CWNA Chapter 4 – Radio Frequency Signal and Antenna Concepts

My Notes from chapter 4 of the CWNA study guide

Azimuth and Elevation Charts (Antenna Radiation Envelopes)

  • In either chart, the antenna is placed at the centre of the chart.
  • Azimuth chart = H-plane = top-down view
  • Elevation chart = E-plane = side view

Interpreting Polar Charts

  • Represent the decibel (dB) mapping of the antenna coverage.
  • Each concentric circle on this logarithmic chart represents a change of 5 dB

Beamwidth

  • RF antennas are capable of focusing the power that is radiating from them, however they are not  adjustable
  • Beamwidth is the measurement of how broad or narrow the focus of an antenna is—and is measured both horizontally and vertically.
  • It is the measurement from the centre, or strongest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power (–3 dB)
  • These –3 dB points are often referred to as half-power points
  • Distance between the half-power points is measured in degrees giving the horizontal and vertical beamwidth measurements
  • Calculating beamwidth
    1. First determine the scale of the Azimuth and Elevation charts (complete each separately).
    2. To determine the beamwidth of the antenna, first locate the point on the chart where the antenna signal is the strongest.
    3. Move along the antenna pattern away from the peak signal until you reach the point where the antenna pattern is 3 dB closer to the centre of the diagram. This is why you needed to know the scale of the chart first.
    4. Draw a line from each of these points to the middle of the polar chart.
    5. Measure the distance in degrees between these lines to calculate the beamwidth of the antenna.

Antenna Types

  • Omnidirectional Antennas
    • Radiate in all directions
    • The Dipole antenna, these days built into the AP
    • Typically found in point-to-multipoint environments
    • With higher-gain omnidirectional antennas, the vertical signal is decreased and the horizontal power is increased
    • Antennas are most effective when the length of the element is an even fraction (such as 1/4 or 1/2) or a multiple of the wavelength (λ).
  • Semi directional Antennas
    • Three types of antennas fit into the semi directional category
      • Patch
        • can be used effectively in libraries, warehouses, and retail stores with long aisles of shelves
        • Before 802.11 MIMO these antennas were used indoors to reduce reflections and hopefully from this the amount of multipath
        • have a horizontal beamwidth of 180 degrees or less, a minimal amount of signal will radiate outside of the building
        • Now with MIMO they are commonly used for High-density indoor deployments
      • Panel
        • Similar to Patch, the terminology gets interchanged quite often
      • Yagi
        • Typically used for short to medium distance point to point connections of up to about 3.2kms
        • Higher gain Yagi antennas can be used to cover more distance
  • Highly Directional Antennas
    • Used strictly for point to point communications (typically between buildings)
    • Two types of Highly directional antennas:
      • Parabolic Dish
        • Similar to Cable TV satellite dishes
        • In high wind its highly recommended that a protective cover known as a radome be used to help offset some of the effects of the wind
      • Grid Antennas
        • A grill of a barbecue, with the edges slightly curved inward.
        • Better in high wind environments
  • Sector Antennas
    • special type of high-gain, semi-directional antenna that provides a pie-shaped coverage pattern
    • Individually, each antenna services its own piece of the pie, but as a group, all of the pie pieces fi t together and provide omnidirectional coverage for the entire area
    • Very small back-lobe on these antennas
    • Typically have gain of about 10dBi
    • Historically, sector antennas were used extensively for cell phone communications. With the expansion of 802.11 networks in stadiums and outdoor venues, the use of sector antennas has increased.
  • Antenna Arrays
    • A group of two or more antennas that are integrated together to provide coverage
    • These antennas operate together to perform what is known as beamforming

