Anechoic Chamber and Scatterometer
By Lindsay Smith

Above: A peek into the anechoic chamber. Photo: Lindsay Smith

Looking to get away from the everyday hustle and bustle? Little do most people know there is a room so quiet at the University of Nebraska-Lincoln's engineering complex that one cannot even hear their own echo. If locked inside, they wouldn't be able to call for help and they'd be stuck in the dark until someone comes to find them. But don't worry: the anechoic chamber is really used for testing radio wave antennas by Ezekiel Bahar, professor of Electrical Engineering.

"Lehman Chambers built the chamber about 12 years ago," Bahar said.

Anechoic means "against echoes." The chamber cost approximately $100,000 to build and is located on the third floor of Scott Engineering Center between rooms 318 and 320. It has a nine foot ceiling and is 27-1/4 feet by 15-1/4 feet, approximately 416 square feet total. The chamber walls, ceiling and doors are lined with a copper sheet. The copper keeps outside electromagnetic waves from disrupting test results. The chamber also has pyramids made of Microwave Absorbing Material (MAM) that line the walls, floor, and ceiling.

These pyramids capture the electromagnetic waves and absorb them, so there are practically no reflected or echo waves. These pyramids are used to make the chamber seem infinitely large and are made of a stiff carbon impregnated foam material. The chamber reduces the amount of space needed to run tests that usually require very large spaces, by limiting outside interference. The waves are absorbed as they bounce along the sides of the pyramids and the power of the reflected waves is decreased.

Above: A cross sectional view of the Microwave Absorbing Material with pyramidal shape. The pyramidal design allows for near complete absorption, no matter the direction of propagation of the waves.

"The optimum range of wavelengths that can be absorbed in the chamber is 1-10 GHz," said Bahar.

The anechoic chamber at UNL is mostly used for testing antennas to make sure they are operating properly. The testing can also involve a transmitter, a receiver and a target. The electromagnetic waves are radiated from the transmitter to the receiver via the target.

The walls of the chamber absorb waves not directly scattered to the receiver by the target so that the receiver doesn't pick up any echo that could skew the test results. When a target is characterized, the receiver and the transmitter are usually located nearby and the reflected waves are measured. For example, these experiments can test materials used in stealth aircraft.

The anechoic chamber can also be used to obtain a target's electromagnetic signature. By compiling tables of data on how the waves are reflected off certain objects, different targets can be identified based on how they fit the compiled data. The anechoic chamber can also be used to investigate groundpenetrating radar, which is used to detect buried objects in the presence of clutter. Clutter due to surface or medium irregularities would interfere with reflected waves coming off the buried object.

The scatterometer in the electromagnetic/optics lab is a tool used to measure scattered light from objects of different materials and shapes. The scatterometer at UNL is a unique device that can measure scattered light on the surface of a sphere (4π solid angle) around the tested material. This is made possible because the receiver base can pivot 180 degrees in a horizontal plane around the sample. The receiver also can rotate 180 degrees in a vertical plane around the sample to allow for measurements over a hemisphere. Measurements over the other hemisphere can be taken by rotating the sample 180 degrees. The incident angle can be between 0 and almost 90 degrees.

This device can also be used in experiments to detect differences in the polarization of light reflected off different chemical and biological materials. The reflected light leaves distinct optical foot prints. This process has applications in the detection of biological and chemical weapons of mass destruction. Being able to remotely sense suspicious materials using optical instruments keeps personnel further from potential danger while allowing them to analyze the object.

Other uses of the scatterometer are in the medical and biochemical fields. Tests are being conducted elsewhere to use polarized light to distinguish between pre-cancerous and normal tissue.

The optical polarimetric scatterometer uses electromagnetic waves at optical wavelengths. Polarization is characterized as linear, circular, or in general elliptical. The scatterometer provides more information about the target than familiar radar. The laser sources in the scatterometer operate at ~ 1 micron and 0.66 microns.

"There are numerous research opportunities available to students at UNL, even at the undergraduate level," said Bahar. "I urge students to just ask their instructors. The very accessible faculty is more than willing to help students conduct undergraduate research."

The scatterometer at UNL is one of only three in the country and UNL had it fabricated to their specifications. The others are at U.S. Navy and NASA laboratories.

The versatility of research equipment at UNL is astounding. What is even better is that the faculty encourages students to use the scatterometer or anechoic chamber for research. Just ask Dr. Bahar.