This paper directly compares a laboratory focused beam system to an alternative measurement system based on recently developed RF spot probes.
In a laboratory, microwave characterization of materials is often accomplished with a free-space focused beam, which uses either lenses or shaped reflectors to focus RF energy onto a specimen. For the 2-20 GHz band, 60 cm diameter lenses can be spaced 77 cm from the specimen to form a Gaussian beam, simulating a plane-wave at the specimen location. An alternative method uses dielectrically loaded antennas near a specimen, which is a more compact and lower cost fixture. That said, the probe method has the disadvantage of slightly reduced accuracy. These are specially designed antennas encapsulated in a dielectric and optimized to provide a small illumination spot 7 to 8 cm in front of the probe. The resulting comparisons show that the spot probe method can be ‘almost’ as good as the higher fidelity, laboratory focused beam method.
