Evaluate Antenna Measurement Methods

Oct. 14, 2010
For UHF antennas at frequencies from 100 to 300 MHz, an anechoic chamber properly equipped with the right signal generation and analysis equipment provides an ideal test solution.

Matching an antenna to an application requires accurate antenna measurements. Antenna engineers need to determine how an antenna will perform in order to ascertain whether it is ideal for a given application. That means taking antenna pattern measurements (APM) and hardware in loop (HiL) measurementssomething that has garnered increasing interest from the defense community over the past five years. While there are a number of different ways to make such measurements, there is no ideal method to use in every scenario. For low frequencies below 500 MHz, for example, the method of choice is often the use of a tapered anechoic chambera technology that has been around since the 1960s. Unfortunately, most modern antenna test engineers are unfamiliar with this economical technology and do not fully understand its limitations, especially beyond 1 GHz. As a result, they have been unable to utilize it to its greatest benefit.

With growing interest in antenna measurements at frequencies as low as the 100- MHz range, it is now more critical than ever before for antenna test engineers to understand the benefits and limitations of antenna measurement methods like the tapered anechoic chamber. When testing an antenna, the antenna test engineer must typically measure a number of parameters such as the radiation pattern, gain, impedance, or polarization characteristics. One of the techniques used to measure antenna patterns is the far-field range where an antenna under test (AUT) is placed in the far-field of a transmit range antenna. Other techniques include the near-field and reflector ranges. The choice of which antenna range to use depends on the antenna being measured.

To better understand the selection process, consider that a typical antennarange measurement system can be divided into two separate parts: the transmit site and the receive site. The transmit site consists of the microwave transmit source, optional amplifiers, transmit antenna, and communications link to the receive site. The receive site consists of the AUT, a reference antenna, receiver, local-oscillator (LO) signal source, RF downconverter, positioner, system software, and a computer.

On a traditional far-field antenna range, the transmit and receive antennas are on the far field of each other typically separated by enough distance to simulate the intended operating environment. The AUT is illuminated by a source antenna at a distance far enough away to create a near-planar phase front over the AUT's electrical aperture. Far-field measurements can be performed on indoor and outdoor ranges. Indoor measurements are typically made in an anechoic chamber, with either rectangular or tapered shape, that is specifically designed to reduce reflections from the walls, floor, and ceiling (Fig. 1). In a rectangular anechoic chamber, a wall absorber is used to reduce reflections. In a tapered anechoic chamber, the geometry of the taper is used to create illumination.

Near-field and reflector measurements can also be performed on indoor ranges, most typically, a nearfield or compact range. On a compact range, a reflector creates a plane wave that simulates far-field behavior. This allows the antenna to be measured at range lengths shorter than their farfield distance. On a near-field range, the AUT is placed in the near-field and the fields on a surface close to the antenna are measured. The data is then mathematically transformed to obtain far-field behavior (Fig. 2). Figure 3 shows the plane wave created by the reflector on the quiet zone in a compact range.

In general, antennas that are under 10 wavelengths (electrically small to medium antennas) are most easily measured in the far-field ranges since the far-field condition is usually met within a manageable distance. For electrically large antennas, reflectors and arrays (more than 10 wavelengths), the far-field is often many wavelengths away. Consequently, the near-field or compact ranges offer a more viable measurement option, despite any increased cost from the reflector and measurement system.

Suppose an antenna test engineer wants to make measurements at low frequencies. This is particularly interesting to the defense community investigating things like the use of antennas at low frequencies to better penetrate structures such as in ground-penetrating radar (GPR) systems, for radiofrequency- identification (RFID) tags operating in the 400- MHz range, and to enable more efficient radios, such as software-defined radios (SDRs) and digital cognitive radios. In this case, anechoic chambers provide an adequate environment for indoor far-field measurements.

The two common styles of anechoic chamber are rectangular and taperedthe so-called direct illumination methods. Each has a different physical footprint and therefore, different electromagnetic behavior. While the rectangular anechoic chamber is a true free-space condition, the tapered chamber uses reflections to create free-space-like behavior. Since reflected rays are used though, the result is a quasi-free, rather than a true free space.

Rectangular chambers are known for being easy to fabricate, physically very large at low frequencies and tend to operate better as the frequency increases. In contrast, tapered chambers are complicated to fabricate and are longer, but physically smaller in width and height, than the rectangular chamber. As frequency increases (e.g., beyond 2 GHz), a great deal more care must be placed on the tapered chamber's operation in order to ensure it achieves adequate performance.

