CWNA Chapter 2 – Radio Frequency Fundamentals

My Study notes from Chapter 2 of CWNA Study Guide

RF Characteristics

  • Wavelength
    • A wavelength is the distance between the two successive crescents (peaks) or two successive throughs (valleys)

Wavelength.png

  • Frequency
    • Is how often an RF signal cycles in a certain time period
    • Standard measurement of frequency is hertz (Hz). An event that occurs once in a second has a frequency of 1 Hz.
      • 1 Hertz (Hz) = 1 Cycle per second
      • 1 Kilohertz (KHz) = 1000 cycles per second
      • 1 megahertz (MHz) = 1,000,000 (million) cycles per second
      • 1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second

 

Frequency.png

  • Amplitude
    • The height of the wave.
    • Can be characterised simply as the signal’s strength or power

 Amplitude.png

  • Phase
    • Is not a property of just one signal but instead involves the relationship between two or more signals that share the same frequency.

RF Behaviours

  • Wave Propagation
    • The way the wave moves is known as wave propagation.
    • This can vary drastically depending on the type of material the signal is traversing.
  • Absorption
    • Most common RF behaviour
    • If signal does not bounce off an object, move around an object, or pass through an object, then 100% absorption has occurred.
    • Most materials absorb some RF signal, the amount depends on the material.
  • Reflection
    • Most important RF behaviour to be aware of.
    • Causes serious performance problems in legacy 802.11a/b/g WLANs
    • When wave hits smooth object that is larger than the wave itself, dependant on the material the signal may bounce in another direction.
    • Two Types:
      • Sky wave reflection: Signals below 1 GHz – signal bounces off charged partials in the ionosphere in the earth’s atmosphere.
      • Microwave: 1GHz – 300 GHz, can bounce off smaller objects like a metal door
  • Scattering
    • Described as multiple reflections
    • Is reflection off an object with multiple sides (think mirror ball)
    • Two types:
      • Lower level has lessor effect on signal quality and strength
      • Second type occurs when RF signal encounters some type of uneven surface and is reflected into multiple directions (Chain link fences, wire mesh in stucco walls)
  • Refraction
    • Signal is bent into behaviour known as refraction.
    • when signal passes through a medium with different density thus causing the wave to change direction.
    • Three most common causes:
      • Water Vapour
      • Changes in air temperature
      • Changes in air pressure
  • Diffraction
    • Signal is bent around an object
    • Sitting directly behind the object is an area known as the RF shadow. Depending on the change in direction of the signal this area can become a dead zone of coverage.
  • Loss (Attenuation)
    • Decrease in amplitude
    • Signal may lose strength on the wire or in the air
    • Signal can be absorbed into materials it passes through.
  • Free Space Path Loss (FSPL)
    • Because of the laws of physics the signal will attenuate as it travels despite the lack of attenuation caused by obstructions, absorptions, reflection, diffraction and so on.
    • Loss of signal strength caused by the natural broadening of the waves.
  • Multipath
    • Occurs when 2 or more signals arrive at the receiving station at the same time or within nanoseconds of each other.
    • Four results of multipath
      • Upfade
        •  Multiple RF signal paths arrive at the same time and are in phase (phase differences of 0 to 120 degrees) will cause upfade
        • Results in increased signal strength
      • Downfade
        • Multiple RF signals arrive at the same time but are out of phase (phase difference of 121 to 179 degrees)
        • Results in decrease signal strength
      • Nulling
        • This is signal cancellation
        • Multiple RF signals arrive at the same time and are 180 degrees out of phase of each other
      • Data Corruption
        • Multiple signals arriving but not at the same time the receiver might have trouble demodulating the signal.
  • Gain (Amplification)
    • Increase in amplitude or signal strength
    • Two types:
      • Active – increase to signal on the transmitter or transceivers side through the use of an amplifier. IE more power is applied
      • Passive – done by focusing the antenna, the inner workings of the antenna make the signal stronger.