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.
1.2 Radio Systems and Antennas
A radio system is generally considered as an electronic system that employs radio waves, a type of EM wave up to GHz frequencies. An antenna, as an essential part of a radio system, is defined as a device that can radiate and receive EM energy in an efficient and desired manner. It is normally made of metal, but other materials may also be used. For example, ceramic materials have been employed to make dielectric resonator antennas (DRAs). There are many things in our lives, such as a power leads that can radiate and receive EM energy but cannot be viewed as antennas because the EM energy is not transmitted or received in an efficient and desired manner or because they are not a part of a radio system, thus they cannot be called antennas.
Since radio systems possess some unique and attractive advantages over wired systems, numerous radio systems have been developed. TV, radar, and mobile radio communication systems are just some examples. The advantages include at least:
Mobility: it is essential for mobile communications;
Good coverage: the radiation from an antenna can cover a very large area that is good for TV and radio broadcasting and mobile communications;
Low pathloss: this is distance (and frequency) dependent. Since the loss of a transmission line is an exponential function of the distance (the loss in dB = distance × per unit loss in dB) and the loss of a radio wave is proportional to the distance square (the loss in dB = 20 log10 (distance)), thus the pathloss of radio waves can be much smaller than that of a cable link. For example, assume that the loss is 10 dB for both a transmission line and a radio wave over 100 m, if the distance is increased 10 times to 1000 m, the loss for the transmission line becomes 10 × 10 = 100 dB but the loss for the radio link is just 10 + 20 = 30 dB, which is much smaller than 100 dB! Therefore, the radio communication system is extremely attractive for long‐distance communication. It should be pointed out that optic fibers are also employed for long‐distance communications since they are of very low loss and UWB – but it is for point‐to‐point communications and fibers/cables normally need to be buried in subsurface, which could be costly in practice.
Figure 1.5 illustrates a typical radio communication system. The source information is normally modulated and amplified in the transmitter and then passed on to the transmit antenna via a transmission line, which has a typical characteristic impedance (which will be explained in the Chapter 2) of 50 Ω. The antenna radiates the information in the form of an EM wave in an efficient and desired manner to the destination, where the information is picked up by the receiver antenna and passed on to the receiver via another transmission line. The signal is demodulated, and the original message is then recovered at the receiver.
Figure 1.5 A typical radio system
Thus, the antenna is actually a transformer that transfers electrical signals (voltages and currents from a transmission line) into EM waves (electric and magnetic fields) or vice versa. For example, a satellite dish antenna receives the radio wave from a satellite and transfers it into electrical signals which are output to a cable to be further processed. Our eyes may be viewed as another example of antennas. In this case, the wave is not a radio wave but an optical wave, another form of EM wave that has much higher frequencies.
Now it is clear that the antenna is actually a transformer of voltage/current to electric/magnetic field; it can also be considered as a bridge to link the radio wave and transmission line. An antenna system is defined as the combination of the antenna and its feed line. As an antenna is usually connected to a transmission line, how to make this connection is a subject of interest since the signal from the feed line should be radiated into the space in an efficient and desired way. Transmission lines and radio waves are in fact two different subjects in engineering. To understand the antenna theory, one has to understand transmission lines and radio waves, which will be discussed in detail in Chapters