The first chapter of the book examines the units of radio receivers, the design of which is determined by the function assigned to them: a receiver for household, professional, mobile communication, and radio access systems. Moreover, the design of receivers can differ significantly depending on the operating frequency range, design, manufacturing technology, the element base used—while maintaining the function of individual nodes, and even with the same type of structure chosen such as direct conversion, superheterodyne, with a low intermediate frequency, with a digital intermediate, and so on [31]. Regardless of the architecture of the receiver, each of them uses an antenna that converts the electromagnetic field near the antenna into an electrical radio signal at the receiver input. The equivalent antenna circuit, describing its properties, is modified depending on the operating range, frequency band, and antenna design. In modern mobile communication and radio access systems, the number of antennas in one subscriber terminal can increase to two, four, or more, depending on the type of network being organized, and in a base station, it can be an array of antennas. The input circuit (INс) of receiver, a unit built between the antenna and the first active element of the radio frequency (RF) front-end, separates the useful signal from a variety of discrete and distributed interfering influences at the receiver input. Its design is significantly complicated in mobile and radio access systems when the mobile station is implemented as a transceiver providing two-way communication with a frequency or time duplex. In this case, the input circuit is complemented by an input radio frequency module (front-end module), which switches the antenna (antennas) to the transmitter output or the receiver input while maintaining and even complicating the voltage-controlled (VC) structure, especially when operating in multistandard mode. The material in this chapter allows the reader to evaluate the effect of antenna parameters on the properties of the input circuit and ways to optimize its structure, as well as the design and parameters of antennas for receivers of wireless systems for various purposes in a complex electromagnetic environment.