The Basic Rules for Anechoic Chamber Design

Key Takeaways

• An anechoic chamber is an indoor range that offers non-reflective free space utilized as a controlled laboratory environment. 

• Irrespective of the chamber type, the mechanical, electrical, acoustical, and aerodynamic design considerations need to be satisfied for the anechoic chamber to function as required. 

• The inclusion of wedges inside the anechoic chamber surface can increase the noise reduction value. 

The design and construction of an anechoic chamber are crucial for achieving error-free measurements

Antenna measurements play an important role in antenna design. The antenna testing setup utilizes the antenna under test and reference antenna to determine the parameters of the test antenna. Typically, antenna measurements are performed in a room with no echo and no reflections. To accurately measure the performance, gain, and pattern characteristics of the antenna under test, the testing setup is kept inside an anechoic chamber, which is free of fields, reflections, and echoes.

The design and construction of the anechoic chamber are crucial for achieving error-free antenna measurements. To ensure the best testing environment, engineers should follow the basic rules for anechoic chamber design that satisfy the SAE, ISO, ANSI, or ASTM standards that are applicable to the given device under test.  In this article, we will explore anechoic chamber designs further. 

The Importance of Testing Location

Outdoor ranges with no boundaries for producing reflections are the optimal testing environments for antenna testing as well as any other RF device testing. However, the measurements from outdoor testing are influenced by external factors such as pressure, temperature, humidity, and wind. The effect of these physical parameters on test measurements is significant enough to disturb the accuracy and reliability of the test results.

Indoor testing locations are designed for performing device testing, especially those utilizing electromagnetic energy. Indoor ranges provide a secure setup for device testing with minimal electromagnetic interference. These indoor ranges are free from radiation, reflection, echo, and interference. Types of indoor ranges include reverberation chambers, semi-anechoic chambers, or fully anechoic chambers. In this article, we will focus on anechoic chambers and their design.

Anechoic Chambers 

An anechoic chamber is an indoor range that offers non-reflective free space. These chambers are used as controlled laboratory environments. The purpose of anechoic chambers is to provide an isolated room free of electromagnetic signals.

The most ideal testing environment is outer space, and anechoic chambers can simulate device testing in outer space. Anechoic chambers are free space ranges, where signal reflections from the neighboring objects are suppressed for accurate test measurements. They can be used for testing antennas and RF devices in any weather condition. They are unaffected by external conditions such as temperature, pressure, wind, and humidity, which helps ensure the accuracy of test results. 

Anechoic chambers form screened rooms that are fully or partially covered with electromagnetic absorbers in the form of wedges. They create an enclosure with high shielding attenuation against electromagnetic interference. Anechoic chambers absorb 100% of ambient emissions but reflect the minimum amount of internal radiation.

Anechoic chambers can be classified into two types based on the geometrical optics techniques employed for reducing the specular reflections in the chamber design.

1. Rectangular anechoic chamber - The floor, ceiling, end, and center parts of the side walls are covered with pyramids. The rest of the chamber is covered with wedges. 
2. Tapered anechoic chamber - Used for frequencies less than 1GHz. The structure of the chamber is designed as a rectangle at the upper end and as a pyramid at the lower end to minimize high-level reflections. 

The Basic Rules for Anechoic Chamber Design

Irrespective of the chamber type, there are certain basic rules for anechoic chamber designs that must be satisfied for the anechoic chamber to function as required. Design considerations include mechanical, electrical, acoustical, and aerodynamic aspects. The factors that need to be considered are the dimensions of the anechoic chamber, internal working measurement distances, internal ambient noise levels, ventilation, flooring, ceiling, and walls. A few of the basic design rules are:

• Chamber size is the most significant factor in anechoic chamber design. For effectiveness, the chamber size should follow the guidelines for minimum dimensions. The chamber size is dependent on the size and type of the device under test, frequency, required measurement distance, wedge depth, and wall thickness. 
• The construction material of the walls, floor, and ceiling should have adequate transmission loss. 
• The structure of the anechoic chamber needs to be designed in consideration of outside noise levels and the lowest noise levels expected from the device under test.
• The inclusion of wedges inside the anechoic chamber surface can increase the noise reduction value. 
• The floors and ceiling need to be covered with wedges for minimum reflections and interferences. It is better to keep the same absorber in the ceiling and floor as the side walls. In a rectangular anechoic chamber, maintaining equal height and width of the chamber can help keep reflections from both at the same level. 
• The anechoic chamber needs to be free from fields, and the absorption characteristics of the walls, ceiling, and floors can ensure this. An absorber material with an absorption coefficient above 0.99 should be selected for the cut-off frequency of the anechoic chamber. 
• Wedges made of fiberglass, foam, or perforated metal can be used. Wedges are usually made one wavelength in thickness.  

Following the basic rules for anechoic chamber design can improve the effectiveness of the test measurements performed inside the chamber. The physical characteristics of a properly designed anechoic chamber will ensure an ideal testing environment.

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