High mutual coupling reduction between microstrip patch antennas using novel structure

Abstract

In this paper, a novel parasitic configuration to reduce the mutual coupling between two adjacent microstrip patch antennas is presented. It is shown that mutual coupling between the antenna elements can be reduced significantly by two simple slots structure on the ground plane and several shorted simple conducting strips between patch antennas. The structure has been constructed and tested. Both simulated and measured results are presented. The results are shown that more than 7 dB reduction within operational frequency bandwidth with the maximum 41 dB mutual coupling at center frequency can be achieved. The measurement results prove the high efficiency of this configuration in multi antenna systems.

Introduction

Microstrip patch antennas have demonstrated to be one of the most versatile antennas in recent years. Nowadays, patches are probably the most used antennas in compact commercial designs. The main advantages of these antennas are high efficiency, low weight, easy manufacturing, and low cost. However, as typical disadvantages, they exhibit a narrow bandwidth and high mutual coupling when used as array elements [1], [2]. Mutual coupling is an unavoidable phenomenon in multiantenna systems, which degrade the system performance such as impedance mismatching, high side-lobe level, and deviation of the radiation pattern from the desired one, and the scan blindness in a scanning array [3], [4]. In antenna array system when two antennas are placed close to each other interaction takes place between them. This not only increases the transmission coefficient but also distorting the radiation pattern, and deteriorates the signal plus interference to noise ratio. Mainly surface waves and near fields can lead to coupling between the antenna elements. The near-field coupling is strong in situations where the antennas are printed on dielectric substrates with very low permittivity [5]. In such scenarios, the coupling can result in severe degradation of the antenna’s radiation characteristics. While surface waves are weakly excited in very thin grounded dielectric substrates, space-waves dominate and show strong coupling when antennas are in close proximity [6].

In recent years, several methods have been presented to reduce high mutual coupling between microstrip patch antennas, such as application of electromagnetic band gap (EBG) structures [7], [8], [9], [10], [11], [12], defected ground structures (DGS) [13], [14], [15], [16], [17], parasitic elements [18], [19], and split-ring resonators (SRRs) [20], [21]. DGS, EBG, parasitic structures, or the SRRs created on the substrate between array elements effectively disturbs the current distribution in the elements and thus introduces high line inductance and capacitance in the microstrip line [22]. Thus, it obtains wide stop band and compact size, which meet emerging application challenges in wireless communication. In [10] a novel configuration of uniplanar compact electromagnetic band-gap (UC-EBG) structure to reduce mutual coupling is given. The reduction in mutual coupling in this case is 10 dB. The structure reported in [11] uses of one pair of slit two walls in air between microstrip antennas resulting in mutual coupling improvement of 20 dB. In [15] by using two E-shaped DGS placed back to back on the ground plane 15 dB reduction of mutual coupling is achieved. By inserting a simple U-shaped microstrip between the two patches mutual coupling is improved by about 15.5 dB in [19]. Reduction of more than 30 dB in mutual coupling is obtained in [21] when two rows of folded split-ring resonators (FSRRs) are placed in between the two patch antennas. In all the above works establishing an efficient mutual coupling reduction technique while preserving the radiation pattern with low distortion, low complexity of fabrication process and cost is still highly challenging.

In this paper, a novel parasitic structure to reduce mutual coupling of two probes fed microstrip rectangular patch antennas is presented. The mutual coupling of elements is parametrically investigated for H-plane coupling reduction, because all elements of the proposed structure are located in H-plane. The major differences of the proposed method compared to the prior arts were using a few simple techniques simultaneously to reduce mutual coupling between the antenna elements. These techniques consist of using two simple slots etched into the ground plane, five narrow conducting strips of different lengths in between the two patch antennas, and using vias at the ends of each of the conducting strips to connect the ground plane. This research was mainly conducted to suppress mutual coupling between two-element structures with linear polarization which two antennas need isolation as high as possible between transmitter and receiver printed together without any extra cost. Also, this new structure is useful for mutual coupling reduction in integrated transceivers, compact linear arrays, MIMO antennas, radar arrays, and Altimeter system applications. It is noteworthy the antenna presented in this paper suitable for Altimeter system applications and the 4.32 GHz frequency is the operating frequency for the Altimeter system applications. The approach and design of a prototype are investigated in Section 2, and simulated and experimental results are presented in Section 3. Finally, concluding remarks are presented in Section 4. In comparison with similar works, these proposed antennas offer maximum reduction of mutual coupling, small dimensions, high gain, and simple structure.