ITNSA0 - Lesson 1 - Radio Frequency (RF) Basics PDF

Topic 1 – Radio Frequency (RF) Basics WIRELESS NETWORKS & SECURITY  Learning Outcomes Describe the concepts of wireless communication Manage wireless components Radio Frequency (RF) signals Radio frequency signals are...

Topic 1 – Radio Frequency (RF) Basics WIRELESS NETWORKS & SECURITY  Learning Outcomes Describe the concepts of wireless communication Manage wireless components Radio Frequency (RF) signals Radio frequency signals are electromagnetic signals that are radiated from an antenna element when an alternating current is applied to it. According to the theory, a time-varying electric field (caused by the applied alternating current) generates a magnetic field and vice versa.  Amplitude The first property of an electromagnetic wave that needs to be discussed is amplitude. Amplitude is the measurement of the distance between the peak (highest amplitude) or trough (lowest amplitude) and the base of each wave. The amount of power that is used to transmit a signal equates to the amount of amplitude it will have. A high amplitude usually translates into a strong signal that can travel long distances in terms of wireless networks. Amplitude is often discussed as transmit amplitude and received amplitude. Transmit amplitude refers to the amplitude of the signal at the time the signal was initially transmitted. Received amplitude refers to the amplitude of a signal when it is received by a receiver.  Frequency Frequency is the rate at which an event occurs in a given unit of time. In the case of radio signals, frequency is the number of waves that occur every second. Frequency is measured in Hertz (Hz).  Wavelength It is possible to work out the length (measured in metres) of a specific wave if you know its frequency. Electromagnetic energy travels at the speed of light in a vacuum. The accepted speed of electromagnetic energy is approximately 300 000 000 m/s. For example, 300 000 000 m/s divided by waves in the 2.4 GHz frequency band (2 400 000 000 Hz) have a wavelength of 0.125 m. Electromagnetic energy is slowed down by the earth’s atmosphere and other obstacles that may reflect, refract, or absorb the signal (the signal retains the same frequency but has a shorter wavelength). Phase It is possible to compare two electromagnetic waves that have the same frequency and decide if these are synchronized. The result of the comparison is the phase of the waves. Phase is measured in degrees (like an angle).  Polarisation In general, the polarity of an object is its physical alignment. The physical alignment of the radiating element in an antenna is important for effective communication between wireless devices. The reason for this is that when an electric current passes through a conductor, the magnetic field that is formed is always perpendicular to that of the electric field.  Polarisation If the position of antenna A is vertical (vertical polarisation) and the position of antenna B is horizontal (horizontal polarisation), then antenna B will only receive a very small portion of the electric field (because it is perpendicular to antenna A).  Radio Frequency Behavior As radio signals travel, these may come across objects that destroy, degrade, or change them in some way. These signals may exhibit what is known as loss or gain. As the names might suggest, gain is the increase in the amplitude of an electromagnetic wave while loss is the decrease of the amplitude. Gain can either be a passive or an active process. A passive gain could be the combination of in-phase waves. Loss can be caused by the combination of out-of-phase waves. The amount of loss that occurs from this phenomenon depends on the degree of phase variation.  Radio Frequency Behavior In a wireless network, each device has a sensitivity level that determines how well it can detect signals against background noise. If there’s too much signal loss, the device can't tell the difference between the noise and the signal, leading to errors. On the other hand, if the signal is too strong, it can raise the noise level, making it harder for the device to detect the signal. The following explains some issues that can affect radio signals: Reflection: Radio frequency (RF) signals can bounce off objects like buildings and metal surfaces. If the surface is smooth, the signal may reflect well, but if it’s rough, the signal can scatter in different directions, weakening it. Inside buildings, signals can reflect off various objects, like metal blinds and doors, causing a phenomenon called multipath. This can lead to significant signal loss, as reflected waves might cancel out the main signal, creating areas with no signal, known as dead zones.  Radio Frequency Behavior Scattering: When an RF signal hits uneven surfaces or small objects, it creates multiple reflections. This happens because the signal bounces off each object it encounters. Refraction: This occurs when an RF signal passes through a denser material (like water or cold air), causing the signal to bend and change direction. Weather changes can affect this bending outdoors. Absorption: Sometimes, RF signals are absorbed by materials like concrete, preventing them from reaching their target. Diffraction: This is the bending and spreading of waves when they encounter an obstruction. Unlike refraction, where the signal bends by passing through a dense medium, diffraction happens when the signal bends around large obstacles.  Voltage Standing Wave Ratio Basic Electron Theory: 1. Matter Composition: All matter is made of molecules, which consist of atoms. Atoms have negatively charged electrons and positively charged protons. Protons are found in the nucleus, while electrons orbit around it. 2. Conductors: Good conductors (like copper) have free electrons that can move easily. When these electrons leave their positions, nearby electrons fill the gaps. This constant movement creates billions of electrons flowing randomly. 3. Electric Flow: When a negative charge is applied to one end of a conductor and a positive charge to the other, electrons flow from the negative side to the positive side. This happens because like charges repel and opposite charges attract.  