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Work on antennas started many years ago. The first well‐known satisfactory antenna experiment was conducted by the German physicist Heinrich Rudolf Hertz (1857–1894), pictured in Figure 1.1. The SI (International Standard) frequency unit, Hertz, is named after him. In 1888, he built a system, as shown in Figure 1.2, to produce and detect radio waves. The original intention of his experiment was to demonstrate the existence of electromagnetic radiation. In the transmitter, a variable voltage source was connected to a dipole (a pair of 1 m wires) with two conducting balls (capacity spheres) at the ends.

Figure 1.1 Heinrich Rudolf Hertz

Figure 1.2 1887 experimental setup of Hertz's apparatus

The gap between the balls could be adjusted for circuit resonance as well as for the generation of sparks. When the voltage was increased to a certain value, a spark or break‐down discharge was produced. The receiver was a simple loop with two identical conducting balls. The gap between the balls was carefully tuned to receive the spark effectively. He placed the apparatus in a darkened box to see the spark clearly. In his experiment, when a spark was generated at the transmitter, he also observed a spark at the receiver gap at almost the same time. This proved that the information from location A (the transmitter) was transmitted to location B (the receiver) in a wireless manner – electromagnetic (EM) waves! The information in his experiment was actually in binary digital form by tuning the spark on and off. This could be considered as the very first digital wireless system that consisted of two of the best‐known antennas: the dipole and the loop. For this reason, the dipole antenna is also called Hertz (dipole) antenna (Figure 1.2).

While Heinrich Hertz conducted his experiments in a laboratory and did not quite know what radio waves might be used for in practice, Guglielmo Marconi (1874–1937, pictured in Figure 1.3), an Italian inventor, was the man who developed and commercialized wireless technology by introducing a radiotelegraph system, which served as the foundation for the establishment of numerous affiliated companies worldwide. His most famous experiment was the transatlantic transmission from Poldhu, UK, to St Johns, Newfoundland, in the USA in 1901 employing un‐tuned systems. He shared the 1909 Nobel Prize in Physics with Karl Ferdinand Braun ‘in recognition of their contributions to the development of wireless telegraphy’. Monopole antennas (near quarter wavelength) were widely used in his experiments, thus vertical monopole antennas are also called Marconi antennas.

Figure 1.3 Guglielmo Marconi.

Source: https://commons.wikimedia.org/wiki/File:Marconi_1909.jpg#/media/File:Marconi_1909.jpg

During World War II, battles were won by the side that was first to spot enemy airplanes, ships, or submarines. To give the Allies an edge, British and American scientists developed radar technology to ‘see’ targets from hundreds of miles away, even at night. The research resulted in the rapid development of high‐frequency radar antennas, which are no longer just wire‐type antennas. Some aperture‐type antennas such as reflector and horn antennas were developed, an example is shown in Figure 1.4.

Figure 1.4 A WWII radar.

Source: From ATNF, used with permission

Broadband, circularly polarized antennas, as well as many other types, were subsequently developed for various applications. Since an antenna is an essential device for any radio broadcasting, communication, and radar systems, there has always been a requirement for better or new antennas to meet existing and emerging applications.

For example, the cellular radio communication system is moving to its 5th generation (5G), the operational frequencies are extended from sub‐6 GHz (e.g. 698–960, 1710–2690, and 3300–3800 MHz) to millimeter waves. The number of antennas in both the mobile portable and the base station is increased significantly to form massive multiple input and multiple output (MIMO) system to dramatically increase the communication data rate and capacity. This means massive new challenges to antenna designers: the antennas are to be placed in a relatively small device, such as a smartphone, and need to perform well at different frequencies (including 3G and 4G mobile frequencies) at the presence of other electronic systems (e.g. Wi‐Fi, GPS, cameras, and a large display) and human body/hands. At millimeter waves, the antennas are also expected to produce beaming forming and steering functionalities to combat increased path loss which poses one of the main challenges for 5G mobile antenna design and measurement. The ultrawide band (UWB) wireless system is another example of recent broadband radio communication and positioning systems. The allocated frequency band is from 3.1 to 10.6 GHz. The beauty of UWB system is that the spectrum, which is normally very expensive, can be used free of charge but the power spectrum density is limited to −41.3 dBm/MHz. Thus, it is only suitable for short‐distance applications (like Bluetooth but with a much larger bandwidth). The antenna design for these systems faces many challenging issues.

The role of antennas is becoming increasingly important. In some systems, the antenna is now no longer just a simple transmitting/receiving device, but a device which is integrated with other parts of the system to achieve better performance. For example, the MIMO antenna system has been introduced as an effective means to combat the multipath effects in the radio propagation channel and increase the channel capacity, where several co‐ordinated antennas are required.

Things have been changing quickly in the wireless world. But one thing has never been changed since the very first antenna was made, that is, that the antenna is a practical engineering subject! It will remain as an engineering subject. Once an antenna is designed and made, it must be tested. How well it works is not just determined by the antenna itself, it also depends on the other parts of the system and the environment. The standalone antenna performance can be very different from that of an installed antenna. For example, when a mobile phone antenna is designed, we must take the case and other parts of the phone, even our hands, into account to ensure that it will work well in the real world. The antenna is an essential device of a radio system, but not an isolated device! This makes it an interesting and challenging subject.