Rf Waves Help
An EM disturbance is called a radio-frequency (rf) wave if its wavelength falls within the range of 100 km to 1 mm. This is a frequency range of 3 kHz to 3000 GHz.
Formal RF Band Designators
The rf spectrum is split into eight bands , each representing one order of magnitude in terms of frequency and wavelength. These bands are called very low, low, medium, high, very high, ultrahigh, superhigh , and extremely high frequencies . They are abbreviated, respectively, as VLF, LF, MF, HF, VHF, UHF, SHF, and EHF. These are depicted in Table 18-1 in terms of the frequency and the free-space wavelength.
Table 18-1 The Bands in the Radio-frequency (rf) Spectrum. Each band spans one mathematical order of magnitude in terms of frequency and wavelength.
These bands have alternative names. Energy at VLF and LF is sometimes called longwave radio or long waves . Energy in the HF range is sometimes called shortwave radio or short waves (even though the waves aren’t short compared with most EM waves in wireless communications used today). Superhigh-frequency and extremely-high-frequency rf waves are sometimes called microwaves .
Radio-frequency waves propagate through the Earth’s atmosphere and through space in various ways, depending on the wavelength. Some waves are affected by the ionosphere; this is especially true at VLF, LF, MF, and HF. The troposphere can bend, reflect, or scatter waves at VHF, UHF, SHF, and EHF.
The atmosphere of our planet becomes less dense with increasing altitude. Because of this, the energy received from the Sun is much greater at high altitudes than it is at the surface. High-speed subatomic particles, UV rays, and x-rays cause ionization of the rarefied gases in the upper atmosphere. Ionized regions occur at specific altitudes and comprise the ionosphere . The ionosphere causes absorption and refraction of radio waves. This makes long-distance communication or reception possible at some radiofrequencies.
Ionization in the upper atmosphere occurs in four fuzzy layers. The lowest region is called the D layer . It exists at an altitude of about 50 km (30 mi) and ordinarily is present only on the daylight side of the planet. This layer does not contribute to long-distance radio communications, and sometimes impedes them. The E layer , about 80 km (50 mi) above the surface, also exists mainly during the day, although nighttime ionization is sometimes observed. The E layer can facilitate medium-range radio communications at certain frequencies. The uppermost layers are called the F1 layer and the F2 layer . The Fl layer, normally present only on the daylight side of the Earth, forms at about 200 km (125 mi) altitude. The F2 layer, which exists more or less around the clock, is about 300 km (180 mi) above the surface. On the dark side of the Earth, when the F1 layer disappears, the F2 layer is sometimes called simply the F layer .
Figure 18-3 illustrates the relative altitudes of the ionospheric D, E, F1, and F2 layers above the Earth’s surface. All these layers have some effect on the way radio waves travel at very low, low, medium, and high frequencies. Sometimes, ionospheric effects can even be observed into the VHF portion of the radio spectrum. These layers not only make long-distance wireless communications possible between points on the Earth’s surface; they also prevent radio waves at frequencies below approximately 5 MHz from reaching the surface from outer space.
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