Radio frequency3 Hz -300 GHz 100 000 km -1 mm Microwave 300 MHz -300 GHz 1 m -1 mm Millimeter (mm) band 110 -300 GHz 2.7 mm -1.0 mm Infrared 300 GHz -400 THz 1 mm -750 nm Far infrared 300 GHz -20 THz 1 mm -15 µm Long-wavelength infrared 20 THz -37.5 THz 15-8µm Mid-wavelength infrared 37.5 -100 THz 8-3 µm Short-wavelength infrared 100 THz -214 THz 3-1.4 µm Near infrared 214 THz -400 THz 1.4 µm -750 nm Visible 400 THz -750 THz 750 -400 mm Ultraviolet 750 THz -30 PHz 400 -10 nm X-Ray 30 PHz -30 EHz 10 -0.01 nm Gamma Ray > 15 EHz < 0.02 nm Gigahertz, GHz = 10 9 Hz; terahertz, THz = 10 12 Hz; pentahertz, PHz = 10 15 Hz; exahertz, EHz = 10 18 Hz.Propagating RF signals in air are absorbed by molecules in the atmosphere primarily by molecular resonances such as the bending and stretching of bonds which converts EM energy into heat.The transmittance of radio signals versus frequency in dry air at an altitude of 4.2 km is shown in Figure 1-1 and there are many transmission holes due to molecular resonances.The lowest frequency molecular resonance in dry air is the oxygen resonance centered at 60 GHz, but below that the absorption in dry air is very small.Attenuation increases with higher water vapor pressure peakin at 22 GHz and broadening due to the close packing of molecules in air.The effect of water is seen in Figure 1-2 and it is seen that rain and water vapor have little effect on cellular communications which are below 5 GHz except for millimeter-wave 5G.RF signals diffract and so can bend around structures and penetrate into valleys.The ability to diffract reduces with increasing frequency.However, as frequency increases the size of antennas decreases and the capacity to carry information increases.A very good compromise for mobile 1.7.1 Exercises By Section † challenging