Radiofrequency

RF , also called radio spectrum , radio waves or RF , applied to the lower energy portion of the electromagnetic spectrum, located between about 3 Hz to about 300 GHz. The hertz is the unit of measurement of the frequency of the waves, and corresponds to one cycle per second.2 The electromagnetic waves in this region of the spectrum can be transmitted by applying the alternating current originating from a generator to an antenna.

Summary

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  • 1 The Radio Waves
  • 2 Radio wave propagation
  • 3 Transmission and reception
  • 4 Absorption of radio waves
  • 5 Reflection of radio waves
    • 1 The polarization of the waves
    • 2 Speed ​​of propagation
    • 3 Wavelength
    • 4 Frequency units
  • 6 Sources

Radio Waves

Radio waves have lengths ranging from just a few millimeters (tenths of inches), and can be so extensive that they can reach hundreds of kilometers (hundreds of miles). In comparison, visible light has wavelengths in the 400-700 nanometer range, about 5,000 less than the wavelength of radio waves. Radio waves range in frequencies between a few kilohertz (kHz or thousands of hertz) and a few terahertz (THz or 1012 hertz). “Far infrared” radiation follows radio waves in the electromagnetic spectrum, far IRs have slightly more energy and shorter wavelengths than radio waves. Microwaves, which we use for cooking and communications, are short radio wavelengths,

Various radio wave frequencies are used for television and FM and AM radio broadcasts, military communications, cell phones, radio amateurs, wireless computer networks, and numerous other communications applications. Most radio waves pass freely through Earth’s atmosphere. However, some frequencies can be reflected or absorbed by charged particles from the ionosphere.

Radio wave propagation

In order to install a wireless network and, in particular, to locate the access points in order to obtain the maximum possible range, you must know some data regarding the propagation of radio waves. Radio waves (abbreviated RF for Radio Frequency) propagate in a straight line in several directions at the same time. In a vacuum, radio waves propagate at 3,108 m / s.

In any other medium, the signal becomes weaker due to

  • reflection
  • refraction
  • diffraction
  • absorption

Transmission and reception

A radio wave originates when a charged particle (for example, an electron) is excited at a frequency located in the radio frequency (RF) zone of the electromagnetic spectrum. Other types of emissions that fall outside the RF range are gamma rays, X-rays, infrared rays, ultraviolet rays, and light.

When the radio wave acts on an electrical conductor (the antenna), it induces in it a movement of the electrical charge (electric current) that can be transformed into audio signals or other types of information-carrying signals.

The function of the emitter is to produce a carrier wave, the characteristics of which are modified based on the signals (audio or video) to be transmitted. It propagates the carrier wave thus modulated. The receiver picks up the waveform and “demodulates” it to deliver only the transmitted signal to the auditor viewer.

Radio wave absorption

When a radio wave hits an obstacle, some of its energy is absorbed and converted to another type of energy, while another part is attenuated and continues to spread. Another part may be reflected. Attenuation occurs when the energy of a signal is reduced at the time of transmission. Attenuation is measured in bels (symbol: B) and is equivalent to the base 10 logarithm of the transmission output current divided by the input current. Decibels (symbol: dB) are generally used as the unit of measurement. Each decibel is one-tenth of a bel. Being a bel 10 decibels, the formula would be:

R (dB) = (10) * log (P2 / P1)

When R is positive, it is called amplification, and when it is negative, it is called attenuation. In cases of wireless transmissions, attenuation is more common. Attenuation increases with increasing frequency or increasing distance. Also, when the signal collides with an obstacle, the attenuation value is highly dependent on the type of material in the obstacle. Metal obstacles tend to reflect a signal, while water absorbs it.

Radio wave reflection

When a radio wave collides with an obstacle, part or all of the wave is reflected and a loss of intensity is observed. The reflection is such that the angle of incidence equals the angle of reflection.

By definition, a radio wave is capable of propagating in various directions. After mirroring multiple times, a source signal can reach a station or access point after taking many different routes (multipath calls). The time difference in propagation (called propagation delay) between two signals taking different routes can interfere with reception, since the received data streams overlap each other.

This interference increases as the transmission speed increases, as the reception intervals of the data streams become shorter and shorter. Therefore, multipath limits the transmission speed in wireless networks.

To overcome this problem, Wi-Fi cards and access points use two antennas per emitter. Using an Automatic Gain Controller (AGC), which immediately switches from one antenna to another based on signal strength, the access point can distinguish between two signals coming from the same station. The signals received by these two antennas are said to be uncorrelated (they are independent) if there is a Lambda / 2 difference (6.25 cm at 2.4 GHz).

The polarization of the waves

Polarization is used to indicate the plane of oscillation of the electric field of an electromagnetic wave. As an example, a vertical transmission antenna produces (in greater quantity) a vertically polarized radio wave, that is to say with the electric field oscillating in the vertical plane, and therefore with the magnetic field oscillating in the horizontal plane.

Propagation speed

Radio waves propagate at the speed of light. It is practically constant and its value is 300,000,000 meters per second or 162,000 nautical miles per second.

Wavelength

The wavelength of a radio wave is defined as the distance the radio wave travels during one cycle. The wavelength is normally expressed in meters, except if it is less than one meter, in which case it is expressed in centimeters or in millimeters.

Frequency units

Frequency is measured in Hertz (Hz). One Hertz is equivalent to cycling one second. Radio frequencies are high and by international agreement the following multiples are usually used:

Kilohertz (kHz): 1,000 Hz Megahertz (MHz): 1,000,000 Hz Gigahertz (GHz): 1,000,000,000 Hz Terahertz (THz): 1,000,000,000,000 Hz

 

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