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

Wavelength=300,000,000ms/2,400,000,000HzWavelength = 0.125 meterWavelength = 12.5 centimeter

Frequency (Hz)

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.

Reflected RF

Refracted RF

Incoming RF signal

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.