Low Frequency 3-D Electromagnetic Analysis of Anechoic Chamber Performance
The advantages of anechoic chambers utilized for antenna measurements, as compared with conventional outdoor antenna test ranges, such as security, interference-free radiation and immunity to weather, are well known. Typical anechoic chambers comprise a shielded enclosure with the internal metallic surfaces covered by absorbing materials, a source antenna to illuminate the device under test (DUT), and positioning equipment on which the DUT is installed and rotated to acquire antenna pattern data. The dimensions of the shielded enclosure are primarily determined by the lowest operating frequency, and thus the required size will be significantly increased if the chamber operation is required down to VHF and/or UHF frequency bands. This in turn increases the construction and absorbing material costs. As a result, the advantage of an indoor chamber is often questioned when considering the VHF or UHF band as the lowest operating test frequency range. The main concern in this case is the projected amount of real estate required for construction of the chamber. A trade-off exists between the desire to lower the cost by reducing chamber size and the desire to ensure minimum performance levels by increasing the chamber size.
In order to achieve a minimum level of accuracy in the antenna measurement, a corresponding minimum level of DUT test zone illumination quality and overall chamber reflectivity needs to be achieved. This requires that turn-key chamber suppliers need to be able to accurately analyze the chamber performance. The ability to accurately characterize chamber performance results in an optimum design implemented at minimum cost.
To date, the method most often implemented in the industry to analyze anechoic chambers has been based on ray tracing. This method suffers from poor accuracy in cases where the room dimensions are only a few wavelengths at the lowest operating frequency. Several factors contribute to poor accuracy for these cases:
- The specular region, where reflections significantly impact the quality of the DUT test zone, has characteristic dimensions that encompass a few wavelengths of the wall surface covered by the absorbing materials, which at VHF and UHF frequencies may exceed side wall, floor and ceiling characteristic dimensions.
- As the effective specular area is large, it is difficult to define the correct incidence angle of the specular illumination for the ray tracing analysis.
- It is difficult to accurately predict or measure the reflection coefficient of the wall absorber at off-normal incidence at low frequencies.
- The phase of the reflection coefficient of the absorber material is difficult to accurately measure. Hence, only RMS or RSS fields can be calculated in the DUT test zone, thus delivering only approximate analysis results.
- In some cases assumptions are made for the reflectivity at off-normal incidence angles at VHF and UHF bands, based on similarity to reflectivity degradation curves at off-normal incidence at higher frequencies (> 2 GHz).1 However, this extrapolation has proven to be inaccurate at a number of installed anechoic chambers.
A useful improvement to the ray tracing method is the aperture integration method, in which the fields reflected from side walls into the test zone are calculated using Kirchhoff integration over apertures (portions of the side walls, floor or ceiling), and the field in the test zone is obtained as a sum of these reflected fields together with the direct radiation of the source antenna.
Although this method is better suited for analysis of anechoic chambers, the available accuracy is still limited and is determined by the information available on the reflectivity (magnitude and phase for two orthogonal polarizations) of the absorbing materials used in the chamber at both normal and off-normal incidence, which is (as indicated earlier) too problematic at the VHF and UHF bands. In addition, neither the aperture integration nor ray tracing methods take into account multiple reflections within the chamber, which is typically a significant effect at these frequencies.
Impact of Inaccurate Chamber Analysis
Lack of accuracy in chamber performance prediction may lead to non-optimum chamber design. However, less than optimum performance is not always easy to identify. For example, field probing of the test zone may not show the full extent of field variations and ripples originating from reflections within the chamber interior due to the long period of the interference pattern produced by the reflections and direct radiation at low frequencies.
In order to ensure adequate chamber performance, field probing should be performed over as broad a frequency range as possible. Smooth changes of the field in the test zone over a broad frequency range are a positive sign, indicating that the chamber performs well. Small variations of the recorded signal over a broad frequency range for multiple matched transmit/receive polarizations is another positive sign of excellent chamber performance.
