Measuring antenna patterns can be difficult at any frequency--reflections can distort patterns and the antenna position often has to be changed to fully characterize it and the shorter wavelengths at microwave frequencies require more precise positioning. Now a team of researchers at the National Institute of Standards and Technology Physical Measurement Laboratory (PML) has developed a way to measure a probe antenna's position to within 50 micrometers while it is moving. This accuracy is required to calibrate antennas for use in the 50 to 500 GHz band, a region of the spectrum already being used by earth observation satellites for climate studies and imaging--the water vapor signal at 183 GHz for example —and at which the lower end of the band is seeing increasing use for short-range telecommunications as lower frequencies become more congested.
Team member David Novotny of the Radio-Frequency Fields Group in PML's Electromagnetics Division observed that currently no international reference standards exist for antennas at such frequencies, leaving researchers with no way to compare and validate measurements at different facilities. He hoped that his facility would be able “to measure probe antennas that are then used by others to characterize systems to a common baseline.”
The system the PML team developed uses a precision three-meter industrial robot arm combined with a metrology-grade laser tracker and other apparatus to provide accuracy within 50 micrometers. One of the goals of ongoing research with the new technology is to improve its accuracy.
Team member Jason Coder explained “your allowable error is proportional to the wavelength.”
At 500 GHz (a wavelength of approximately 0.6 mm), it’s necessary to know the probe position to within 15 to 20 micrometers and to ensure that the probe is normal to the scan surface within 0.05 degrees. Moreover, a comprehensive characterization will require knowledge of the exact relative distance between the antennas as well as the relative alignment of their apertures.
Coder added that the robot-arm/laser tracker system is particularly well suited to instrument clusters for orbiting satellites.
“Often the chief item of interest isn’t the antenna and its circuitry per se, but how multiple systems perform in concert and we can accommodate a much larger test payload than a single antenna,” said Coder. “With the new facility, if a customer was interested, we could put the entire small satellite — up to two meters in diameter — in here and characterize the system as a whole, which would be a huge advantage.”
For more information and pictures of the measurement system see High-Tech Measurements for High-Frequency Antennas