ABSTRACT

It is a very common practice to over specify the Quiet Zone performance requirements for an anechoic chamber. Very often what is done is a person who is in need of a chamber contacts someone with a similar facility, often a supplier or a customer, and simply patterns their performance requirement after what the other guy has done. This often results in a chamber, which is specified to a tighter performance requirement than is actually needed to perform the particular measurements required and can cost thousands of dollars more than is necessary.

Qualcomm had a requirement to build a chamber for the evaluation of various antenna designs for mobile communication equipment. Due to building and space limitations the "ideal" size for a chamber operating in the 800 Mhz to 6.0 Ghz was not available. Qualcomm worked with AEMI to define the performance parameters to provide them with the best performing chamber that could be built within the restricted space available.

Once the design parameters were defined adequately the chamber deign was developed and the chamber was built. Once the chamber was built Qualcomm went about defining the best test methods and parameters that could be achieved given the performance limitations that were evident in the design due to the compromises that had to be made in the limited space available to accommodate the chamber.
This paper will discuss the design process, the design limitations and the methods used to overcome the performance compromises made in the development of the chamber and its intended purpose.

Keywords: Absorber Material, Anechoic Chamber, Antenna Measurements, Range Equation, Phase Taper.

1.0 Introduction

This paper describes a joint effort between Advanced ElectroMagnetics, Inc and Qualcomm, Inc. to develop and build an antenna measurement facility for the development of mobile communication antennas.

Due to the fact that Qualcomm was in a leased facility and was growing at an incredible rate, laboratory space was at a premium. The space made available for the Anechoic Chamber was smaller that what would be ideal for the frequencies to be tested.

Given the limitation in space available, it was our task to define the minimum chamber performance levels that would allow accurate measurement of the subject antennas. Once this was established the chamber design would be completed, the shielded enclosure built, the absorber installed and finally the chamber would be evaluated and the measurement system would be installed to accomplish the accurate measurement of the antennas developed for Qualcomm's requirements.

Once the chamber performance and design were established, the building was prepared to accommodate the chamber. The shielded enclosure was constructed, under separate contract, the absorbers were manufactured, the absorbers were installed, the chamber was tested, and finally the measurement equipment was installed and calibrated.

As final check, a "golden" antenna was used to evaluate the effectiveness of the chamber to correlate to existing data. Figure 1 shows the schematic of the chamber and instrumentation.

2.0 Chamber Performance Criteria

Due to the type of measurements that were to be conducted in this chamber, the following quiet zone performance levels were established.

Given the fact that office space at Qualcomm was at a premium the maximum chamber dimensions were set at 30' long x 12' wide x 10' high. The target length was determined to be 20' for these chamber dimensions. With these chamber dimensions the far-field equation (2D2/l) provides us with the following antenna apertures that can be tested in this chamber while maintaining the standard 22.5 degrees of phase taper across the aperture of the antenna under test.

3.0 Chamber Description

Based on the requirements listed in Section 2, Qualcomm contacted Lindgren RF Enclosures to build the 100 dB shielded enclosure. Utilizing standard modular shielded construction techniques, which incorporate a hat and flat clamp mechanism to join together double skin panels, the modular panel construction consists of zinc coated, 60 gauge steel laminated to both sides of a high-density particleboard core. The hat and flat clamping mechanism consists of cold rolled steel, which is designed to provide uniform clamping pressure along the whole perimeter of the attached panel. The fasteners used to clamp the shield together are hardened zinc plated TORX screws placed on nominal 100mm centers. The corner intersections of the enclosure are finished and sealed with cast bronze caps that are precision machined to match the framing members. The enclosure is assembled on an underlayment consisting of a polyethylene vapor barrier and a 3mm thick dielectric hardboard. This construction can be seen in Figure 2.

4.0 Chamber Configuration

As is typical in the establishment of the absorber design, the lowest frequency of use of the facility determines the largest absorbers to be utilized in the design as well as the overall absorber thickness and placement. For this design the -25 dB @ 825 MHz drives the chamber design.