Beamforming

  • Beamforming is a method of concentrating RF energy.
  • Concentrating the signal means the signal will be greater than the SNR  at the receiver therefore providing better transition
  • Three types of Beamforming:
    • Static Beamforming
      • Static beamforming is performed by using directional antennas to provide a fixed radiation pattern.
      • Uses multiple directional antennas clustered together but all pointing away from a central point
      • Also known as a indoor sectorized array
    • Dynamic Beamforming
      • Focuses RF energy in a direction in a particular shape
      • The radiation pattern of the signal can change on a frame by frame basis
      • Provides optimal power and signal for each transition
      • Uses an adaptive antenna array that manoeuvres the beam in the direction of the receiver dynamically
      • Also known as smart antenna technology or beamsteering
    • Transmit Beamforming
      • transmitting multiple phase-shifted signals with the hope and intention that they will arrive in-phase at the location where the transmitter believes that the receiver is located.
      • Does not change antenna radiation pattern and an actual directional beam does not exist
      • Not really an antenna technology it’s a digital signal processing technology
      • Two types of transmit beamforming
        • Implicit TxBF
          • Uses an implicit channel-sounding process to optimize the phase differentials between the transmit chains
        • Explicit TxBF
          • Requires feedback from the stations in order to determine the amount of phase-shift required for each signal
          • 802.11ac defines Explicit TxBF requiring the use of channel measurement frames and both the transmitter and receiver to support beamforming

Visual Line of Sight (Visual LOS)

  • When light travels from one point to another it travels across what is perceived to be an unobstructed straight line which is known as Visual Line of Sight
  • Has no bearing on whether or not a RF transition will be successful or not

RF Line of Sight

  • An additional area around the visual LOS needs to remain clear of obstacles and obstructions. This area around the visual LOS is known as the Fresnel zone and is often referred to as RF line of sight

Fresnel Zone

  • Imaginary, elongated, football-shaped area  that surrounds the path of the visual LOS between two point-to-point antennas.
  • Exists above, below and to the sides of the visual LOS (in a 360 degree fashion)

Fresnel Zone.png

  • Theoretically, there are an infinite number of Fresnel zones
  • Closest to the centre is known as the First Fresnel Zone, next closest is Second Fresnel Zone and so on
  • Only the first two really need to be worried about
  • If First Fresnel Zone becomes obstructed (even partially) it will have a negative influence on the RF Communications
  • Under no circumstances should you allow any object or objects to encroach more than 40 percent into the first Fresnel zone of an outdoor point-to-point bridge link, this will result in an unreliable P2P link, even less than 40% is likely to impair the performance of the link
  • The typical obstacles that you are likely to encounter are trees and buildings
  • Important to periodically check, as trees grow and buildings get built
  • Formulas for calculating the First Fresnel Zone radius:
    • Calculate it at the middle of the P2P link:

equation-1

    • For the optimal clearance that you want along the signal pathequation2
      • D = Distance of the link in miles
      • F = transmitting frequency in GHz
      • D = Distance of the link in mile
      • F = transmitting frequency in GHz
    • Calculate Fresnel Zone radius at any point along the connection:

equation3.png

      • N = which Fresnel Zone you are calculating (usually 1 or 2)
      • d1 = distance from one antenna to the location of the obstacle in miles
      • d2 = distance from the obstacle to the other antenna in miles
      • D = total distance between the antennas in miles (D = d1 + d2)
      • F = transmitting frequency in GHz
  • Smaller beamwidth does not equal smaller Fresnel Zone, Fresnel Zone is related to the frequency being used
  • The second Fresnel zone is then the area beyond the first Fresnel zone, where the waves are out of phase with the point source signal. All of the odd-numbered Fresnel zones are in phase with the point source signal, and all of the even-numbered Fresnel zones are out of phase

Earth Bulge

  • Recommended to take into account on P2P links over 7 miles
  • After 7 miles the Earth itself starts to impede on the Fresnel Zone
  • Formula to calculate the increased height you need to raise your antennas to account for the Earth Bulge:

equation4.png

      • H = height of the earth bulge in feet
      • D = distance between the antennas in miles
  • Formula to work out antenna height taking into account Fresnel Zone and Earth Bulge

equation5.png

      • OB = obstacle height
      • D = distance of the link in miles
      • F = transmitting frequency in GHz

Antenna Polarization

  • As waves radiate from an antenna, the amplitude of the waves can oscillate either vertically or horizontally
  • Important that the transmitting and receiving antenna are oriented the same way
  • Most indoor APs with low gain Omni directional antennas should be polarised vertically when mounting to the ceiling
  • Laptop manufacturers build antennas into the sides of the monitor. When the laptop monitor is in the upright position, the internal antennas are vertically polarized as well.
  • When aligning a point-to-point or point-to-multipoint bridge, proper polarization is extremely important. If the best received signal level (RSL) you receive when aligning the
  • antennas is 15 dB to 20 dB less than your estimated RSL, there is a good chance you have cross-polarization.
  • If this difference exists on only one side and the other has higher signal, you are likely aligned to a side lobe.

