Designing Filtering Antennas for 5G Applications

Introduction

The modern world relies greatly on systems of wireless communications for a variety of services, ranging from Wi-Fi to satellite communications to FM radio. All of these systems generally contain three discrete blocks, each with a specific functionality. A transmitter/receiver which is the antenna, a signal selection device which isolates the frequency of interest known as a filter, and a suitable amplification stage. Figure 1 presents a sample block diagram of the RF transceiver frontend.

Evidently, when the device is transmitting, the filter is responsible for rejecting out-of-band frequencies so that the signal will be amplified and reach the antenna as a last step. Reversely, while receiving the antenna only forwards the signal to the filter stage, which once again rejects all the unwanted frequencies and forwards the signal to the amplification stage usually performed by an LNA, or low-noise amplifier. Aside from these fundamental blocks, the signal also undergoes frequency down- or up-conversion and digital processing at supporting circuit blocks, but the key outtake is that a proper impedance matching between the antenna and filter stages is of crucial importance, as any mismatches lead to severe performance degradation. In the conventional approach of circuit design, this comes with the unfortunate effect of increasing the overall circuit size.

Figure 1.

Graphical representation of the RF transceiver block diagram and its fundamental blocks

It is understandable, that in principle such systems are much more complex, but without these fundamental blocks wireless communication could not be established. However, the rapid rise of the Internet-of-Things (IoT) devices and in general the handheld consumer electronics, makes the need for miniaturization apparent. After years of scientific research and technological advances such as Micro-Electro-Mechanical Systems (MEMS) design which enable new fabrication techniques, the discrete blocks design tends to become obsolete, in favor of a seamless integration of the three blocks on a joint module.

The driving force behind all this research into the integration of the RF front end is simply owed to power constraints. The vast majority of the new devices have to rely exclusively on a battery for their functionality, sometimes non-replaceable and non-chargeable, in an effort to save space in the design. Therefore, its capacity is limited, and the entire system must work with the least possible power consumption to prolong its operational time. Even though there are multiple constraints to trying to minimize power consumption, one of the most straightforward ways is to reduce losses.

Attenuation -or loss- generally occurs due to four major causes. Thermal dissipation, path from transmitter to receiver, coupling imperfections and impedance mismatches. The possible elimination of the last two, is the reason why emphasis is being given to the codesign of multiple blocks using the same materials and techniques on the same substrate, and the general tendency is to integrate the filter and the antenna blocks, into a filtering antenna. By eliminating the path between these two stages, it is possible to remove, or significantly diminish, transition loss, insertion loss and external noise, which are primarily associated with the imperfections of physical coupling of the path to the components.

A filtering antenna is a structure that creates a single module, in contrast to the conventional fabrication of an antenna and a filter separately, matched to a pure resistive impedance, such as a 75Ω or 50Ω load, and a direct connection between the two as shown in Figure 2.

Figure 2.

Design of antenna and filter (a) with the conventional approach and the use of a matching network and (b) with the new approach