Antenna Test Method Selection and Evaluation

Matching an antenna to an application requires accurate antenna measurements. Antenna engineers need to determine how an antenna will perform in order to determine if it is suitable for a particular application. This means using antenna pattern measurement (APM) and hardware-in-the-loop (HiL) measurement techniques, which have seen increasing interest in the defense sector over the past five years. While there are many different ways to perform these measurements, there is no one ideal method that works for every situation. For example, low-frequency antennas below 500 MHz are typically measured using an anechoic chamber, a technology that has been around since the 1960s. Unfortunately, most modern antenna test engineers are not familiar with this very economical technology, nor do they fully understand its limitations, especially above 1 GHz. As a result, they are unable to use this technology to its full potential.

　　With the growing interest in antenna measurements down to 100MHz, it is becoming increasingly important for antenna test engineers to understand the advantages and limitations of various antenna test methods, such as conical microwave chambers. When testing antennas, antenna test engineers typically need to measure many parameters, such as radiation patterns, gain, impedance, or polarization characteristics. One of the techniques used to test antenna patterns is far-field testing, where the antenna under test (AUT) is mounted in the far field of the transmitting antenna. Other techniques include near-field and reflector testing. The antenna test field used depends on the antenna to be tested.

　　To better understand the selection process, consider this: A typical antenna measurement system can be divided into two separate parts, the transmit station and the receive station. The transmit station consists of a microwave transmission source, optional amplifier, transmit antenna, and a communication link to the receive station. The receive station consists of the AUT, reference antenna, receiver, local oscillator (LO) signal source, RF downconverter, positioner, system software, and computer.

　　In a traditional far-field antenna test range, the transmit and receive antennas are located in the far field of each other, usually far enough apart to simulate the desired operating environment. The AUT is illuminated by the source antenna far enough away to produce a nearly planar wavefront on the electrical aperture of the AUT. Far-field measurements can be made in indoor or outdoor test ranges. Indoor measurements are usually made in a microwave anechoic chamber. These chambers can be rectangular or conical and are specifically designed to reduce reflections from walls, floors, and ceilings (Figure 1). In a rectangular microwave anechoic chamber, a wall absorbing material is used to reduce reflections. In a conical microwave anechoic chamber, a cone shape is used to produce illumination.

Figure 1: These are typical indoor direct-beam measurement systems, showing a conical (left) and rectangular (right) test field.

　　Near-field and reflection measurements can also be performed in an indoor test range, and are typically near-field or compact test ranges. In a compact test range, a reflecting surface generates a plane wave that simulates the far-field behavior. This allows antenna measurements to be made in a test range that is shorter than the far-field distance. In a near-field test range, the AUT is placed in the near field and the field on the surface close to the antenna is measured. The measurement data is then mathematically transformed to obtain the far-field behavior (Figure 2). Figure 3 shows a plane wave generated by a reflecting surface on the quiet zone in a compact test range.

Figure 2: In a compact test field, a flat waveform is produced by a reflection measurement.

　　Generally speaking, antennas under 10 wavelengths (small and medium-sized antennas) are easiest to measure in a far-field test range because far-field conditions can often be easily met at a manageable distance. For electrically large antennas, reflectors, and arrays (greater than 10 wavelengths), the far field is often many wavelengths away. Therefore, near-field or compact test ranges can provide a more viable measurement option, regardless of the increased cost of the reflector and measurement system.

　　Suppose an antenna test engineer wants to make measurements at low frequencies. This is of particular interest to the defense sector, which needs to investigate things like using antennas at low frequencies to better penetrate structures in ground penetrating radar (GPR) systems (for radio frequency identification (RFID) tags operating in the 400MHz range), as well as to support more efficient radios such as software defined radios (SDRs) and digital remote sensing radios. In this case, a microwave anechoic chamber can provide a good enough environment for indoor far-field measurements.

　　Rectangular and conical are two common types of microwave anechoic chambers, the so-called direct illumination method. Each chamber has different physical dimensions and therefore different electromagnetic behavior. Rectangular microwave anechoic chambers are in a true auto-space state, while conical chambers use reflections to form free-space-like behavior. Because reflected rays are used, the final result is quasi-free rather than truly free space.

　　As is known, rectangular chambers are relatively easy to manufacture, have very large physical dimensions at low frequencies, and perform better as the frequency increases. Conical chambers, on the other hand, are more complex to manufacture and are longer, but are smaller in width and height than matrix chambers. As the frequency increases (e.g., above 2 GHz), the operation of the conical chamber must be handled with care to ensure adequate performance.

　　The difference between rectangular and tapered chambers can be better understood by examining the absorbing measures used in each chamber. In a rectangular chamber, the key is to reduce the reflected energy in the chamber area called the Quiet Zone (QZ). The Quiet Zone level is the difference in dB between the reflected ray entering the Quiet Zone and the direct ray from the source antenna to the Quiet Zone. For a given Quiet Zone level, this means that the normal reflectivity of the back wall must be equal to or greater than the Quiet Zone level to be achieved.