Antenna Diversity

  • Antenna diversity exists when an access point has two or more antennas with a receiver functioning together to minimize the negative effects of multipath.
  • When the access point senses an RF signal, it compares the signal that it is receiving on both antennas and uses whichever antenna has the higher signal strength to receive the frame of data.
  • Only one antenna is operational at any given time, it can’t send on one and receive on the other

Multiple-input, Multiple-output (MIMO)

  • More sophisticated form of antenna diversity
  • MIMO systems take advantage of multipath
  • Can receive or transmit on multiple antennas concurrently
  • 802.11n and 802.11ac radios use MIMO technology.
  • MIMO Antennas
    • Indoor MIMO Antennas
      • Not normally a decision to make as vendors on most APs already have integrated MIMO antennas into the AP with no antennas protruding
      • If the AP has external antennas the these should be installed as per the vendors recommendations if this isn’t specified then you should align them slightly off parallel with each other.
    • Outdoor MIMO Antennas
      • This benefit may not be realized if the environment does not have reflective surfaces that induce multipath

Antenna Connection and Installation

  • Voltage Standing Wave Ratio
    • Measurement of the change in impedance to the AC Signal
    • A standard unit of measurement of electrical resistance is the ohm
    • VSWR is therefore a ratio of impedance mismatch, with 1:1 (no impedance) being optimal but unobtainable, and typical values range from 1.1:1 to as much as 1.5:1.
    • VSWR military specs are 1.1:1.
      If VSWR is large, this means that a large amount of voltage is being reflected back toward the transmitter.
    • Understand that VSWR may cause decreased signal strength, erratic signal strength, or even transmitter failure
    • Make sure that the impedance of all of the wireless networking equipment is matched, will help minimise VSWR
  • Signal Loss
    • When connecting an antenna to a transmitter, the main objective is to make sure that as much of the signal that is generated by the transmitter is received by the antenna to be transmitted.
    • To achieve this we need to pay attention to the connectors and cables between the AP and the antenna. If inferior components are used these will more than likely result in the AP functioning below its optimal capacity
  • Antenna Mounting
    • Proper installation of an Antenna is one of the most important tasks to ensure optimal function of the network. Key areas to achieve this are:
      • Placement
        • Dependant on the antenna type
        • Omni antennas normally in the middle of the area to be covered.
          • Remember that high gain Omni antennas not to be placed too high above ground because of the narrow vertical coverage
        • When installing directional antennas ensure you know the horizontal and vertical beamwidths so you can properly aim the antenna
          • If the power is too high you can overshoot your coverage area which can be a security risk. Lower the transmit power
        • Ensure when mounting outdoor directional antennas that you have ensure that you have calculated the fresnel zone and that its clear
      • Indoor Mounting Considerations
        • Two common concerns are aesthetics and security.
      • Outdoor Mounting Considerations
        • If the antenna is being installed in a windy location (and what rooftop or tower isn’t windy?), make sure that you take into consideration wind load and properly secure the antenna
      • Appropriate us and environment
        • Make sure you are not using indoor Aps and antennas outdoors.
        • Outdoor Aps and antennas are designed to withstand hasher environmental (temp, wind, rain etc.)
      • Ingress protection rating
        • Ingress Protection Rating is sometimes referred to as the International Protection Rating and is commonly referred to as the IP Code
        • he IP Rating system is published by the International Electro technical Commission (IEC).
        • The IP Code is represented by the letters IP followed by two digits or a digit and one or two letters, such as IP66.
        • First Number is protection against solids. These go from 0 to 6 (zero for no protection, 6 full protection)
        • Second number is protection against water These go from 0 to 8 – no protection (0), dripping water (1), water splashing from any direction (4), powerful water jets (6), and immersion greater than one meter (8)
      • NEMA Enclosure Rating
        • NEMA Enclosure Rating is published by the United States National Electrical Manufacturers Association (NEMA)
        • Similar to IP ratings but also take into account other features such as  corrosion resistance
      • ATEX Directives
        • Organizations in the European Union must follow these directives to protect employees
        • There are two ATEX directives:
          • ATEX 95 This pertains to equipment and protective systems that are intended to be used in potentially explosive atmospheres.
          • ATEX 137 This pertains to the workplace and is intended to protect and improve the safety and health of workers at risk from explosive atmospheres.
      • National Electrical Code Hazardous Locations
        • The National Electrical Code (NEC) is a standard for the safe installation of electrical equipment and wiring
      • Orientation and Alignment
        • Before installing an antenna, make sure you read the manufacturer’s recommendations for mounting it.
        • Weatherproof the cables and connectors and secure them from movement.
        • Document and photograph each installation of the access point and antennas. This can help you troubleshoot problems in the future and allows you to more easily determine if there has been movement in the installation or antenna alignment
      • Safety
        • Be careful when working with your antenna or near other antennas. Highly directional antennas are focusing high concentrations of RF energy. This large amount of energy can be dangerous to your health.
        • Do not power on your antenna while you are working on it
        • Ensure Aps and antennas are mounted correctly
      • Maintenance
        • Advisable to periodically perform a visual inspection of the antenna and, if needed, verify its status with the installation documentation
        • Two types of maintenance:
          • Preventive
          • Diagnostic

