Techniques for Characterizing Microwave Printed Antennas
- Created: Monday, 01 December 2003
Slot-line and other printed antennas can be characterized quickly and inexpensively.
The combination of a de-embedding technique and a direct on-substrate measurement technique has been devised to enable measurement of the electrical characteristics (impedances, scattering parameters, and gains) of microwave printed antennas that may be formed integrally with feed networks that include slot lines, coplanar striplines, and/or coplanar waveguides. The combination of techniques eliminates the need for custom test fixtures, including transitions between (1) coaxial or waveguide feed lines in typical test equipment and (2) the planar waveguide structures of the printed circuits under test. The combination of techniques can be expected to be especially useful for rapid, inexpensive, and accurate characterization of antennas for miniature wireless communication units that operate at frequencies from a few to tens of gigahertz.
Both techniques involve the use of an automatic network analyzer (ANA) coupled with a wafer probe station and a pair of ground-signal microwave probes. The de-embedding technique includes the established through-reflect-line (TRL) calibration technique, which involves the fabrication and testing of standard calibration structures on the same substrate alongside an antenna that one seeks to characterize (for example, see Figure 1). The dimensions of the calibration structures are related to those of the antenna in a predetermined way. For the TRL measurements, the ANA is operated under the control of de-embedding software developed by the National Institute of Standards and Technology (NIST). This software processes the TRL measurement data to establish an electrical reference plane in the antenna, to which plane all de-embedded scattering parameters and impedances are meant to be referred. Thereafter, the de-embedding software can be used to obtain the reference-plane characteristics of the antenna from ANA measurements taken at the input terminals of the slot-line feed.
The direct on-substrate measurement technique involves the calibration of the ground-signal microwave probes to their tips. This calibration is done by use of the ANA with an open circuit, a short circuit, and a matched load as standards. The standards for direct measurements are provided by the probe manufacturer on an impedance standard substrate. Calibrated probes are then put in contact with the input terminals of the slot-line feed of the antenna, and the antenna is excited via the probes. The direct on-substrate measurement technique is best suited for designs in which the slot-line feeds are short enough to interfere only minimally with the probes.
Figure 2 depicts a setup for measuring the absolute gain of a pair of identical antennas. In this case, the antenna input and output measurements are made by the direct on-substrate probe measurement technique, the antennas are oriented facing each other and matched in polarization, and the antennas are placed far enough apart that, to a close approximation, far-field radiation conditions prevail. The gain is calculated from the geometric parameters of the setup, and from the transmitted and received power levels measured by the ANA via the probes connected to the terminals of the transmitting and receiving antennas, respectively.
This work was done by Rainee Simons of NYMA/Federal Data Corp. and Richard Q. Lee of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Computers/Electronics category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4—8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17040.