TRW, working with Nearfield Systems Inc., is building a state-of-the-art near-field antenna measurement system to test the Astrolink payload antenna system. Astrolink is the next generation broadband satellite network that will deliver high speed Internet connections to the business desktop. TRW is building the Astrolink on-board communications payload which includes the antenna system. For this multi-reflector antenna payload, TRW is building a 40 ft. x 30 ft. horizontal near-field measurement system to operate from 1 to 50 GHz using NSI’s high speed Panther receiver and Keysight Technologies high speed VXI microwave synthesizers. The system will be capable of performing conventional raster scans, as well as directed plane-polar scans tilted to the plane of a specific reflector. Completion of the range is scheduled for the first quarter 2001.
This paper will describe the near-field antenna measurement system that will test the Astrolink antenna payload and provide an overview of the specifications and test requirements for this test system. This paper will also describe the tilted plane-polar scanning capability, the 1 to 50 GHz RF subsystem, and the facility plans and progress.
Keywords: Near-field, Probe, Scanner, Stepper Motor, Plane Polar Scan, Measurement workstation PC
As commercial satellite payloads grow in complexity and capability, the measurement systems that verify that performance necessarily needs to grow as well. It is to that end that a large horizontal near-field antenna measurement range is being constructed. The Astrolink payload includes several apertures that must be tested in rapid succession and with great accuracy.
In order for such a facility to appear, much planning for building locations and project support must occur. This has been the case as a new building annex is being constructed to house this near-field range. Consideration for assembly and integration are key as well as the flexibility for growth contingency.
We have seen a marked rise in the number of horizontal planar near-field ranges within the last 5 years as satellite manufacturers have found that technique especially attractive for accuracy, throughput and placing the AUT in a uniform 1G environment. This technique is attractive to us especially if weight offloading devices are required to place the AUT in a zero gravity configuration.
This horizontal near-field range will be located in a newly constructed annex to the Building M1/M2’s Satellite Space Vehicle Production Facility. The annex will be immediately adjacent to a high bay area that will be used to integrate and test the rest of the Astrolink payload system.
The range occupies one end of the building with the remainder of the annex dedicated to the assembly and integration of the Antenna payload. The temperature will be controlled to ±2°F to minimize measurement error due to temperature drift. Of the six temperature sensors in the room, we will average the four surrounding the antenna range for feedback control.
A canopy with microwave absorber will be constructed above the range with an integrated deluge sprinkler system connected to a VESDA type fire detection system. A protective net will be installed just underneath the absorber to protect the test articles from any absorber pieces that may come loose over time. This protective netting is RF transparent and is strong enough to hold detached absorber.
An overhead 5 ton crane is available for use over the assembly and integration area.
The steel support frame for the near-field scanner was designed by Stephen Woolley & Associates and W.E. O’neil Construction Company is providing the construction and installation of this structure. The steel support frames were designed to have no more than 0.005" of deflection as the scanner travels back and forth upon it. Cross bracing the support frame adds needed rigidity and earthquake safety. A mounting platform will be provided for the Z-plane laser and the X-Y axis laser interferometer package.
This support frame has catwalks on 3 sides of the scanner and access is via a ladder. A swing out jib crane is available to lift heavy items up onto the catwalk as required. It has a 500 pound capacity.
The near-field system, which includes the scanner, PC system, software and RF hardware, is provided by NSI (Nearfield Systems Inc.).
The new range must provide the capability of measuring electrically large aperture antennas, common to many spacecraft payloads. The ranges will be required to accommodate antennas when mounted to spacecraft simulators or the actual spacecraft hardware itself. It is a PC based system and will support raster and plane polar scanning for data acquisition.
Mechanical Subsystem - Scanner
The 40 ft. x 30 ft. scanner is shown in Figure 4, as it will be mounted on top of the steel support structure in the satellite test facility. The scanner axes are driven by stepper motors: one motor for the 30’ Y-axis, two motors to drive either side of the Y-axis bridge over the 40 ft. X-travel, one motor for the 40 in. Z-axis travel, and various additional motors for POL-axis, Probe Tilt and Z-Retroreflector tracking. The stepper motors are powered by the Antenna Range Controller (ARC) box. This box contains the motor drivers and receives the computer commands. The scanner has 6-axes of motion. In addition to the X, Y, Probe Z and Probe Roll axes, there are two additional remotely controlled Probe motion stages that provide an additional Probe Az and Probe El motion. The linear Z-axis translator allows the probe to move in and out along Z. This allows the operator to easily check for probe/AUT interaction. A rotation stage is also provided that allows the system to take dual polarization data. This is essential to check cross-polarization levels. These two stages are also stepper motor driven.
The probe tilt motor stages will allow the scanner to take tilted plane scans with arbitrary probe tilt.
The scanner will be mated to the TRW provided steel supports and is leveled with fixators.
NSI also added two of their laser optics systems for increased accuracy. The first system uses laser interferometers for X and Y position feedback and optical sensors for straightness measurement of the X and Y axes (Patent No. 5,408,318). The second system is a spinning Z-plane laser and sensor that can monitor the out-of-plane motion of the probe. This can be run either in an active (probe travel is corrected on the fly) or passive mode (interpolated data is used to perform phase correction).
There are two PC's that are provided with the system: a measurement workstation PC and a data processing PC. In addition to these PC's, an HP Unix workstation will be available to perform other types of analysis related to the testing of the various types of antennas.
Both the measurement workstation and processing computers are Pentium PC’s running at 600 Mhz. The processing PC does not contain any interfaces to the range system. It is networked to the measurement workstation PC to increase range productivity including additional capability for processing data files. The measurement workstation PC controls the receiver, synthesizers, scanner motion control and laser optics. The lasers are connected to the PC via a DSP card which also handles the timing logistics during a near-field measurement.
