Radio Frequency Fundamentals

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					Radio Frequency
Fundamentals

    Wireless Networking Unit
Radio Frequency Signals
   Radio Frequency (RF)
       RF signals are high frequency alternating
        current (AC) signals composed of
        electromagnetic energy.
       Imagine dropping a rock into a still pond
        and watching the concentric ripples flow
        away from the point where the rock hit the
        water. This is how RF waves exit an
        antenna.
RF Properties
   All radio frequency signals have the
    following properties:
       Amplitude,
       Frequency,
       Wavelength,
       Phase, and
       Polarity,
RF Properties
   Amplitude
       RF energy is analogous to sound waves.
        Sound waves are changes in the pressure
        of the air. The cone of a loudspeaker
        creates high and low sound waves by
        moving the air back and forth.
       With RF waves, electrons vibrating in an
        antenna cause waves of high and low
        pressure.
RF Properties
   Amplitude
       In the same way that we can measure the
        change in air pressure from a passing
        sound wave, we can measure the change
        in RF energy caused by a passing RF wave,
       The change in RF energy is know as
        “Amplitude” of a signal,
            Higher amplitude signals are more likely to
             show a higher signal strength,
RF Properties
   Amplitude
       Examples
           FM Radio Stations transmit @ 6000 to 100,000
            watts,
           Microwave Ovens @ 700- 1000 watts,
           Cell phones @ tenths of a watt to 1 watt,
           Wireless 802.11 networks @1-200 milliwatts
                1 milliwatt = 1/1000 of a watt
RF Properties
   Amplitude
       Amplitude is the most basic quality of an
        RF signal
       The higher the amplitude of an RF signal
        the further it will travel before becoming
        weakened to the point of being un-
        receivable,
RF Properties
   Frequency
       Alternating Current (AC) signal
            Since amplitude is the most basic quality of an
             RF signal, information is conveyed by changing
             the amplitude of the RF signal over time.
       Direct Current (DC) signal
            A signal whose amplitude doesn’t change at all
             over time is referred to as a direct current
             signal,
RF Properties
   Frequency is measured in “Hertz”,
       Transmission and reception are easier when the
        signal oscillates with a more or less regular
        rhythm.
       The time between one peak in the signal’s
        amplitude and the next peak is constant from
        peak to peak.
       The number of times per second that the signal’s
        amplitude peaks is the frequency of the signal.
       802.11 transmissions operate at frequencies of
        around 2.4 to 5.8 GHz
            1 GHz – 1,000,000,000 Hz or 1 billion cycles per second
RF properties
   Modulation
       In reality an 802.11 signal is not perfectly
        fixed at one particular frequency, but
        modulates slightly around a central
        frequency,
       Since a change in the signal is required to
        convey information the slight modulations
        around the central frequency are
        interpreted as ones and zeros,
RF Properties
   Frequency bands
       802.11 is constrained by the FCC’s limits on
        what frequencies can be used:
            2.4 GHz band is used for:
                 802.11
                 802.11b
                 802.11g
                 802.11n
            5 GHz band is used for:
                 802.11a
RF Properties
   Wavelength
       The wavelength of an RF signal is a
        function of the signal’s frequency and it’s
        speed through space. If a signal’s wave
        front it traveling through space at a certain
        speed, and we know the amount of time
        between each peak, then we can calculate
        how far the signal will have traveled from
        one peak to the next. That distance is the
        signal’s wavelength.
RF Properties
   Wavelength
       RF energy travels at the speed of light,
        approximately 300,000,000 meters per
        second,
       A signal traveling through an Ethernet
        cable will travel at about two-thirds the
        speed of light,
    RF Properties
   If we assume that an RF signal is traveling
    at the speed of light, then its wavelength
    and frequency can be calculated:
   Wavelength (m) = 300,000,000 m/s
                        Frequency (Hz)
    Wavelength=300,000,000ms/2,400,000,000Hz
    Wavelength = 0.125 meter
    Wavelength = 12.5 centimeter
RF Properties
   By rearranging the formula, we can
    calculate frequency from wavelength:
   Frequency (Hz) = 300,000,000 m/s, or
    the speed of light,
RF Properties
   Wavelength
       Practical Use
            The most direct way that we interact with
             wavelength is through the antennae on most
             802.11b access points (AP)
                 Antennae are most receptive to signals that have a
                  wavelength equal to the length of the antenna’s
                  element.
                 Antenna elements of one-half and one-quarter
                  wavelength are the next best choice.
RF Properties
   Phase
       Phase is a method of expressing the relationship
        between the amplitudes of two RF signals that
        have the same frequency.
       Phase is measured in degrees (like the degrees of
        a compass)
            If two signals are aligned so that they both reach their
             peak at the exact same time, we say that they have zero
             degrees of phase separation. They are completely in
             phase.
            If the signals are aligned so that on reaches its peak at
             the exact same time that the other reaches its trough
             (lowest amplitude) we say that they have 180 degrees of
             phase separation.
RF Properties
   To the Wireless Lan Engineer, phase is
    important because two signals that are
    in phase add their energy together,
    resulting in a stronger signal.
   Two signals that are 180 degrees out of
    phase, completely cancel each other
    out.
RF Properties
   Polarization
       A radio wave is actually made of up of two
        fields:
            One electric,
            One magnetic,
       The sum of these two field is called the
        “electromagnetic field”,
       When energy is transferred back and forth
        from one field to the other it is called
        “Oscillation”
RF Properties
   E-Plane
       The plane that is parallel with the antenna
        element is referred to as the “E-Plane”,
   H-Plane
       The plane that is perpendicular to the
        antenna element if referred to as the “H-
        plane”,
RF Properties
   Gain
       Is the term used to describe an increase in
        an RF signal’s amplitude.
   Loss
       Loss describes a decrease in signal
        amplitude.
            Cable resistance can cause loss of signal, since
             resistance coverts electrical signals to heat,
RF Properties
   Reflection
       Reflection occurs when a propagating
        electromagnetic wave strikes an object that
        has very large dimension in comparison to
        the wavelength of the propagating wave.
            If the surface is smooth, the reflected signal
             may remain intact, though there may be some
             loss due to absorption.
RF Properties
   Refraction
       Refraction describes the bending of a radio
        wave as it passes through a medium of
        different density.
    Incoming RF signal

                                          Reflected RF




                                   Refracted RF
RF Properties
   Diffraction
       Diffraction occurs when the radio path between
        the transmitter and receiver is obstructed by a
        surface that has sharp irregularities or an other
        wise rough surface
            Diffraction is commonly confused with refraction.
            Diffraction describes a wave bending around an obstacle,
             whereas refraction describes a wave bending as it ravels
             from a medium of one density to a medium of another
             density (fog).
RF Properties
   Absorption
       Absorption occurs when the RF signal
        strikes an object and is absorbed into the
        material in such a manner that it does not
        pass through, reflect off, or bend around
        the object.

				
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posted:9/12/2012
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