In recent years, multiple-input-multiple-output (MIMO) antennas with the ability to radiate waves in more than one pattern and polarization play a great role in modern telecommunication systems. This paper provides a theoretical review of different mutual coupling reduction techniques in MIMO antenna systems. The increase in the mutual coupling can affect the antenna characteristics drastically and therefore degrades the performance of the MIMO systems. It is possible to improve the performance partially by calibrating the mutual coupling in the digital domain. However, the simple and effective approach is to use the techniques, such as defected ground structure, parasitic or slot element, complementary split ring resonator, and decoupling networks which can overcome the mutual coupling effects by means of physical implementation. An extensive discussion on the basis of different mutual coupling reduction techniques, their examples, and comparative study is still rare in the literature. Therefore, in this paper, different MIMO antenna design techniques and all of their mutual coupling reduction techniques through various structures and mechanisms are presented with multiple examples and characteristics comparison.

In the last few years, several antenna designs of rectenna that meet various objectives have been proposed for use in RF energy harvesting. Among various antennas, microstrip patch antennas are widely used because of their low profile, light weight, and planar structure. Conventional patch antennas are rectangular or circular in shape, but variations in their basic design are made for different purposes. This paper begins with an explanation and discussion of different designs, put forward with an aim of miniaturization, harmonic rejection, and reconfigurability. Finally, microstrip patch structured rectennas are evaluated and compared with an emphasis on the various methods adopted to obtain a compact rectenna, harmonic rejection functionality, and frequency and polarization selectivity.

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Absorption of radio frequency signal by atmospheric rain and losses is prevalent at frequencies above 10 GHz. This study presents studies on rain attenuation at 18 and 38 GHz microwave link systems, which specifies the minimum performance parameters for terrestrial fixed service digital radio communication. The 3.2 km experimental link at 18 and 38 GHz are used for studying rain propagation effects. The received signal data for rain attenuation and rain rate were collected at 10 s intervals over a 3‐year period from 2013 to 2015. The record shows rain rate of about 50 mm/h and attenuation values of 33.38, 21.88 and 20.89 dB occurred for 0.01% of the time under 18 GHz horizontal, vertical and 38 GHz vertical polarisation, respectively. This study highlights the discussion and comparison of ITU‐R P.530‐16, Moupfouma, Da Silva Mello and Abdulrahman models. Relative error margin of about 71, 60, 38%; 64, 49, 42%; 3, 38, 42% were obtained for 0.1, 0.01, 0.001% of the time in 18 GHz horizontal, vertical and 38 GHz vertical polarisation, respectively, from ITU‐R P. 530‐16 which have been analysed along with suitable approach to characterise rain attenuation in microwave link whose efficiency have been compared with prominent rain attenuation models.

The growth of wireless communications in recent years has made it necessary to develop compact, lightweight multiband antennas. Compact antennas can achieve the same performance as large antennas do with low price and with greater system integration. Dual-frequency microstrip antennas for transmission and reception represent promising approach for doubling the system capacity. In this work, a miniaturized dual band antenna operable at 2.45 and 5.8 GHz is constructed by modifying the standard microstrip patch antenna geometry into a fractal structure. In addition to miniaturization and dual band nature, the proposed antenna also removes unwanted harmonics without the use of additional filter component. Using a finite-element-method-based high frequency structure simulator (HFSS), the antenna is designed and its performance in terms of return loss, impedance matching, radiation pattern, and voltage standing wave ratio (VSWR) is demonstrated. Simulation results are shown to be in close agreement with performance measurements from an actual antenna fabricated on an FR4 substrate. The proposed antenna can be integrated with a rectifier circuit to develop a compact rectenna that can harvest RF energy in both of these frequency bands at a reduction in size of 25.98% relative to a conventional rectangular patch antenna.