The NSI software is their latest NSI97 version, which is a Windows 2000-based system. TRW is using its file transfer utility, which will convert the processed far-field data to the appropriate TRW format for further processing on UNIX workstations.
The NSI software includes a homogeneous coordinate transform for the plane-polar scanning, which requires simultaneous motion of up to 3 scanner axes (see Figure 10).
NSI has included their Motion Tracking Interferometer (MTI®) (Patent No. 5,419,631) which will actually correct solid-body rotation errors in the XY plane due to thermal or seismic events.
The RF equipment, shown in Figure 7, is comprised of an NSI high speed Panther receiver with dual Keysight VXI synthesizers and remote mixers. This configuration allows multi-frequency, multi-beam scanning with frequency, AUT ports, or polarization in the inner loop. The Panther receiver is capable of up to 80,000 amplitude and phase measurements per second using the NSI High Speed Beam Controller (HSBC), which is used to control the sequencing of frequency, AUT port and polarization steps (see figure 8). The Panther receiver is capable of measuring 68 dB of dynamic range with a sensitivity of -90 dBm (single sample). The Panther receiver is interfaced directly to the Windows PC using a plug-in DSP card. The VXI synthesizers are interfaced to the PC using an IEEE-1394 Firewire card. An HP 16-port PIN diode switch is also provided to allow multi-port AUT measurements on the fly. The NSI software is flexible enough to place any frequency, AUT port or polarization in any loop position in the scan generator. The HSBC is capable of up to 9,000 arbitrary measurements at each position trigger with a timing resolution of one (1) microsecond.
The mixers are Keysight H50 coaxial mixers that provide performance to 50 Ghz. Frequencies above 20 Ghz will require the use of active multipliers.
The probes initially provided with the range are twelve (12) open-ended waveguide probes and four (4) dual pol probes. Additional dual port probes can be provided later by individual projects that use the range per their specific requirements.
RF Cabling for the range is a combination of Andrews Heliax® cable for the fixed sections and flexible Gore cables for the moving portions. The cable movement is handled via cable tracks - one for the X-axis and two for the Y-axis.
RF absorber is provided on the ceiling of the range and on the side walls 10’ down. Mobile absorber panels will be available to move around the AUT to reduce multipath effects if required. All steel support structure will be covered with absorber as well. The NSI scanner itself has easily removable absorber panels integrated into its design to make installation simple (see Figure 9).
Since the AUT (Antenna Under Test) will be in a "Cup Up" orientation, range tooling will be built to accommodate this configuration. Special considerations to make the support tooling stiff were given to the design of the equipment. In order to check range multipath effects, adjustments for AUT height will also be available.
Alignment in the Range
The AUT must be aligned to the range in two directions: theta and phi. The theta angle of zero degrees is orthogonal to gravity. Therefore, AUT alignment to the scanner, which itself is aligned to gravity is accomplished with a precision level or by Theodolites. Phi rotation alignment brings the AUT axes into alignment with the scanner’s X and Y axes. The scanner’s X and Y axes are aligned orthogonal to one another at installation. The phi alignment of the AUT is performed by autocollimating a Theodolite on an optical cube attached to the AUT during the integration of the AUT. The alignment technician then rotates the AUT until it is brought into alignment with the Theodolite and hence, the scanner.
After the installation of the near-field system is accomplished, NSI will perform its own validation tests on the range. In addition to their own procedure, we will also require them to measure an antenna with known performance characteristics as a check of the overall performance integrity. Standard self-comparison testing will of course be performed to evaluate the error budget for the ranges.
A description of the new TRW near-field range facility has been described. We look forward to using this range to its full potential while examining methods for increasing throughput. The increased speed and efficiency of this new system will greatly improve our ability to measure many antennas, acquire the large amounts of data required by each program and fulfill the aggressive testing schedules required of new projects.
The authors would like to thank Mitch Otera and Rob Tyrrell for their managerial and procurement support throughout this project and Greg Hindman of NSI for the detailed information regarding his company's near-field antenna ranges.
 Fooshe, D. and Slater, D., Digital Receiver Technology for High-Speed Near-field Antenna Measurements, 1999 AMTA Symposium Proceedings, p. 394
 McCabe, B. and Williams, B., Validation of a 40’ by 22’ Nearfield Range at Hughes Space and Communications Company, 1998 AMTA Symposium Proceedings, p. 259
 Way, J., HSC’s New Near-field Measurement Facility, 1996 AMTA Symposium Proceedings, p. 230
 Hindman, G. and Masters, G., Implementation of a 22’ x 22’ Planar Near-field System for Satellite Antenna Measurements, 1993 AMTA Symposium Proceedings, p. 173
 Slater, D., Near-Field Antenna Measurements, Artech House, Norwood, MA, 1991
 A.C. Newell, Error Analysis Techniques for Planar Near-field Measurements, IEEE Trans. A/P; Vol. AP-36, No. 6, June 1988
Figure 1 TRW Building M1 Annex Plan View
Bottom left is the integration area, Top left is the near-field range area with isolated seismic slab shown. To the right is an airlock for ingress and egress from M1.
Figure 2 TRW Horizontal Near-field Facility
Figure 3 TRW Horizontal Near-field Facility
Figure 4. TRW 40x30 Scanner Mounted on Support Structure
Figure 5. X-beam at NSI Factory
Figure 6. Range Y-Bridge at NSI weld shop (placeholder)
Figure 7. RF Subsystem @ NSI Factory
Figure 8. NSI Panther Receiver and High Speed Beam Controller
Figure 9. Absorber Panel Mounting on Bridge
Figure 10. Homogeneous Coordinate Transform Provides Plane Polar Scan Capability