The millimetre-wave (mm-wave) band is expected to be one of the solutions for future wireless communication systems, as it provides wide bandwidth, enhancing the data transmission rate. Therefore, high-directive antennas, which are principal elements for mm-wave wireless communications, are used since they can overcome the negative effects of high path losses. These high-gain antennas can be used in various applications, including point-to-point communication, automotive radar, and imaging. The main goal of this project is to design transmit-array (TA) antennas that operate at the mm-wave band (around 28 GHz) with high aperture efficiency, high gain, wide bandwidth, and a low-profile TA surface. Basically, the aperture efficiency of the TA antenna depends on the unit cell (UC) characteristics and the feed antenna radiation pattern. 
	The UC characteristics, including the transmission coefficient, bandwidth, and phase range, all play an essential role in designing efficient TA antennas. Therefore, the unit cells of the TA surface are designed to have a wide bandwidth with a stable broadside radiation pattern and a full phase range (0 ͦ-360 ͦ). Such unit cells are designed using two techniques:
1) Capacitive feeding to extend bandwidth and 2) differential feeding to minimize cross-polarization and enforce a broadside radiation pattern that, enhancing the antenna gain.
Simulated results show that the unit cells have achieved a 360 ͦ phase range and a maximum element loss of 0.5 dB. In addition to the design’s high performance UC, a wideband feeder antenna is required to illuminate the TA surface. Thus, a wideband patch antenna which is a part of the UC is used to illuminate the TA surface. Also, a hybrid antenna with a high data rate has been designed and fabricated to excite the TA aperture. The designed hybrid antenna has a 14.5 dBi maximum realized gain at 30 GHz, and its return loss bandwidth is 34.48%. Results show that the TA’s aperture efficiency depends on the illuminating source’s radiation pattern, such as its amplitude and phase distribution over the TA surface.
	Experiments show efficient illumination of TA surface requires a specific radiation pattern; therefore, a circular horn antenna has been designed with high acceptable amplitude tapering, uniform phase distribution, symmetric radiation pattern, and low sidelobe level. This utilized horn antenna has a particular radiation magnitude distribution, and the radiation pattern model is sec⁡θ, which makes it capable of dealing with the relative difference in path loss and resulting in a good tapering efficiency.
The TA antenna’s surface and the feeding antennas have been designed, fabricated and measured. The TA antenna measured results show a maximum gain of 31.15 dBi, with a 1-dB gain bandwidth of 12.7%, while the 3-dB gain bandwidth is 21%, around 28 GHz. On the frequency range from 25 to 31.5 GHz, the aperture efficiency is better than 50%, and the cross-polarization level is less than -37 dB. Furthermore, factors that affect the TA antenna’s performance are studied and summarized in this work. These factors are mutual coupling, phase errors in TA design, quantization error, phase range error, feed antenna, TA antenna shape, and incident angle approximation. 
	Circular polarization antennas have advantages over linear polarization antennas because rapid alignment is not required between transmitter and receiver antennas, reducing polarization mismatching error. The Faraday rotation effect also harms the linear polarized waves. Moreover, circularly polarized wave energy is on both planes, suppressing interference. A circular polarizer is used to convert the LP incident signal to CP signals, in which the incident electric field is resolved to its orthogonal components, introducing a 90 ͦ phase shift. The proposed polarizer contains two layers of the Jerusalem cross (JC). The JC UC simulated results show equal orthogonal retransmitted electric fields with 90 ͦ phases. The designed CP transmit-array results show a maximum realized gain of 30.5 dBi, with an AR bandwidth of 23%. Finally, a general TA antenna design methodology is also provided.