Microwave Journal
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Online Spotlight: The MIT Lincoln Laboratory RF Systems Test Facility for Rapid Prototyping

March 7, 2021

The current capabilities of the RF Systems Test Facility at MIT Lincoln Laboratory for rapid prototyping and measurements of antennas, radar targets and electromagnetic systems are described. The facility has six shielded anechoic chambers consisting of a low frequency tapered chamber, mmwave chamber, small near-field scanner, system test chamber, compact range and large near-field scanner. An on-site machine shop, RF laboratory and system integration laboratory have significant roles in the rapid prototyping process at the RFSTF. RF measurements of phased arrays and other types of RF measurements are described.

I. INTRODUCTION

The MIT Lincoln Laboratory RF Systems Test Facility (RFSTF) depicted in Figure 1 is a research and development rapid prototyping facility with resources to design, fabricate and measure antennas, radar targets and electromagnetic systems for surface, airborne and space applications. The initial operating capability of the RFSTF was established in 2004 with four anechoic chambers as previously described.1-3 Since then, two planar near-field scanning anechoic chambers and spherical near-field scanning capability have been added to the facility. The RFSTF currently comprises six anechoic chambers, a system integration laboratory (SIL), high-bay staging area/rapid prototype shop, and the RF Laboratory. The RFSTF is co-located with the MIT Lincoln Laboratory Flight Test Facility, which allows rapid integration of RF sensors with airborne platforms.

The six shielded anechoic chambers (tapered, mmwave, small near-field scanner, system test, compact range and large near-field scanner) enable antenna, radar cross section (RCS), and electromagnetic systems measurements over a wide frequency range. The three large system test, compact range and large planar near-field chambers can accommodate large, heavy, test articles, making use of an overhead crane (system test chamber), rolling gantry with crane (compact range chamber), and reconfigurable rolling cart (large near-field chamber). The system test rectangular chamber provides far-field and spherical near-field scanning capabilities up to 20 GHz. Antenna and RCS measurements are performed in the compact range from UHF up to 100 GHz with a blended rolled-edge reflector and multiple feed horns. The large near-field scanner chamber is configured to calibrate and measure gain radiation patterns of large phased array antenna apertures up to 50 GHz. In the tapered chamber, a dual-polarized ultrawideband feed allows measurements from 250 MHz up to 3 GHz, and higher frequencies. The rectangular mmwave chamber operates up to 100 GHz. The small near-field scanner chamber is used primarily for calibrating and testing small phased array antenna panels or reflector antennas up to 26 GHz. Rapid prototyping of antennas, radar targets and electromagnetic systems is facilitated with electromagnetic simulation software.

This article is organized as follows. Sections II to VII describe each of the RFSTF chambers. Example RFSTF measurements of phased arrays and other antennas are described. Section VIII describes the rapid prototyping shop, RF lab and system integration lab. Section IX is a summary.

Figure 1

Figure 1 RF Systems Test Facility at MIT Lincoln laboratory.3

II. LOW FREQUENCY TAPERED CHAMBER

The RFSTF low frequency tapered chamber (see Figure 2) is EMI shielded and has a range length of R = 8.5 m to the center of the test zone, with a rectangular section 4.3 by 4.3 m in cross section by 5.1 m in length. The tapered chamber uses an ultrawideband dual-polarized conical tip feed4 developed by the Ohio State University that is designed to cover the 250 MHz to 3 GHz band and higher frequencies. The walls of the tapered chamber are covered with wedge absorber varying in height of 20 cm near the conical tip to 46 cm approaching the test zone. The test zone region uses 0.6 m pyramidal absorber. The back wall is covered with 1.8 m pyramidal absorber. The shielded conical tip region is 1.8 m in length with a 30 degree full cone angle. The conical tip houses two wideband 180 degree hybrids that provide a balanced feed to an orthogonal pair of ultrawideband flared launchers. Wedge absorbers provide a smooth transition toward the dual-polarization feed region. A custom RF switch box is used to manually control the feed excitation for vertical or horizontal polarization.

Figure 2

Figure 2 RFSTF tapered chamber with L-band 3D printed and copper plated conformal array antenna under test.5 The wideband dual-polarized feed can be seen at the conical tip.3

The tapered chamber antenna under test (AUT) positioner consists, in part, of a 0.61 m manual linear slide along the boresight axis over a lower azimuth positioner. A mast is mounted above the linear slide and an azimuth (polarization) over elevation positioner is mounted at the top of this mast. In some measurements requiring only azimuth rotation, the AUT is supported on a foam column mounted above the linear slide. Gain pattern measurements of the 3D printed copper plated patch array (see Figure 2) have previously been described.5

III. MMWAVE CHAMBER

The mmwave rectangular anechoic chamber (see Figure 3) is EMI shielded and is used for testing small antennas (typically feed horns for reflector antennas) and small arrays from approximately 4 to 100 GHz. The chamber has interior dimensions of 9.1 m (length) by 6.1 m (width) by 5.5 m (height) and a range length of R = 6.7 m between the source antenna and the AUT.

