permittivity with losses
We will study how the electromagnetic wave behaves as is incident on to a wall of this type of absorber.
At the tip of the absorber
The wave impedance is that of air
NO SUDDEN CHANGE IN WAVE
IMPEDANCE = LOW
REFLECTIVITY
Along the length of the pyramid the wave impedance falls between those two values.
At the base of the pyramid
The wave impedance becomes
Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the height
Let’s assume a length of 30cm
The wavelength at 3GHz is
10cm
And at 10GHz is
30mm
For 3GHz
Wavelength at 3GHz
Approximate thickness of equivalent solid material
Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the height
For 10GHz
Wavelength at 3GHz
Approximate thickness of equivalent solid material
In practice the reflection coefficient may not be as small as this but it will be significantly
Smaller than at 3GHz
The metal boundary may be modeled as a lossy dielectric with an effective dielectric constant, εeff , and an effective conductivity, σeff
AEP 12
AEP 18
AEP 18
WORK IN PROGRESS…
(help??)