This compact, lightweight, dual- frequency antenna feed developed for future soil moisture and sea surface salinity (SSS) missions can benefit future soil and ocean studies by lowering mass, volume, and cost of the antenna system. It also allows for airborne soil moisture and salinity remote sensors operating on small aircraft. While microstrip antenna technology has been developed for radio communications, it has yet to be applied to combined radar and radiometer for Earth remote sensing.
The antenna feed provides a key instrument element enabling high-resolution radiometric observations with large, deployable antennas. The design is based on the microstrip stacked-patch array (MSPA) used to feed a large, lightweight, deployable, rotating mesh antenna for spaceborne L-band (≈1 GHz) passive and active sensing systems. The array consists of stacked patches to provide dual-frequency capability and suitable radiation patterns. The stacked-patch microstrip element was designed to cover the required L-band center frequencies at 1.26 GHz (lower patch) and 1.413 GHz (upper patch), with dual-linear polarization capabilities. The dimension of patches produces the required frequencies.
To achieve excellent polarization isolation and control of antenna sidelobes for the MSPA, the orientation of each stacked-patch element within the array is optimized to reduce the cross-polarization. A specialized feed-distribution network was designed to achieve the required excitation amplitude and phase for each stacked-patch element.
The patches are thin copper/Kapton layers bonded to Astro-Quartz layers. As illustrated in the figure, three copper/Kapton/Astro-Quartz layers are built to function as the upper patch, lower patch, and ground plane. The lower radar patches sit on a honeycomb dielectric structure above the conducting ground plane. The honeycomb is filled mostly with air and, therefore, introduces only a small loss at L-band frequencies. On the top of the radar patches sits another honeycomb dielectric structure to support the radiometer patches. All of the layers and the honeycombs are drilled to allow attachment to the feed wires to the lower patch (radar). The lower patch is fed through the ground plane, while the upper patch acts as a parasitic patch to introduce the 1.413 GHz.
A seven-element stacked patch array with elements forming a hexagonal pattern is the most suitable for space applications; however, a 16-element array with a 4×4 rectangular configuration is better for airborne and ground applications.