Figure 3

Figure 3 RFSTF mmwave chamber with horn AUT.

To achieve good RF performance up to 100 GHz, the central portions of the mmwave chamber walls, floor and ceiling are covered with unpainted 30.48 cm pyramidal absorber. Outside the central region, a blue-painted 20.32 cm pyramidal absorber is used. The AUT positioner is a polarization-over-elevation positioner mounted on a tower attached to a 0.61 m manual linear slide along the boresight axis over an azimuth positioner, which is the same configuration used in the RFSTF tapered chamber. The AUT positioner is optically aligned to facilitate accurate measurements of mmwave antennas. A tracking laser interferometer was used in the optical alignment process to align the lower azimuth, slide, elevation and polarization positioners. To maintain a fully shielded chamber, the source antenna horn is located in a shielded metal box with the open end facing the AUT. The source antenna is accessed from the control room and its position is adjusted in range and height with a dual-slide mechanism.

IV. SMALL NEAR-FIELD SCANNER

In the small near-field chamber, a 1.52 m by 1.52 m vertical planar near-field scanner (Nearfield Systems, Inc.) is typically used to calibrate and measure gain radiation patterns of small phased array aperture antennas. The scanner has a four-axis automated control for x, y, z Cartesian coordinates and probe polarization for vertical and horizontal polarization measurements. The scanner is in a shielded anechoic chamber with interior dimensions of 3.7 by 3.7 by 3.7 m, with the walls, floor and ceiling covered with 30.5 cm pyramidal absorber. Figure 4 is a photograph of a rectangular waveguide probe positioned in front of an S-band phased array of dual-polarized square patch antennas.

Figure 4

Figure 4 RFSTF small near-field scanner chamber with S-band dual-polarized phased array microstrip patch antenna panel under test.3

The probe antenna and vertical planar Cartesian coordinate scanner are surrounded by pyramidal absorber. The probe moves parallel to the array face, horizontally in the x direction and vertically in the y direction; and it moves perpendicular to the array face in the z direction as well. The rectangular probe shown is linearly polarized and is rotated by means of a polarization positioner to measure the voltage response to horizontal and vertical electric-field components for the AUT. The radiation pattern of the near-field probe with surrounding absorber is pre-measured and used to compensate and generate the far-field radiation pattern of the AUT. The antenna positioner in this chamber is also designed to rotate in azimuth to allow cylindrical near-field scanning. In this case, the probe scanner moves the near-field probe in the y direction and the AUT is rotated in the azimuth angle φ.

V. SYSTEM TEST CHAMBER

The general-purpose rectangular System Test Chamber, shown in Figure 5, is 18.3 m long by 12.2 m wide by 10.7 m high. It is RF shielded and covered uniformly with 1.22 m pyramidal absorber for RF testing typically from 150 MHz to 20 GHz. This chamber is used, in part, for standard far-field antenna testing and spherical near-field testing using an MI Technologies control and software system. A dipole array AUT and source antenna tower are also shown in Figure 5. The AUT positioner consists of an azimuth over elevation positioner on a tower mounted on a manual linear slide above a lower azimuth positioner over an automatic linear slide. The source tower is a linear positioner with a lifting capacity of 385 kg and has a polarization positioner with a DC to 26.5 GHz rotary joint.

Figure 5

Figure 5 System Test Chamber with source tower and dipole array AUT.

To achieve accurate spherical near-field measurements, the coordinate systems of the source positioner and AUT positioner are laser aligned. The RF measurements system includes a 16-channel 0.1 to 20 GHz SP16T switch (multiplexer) that is used to collect measured AUT data from multiple antenna elements or from subarrays.

The facility is designed so that ground vehicles can be driven into the chamber for RF electromagnetic interference (EMI) testing and other RF measurements (see Figure 6). Pipe penetrations through one of the chamber walls are provided to allow vehicle exhaust gases to be removed from the anechoic chamber. The pipe penetrations can be closed to maintain full chamber RF shielding when not in use. Additionally, air vehicles (see Figure 7)6 or other test articles such as large radome panels (see Figure 8) can be suspended from an overhead crane for various RF measurements. For insertion loss measurements, the radome panel is first suspended well above the transmit and receive horns and then a baseline S21 measurement is made. The radome panel is then lowered between the two horns and relative insertion loss is then measured.

