Motion system adds antenna accuracy
Positioning radio antennas requires a motion control system with high repeatability and minimal backlash. Nearfield Systems Inc. (NSI), located near Los Angeles, manufactures equipment used to measure performance characteristics of radio antennas, including antennas used for personal communications systems, military radar and communications, and spacecraft antennas. For a recent model, NSI used a unique motion control system from Nexen Group, located in Vadnais Heights, MN.
Nearfield designed the Spherical Near-field Arch Roll Scanner, model NSI-700S-300, to measure the radiation characteristics of arrays emitted from antennas that require full-hemisphere coverage. To allow the sensitive antennas to remain stationary throughout the testing process, the system required a sensing probe that could travel above a hyper-hemispheric surface surrounding the antenna.
“We required a motion control system that would provide high repeatability and minimal backlash,” said Hulean Tyler, a mechanical designer and project manager at NSI. “Our system samples energy in the air. We need extremely accurate positional repeatability. If the probe doesn’t return to the exact same position in which it started, it will result in a different measurement.
“To build this system, we needed a very long curved rack on the theta access,” Tyler said. “Standard rack vendors told us they would construct a straight rack and then bend it to meet our requirements. However, that didn’t seem like an efficient method of achieving precision and repeatability.”
Tyler contacted Nexen Group, a manufacturer of precision motion control solutions, after seeing the Nexen Roller Pinion Gear (RPG) system tested at a trade show several years earlier.
Nexen’s RPG system has a ring gear and roller pinion technology to maintain accurate positioning at peripheral speeds as high as 11 m/sec (36.1 ft/sec). Even at these high speeds, the extremely low friction design does not create heat or wear on the components. The RPG system has bearing supported rollers that move smoothly along the face of each tooth. By maintaining opposing contact with two or more teeth at all times, the RPG system delivers backlash-free performance in both directions. Solid and segmented ring gears (up to 1 m dia.) provide easy integration and unlimited diameters. (Ring gears larger than this become segmented and can be constructed in virtually any diameter.) This flexibility met NSI’s constraints.
“We were locked into a very tight room,” Tyler said. “The diameters couldn’t change, so the ability to create custom diameters—that were large enough to meet our requirements—was very important.”
With the new system, the NSI-700S-300 dual-axis probe motion design delivers high-accuracy electromagnetic field measurement beyond ±90 degrees—at high speeds and while the antenna remains stationary, said to be ideal for wide-angle satellite and radar antenna testing. These devices can’t be tested with a planar near-field technique, because this scanning method does not allow reliable side-lobe coverage beyond approximately ±75 degrees. While traditional spherical near-field systems solve this limitation, they require antennas to move along either one or two axes, which can prove difficult for large and sensitive satellites and antennas.
Instead, the NSI-700S-300’s probe is precisely positioned by the RPG and augmented by dynamic motion compensation. The arch rotates at 30 degrees per second throughout the measurement process. The near-field probe detects the fields radiated by and in close proximity to the antenna. A spherical wave expansion allows for determination of far-field data, providing total characterization of the antenna’s performance—including antenna gain, side-lobe structure, beam pointing, and cross polarization.
The stationary antenna testing system simplifies setup and costs. It eliminates the need for expensive slip-ring assemblies to the antenna, and it simplifies the alignment procedures. The equipment can be simply set up in the manufacturing assembly area and rolled to the test range area. The NSI-700S-300 also allows the antenna to be measured in the same orientation in which it will be used, so the flexure that results from a changing gravity vector does not distort the measurements.
The first system was installed in September 2009 and the second in January 2010.
Nexen engineers produced drawings and checked the system, Tyler said. Responses to the first installations have been positive, from other NSI customers impressed with the “overall capabilities and measurement accuracy of the scanner,” said Jesus Aguilar, regional sales manager at NSI.
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