Antenna Accessories

  • Cables
    • Improper installation or selection of cables can detrimentally affect the RF communications more than just about any other component or outside influence
    • Make sure you select the correct cable.
    • Make sure the cable you select will support the frequencies that you will be using
    • Cables introduce signal loss into the communications link
    • Attenuation increases with frequency. If you convert from a 2.4 GHz WLAN to a 5 GHz WLAN, the loss caused by the cable will be greater.
    • Either purchase the cables pre-cut and preinstalled with the connectors or hire a professional cabler to install the connections (unless you are a professional cabler).
  • Connectors
    • Many different types of connectors for 802.11 equipment
    • FCC requires that amplifiers have unique connectors or electronic ID systems to prevent noncertified antennas from being used
    • RF connectors need to be of the correct impedance to match the other RF equipment
    • RF connectors on average add about 1/2 dB of insertion loss
  • Splitters
    • Takes an RF signal and divides it into 2 or more separate signals
    • Only used in special unique situations. Sector antennas are one such situations
  • Amplifiers
    • Provides an overall increase in gain to the signal, normally refered to as active gain
    • Can be unidirectional or bidirectional
    • The amplifier’s increase in power is created using one of two methods:
        • With the fixed-gain method, the output of the transceiver is increased by the amount of the amplifier.
      • Fixed-Output
        • A fixed-output amplifier does not add to the output of the transceiver. It simply generates a signal equal to the output of the amplifier regardless of the power generated by the transceiver.
    • Mainly used to account for signal loss through the cable
  • Attenuators
    • Some situations you need to decrease the signal being radiated
    • Available as fixed or variable dB loss
  • Lightning Arrestors
    • The purpose of a lightning arrestor is to redirect (shunt) transient currents caused by nearby lightning strikes or ambient static away from your electronic equipment and into the ground
    • Not capable of protecting against a direct strike
    • Should be installed between the transceiver and the antenna
  • Grounding Rods and Wires
    • When lightning strikes an object, it is looking for the path of least resistance, or more specifically, the path of least impedance.
    • Grounding rods and wires are also used to create what is referred to as a common ground

Regulatory Compliance

  • In order for an access point manufacturer to sell its product within a country or region, it must prove that its product operates within the rules of the relevant regulatory domain, such as the FCC.
  • Violation of regulatory domain rules could result in a manufacturer being fined or even denied the right to sell its product within the country or region where the violation occurred. Therefore, most manufacturers will not sell or support an antenna that is not on their list of approved antennas.
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