Figure 6

Figure 6 EMI testing in the system test chamber.3

Figure 7

Figure 7 Air vehicle with belly-mounted ultrawideband VHF/UHF dipole array antenna and tail-mounted dipole suspended from the overhead crane during RF mutual coupling measurements in the system test chamber.6

Figure 8

Figure 8 Radome panel insertion loss measurements setup at C-band utilizing the overhead crane in the system test chamber.3



VI. ROLLED-EDGE COMPACT RANGE

The RFSTF compact range facility shown in Figure 9 has an EMI shielded anechoic chamber with interior dimensions of 20.1 m (length) by 13.4 m (width) by 11.6 m (height). The adjacent antenna/target handling room (antechamber) is 9.1 m long by 13.4 m wide by 11.6 m high. Electrically controlled large double doors provide a test article access opening 9.1 m high by 4.7 m wide. The anechoic chamber temperature is maintained in the range of approximately 20 to 22 °C to ensure a stable environment for maintaining the reflector surface shape and for RF stability of the chamber for RCS background subtraction.

Figure 9

Figure 9 Compact range with low RCS ogive positioner.

The RFSTF compact range reflector antenna was designed by the Ohio State University and uses a blended rolled-edge design1-3, 7-10 to reduce diffraction from the edges of the reflector, which improves target zone amplitude and phase characteristics. The reflector is offset-fed to reduce feed blockage effects. Pulse gating is used to reduce multipath and further improve target zone characteristics. The rolled-edge parabolic reflector, feed and target support structures are located on a common isolated concrete slab to mitigate potential vibration effects. The walls, floor and ceiling of the chamber are covered with 0.91 m pyramidal absorber between the reflector and feed locations. From the feed to the back wall (behind the test article positioner), wedge absorber is used, varying in height from 0.9 to 1.1 m in a Chebyshev pattern.11, 12 The back wall is covered with pyramidal absorber in a Chebyshev pattern with absorber height varying from 0.91 to 1.1 m.

The offset-fed blended rolled-edge reflector antenna was provided by MI Technologies, 9, 10 and it is a composite structure with a projected aperture of 7.3 by 7.3 m. The reflector has a central parabolic section 3.048 by 3.048 m with a blended section to generate a 3.7 by 3.7 by 3.7 m quiet zone. The focal length of the offset parabolic reflector is 7.315 m. The electrically conductive surface of the reflector is a silver-based paint applied in multiple layers. The measured surface resistivity of the reflector is 0.11 ohms/square maximum over the surface of the reflector, which provides a highly electrically conductive surface. Reflector surface accuracy measurements were performed using a tracking laser interferometer.10

A gantry with a crane and scissors lift is used to move personnel, equipment and test objects to and from the test positioner. The crane has a lifting capacity of 1134 kg. The scissors lift has a lifting capacity of 454 kg for personnel and equipment. The gantry is electrically powered and rides on two rails recessed in the floor spaced approximately 4.1 m apart. Stairs next to the gantry allow personnel to access the gantry in the antechamber. A ladder is attached permanently to the gantry to allow additional access inside the test chamber.

Seven dual linearly polarized feed antennas (see Figure 10) are used to illuminate the reflector in the frequency bands listed in Table I. The feed antennas are located on individual vertical slides on a platform that moves horizontally to allow the phase center of each horn to be placed at the focal point of the reflector. An absorber fence is placed behind the focal point feed location to reduce backlobe radiation from the feed into the target zone. A network analyzer-based signal source and receiver system with time gating, along with a 24-channel switch matrix, allows measurements of individual antenna elements in an array or of multiple subarrays for digital beamforming.

Figure 10

Figure 10 RFSTF compact range feeds. The 2 GHz to 18 GHz feed shown on the upper right has been moved up to the focal point of the reflector.

TABLE I
COMPACT RANGE DUAL-POLARIZED FEED ANTENNAS

Table 1

The antenna positioner equipment normally used in this chamber is configured as follows: A lower azimuth over elevation positioner is located in a 1.37 m deep pit with a diameter of 3.35 m. On top of this positioner is a work platform and spacers upon which an automated 1.8 m linear slide is mounted above the floor absorber. The upper polarization-over-elevation positioner is mounted on a fiberglass tower that is attached to the linear slide.

