Radio Frequency Fundamentals
- What is Radio Frequency?
- Radio Frequency Characteristics
- Radio Frequency Propagation
What is a Radio Frequency?
Radio frequency (RF) is a form of electromagnetic energy typically used in wireless communications systems. RF signals are generated by a transmitting device as an alternating current (AC) which produces electromagnetic waves as it radiates away from the device’s antenna. Electromagnetic waves are made of two distinct waves, electric and magnetic, that oscillate at right angles to each other and to the direction of the wave.
Some of the most common applications for radio frequency communications include;
- Radio and television broadcasting
- Amateur and CB radio
- Navigation and air traffic control
- Cellular Telephony
- Remote controlled toys
- Wi-Fi, Bluetooth, and Zigbee data transmissions
Other forms of electromagnetic radiation include microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.
Radio waves are non-ionizing radiation, which means they lack the energy to remove electrons from atoms. As a result, they are less harmful to living organisms than other forms of electromagnetic radiation, such as X-rays and gamma rays.
Radio Frequency Characteristics
The wavelength of an RF signal is the distance between the two consecutive corresponding points of the same wave – such as two peaks or two troughs. This is the distance an RF signal travels in a single cycle (also known as a full cycle).
RF wavelengths are represented by the greek symbol λ (lambda) and is usually measured in metres. The accepted minimum and maximum radio frequency wavelength are typically between one millimetre and 100 kilometres.
The formula for wavelength is:
λ = v/f
λ = Wavelength in metres
v = speed of light – 299,792,458 m/s
f = frequency
In the context of RF, frequency refers to the oscillation rate or the number of times a waveform changes direction (up/down) or completes a full cycle in one second. Or, more specifically – how many times an AC current moves back and forth between its positive and negative crests in a one-second timeslot.
The frequency of an RF wave is inversely proportional to its wavelength. In other words, as the size of the wavelength increases, the frequency decreases, and vice versa.
Frequency is measured in hertz (Hz) where:
1 hertz (Hz) = 1 cycle per second
1 kilohertz (KHz) = 1,000 cycles per second
1 megahertz (Mhz) = 1,000,000,000 cycles per second
1 gigahertz (GHz) = 1,000,000,000,000 cycles per second
Putting this into the context of 802.11 Wi-Fi technologies: 2.4 GHz radios oscillate 2.4 billion times per second. 5 GHz radios oscillate 5 billion times per second. And 6 GHz radios oscillate 6 billion times per second!
The unit for frequency hertz is named after the German physicist Heinrich Hertz, who made important discoveries in electromagnetism, including the existence of electromagnetic waves.
Amplitude refers to the maximum height of a radio wave, measured from the baseline to the peak of the waveform. Amplitude is directly related to the power of an RF signal; the greater the amplitude, the more power the waveform carries at a given point in time.
Amplitude is measured in Watts (W). Typical 802.11 Wi-Fi radios transmit RF signals between 1mW and 4W.
Phase refers to the relationship between two RF signals operating on the same frequency detected by a receiving radio. If two signals are in phase, their peaks and troughs will occur simultaneously. If two signals are out of phase, the peaks and troughs occur at different times.
The phase of two RF signals is typically measured in degrees. 0 degrees of phase separation means that two signals are completely in phase, and the receiving amplitude is combined to create a stronger signal. 180 degrees of phase separation means that the two signals are completely out of phase and will cancel each other out; there is no received signal. Different levels of phase separation will determine whether a receiving signal is increased or decreased at the receiver. See Multipath for more details.
Radio Frequency Propagation
It is important to understand how RF signals travel through and interact with different mediums, including the air itself. A good analogy for how an RF signal travels without obstruction is shown in the image below. When an object is dropped into a body of water, it creates waves that travel in all directions away from the object. The waves are strongest near the centre and weaken as they move away. An RF signal acts the same way; the transmitting device emits the signal, and the power or amplitude of the signal decreases as it moves away.
Reflection is the behaviour in which RF signals bounce off a smooth object larger than the wave itself and is an important phenomenon because it can affect the strength and reliability of RF signals.
Common RF reflection sources include metal surfaces, such as metal doors, windows, or walls; glass surfaces, such as windows or glass partitions; smooth, flat surfaces, such as plaster or drywall; water, such as pools or ponds; snow or ice; trees or other vegetation; buildings or other structures; and vehicles, such as cars, trucks, or aeroplanes.
Refraction is the bending of an RF wave as it passes through one medium to another. When an RF signal passes through a medium with a different refractive, it will change direction and speed. This can affect signal propagation and the ability to be detected by receiving antennas.
Common sources of RF refraction include water vapour, changes in air pressure, and changes in air temperature.
Diffraction is the phenomenon that occurs when an RF wave is bent around an object that it encounters. Much like refraction, diffraction can cause the RF signal to spread out in an unpredictable manner and affect the received signal.
Scattering occurs when an RF signal encounters many multiple reflective surfaces, and the signal is scattered or deflected in various directions.
Scattering can be caused by various factors, including atmospheric conditions, obstacles in the path of the RF wave, and the presence of particles such as dust or moisture in the air.
Absorption is one of the most common behaviours of an RF signal and occurs when a signal encounters an object or material that absorbs the wave’s energy. This absorption of energy causes a reduction in the amplitude of the signal as it is detected by a receiving radio.
Most materials will absorb some amount of an RF signal, with the amount of absorption varying depending on the type of material and the signal’s frequency.
Attenuation, or loss, is the reduction in the amplitude of an RF signal as it propagates through the air and other mediums or obstacles. Different materials will cause RF signals to attenuate at different levels. For example, a section of drywall could attenuate a signal by 3dB (half the amount of power is lost), while a 200mm block of concrete could attenuate a signal by 45dB (likely unrecognisable by the receiving radio).
Free Space Path Loss
Free space path loss is a naturally occurring phenomenon that describes the attenuation or loss in RF signal as it propagates through free space. The phenomenon is due to the inverse square law, which states that the amplitude of an RF wave decreases in proportion to the square of the distance between the transmitter and receiver.
This gives the formula:
FSPL = 32.44 + 20 log(d) + 20 log(f)
FSPL = path loss in dB
d = distance in kilometres between antennas
f = frequency in MHz
Multipath occurs when an RF signal takes multiple paths to reach the receiving radio caused by any combination of the above propagation phenomenon. In an 802.11 wireless network, multipath can have positive and negative effects.
One of the main advantages of multipath in an 802.11 wireless network is that it can increase the received amplitude of the signal helping to deliver data and maintain a stable connection. By taking multiple paths to the destination, the signal can reach the receiver even if one of the paths is degraded or blocked. However, multipath can also have negative effects on an 802.11 wireless network. For example, when multiple copies of the signal arrive at the receiver with different time delays (out of phase), this can cause interference and reduce the overall signal-to-noise ratio (SNR).
In the context of radio frequency transmission, gain refers to the increase in the amplitude of a signal. Gain is expressed in decibels (dB), and a higher gain means that the signal will be stronger and have a greater range. There are two types of gain – active and passive.
Active gain refers to the increase in a signal’s amplitude using a device that requires its own power source. In 802.11 networks, this is commonly the transceiver itself or an amplifier placed between the transceiver and the antenna.
Conversely, passive gain refers to increasing a signal’s amplitude using a device that doesn’t require its own power source. This is typically done via an antenna connected to the transmitting or receiving device. Some antennas are created to redirect or focus the radio’s emitting energy more toward one direction than another.
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