First, the back wall absorbers are chosen to provide an efficient termination to the range. In order to terminate this range AEP-36 absorbers were chosen which provides -30 dB absorption at 825 MHz and -45 dB absorption at 2.0 GHz thus providing a 5 dB safety margin.

With the back wall established the transmit wall is next. Due to the fact that we are using a standard gain horn we can be assured that the front to back ratio is in the order of 10 dB to 15 dB. This allows the absorbers mounted on the transmit wall to be required to absorb only -15 dB at 825 MHz. In order to provide a decent safety margin on the transmit wall treatment, it was decided to utilize AEP-24 absorbers which provides for -25 dB absorption at 825 MHz.

The next design parameter to be established is the size of the absorbers to be utilized in the specular region of the sidewalls, floor, and ceiling. For this analysis the smallest cross sectional dimension is used, which in this case is the height dimension of 10 feet. Utilizing standard ray tracing techniques it was determined that the materials required in the specular region must attenuate -25 dB at an angle of 60 degrees. This translates into absorber thickness of 1.5 lambda at 825 MHz, which in turn defines the specular absorber to be a minimum of 18" thick. The specular absorbers are to be AEP-18 with the balance of the sidewalls, floor, and ceiling to be treated with AEP-12 absorbers. See Figure 3 for the absorber layout. Figure 4 shows the chamber and instrumentation.

5.0 Microwave Instrumentation

The RF equipment consists of an HP 8720D Vector Network Analyzer and a 1-watt solid-state amplifier (AR model 1S1G4) to improve dynamic range. The transmit source antenna is a quad-ridged dual-linear horn built by Condor Systems (model AS-48410), covering 0.7 to 4.5GHz. For Handset EIRP and Sensitivity tests a Tektronix CMD-80 Base Station Simulator and/or an Agilent 8960-Series-10 E5515C Wireless Communications Test Set are being used.

6.0 Positioning Equipment

The antenna positioner system consists of a roll-over-azimuth positioner system mounted on a dielectric model tower built by Orbit/FR, shown in Figure 5. The azimuth positioner (axis ÔA') consists of an Orbit/FR Al-560, which is mounted on top of Orbit/FR Al-4701 motorized linear slide. The roll (axis ÔB') axis is a belt-pulley system, driven by an AL-360 positioner. The roll positioner is as low profile as possible and mounted well below the horizontal antenna boom to minimize RF interference. The horizontal boom consists of low profile dielectric materials with interchangeable assemblies. The model tower can support 40lb loads such as a phantom head. See Figure 6.

3D measurements are performed so that the elevation angle (axis ÔA') is changed in discrete steps and at each of these elevation positions the DUT is rotated 360 degrees in azimuth by the Axis ÔB' roll positioner. The data is sampled in intervals of azimuth, typically every 4 degrees. The transmit source horn is equipped with an Orbit/FR Al-360 roll positioner for spinning linear or changing source polarization. All positioners are controlled by an Al-4806-3A Positioner Controller.

7.0 Measurement Software

The antenna measurement system is controlled by Orbit/FR 959-Plus software. The computer is a Compaq Pentium III 500MHz with Dual-Processors, running Windows NT 4.0. A National Instruments I-EEE-488.2 card is used to control the Network Analyzer and Positioner Controller. Post processing of antenna data is performed using MATLAB scripts to calculate gain, directivity, efficiency, average gain, and 3D radiation patterns.

8.0 Overall System Performance

The chamber was tested for shielding effectiveness and Free Space VSWR. The shielding effectiveness met the 100dB requirement. Free Space VSWR method was used to assess the quiet zone. The probe (horn antenna) was traversed through a 1.2-meter area to measure a 1-meter quiet zone. The test range distance is 13 feet. The probe was raster scanned in two-dimensions within the quite zone. The scan was repeated at different look angles and amplitude data was recorded. This method measures the standing wave set up by the interference of direct signal and the vector sum of the signals reflected from the chamber absorbers. Measurements were performed from 824 to 2000MHz at both vertical and horizontal polarizations. At PCS band the results showed reflectivity of approximately -35dB for both polarizations. For AMPS band the reflectivity in vertical polarization was approximately -25dB, while the horizontal polarization reflectivity was approximately -20dB at the low end of the band.