Figure 11 shows an example of an ultrawideband dipole array panel under test in the compact range. Once this antenna panel measurement was completed, the panel and two others, were then mounted on the belly of an aircraft on the Lincoln Laboratory Flight Test Facility side of the hangar.3, 6 For radar cross section and some antenna measurements, the slide positioner, fiberglass tower, and upper positioner are removed using the gantry crane, and the lower azimuth over elevation positioner is used with one of a number of foam columns or the low RCS metal ogive (see Figure 9) to support and rotate test articles. The metal ogive mount has a roll axis stinger on top of an azimuth/elevation positioner.

Figure 12 shows a prototype inflatable dual-reflector antenna with feed horn under test on the ogive mount. This dual-reflector antenna is designed to be fed with a horn or with a phased array to provide limited electronic steering for space applications.13 Figure 13 shows an ultrawideband dipole array under test on one of the foam columns.6 In some cases during antenna or RCS testing on foam columns, the article under test can be secured with an electrically non-conducting tether (rope or harness) that is passed through a temporary hole/tube in the chamber ceiling, in which the tether is rigidly attached to structure above the chamber.

Figure 11

Figure 11 Ultrawideband dipole array panel under test on the compact range antenna positioner.3, 6

Figure 12

Figure 12 Compact range with 2.4 m diameter inflatable dual-reflector antenna13 under test with an X-band feed horn on the ogive positioner.

Figure 13

Figure 13 Compact range with ultrawideband UHF dipole array AUT on a foam column.6

VII. LARGE NEAR-FIELD SCANNER

In the large near-field chamber, an Orbit/FR 5.5 by 5.5 m vertical planar near-field scanner is used to calibrate and measure gain radiation patterns of large phased array aperture antennas. Operation of this large scanner is similar to the small near-field scanner described in the previous section. The scanner has four-axis automated control for x, y, z Cartesian coordinates and probe polarization (vertical and horizontal polarization measurements). The anechoic chamber interior dimensions are approximately 9.1 m (length) by 8.2 m (width) by 7.6 m (height). An overhead crane installed in the chamber has a lifting capacity of 3,629 kg (8,000 pounds). The walls, floor, and ceiling are covered with 46 cm pyramidal absorber with unpainted tips. The scanner is optically aligned and designed to perform accurate scans with multiple near-field probes from about 400 MHz to 50 GHz. An antenna assembly area with overhead crane is located adjacent to the anechoic chamber. The test antenna is moved into and out of the chamber on a general-purpose wheeled cart mounted on rails.

An example of a large phased array antenna setup for RF measurements is shown in Figure 14. The near-field probe is moved in front of each antenna element and the element phase shifters are cycled through their states, to provide a phase calibration look up table followed by offline processing for far-field radiation patterns.14-16 A standard gain reference horn mounted near the upper left of the array aperture is used to provide accurate gain measurements.

Figure 14

Figure 14 RFSTF large near-field scanner chamber with S-band multifunction phased array radar antenna.14- 16

VIII. RAPID PROTOTYPING SHOP, RF LABORATORY, AND SIL

The rapid prototyping shop (see Figure 15) has a wide variety of machining tools and 3D printers to fabricate antennas, target-mounting fixtures, and other mechanical pieces necessary to aid and assist in development of new hardware and testing in the facility. The prototyping shop is located in a high-bay area with a large capacity overhead crane, allowing for a wide range of mechanical work to be performed on larger devices and systems. The RF Laboratory (see Figure 16) and SIL (see Figure 17) are general-purpose areas that can be configured to support the needs of many different projects including RF assembly and test as well as development of advanced signal processing techniques and algorithms. The SIL will typically contain racks with transmitters and receivers and is directly connected to the System Test Chamber through a cable trough system allowing transmit and receive signals to be passed to and from the AUT.

Figure 15

Figure 15 RFSTF rapid prototyping shop.

Figure 16

Figure 16 RF Laboratory at the RF Systems Test Facility.

Figure 17

Figure 17 Reconfigurable SIL, which is connected to the System Test Chamber for radar, communications and other electromagnetic systems testing.

IX. SUMMARY

The RF Systems Test Facility at MIT Lincoln Laboratory provides rapid prototyping and RF measurements capabilities for numerous types of antennas, radar targets and electromagnetic systems.17 The facility has six shielded anechoic chambers consisting of a low frequency tapered chamber, mmwave chamber, small near-field scanner, system test chamber, compact range and large near-field scanner. The on-site machine shop, RF lab and system integration lab have significant roles in the rapid prototyping process at the RFSTF.

ACKNOWLEDGMENT

Technical discussions with Jeffrey S. Herd, Michael W. Shields, Paul A. Theophelakes, Edward D. Martin, M. David Conway, and Alexander F. Morris are sincerely appreciated.

This material is based on work supported by the United States Air Force under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the United States Air Force.

References

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