　　The side walls are critical because the reflection in the rectangular chamber is an oblique incidence, which compromises the efficiency of the absorber. However, due to the gain of the source antenna, less energy hits the side walls (floor and ceiling), so the gain difference plus the oblique incidence reflectivity must be greater than or equal to the quiet zone reflectivity level.

　　Typically only the sidewall areas where specular reflections occur between the source and the quiet zone require expensive sidewall absorbers. In other cases (such as at the transmitting endwall behind the source), shorter absorbers can be used. Wedge-shaped absorbers are often used around the quiet zone to help reduce any backscatter and prevent it from negatively affecting the measurement.

　　What absorbers are used in conical chambers? The original purpose of developing this type of chamber was to circumvent the limitations of rectangular chambers at frequencies below 500MHz. At these low frequencies, rectangular chambers have to use less efficient antennas, and the thickness of the side wall absorbers must be increased to reduce reflections and improve performance. Likewise, the size of the chamber must be increased to accommodate the larger absorbers. Using smaller antennas is not a solution, because the lower gain means that the side wall absorbers must still be increased in size.

　　The tapered chamber does not eliminate specular reflections. The cone shape brings the specular area closer to the feed (the aperture of the source antenna), so the specular reflections become part of the illumination. The specular area can be used to create illumination by forming a set of parallel rays incident on the quiet zone. As shown in Figure 3, the resulting quiet zone amplitude and phase tapers are close to what would be expected in free space.

Figure 3: Plane waves generated by reflecting surfaces in the quiet zone in a compact test range.

　　The illumination mechanism of the conical chamber can be explained more clearly using array theory. Consider the feed to consist of a real source antenna and a set of images. If the image is far away from the source (electrically), then the array factor is irregular (e.g., has many ripples). If the image is closer to the source, then the array factor is an isotropic pattern. To an observer at the AUT (in the far field), the source he sees is the pattern of the source antenna plus the array factor. In other words, the array will look like a single antenna in free space.

　　In a tapered chamber, the source antenna is critical, especially at higher frequencies (e.g., above 2 GHz), where chamber behavior is more sensitive to small changes (Figure 4). The angle and treatment of the entire cone are also important. The angle must remain constant, as any change in the angle of the cone portion will cause illumination errors. Therefore, maintaining a consistent angle when measuring is key to achieving good tapered performance.

Figure 4: In a typical conical anechoic chamber, the layout of the absorber appears simple, but the area closer to the source antenna (the conical dark area) is very important.

　　As with rectangular chambers, the reflectivity of the receiving wall absorber in a tapered chamber must be greater than or equal to the required quiet zone level. Side wall absorbers are less critical, since any rays reflected from the side walls of the cube portion of the chamber will be further absorbed by the back wall (which has the best performing absorber). As a general "rule of thumb," the reflectivity of the absorber on the cube is half that of the back wall absorber. To reduce potential scattering, absorbers can be placed at 45-degree angles or in a diamond pattern, although wedge-shaped materials can also be used.

　　The table provides the characteristics of a typical conical microwave chamber, which can be used to compare with a typical rectangular chamber. Less tapered absorber material means a smaller chamber, and therefore lower cost. Both chambers provide essentially the same performance. However, it should be noted that in order to achieve the same performance as the conical chamber, the rectangular chamber must be larger, with longer absorbers and more absorbers.

Figure 5: A small 200MHz to 40GHz conical chamber for antenna testing.

　　While it is clear from the previous discussion that conical chambers offer advantages over rectangular chambers at low frequencies, measured data demonstrates the true usability of conical chambers. Figure 5 shows a small conical chamber from 200 MHz to 40 GHz with dimensions of 12 x 12 x 36 feet and a quiet zone of 1.2 meters. A dual-ridged broadband horn antenna was used to illuminate the quiet zone at lower frequencies. The quiet zone was then measured using an Agilent N9030A PXA spectrum analyzer with a log-periodic antenna. The reflectivity measured at 200 MHz was greater than 30 dB (see Figure 6). Figures 7 and 8 show the source antenna on top of the feed and the scanning antenna in the quiet zone, respectively.

Figure 6: As can be seen from the figure, the reflectivity measured at 200MHz is greater than 30dB.

Figure 7: This test uses a dual-ridged speaker as the source.

　　There are many different methods like APM and HiL to perform antenna measurements. The trick to the measurement is to choose the right antenna test field, depending on the antenna to be tested. For medium-sized antennas (10 wavelengths in size), a far-field test field is recommended. On the other hand, conical chambers provide a better solution for frequencies below 500MHz. They can also be used for frequencies above 2GHz, but they require extra care to ensure adequate performance. By understanding the correct use of conical microwave chambers, today's antenna test engineers have a very useful tool at their disposal for antenna measurements in the 100MHz to 300MHz and UHF range.

Figure 8: This test uses a log-periodic antenna to scan the QZ to measure reflectivity.