Size-reduction and suppression of cavity-resonances in hybrid mm-wave antennas for polarimetric measurements

Abstract

Size and feed structure are some of the important constraints for using antenna-elements in multi-element two-dimensional arrays, where easy planar integration and appropriate matching with transceiver chips are essential. This applies especially when differential signaling and adaptable polarization are required. Based on a balanced-fed patch-excited cavity-backed horn antenna (hybrid antenna), feeding concepts, and approaches to reduce size are discussed and evaluated in this paper. The influence of the substrate-integrated cavity is analyzed and methods to overcome the restrictions are presented, together with simulated and measured results. The antenna elements are evaluated with regard to their use in multistatic and polarimetric sparse arrays, which will be briefly introduced. The optimized antennas achieve 10 dB return loss, a gain of more than 5 dBi as well as symmetric and homogeneous radiation patterns in amplitude and phase with low crosspolarization in the desired frequency-band of operation between 70 and 80 GHz. I . I N T R O D U C T I O NImaging systems in the millimeter-and microwave regimes are attracting particular attention due to their use in security, medical, and non-destructive-testing systems [1][2][3][4]. Current advances in the semiconductor technology have made integrated radiofrequency (RF) components at millimeter-wave frequencies available in high quantity [5], allowing for high integration levels and utilizing digital beamforming. An overview of these advanced imaging systems and reconstruction methods is given in [6].To benefit from the high transceiver count required for digital beamforming in multistatic sparse arrays, the cost aspects, space, and integration efficiency also of the antenna array have to be ensured. This becomes much more difficult at millimeter-wave frequencies and for radar applications, where radiation characteristics and the achievable resolution due to bandwidth and the short wavelength are challenging aspects [1]. Even more sophisticated systems using sparse periodic arrays with reduced number of antennas rely on appropriately designed antennas [4,7], especially if polarization is used for further examination [3,8].Using a planar, balanced-fed antenna is the first step to an easy integration with differential interfaced monolithic microwave integrated circuits (MMICs) [5]. In [9,10], the concept of a patch-excited horn antenna, hereinafter referred to as the hybrid antenna, is introduced. For achieving a higher and less extensive integration level, this design has to be reduced in size and complexity, while maintaining the performance of the antenna. Challenges are the feeding structure and the performance in proximity to other antennas. After introducing the concept of the antenna and array in Section II, the relevant design improvements of the antenna are presented in Section III. Continuing with the optimization of the structure in Section IV, simulated and measured results of the antennas and arrays are presented and discussed in Section V. I I . A N T...