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In this paper, the design and implementation of a compact one-port Ka-band (26.5-40 GHz) vector reflectometer or network analyzer (VNA), operating in full Ka-band frequency range, are presented. The design follows that of a recently developed VNA but with the goal of producing a wideband version. The most critical component of this design is a wideband electronically controlled phase shifter, designed using nonuniformly spaced p-i-n diode-loaded slots incorporated in the broad wall of a rectangular waveguide. Nonuniform placement of the slots is shown to significantly reduce unwanted resonances within the operating bandwidth. Also, the new design provides the possibility of using a more accurate three-port S-parameter characterization instead of the previously used two-port approach. The design foundation and measurement results are presented.Index Terms-Microwave measurements, millimeter-wave imaging, millimeter-wave measurements, phase shifters, reflectometry.

Recent advances in several technological fronts have made it possible to design and develop real-time microwave imaging systems (cameras) for use in a wide range of applications. Proper design of the individual antenna array element in these systems is of critical importance. Furthermore, wideband versions of these imaging systems, which are capable of one-sided operation and in conjunction with advanced synthetic aperture radar imaging algorithms, can produce high-resolution 3-D holographical images. In this paper, an orthogonally fed, small elliptical dual varactor-loaded slot antenna, operating in K-band (18-26.5 GHz), is designed for use in such imaging systems. The feed design uses a coplanar waveguide with a 90°bend without a direct dc connection to the antenna. This feed structure accommodates effective placement of required switches while enabling one-sided measurement. Finally, the slot is loaded with two varactor diodes resulting in 3.5 GHz of bandwidth suitable for high-resolution microwave 3-D holographical imaging. The design of this slot and a (1/4) array of such slots is presented. Two arrays are fabricated and used independently, as well as simultaneously, to form (mono-and bistatic, respectively) imaging results.Index Terms-Antenna array feeds, antenna arrays, microwave imaging, millimeter wave imaging, nondestructive testing (NDT), synthetic aperture imaging.