Voltage Standing Wave Ratio Key Terms: Voltage (V): The difference in electric charge between the two ends of a wire. Higher voltage means stronger forces pushing electrons. Current (A): The flow of electrons through a conductor, measured in amps. There are two types: Direct Current (DC): Electrons flow steadily from negative to positive. Alternating Current (AC): Electrons flow back and forth, changing direction, which is used in radio signals. Resistance (R): Measures how much a material opposes electron flow. Good conductors have low resistance; poor conductors (like glass) have high resistance. Resistance is measured in ohms. Impedance is a broader term that includes resistance from an AC's magnetic field.  Voltage Standing Wave Ratio VSWR (Voltage Standing Wave Ratio): VSWR measures how well impedances match in a circuit. A ratio of 1:1 is perfect, indicating no reflected power. A VSWR of 1.5:1 shows some mismatch. In wireless networks, it’s essential to have a VSWR as close to 1:1 as possible to avoid power loss and protect equipment.  Intentional radiators and equivalent isotopically radiated power Every country has an authority that regulates radio technology. In the USA, it's the Federal Communications Commission (FCC), and in South Africa, it's the Independent Communications Authority of South Africa (ICASA). These authorities enforce strict rules that must be followed. For example, in the USA, the maximum equivalent isotopically radiated power (EIRP) for devices that transmit or receive radio signals is four watts. EIRP measures the actual power output at the antenna. To ensure your wireless network meets local regulations, you need to understand how to calculate power output levels in your RF circuit.  Some radio frequency mathematics There are several key areas in an RF circuit in which you will need to calculate the power levels. These key areas are sectioned and numbered in the figure below:  Some radio frequency mathematics To ensure that your wireless LAN meets the power limits set by your country's communication authority, you need to consider the following areas: 1. The power of devices like access points and network interface cards. 2. Any gain or loss in devices like cables, amplifiers, and connectors between the transmitting and receiving devices. 3. The power level at the last connector before the antenna. 4. Equivalent Isotopically Radiated Power (EIRP), which is the total power radiated by the antenna. 5. The difference in power between the transmitted and received signals. Once you understand these points in your RF circuit, the next step is to learn the industry measurement units used.  Watts (W) The watt, named after James Watt, is a unit of power established in 1889. It measures how quickly energy is used or converted, like in electric circuits. In wireless networks, it indicates the power devices use to create radio signals. One watt equals one ampere multiplied by one volt. For example, a light bulb using 0.5 A at 120 V consumes 60 watts (0.5 A × 120 V). Most wireless devices use between 15 and 100 milliwatts (mW), where 1 mW equals 0.001 W. This power is suitable for most indoor setups, but long-distance connections may need more power.  Decibel (dB) Before it is possible to understand the decibel properly, you need to understand the concept of logarithms. A logarithm, by definition, is the exponent that is required to produce a given number. A logarithm can be noted mathematically as follows: The variable B in the above notation represents the base of the numbering system (e.g. binary, octal, decimal, hexadecimal, etc.). This Learning Manual will only deal with the decimal numbering system, i.e. to the base of 10. A represents any number for which you want to find the logarithm and C represents the logarithm of A. For example, if log10 1 000 = 3, then it is the inverse function of 103 = 1000  Decibel (dB) The bel is a logarithmic unit of measure that was named in honor of Alexander Graham Bell by engineers at Bell Laboratories. A bel is considered to be a relative measurement because it measures the difference between any two values. For example, if the signal strength of an electromagnetic wave is one bel greater than the signal strength of another, then the former is ten times the strength of the latter. You should use the following formula to work out the difference, in bels, between two values A1 and A2:  Decibels relative to a milliwatt (dBm) Calculating in decibels (dB) can be simpler than switching between milliwatts and decibels. However, decibels measure differences, not fixed values, so you need a reference point. A common reference is 1 mW, leading to the term dB-milliwatt (dBm). For instance, 100 mW equals 20 dBm because it's 100 times (or 20 dB) greater than 1 mW. You can also use a formula to find the power level in dBm without relying on the 3s and 10s rule.  Decibels relative to an isotropic radiator (dBi) In wireless networking, an isotropic radiator is a theoretical antenna that spreads energy evenly in all directions, like the sun. Most real antennas focus their signal more in certain directions. This focusing ability is called gain, measured in dBi. Antenna gain is always a positive value because when power is concentrated in one direction, the signal is stronger there than if it were spread out equally. For example, an antenna with a gain of 3 dBi will produce a stronger signal (2 mW) in its preferred direction, while an isotropic radiator would provide 1 mW of power equally in all directions.  Received signal strength indicator (RSSI) Wireless network cards need to accurately measure the strength of radio frequency (RF) signals to make decisions like when to switch connections. The IEEE 802.11 standard defines a method called Received Signal Strength Indicator (RSSI) to help these cards report signal strength to their software. However, how RSSI is implemented can vary by manufacturer. This means that different wireless network cards might measure signal strength differently. The 802.11 standard defines RSSI as an integer value between 0 and 255, allowing for 256 levels of measurement. However, many manufacturers don’t measure all 256 levels; some may only track 101 levels, while others might use a maximum of 31 or 60. This inconsistency makes it hard to relate an RSSI value to a specific power level in milliwatts. For example, an RSSI value of 10 could represent 30 mW for one manufacturer and 60 mW for another.  Group Discussion What are some of the issues that can affect radio signals? Please elaborate on the these. 20 – 25 mins for each group. END OF LESSON 1