A better sense of the differences between the rectangular versus tapered chamber can be made clearer by examining the absorber treatment used in each type of chamber. In a rectangular chamber, the key is to reduce the reflected energy in an area of the chamber known as the quiet zone (QZ). Measured in dB, the QZ level is the difference between the reflected rays that come into the QZ and the direct ray from the source antenna to the QZ. For a given QZ level, that means that the back wall will require normal reflectivity that is equal to or better than the QZ level to be achieved.

Since the reflection in a rectangular chamber is at an oblique incidence, which tends to make the absorber not as effective, the side walls are extremely critical. However, because of the source antenna's gain, there is a lower amount of energy illuminating the side walls (floor and ceiling) so the difference in gain plus the oblique incidence reflectivity must be better than or equal to the level of QZ reflectivity.

A costly side wall absorber is typically needed only in the areas of the side wall where a specular reflection exists between the source and the QZ. In every other instance (e.g., at the transmit-end wall behind the source), a shorter absorber can be used. A wedge absorber is generally used around the QZ area to help reduce any backscattering and prevent it from negatively affecting measurements.

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What about the absorber treatment in a tapered chamber? These chambers were originally developed to avoid the limitations associated with rectangular chambers at frequencies below 500 MHz. At these low frequencies, rectangular chambers are forced to use less efficient antennas and the thickness of their side wall absorber must be increased to reduce reflections and allow for good performance. Likewise, the chamber's size must also increase to accommodate the larger absorber. Using a smaller antenna is not an option, since the lower gain means that the sidewall absorber still has to increase in size.

The tapered chamber does not eliminate specular reflection. The shape of the taper brings the specular area closer to the feed (the aperture of the source antenna) and, as a result, the specular reflection becomes part of the illumination. The specular area is used to generate the illumination by creating a series of parallel rays incident into the QZ. As shown in Fig. 3, the resulting QZ amplitude and phase tapers approach those expected in free space.

A clearer explanation of the tapered chamber's illumination mechanism can be derived by using array theory. Consider that the feed is made of the actual source antenna and a collection of images. If the images are far away (electrically) from the source then the array factor is irregular (e.g., with a lot of ripple). If the images are closer to the source, then the array factor is an isotropic pattern. To an observer at the location of the AUT (in the far field), the source seen is the pattern of the source antenna multiplied by the array factor. In other words, the array will look like the antenna by itself in free space.

In a tapered chamber, the source antenna is very critical, especially at higher frequencies (e.g., above 2 GHz), where the chamber's behavior is much more sensitive to small changes (Fig. 4). The angle and treatment of the entire taper is also important. The angle must be kept constant since any change in the tapered section angle will introduce illumination errors. Maintaining a continuous angle on the treatment is therefore a key to achieving good tapered performance.

As with the rectangular chamber, the receive-end wall absorber in the tapered chamber must have a reflectivity that is better than or equal to the required QZ level. The side wall absorber is not as critical because any ray reflected from the side walls of the chamber's cubical section are further absorbed at the back wall (where the highest performance absorber is located). As a general "rule of thumb," the absorber on the cubical is half the reflectivity of the back wall absorber. To reduce potential scattering, the absorber can be placed in a 45-deg. pattern or diamond arrangement, although a wedge could also be used.

The table offers a characterization of a typical tapered anechoic chamber, which can be used for comparison to a typical rectangular chamber. The smaller volume of the tapered absorber means a small chamber and therefore less cost. Both chambers provide roughly the same performance. Note though that in order for the rectangular chamber to achieve the same performance as its tapered counterpart it must be much larger, with a longer absorber and with much larger absorber volume.

While it is clear from the previous discussion that the tapered chamber offers a number of benefits over the rectangular chamber at low frequencies, measured data demonstrates the tapered chamber's true usability. Figure 5 shows an example of a small 200-MHz-to- 40-GHz tapered chamber, measuring 12 x 12 x 36 ft. and with a 1.2-m QZ. Here, a dual-ridged broadband horn antenna is used to illuminate the QZ at lower frequencies. A log-periodic antenna is then used to measure the QZ using a N9030A PXA spectrum analyzer from Agilent Technologies. The resulting measured reflectivity of better than 30 dB at 200 MHz is shown in Fig. 6. Figures 7 and 8 show the source antenna at the tip of the feed and the scanning antenna in the QZ.

There are many different ways to make antenna measurement like APM and HiL. The trick lies in selecting the right antenna range, a decision that depends on the antenna being measured. For electrically medium antennas (to 10 wavelengths in size), far-field ranges are advisable. Tapered chambers, on the other hand, offer a better solution for frequencies under 500 MHz. They can also be used for frequencies above 2 GHz, although more attention to their operation is required to ensure adequate performance. By understanding the proper use of tapered anechoic chambers, today's antenna test engineers now have access to an extremely useful tool for antenna measurements over the 100-to-300- MHz and UHF range.

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