This paper describes the experience of using a tracking laser interferometer to align and characterize the planarity of a planar near-field scanner. Last year, The Aerospace Corporation moved their planar near-field antenna range into a new larger room with improved environmental controls. After this move, the near-field scanner required careful alignment and characterization. The quality of the scanner is judged by how accurately the probe scans over a planar surface. The initial effort to align the scanner used a large granite block as a planarity reference surface and cumbersome mechanical probe measurements. However, a tracking laser interferometer was used for the final alignment and characterization.
The laser interferometer was included as part of an alignment service purchased from MI Technologies. The tracking laser interferometer emits a laser beam to a mirrored target called an SMR (Spherically Mounted Retroreflector). Encoders in the tracker measure the horizontal and vertical angles while the laser interferometer measures the distance. From these measurements, the three-dimensional SMR location is determined. The laser has the ability to very accurately (within about 0.001 inch) measure the location of the scanning near-field probe.
This paper includes a description of the mechanical alignment of the scanner, the tracking laser interferometer measurements, and the final planarity characterization.
We are witnessing an explosive expansion of RF transmitting and receiving products due to consumer demand for wireless voice and data connectivity and the availability of cost-effective technology to produce such products and services. Developers of these new products are keenly interested in the pattern, gain and polarization of their products. Traditional antenna measurement equipment can provide the needed information but usually at a prohibitively high price since such test equipment was designed for general purpose, very high precision aerospace applications. Many of the features needed in aerospace antenna measurements are not required in wireless applications. For example, aerospace measurements are often made at 35 or 95 GHz while wireless communication devices often work at 800 or 1,900 MHz. In addition, phase and amplitude measurements are made in the aerospace applications while wireless measurements usually only require amplitude information.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations (,)zxyδcan be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence.
This paper describes a method for estimating the Z-position error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
1125 Satellite Blvd. NW,
Ste. 100
Suwanee, GA 30024 USA
+1 678 475 8300
+1 678 542 2601
19730 Magellan Dr.
Torrance, CA 90502 USA
+1 310 525 7000
+1 310 525 7100
C/O AMETEK LAND,
Stubley Lane,
Dronfield, S18 1DJ UK
+44 1246 581500
www.nsi-mi.co.uk
