Frequency-tunable orthogonally-fed elliptical slot imaging array

An orthogonally-fed, small, resonant elliptical slot, loaded by a varactor diode, is designed through full-wave simulation for use in synthetic aperture radar imaging arrays. The varactor diode provides electronic control over the slot’s resonant frequency, allowing it to be tuned over a few GHz within the K-band (18-26.5 GHz) frequency range. The final slot design employs two varactor diodes resulting in 3.5 GHz of tunable bandwidth. The broadest dimensions of the antenna are restricted to being less than λ/2 as a consequence of the required spacing in a 2D imaging array. The feed design uses a coplanar waveguide (CPW) with a 90° bend, RF coupled to the antenna. The manufactured slot is tested using a vector network analyzer (VNA) showing good agreement with the simulation results. Since the feed line is a CPW, one can readily incorporate a commercial surface-mount switch onto the feed board, allowing a single input port to be connected to four antenna elements in the array. Two four-element arrays are manufactured and a one-sided imaging apparatus is formed by properly aligning them. Each array is connected to one port of a two-port VNA, and they are raster scanned over several test samples. Reflection measurements (using one array) and transmission measurements (i.e., bi-static; from one array to the other) are made during the scans, along with some reference measurements. A correction method is developed and implemented to provide a common phase reference between frequencies. The characteristics of the switch dominate the reflection measurements, although this effect is mitigated by applying a proper correction algorithm. The system works best in transmission (bi-static) mode. The data is processed to generate 3D holographic SAR images showing indications of the targets in several test samples.