A novel differential probe design is introduced in this paper for near-field microwave and millimeter wave non-destructive testing (NDT) and imaging applications. In such applications, the variations in the distance between the probing antenna and the structure under inspection, i.e., standoff distance, can potentially mask the signal of interest, and hence, adversely impact the detection capability of the probe. Differential near-field probes and compensation methods were developed in the past to null out the standoff distance variation effect from the measured signal. The available methods, however, suffer from some limitations such as using two balanced apertures or offering limited range of compensation. While the differential probe proposed here exhibits an excellent immunity against standoff distance variation, it overcomes the limitations of the aforementioned methods. The proposed probe is based on electronically modulating the aperture of a rectangular waveguide using PIN diode-loaded dipoles placed symmetrically in the aperture region. It will be shown that the adverse effect of standoff distance variation can be eliminated, or otherwise, significantly reduced by non-coherently subtracting the signals measured at two diferent aperture modulation states.

Meeting Name

IEEE International Instrumentation and Measurement Technology Conference (2008: May 12-15, Victoria, British Columbia, Canada)


Electrical and Computer Engineering

Keywords and Phrases

Computer Networks; Instruments; Measurement Theory; Microwaves; Millimeter Wave Devices; Millimeter Waves; Nondestructive Examination; Semiconductor Diodes; Differential Probes; Loaded Dipoles; Microwave And Millimeter Wave NDT; Modulated Apertures; Near-Field Imaging; Standoff Distance Variations; Electromagnetic Waves

International Standard Book Number (ISBN)


International Standard Serial Number (ISSN)


Document Type

Article - Conference proceedings

Document Version

Final Version

File Type





© 2008 Institute of Electrical and Electronics Engineers (IEEE), All rights reserved.

Publication Date

01 May 2008