The purpose of this invention is to provide a novel, low-loss, dual-band reflectarray element design that is transparent and low-loss across one frequency band (Ka-band in the initial application), and provides the phasing and beam collimation capability across the other band (W-band in the initial application). The surface comprises a periodic lattice of “hybrid loop” reflectarray elements that is phased to collimate a beam in a manner similar to that of a phased array. The reflectarray surface can be designed with arbitrary feed source positioning, which alleviates focal plane line contention between the two bands and enables the shared aperture design approach.
Typically, dual-band, reflector-based radar designs require two distinct apertures (one for each band). To achieve a more compact design and reduce size, weight, and power (SWaP), as well as cost, a dual-band, shared-aperture reflector/reflectarray (with frequency selective surface) was proposed. To take full advantage of the shared-aperture approach, the associated reflectarray element/surface must be simple (e.g. single layer is highly desirable), highly manufacturable, and have low RF loss. The hybrid loop design described in this disclosure meets these criteria, and is therefore a critical enabling technology.
The low-loss, passive reflectarray element for a W-band application uses two distinct elements. A crossed dipole is used for the states with low phase shift, and a crossed loop dipole is used for the states with high phase shift. By optimizing the phase shift performance curves for each element, the reflectarray surface losses are significantly reduced. Furthermore, the hybrid loop element design is realized on a single-layer substrate, enabling a low-cost, low-risk design for a very large reflect-array surface. The current design is tailored for a dual-band (Ka/W) application, wherein the Ka-band passes through the reflectarray surface.
A planar passive dual-polarization Ka/W-band reflectarray prototype was developed as an early pathfinder to support the ACE cloud-profiling radar system. The dual-band reflectarray array design using the hybrid loop element has demonstrated the key electrical and mechanical features required for a viable, full-scale, dual-band (Ka/W) ACE shared aperture approach. These discriminating features are low-loss performance at W-band (94 GHz), and RF transparency at Ka-band (35 GHz). Planar coupons were fabricated and tested to experimentally verify the design.
A reflectarray, by definition, is an array of passive or active elements with either a fixed or variable return phase used to synthesize a far-field antenna pattern. For an application that does not require beam scanning, a passive reflectarray can be used to collimate a source; typically, a microwave feed horn. This allows for a more convenient positioning of the source, or in the case of dual-band systems, allows for a shared primary reflector between the two frequency bands (as opposed to using a separate reflector for each band). Thus, the reflectarray solution enables dual-band capability on a single reflector, which offers significant SWaP advantages, as well as cost and risk reduction.
The reflectarray is realized by etching a periodic array of crossed dipole and crossed-loop dipole elements on a dielectric substrate. When the RF energy from a source (feed horn, array, etc.) is incident on the surface, the element locally delays the energy, and by properly choosing the element to phase the incident wave, a far-field beam can be synthesized.