Wideband SIW Self-Diplexing antenna with simultaneous control of bandwidth and band position

Abstract

This paper presents a wideband self-diplexing antenna in substrate integrated waveguide (SIW) technology. The problem of narrow radiating bands of traditional self-diplexing antennas is first solved by introducing stepped-impedance resonant slots on top of an SIW cavity. Radiation in each band is obtained from two modes of the resonator, which helps in bandwidth improvement. Bandwidths are controllable by the stepped impedance parameters. Good isolation between the ports enables almost independent control of the band position. Further, a third non-radiating slot is introduced in the middle of the structure, which improves input matching at both the ports. A prototype antenna is fabricated for X-band wireless systems. The measured 10 dB bandwidths are 12.65% (9.25–10.5 GHz) and 17.24% (10.6–12.6 GHz) with the boresight gain ranging over 7–9 dBi and 8.4–10.6 dBi, respectively. Even though the band separation is just 100 MHz, measured isolation is at least 20.5 dB. Isolation can be further improved for wider band separation. The present antenna can be directly integrated with microwave circuits because it does not use any ground plane perturbation.

Introduction

In modern wireless systems, dual-band antennas are becoming increasingly popular for its ability to operate at two frequencies [1], [2], [3]. However, since a dual-band antenna is usually a one-port device, one must use a diplexing filter to separate the two frequency components. Thus, overall size of the system increases [4], [5]. A self-diplexing antenna directly separates two signals into two different ports. Thus, it does not require any external filter.

A traditional antenna can be modified to obtain self-diplexing characteristics. For example, self-diplexing antennas are designed using unequal radiating patches at different layers [6], [7], planar-inverted F-antenna using a shorting pin in between two ports [8], rectangular aperture antenna with two asymmetrical T-stubs [9] and patch antenna with two different U-shape slots [10]. However, the antennas in [6], [7], [8], [9], [10] show poor radiation performances in terms of both broadside gain and front-to-back ratio (FTBR).

Recently, self-diplexing antennas are proposed in substrate integrated waveguide (SIW) to obtain higher gain and FTBR. In SIW technology, self-diplexing antennas are usually designed by using different shapes of slot [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. However, a common problem with all of these SIW self-diplexing antennas is their narrow impedance bandwidth, typical 10 dB bandwidth being always<6%.

This paper presents an SIW based self-diplexing slot antenna with greatly improved effective bandwidth. Bandwidth is improved by using linear superposition of two radiating modes of a single stepped impedance slot. The resonant frequencies and hence the bandwidth of each band is controlled by the stepped impedance parameters. Further, a stepped SIW cavity resonator and a third slot is used to improve both impedance matching and isolation between the ports. The antenna offers a measured gain of at least 7 dBi over its entire range of operation. The bandwidth has been improved to 286%-1400% compared to SIW self-diplexing antennas in [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Moreover, this antenna has facility to control of bandwidth and band position simultaneously. The present antenna can be directly integrated with microwave circuits because it does not use any ground plane perturbation.