3-D Electromagnetic Simulation
Given the limitations stated above for the aforementioned analysis techniques at low frequencies, it is obvious that a more rigorous and comprehensive analysis is required. An appropriate analysis is a 3-D electromagnetic simulation as described below.
A short time ago, no standard software package could be utilized to solve the problem of analyzing an anechoic chamber for comprehensive 3-D field quality. Recently, thanks to significant steps made in PC technology and advances in 3-D electromagnetic computer simulation packages utilizing time-domain techniques,2 such analysis is now possible.
ORBIT/FR has implemented a full 3-D simulation for anechoic chambers at low frequencies using a commercial transient solver package available from CST. Typical simulation results are presented below to illustrate the effectiveness of the 3-D time domain technique as applied to the analysis of anechoic chambers at low frequencies.
The primary analyses performed for the chamber include field uniformity over a volume of interest within the chamber, such as the DUT test zone, including subsequent determination of amplitude taper and ripple; phase variation; and induced cross-polarization levels in the chamber fields. These metrics are a function of the following parameters:
- Absorbing material layout and grades
- Source antenna/DUT separation
- Operating frequency
- Source antenna beamwidth
- DUT positioning equipment
- DUT supporting structure geometry and materials
The analysis can provide information leading to the proper selection of source antenna, source antenna/DUT separation, absorber layout and minimum chamber size required to achieve desired performance levels.
Chamber Modeling Methodology
The most efficient way to model the anechoic chamber is to create a database of standard components, including absorbing materials of different types such as pyramids, wedges, hybrids (a combination of pyramids backed by ferrite tiles), walkways, etc. Each type of material may contain different size models. For example, pyramids may be produced with various heights ranging from 2 to 96 inches or more. For UHF and VHF applications the larger heights are utilized, typically from 36 to 96 inches.
Material loading levels are input and stored in the database. These are based on measurements of the material properties ε’(f), ε”(f) , μ’(f), μ”(f) over the desired frequency range. Accurate characterization of the material using these parameters is critical for an acceptable simulation.
Figure 1 Isometric view of the anechoic chamber design (a) and absorber layout on the floor (b).
Typically, the absorber layout contains different types and different grades of absorbing materials, as shown in Figure 1. The design shown is for an anechoic chamber size of H = 6 m, W = 6 m W, L = 10 m, intended to operate down to 150 MHz. The floor contains a “diamond patch” of diagonally oriented pyramidal pieces of absorbing material located in the “specular” region of the floor, and is surrounded by lower grades of pyramids and wedge absorbing materials at the far end of the floor. The positioning system, a fiberglass tower supporting the DUT and walkway are also shown in the layout, and are accounted for in the simulations.
Optimal Selection of Absorber Layout and Grades
Figure 2 Test zone field contour at center of the test zone (a) 35-inch pyramidal absorber; (b) 48-inch pyramidal absorber.
Here, the effect of the absorber layout and absorber grades on the test zone performance is discussed. It is assumed that the basic chamber layout is similar to the one shown. A study and comparison of the test zone performance are made for two cases in which the receive wall and the “diamond patch” are made of 36 and 48 inches high pyramidal absorbers, respectively. The test zone fields for the two cases are presented in Figure 2 in the form of contour plots of the fields in the central cross-section of the test zone. It is evident upon inspection that the field in case b) is more uniform than in case a), indicating that the performance is superior using the 48 inch pyramids.
It is also readily observed that the reflectivity of the side walls (H-plane of the incident field) is inferior to that of the floor and ceiling (E-plane of the incident field), yielding the “ellipse” like field distribution in the test zone.
Effect of Source Antenna/DUT Separation on Test Zone Performance
Figure 3 Test zone field contour as a function of source antenna/DUT separation: (a) 3 m separation; (b) 6 m separation.
