Comparison of Material Measurement Accuracy of RF Spot Probes to a Lens-Based Focused Beam System

Free-Space Focused Beam Technique

This paper is directly comparing the materials measurement accuracy of a standard laboratory-focused beam system to a measurement system based on recently developed RF spot probes.

A popular method for microwave characterization of materials is the free-space focused beam technique, which uses lenses or shaped reflectors to focus energy onto a confined region of a material specimen. In the 4-18 GHz band, 60 cm diameter lenses are typically spaced 30 to 90 cm from the specimen under test to form a Gaussian beam with plane-wave characteristics at the focal point. This method has proved popular because of its accuracy and flexibility. 

Alternate Technique 

Another free-space measurement technique employed by some uses dielectrically loaded antennas placed near a specimen. In this alternate technique, the dielectrically loaded antennas are smaller than lenses, making the hardware more compact and lower cost, however, this is done at the expense of potentially reduced accuracy. 

The Difference Between a Laboratory-Focused Beam System and RF Spot Probes 

The spot probes are specially designed antennas encapsulated in a dielectric and optimized to provide a small illumination spot 3 to 8 cm in front of the probe. Several dielectric, magnetic, and resistive specimens were measured by both systems for direct comparison. With these data, uncertainty analysis comparisons were made for both fixtures to establish measurement limits and capability differences between the two methods. Understanding these uncertainties and measurement limits is key to implementing spot probes in a manufacturing setting for quality assurance applications.