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The high frequencies or short wavelengths of the mmWs make their propagation characteristics very unique. The propagation characteristics directly determine the behaviors of waves propagating to desired destinations through a certain path and media. In a long‐distance wireless communication, radar, or imaging/sensing application, the propagation properties of the wave fully determine the system design requirements, in particular the selection of the adequate operating frequency and bandwidth [2].

As shown in Table 1.2, the dominant propagation modes of the waves vary against operating frequencies. Furthermore, the types of propagation modes determine the distance of wave propagation. It can be found that:

1 the wave mainly propagates in ionospheric modes like a skywave when the frequencies are lower, for instance, at very high frequency (VHF) and below;

2 the wave can propagate in surface modes like a groundwave when the frequencies are at low frequency (LF) to high frequency (HF) bands; and

3 at higher frequencies, typically VHF and above, the wave just travels in direct modes, that is, the line‐of‐sight (LOS), where the propagation is limited by the visual horizon up to about 64 km on the surface of the earth.

Table 1.1 Allocation of the radio frequency bands by ITU.

ITU band number	Designated band	Frequency	Wavelength in air
1	Extremely low frequency (ELF)	3–30 Hz	9993.1–99 930.8 km
2	Super low frequency (SLF)	30–300 Hz	999.3–9993.1 km
3	Ultra low frequency (ULF)	300–3000 Hz	99.9–999.3 km
4	Very low frequency (VLF)	3–30 kHz	10.0–99.9 km
5	Low frequency (LF)	30–300 kHz	1.0–10.0 km
6	Medium frequency (MF)	300–3000 kHz	0.1–1.0 km
7	High frequency (HF)	3–30 MHz	10.0–100.0 m
8	Very high frequency (VHF)	30–300 MHz	1.0–10.0 m
9	Ultra high frequency (UHF)	300–3000 MHz	0.1–1.0 m
10	Super high frequency (SHF)	3–30 GHz	10.0–100.0 mm
11	Extremely high frequency (EHF)	30–300 GHz	1.0–10.0 mm
12	Tremendously high frequency (THF or THz)	300–3000 GHz	0.1–1.0 mm

Note:

1 Hz: hertz

2 k: kilo (103), M: mega (106), G: giga (109), T: tera (1012).

The LOS refers to the waves directly propagating in a line from one transmitting antenna to the receiving antenna. However, it is not necessary for the wave to travel in a clear sight path. Usually, the wave is able to go through buildings, foliage, and other obstacles 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.