Antenna Test Method Selection Evaluation[Copy link]
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 employing antenna pattern measurement (APM) and hardware-in-the-loop (HiL) measurement techniques, which have seen growing interest in the defense community 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 unfamiliar with this very economical technology, nor do they fully understand its limitations, especially above 1 GHz. As a result, they are unable to use it to its full potential. With the growing interest in antenna measurements at frequencies as low as 100 MHz, it becomes increasingly important for antenna test engineers to understand the strengths and limitations of various antenna test methods, such as anechoic chambers. When testing antennas, antenna test engineers typically need to measure a number of parameters such as radiation pattern, 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 site of choice depends on the antenna to be tested. To better understand the selection process, consider this scenario: 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 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 site, the transmit and receive antennas are located in the far field of each other and are usually separated by a sufficient distance to simulate the desired operating environment. The AUT is illuminated by a source antenna that is far enough away to produce a nearly planar wavefront across the electrical aperture of the AUT. Far-field measurements can be made in either indoor or outdoor test ranges. Indoor measurements are usually made in a microwave anechoic chamber. These chambers can be rectangular or conical and are designed to reduce reflections from walls, floors, and ceilings (Figure 1). In rectangular microwave anechoic chambers, a wall absorber is used to reduce reflections. In conical microwave anechoic chambers, a cone shape is used to produce the illumination.
Figure 1: These are typical indoor direct-beam measurement systems, showing conical (left) and rectangular (right) test ranges.
Near-field and reflection measurements can also be made in indoor test ranges, and are usually near-field or compact ranges. In a compact test range, a reflecting surface produces a plane wave that simulates far-field behavior. This allows antennas to be measured 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 a 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 produced by a reflecting surface in a compact test range.
Figure 2: Flat waveform produced by reflection measurement in a compact test range
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 manageable distances. For electrically large antennas, reflectors, and arrays (over 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 reflectors and measurement systems. Suppose an antenna test engineer wants to make measurements at lower frequencies. This is of particular interest to the defense sector, which needs to investigate things like using antennas at lower 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 has different electromagnetic behaviors. Rectangular microwave chambers are in a true free space state, while conical chambers use reflections to create free space-like behavior. Because reflected rays are used, the result is quasi-free space rather than true free space. It is well known that rectangular chambers are easier to manufacture, have very large physical dimensions at low frequencies, and perform better as frequency increases. Conical chambers, in contrast, are more complex to manufacture and are longer, but are smaller in width and height than matrix chambers. As frequencies increase (i.e., above 2 GHz), the operation of conical chambers must be handled with care to ensure adequate performance. The difference between rectangular and conical chambers can be more clearly understood by examining the absorbing measures used in each chamber. In rectangular chambers, the key is to reduce the reflected energy in a region of the chamber 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 required normal reflectivity of the back wall must be equal to or greater than the desired quiet zone level. The side walls are critical because the reflection in a rectangular chamber is at an oblique incidence, which compromises the efficiency of the absorber. However, due to the gain of the source antenna, less energy is striking 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 side wall areas where specular reflections occur between the source and the quiet zone require expensive side wall absorbers. In other cases (such as at the transmitting end wall behind the source), shorter absorbers can be used. Wedge-shaped absorbers are typically 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 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 cone 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 a reflective surface on the quiet zone in a compact test range The illumination mechanism of the cone can be more clearly explained using array theory. Consider the feed to consist of the real source antenna and a set of images. If the images are far away from the source (electrically), then the array factor is irregular (e.g., has many ripples). If the images are 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 very critical, especially at higher frequencies (i.e., above 2 GHz), where the chamber behavior is more sensitive to small changes (Figure 4). The angle and treatment of the entire cone are also important. The angle must be kept constant, because any change in the angle of the cone section will cause illumination errors. Therefore, maintaining a continuous angle when measuring is the key to achieving good tapered performance. Figure 4: In a typical conical chamber, the placement of absorbers may appear simple, but the area closer to the source antenna (the conical dark area) is critical. As with rectangular chambers, the reflectivity of the absorber on the receiving wall in a conical chamber must be greater than or equal to the required quiet zone level. The side wall absorbers are less critical, since any rays reflected from the side walls of the cubic 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. The smaller amount of conical absorber material means a smaller chamber, and therefore a lower cost. Both chambers provide essentially the same performance. However, it should be noted that to achieve the same performance as the conical chamber, the rectangular chamber must be larger, with longer absorbers and a greater amount of absorber material.
Figure 5: A small conical chamber for antenna testing from 200 MHz to 40 GHz
While it is clear from the previous discussion that conical chambers can offer advantages over rectangular chambers at lower frequencies, measured data demonstrates the real usability of conical chambers. Figure 5 shows a small conical chamber from 200MHz to 40GHz 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 200MHz was greater than 30Db (as shown in Figure 6). Figures 7 and 8 show the source antenna on top of the feed and the scanning antenna in the quiet zone, respectively. There are many different methods for antenna measurements like APM and HiL. The trick is to choose the right antenna test site, depending on the antenna to be tested. For medium-sized antennas (10 wavelengths), a far-field test site is recommended. On the other hand, tapered chambers offer a better solution for frequencies below 500 MHz. They can also be used for frequencies above 2 GHz, but some care needs to be taken to ensure adequate performance. By understanding the proper use of a conical anechoic chamber, today’s antenna test engineers have a very useful tool at their disposal for making antenna measurements in the 100MHz to 300MHz and UHF ranges.
Figure 8: This test uses a log-periodic antenna to sweep the QZ to measure reflectivity
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 200MHz was greater than 30Db (as shown in Figure 6). Figures 7 and 8 show the source antenna on top of the feed and the scanning antenna in the quiet zone, respectively. There are many different methods for antenna measurements like APM and HiL. The trick is to choose the right antenna test site, depending on the antenna to be tested. For medium-sized antennas (10 wavelengths), a far-field test site is recommended. On the other hand, tapered chambers offer a better solution for frequencies below 500 MHz. They can also be used for frequencies above 2 GHz, but some care needs to be taken to ensure adequate performance. By understanding the proper use of a conical anechoic chamber, today’s antenna test engineers have a very useful tool at their disposal for making antenna measurements in the 100MHz to 300MHz and UHF ranges.
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