with diffraction or reflection, in particular at lower frequencies such as VHF and below.
On the other hand, like a light wave, also an electromagnetic wave, the mmWs with shorter wavelengths in millimeter orders, in particular, at EHF and above, always propagate in LOS modes. Their propagation is significantly affected by the typical phenomena of reflection, refraction, diffraction, absorption, and scattering so that a clear path without any lossy or/and wavelength comparable obstacles in the traveling path is required. Such a propagation feature of waves will be reflected in the design considerations of antennas in mmW systems.
Besides the blocking of obstacles in the traveling path, the propagation of the mmWs are also affected by the interaction between the waves and the medium, for example, the atmosphere on the earth. Figure 1.1 shows the average atmospheric absorption of the waves at sea level (i.e., a standard atmospheric pressure of 1013.24 millibar), a temperature of 20 °C, and a typical water vapor density of 7.5 g m−3 [3]. The absorption is frequency dependent and ignorable when the frequency is lower than, for instance, 20 GHz with an attenuation less than 0.1 dB km−1 or 50 GHz with an attenuation less than 1.0 dB km−1. This is one of the key reasons that almost all existing long‐distance wireless systems operate at lower frequencies, for instance, sub‐6 GHz bands.
Table 1.2 Dominant propagation modes and typical applications of electromagnetic waves at various frequencies.
| Frequency | Wavelength in air | Dominate propagation modes | Typical applications |
|---|---|---|---|
| Extremely low frequency (ELF): 3–30 Hz | 9993.1–99 930.8 km | Guided between the Earth and the ionosphere | Very long‐distance wireless communication (under water/ground) |
| Super low frequency (SLF): 30–300 Hz | 999.3–9993.1 km | Guided between the Earth and the ionosphere | Very long‐distance wireless communication (under water/ground) |
| Ultra low frequency (ULF): 300–3000 Hz | 99.9–999.3 km | Guided between the Earth and the ionosphere | Very long‐distance wireless communication (under water/ground) |
| Very low frequency (VLF): 3–30 kHz | 10.0–99.9 km | Guided between the Earth and the ionosphere | Very long‐distance wireless communication (under water/ground) |
| Low frequency (LF): 30–300 kHz | 1.0–10.0 km | Guided between the Earth and the ionosphere; ground guided | Very long‐distance wireless communication and broadcasts |
| Medium frequency (MF): 300–3000 kHz | 0.1–1.0 km | Ground guided; refracted wave in ionospheric layers | Very long‐distance wireless communication and broadcasts |
| High frequency (HF): 3–30 MHz | 10.0–100.0 m | Ground guided; refracted wave in ionospheric layers | Very long‐distance wireless communication and broadcasts |
| Very high frequency (VHF): 30–300 MHz | 1.0–10.0 m | Line‐of‐sight refracted in ionospheric | Wireless communication, radio, and television broadcasts |
| Ultra high frequency (UHF): 300–3000 MHz | 0.1–1.0 m | Line‐of‐sight | Wireless communication, television broadcasts, heating, positioning, remote controlling |
| Super high frequency (SHF): 3–30 GHz | 10.0–100.0 mm | Line‐of‐sight | Wireless communication, direct satellite broadcasts, radio astronomy, radar |
| Extremely high frequency (EHF): 30–300 GHz | 1.0–10.0 mm | Line‐of‐sight | Wireless communication, radio astronomy, radar, remote sensing, energy weapon, scanner |
| Tremendously high frequency (THF): 300–3000 GHz | 0.1–1.0 mm | Line‐of‐sight | Radio astronomy, remote sensing, imaging, spectroscopy, wireless communications |
Figure 1.1 The average atmospheric absorption of waves at a sea level at the temperature of 20 °C, standard atmospheric pressure of 1013.24 millibar, and a typical water vapor density 7.5 g m−3 [3].
Figure 1.2 (a) Aperture antennas and (b) microstrip antennas.
The wave attenuation is caused by the absorption of water (H2O) and/or oxygen (O2) in the atmosphere. There are several absorption peaks across the frequency band up to 400 GHz. The lowest two peaks appear around the 25 and 60 GHz bands, respectively. In particular, the attenuation at the 60 GHz band is 10 times that of the 30 GHz band. In addition, the temperature, pressure, and water vapor density also significantly affect the absorption. It suggests that the wave attenuation at the mmW bands may increase greatly when it is raining, snowing, or foggy. Such an observation must be considered in the calculation of link budget of mmW systems. As a result, the selection and design of antennas should meet the requirements of mmW systems with particular attention to uniqueness of wave propagation.
1.3 Millimeter Wave Technology
1.3.1 Important Features
mmW technology has long been developed for various wireless systems in the past decades because of the apparent advantages over the systems operating at the lower frequency bands, that is, their shorter operating wavelength and wider operating bandwidth with the same fractional bandwidth. The shorter operating wavelength is, for instance, good for an imaging system with higher spatial resolution. Physically the resolution limitations in an imaging system restrict the ability of imaging instruments to distinguish between two objects separated by a lateral distance less than approximately half an operating wavelength of waves used to image the objects.
With the shorter operating wavelengths, mmW systems also
