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Mid-air collisions (MACs) are commonly caused by temporary or permanent loss of flight control resulting in catastrophic damages and hence must be countered with suitable technology. The avionic industry has been using Traffic Alert and Collision Avoidance System (TCAS) for avoiding mid-air collisions since 1987. The TCAS either uses an Omni-directional antenna or a directional antenna for the surveillance of other nearby aircraft. The existing TCAS directional antennas, i.e., an array of monopole antennas are characterized by a maximum gain of 3.6 dB, which is regarded as low. As a result, many research has been carried out to find the alternative for the TCAS antenna and have found microstrip patch antenna (MPA) which yield some promising results such as feed flexibility, multiband, beam steering, etc. However, the MPA still suffers from disadvantages such as low gain and narrow bandwidth. Hence, lately, a tremendous amount of work has been carried out to enhance the gain of the MPA. In this paper, we first review the key methods that has been reported so far to improve the gain of the traditional TCAS antenna. We note that, in most of the reported work, conventional techniques such as parasitic patches, and shorting pins are used to improve the gain of MPA, and no substantial work was done using advanced techniques such as electromagnetic-band gap (EBG), metamaterials and metasurfaces. Hence, we further review the work that has been carried out to improve the gain of vertically polarized MPAs using advanced techniques for L- and S-band (up to 3 GHz). Many advanced structures that are easily adapted to enhance the gain of TCAS antenna are thoroughly discussed in this work.

A Review on Gain Enhancement Techniques for Vertically Polarized Mid-Air Collision Avoidance Antenna for Airborne Applications