A dual polarized planar antenna for radar and communication systems.

Abstract

In radar and communication systems, there is a significant advantage in being able to distinguish and receive signals in two orthogoral polarizations. When used in a radar system, a dual polarized element allows for an increased ability to identify targets and provides for reduced susceptibility to jamming and other Electronic Counter Measures (ECM). In a communication system, the dual polarization can be used to reduce the effects of fading from multipath interference, or it can be used for polarization diversity to increase the effective bandwidth of a system by up to 100%. The antenna element presented here consists of two separate antennas: a printed dipole; and a finite-ground slot antenna. These are fabricated together on a single dielectric carrier and they are planar in nature. The element is fed using microstrip-to-slot and microstrip-to-CPW transitions to form dual microstrip feedlines, allowing the antenna to be easily integrated with existing transmit/receive systems. The horizontally polarized signal is generated using a printed dipole antenna fed with a microstrip to slot vertical transition. This element has good radiation properties with cross-pol levels remaining 15 dB below the main lobe and an impedance bandwidth of more than 20% of the center frequency. The slot antenna is embedded within the arms of the printed dipole resulting in a finite-ground slot antenna. It is used in an endfire fashion with the normal radiation null in the plane of the slot removed using a dual phase reflecting surface. The different reflector requirements for the slot and dipole elements were met by corrugating the ground plane to different depths in the plane above and below the slot, resulting in a dual-phase, polarization dependant reflecting surface. The impedance bandwidth, as with most slot antennas, was narrowband and limits the dual-polarized element to less than 5% of the center frequency. In addition, due to the endfire radiation, the cross-pol levels for this element rise quickly in the azimuth plane and will limit the electronic scanning angles for the vertical polarization to +/-20°, although the element performs well for fixed and mechanically steered arrays.