Further investigation involved probing the quiet zone with a sleeve dipole. The probe was positioned through the 14-inch test zone in transversal (cross-range) direction. The amplitude ripple at 825 and 1900 MHz is less than ±0.35dB. The magnitude variation at 1900 MHz is shown in Figure 7.

Scientific Atlanta standard gain horns are used for calibration to establish absolute gain levels. A set of calibrated balanced dipoles (which have broad beamwidths similar to the DUT) are used to validate DUT gain results.

9.0 Summary

So what does all of this have to do with the price of a dB you ask? Well here is what we are driving at. In this instance the chamber performance was specified as -25 dB @ 825 MHz. The size of the chamber was restricted to 30' x 12' x 10'. Given these restrictions the chamber was designed and built with the knowledge that these were not the ideal dimensions for the performance requested. For the sake of this discussion, let's say that the cost to build this facility was as follows: Shielded enclosure $54,000.00 + Anechoic treatment $43,000.00 = $97,000.00 or $3,880.00 per dB. Let us now assume that the performance level had been specified at -30 dB @ 825 MHz.
Thus, we keep the target length at 20' but due to the increased chamber performance requirement, the performance required in the specular region has been increased by 5 dB. This is a very significant increase which is driven by the angle of arrival of the energy in the specular region which will in turn increase the thickness of the absorbers in the specular region which in turn will increase the overall width and height of the chamber. The length will also have to be increased slightly due to the higher performance requirements.

The design will look something like this. In order to increase the performance from -25 dB to -30 dB the back wall absorber will have to be increased from 36" to 48" maintaining the 5 dB safety margin. This increases the length of the chamber by 1' but since the shielded enclosure is made up of modular panels the length would be increased by an increment of 2' thus providing the new length of 32'. In order to provide -30 dB of performance in the specular region, this would require absorber thickness of 3 lambda VS the 1.5 lambda of the original design basically doubling the thickness of the materials placed in the specular area of the chamber. In order to keep the test zone from experiencing aperture blockage from the specular material, the chamber must be widened by 4' and the height must be increased by 4'. Test zone performance and test zone size are shown in Tables 1 and 2.

With these new dimensions the chamber has now grown to 32' long x 16' wide x 14' high. The absorber thickness has significantly increased in size as well. This new chamber design will now cost the following: Shielded enclosure $82,000.00 + Anechoic treatment $59,000.00 = $141,000.00 or $4,700.00 per dB. What we are saying here is: Why spend $141,000 USD for a facility when with careful analysis of your true requirements, it can be built for $97,000.00?

10.0 References

1. Leland H. Hemming, "Electromagnetic Test Facilities. Basic Design Principals." A series of lectures at AEMI 1990.
2. Dr. Ed Joy, Dr. Stan Gillespi and others, "Far Field Anechoic Chamber, Compact Range, and Near Field Antenna Measurements", Short course at Georgia Tech 1998.
3. J. D. Kraus "Antennas" McGraw Hill 1988.
4. M. Skolnik "Radar Handbook."

Gabriel A. Sanchez is the President and Founder of Advanced ElectroMagnetics, Inc. a wholly owned subsidiary of Orbit/FR. Mr. Sanchez received his Associate of Arts Degree from Los Angeles Harbor College in 1972 and his Bachelor of Science degree in Engineering Technology from California Polytechnic University, Pomona in 1974. After a short time as a Quality Assurance Specialist for the Defense Contract Administration he joined Emerson & Cuming in their Gardena, California plant. After four years with Emerson & Cuming he moved to San Diego to join Plessey Microwave in the Kearney Mesa area. Upon the closure of the anechoic product line at Plessey Mr. Sanchez established AEMI in 1980. Mr. Sanchez's duties consist of general management, technical design, sales and marketing for AEMI's absorbers and anechoic chambers. Mr. Sanchez has authored numerous technical papers on absorbing materials and anechoic chamber design. Mr. Sanchez also holds three patents for absorber design, chamber design and measurement techniques. He may be reached at gsanchez@aemi-inc.com.