The separation between the source antenna and the DUT is an important parameter at VHF and UHF frequency bands and can strongly influence the test zone performance. It is well known that at microwave frequencies (> 2 GHz) the absorbing materials perform reasonably well for large incident wave angles such as 65°. However, at VHF and UHF frequencies, the absorber performance at large off-normal angles is sharply reduced. The examples shown in Figure 3 illustrate field contour plots calculated for the following:
- A 3 m source antenna/DUT separation, where the incident wave angle to the side walls, floor and ceiling is approximately 27°.
- A 6 m source antenna/DUT separation, where the incident wave angle to the side walls, floor and ceiling is approximately 45°.
As is readily observed, the longer separation leads to an unexpected and undesired field distribution across the test zone. The field structure results in a local minimum in the center of the test zone, which represents an unstable condition that tends to change rapidly with frequency, and, therefore, results in poorly controlled field uniformity in the test zone.
Effect of Source Antenna Beamwidth (BW) on Test Zone Performance
Another important parameter in the design of the anechoic chamber is the beamwidth of the source antenna. As mentioned previously, the reflectivity in the H-plane of the chamber is typically worse than in the E-plane. Poor reflectivity can be improved to some degree by reducing the beamwidth of the source antenna. However, too large of a beamwidth reduction leads to increased amplitude taper and reduction in the size of the test zone. Moreover, beamwidth reduction is not easy to achieve at VHF and UHF frequencies, since this requires increased dimensions of the source antenna, which is not always practical in an anechoic chamber. Electromagnetic simulation is thus an important tool in analyzing the optimum choice of source antenna prior to anechoic chamber construction.
Figure 4 H-plane patterns of LPD-based source antenna (a) single LPD antenna, 3 dB BW, ≈ 106° and (b) pair of LPD antennas, 3 dB BW, ≈ 75°.
Figure 5 Contour plots of test zone fields obtained using different source antennas: (a) single LPD antenna, 3 dB BW ≈ 106°; (b) pair of LPD antennas, 3 dB BW ≈ 73°.
The simulation examples represent the test zone field contour plots obtained with a source antenna composed of a single log-periodic dipole (LPD) antenna as well as with a pair of LPD antennas separated by a half wavelength. The difference in H-plane patterns of the two source antennas is shown in Figure 4. Inspection of the contour plot lines in Figure 5 reveals that the dual LPD array produces a more uniform field, exhibiting fields with nearly circularly symmetric contour lines.
Effect of Positioning Equipment on Test Zone Performance
Figure 6 Fields in the test zone with positioner, walkways and fiberglass tower included.
Positioning equipment and a walkway absorber path for operator access to the DUT are required elements in any anechoic chamber. As a result of their close proximity to the test zone, both may cause undesired and uncontrolled test zone field perturbations, especially at low frequencies. The effects of these elements on test zone performance are often hard or impossible to estimate prior to their installation. Electromagnetic 3-D computer simulation is a tool capable of taking these elements into account and to estimate the overall test zone performance in advance, prior to the chamber construction. Figure 6 shows how the positioner and fiberglass tower that supports the DUT may affect the test zone field distribution. As is evident, the field may lose important symmetry properties and the peak of the field (or the center of the test zone) moves higher, sometimes above the DUT mounting location. Addition of absorbers on the positioner assembly can often reduce this effect.
Electromagnetic 3-D analysis of anechoic chambers at low frequencies such as the VHF and UHF bands, executed with the aid of time domain software, delivers significant advantages over previously utilized analysis methods, resulting in a much more accurate characterization of test zone fields, thus allowing a more cost-effective and higher performance chamber design.
The performance factors that may be evaluated include:
- Broadband analysis of the effects produced by absorber material layout and various absorber grades on test zone performance
- Definition of the optimal source antenna/DUT separation as a function of frequency
- Optimal choice of the source antenna(s)
- Analysis of the effects produced by DUT positioning equipment and mounts, as well as walkway absorber, on test zone performance.
1. L.H. Hemming, Electromagnetic Anechoic Chambers: A Fundamental Design and Specification Guide, Wiley-Interscience, Hoboken, NJ, 2002.
2. CST Studio Suite 2008.