Radio Frequency and Antenna Radio Frequency and Antenna FundamentalFundamental

Fundamentals of Electromagnetic Waves

Electromagnetic WavesElectromagnetic Waves

An An electromagnetic wave is a fluctuation of energy consisting is a fluctuation of energy consisting of two fields: electric and magnetic. of two fields: electric and magnetic.

These fields oscillate or move back and forth at right angles These fields oscillate or move back and forth at right angles to each other, and the wave moves out from the propagating to each other, and the wave moves out from the propagating antenna in a direction related to the shape of the antenna.antenna in a direction related to the shape of the antenna.

Electromagnetic WavesElectromagnetic Waves

An An electromagnetic wave is a propagating combination of electric and is a propagating combination of electric and magnetic fields.magnetic fields.

The alternating current (AC)The alternating current (AC) in the antenna generates a magnetic field in the antenna generates a magnetic field around the antenna that generates and electric field.around the antenna that generates and electric field.

The electric and magnetic fieldsThe electric and magnetic fields are oscillating perpendicular to each are oscillating perpendicular to each other, and they are both perpendicular to the direction of propagationother, and they are both perpendicular to the direction of propagation

Electromagnetic WavesElectromagnetic Waves

A very specific form of these electromagnetic waves is used to communicate wirelessly in IEEE 802.11 networks.

This form of wave is a radio frequency wave.

An RF-based system is a system that relies on the phenomenon of electromagnetic wave theory to provide data and voice communications wirelessly.

RF Characteristics

All RF waves have characteristics that vary to define the wave. Some of these properties can be modified to modulate information onto the wave. These properties are wavelength, frequency, amplitude, and phase.

WavelengthWavelength

The wavelength of an RF wave is calculated as the distance between two adjacent identical points on the wave.

FrequencyFrequency

Frequency refers to the number of wave cycles that occur in a second.

The impact of frequency usage on WLANs is tremendous. By using different frequencies, you can enable distinct connections or RF links in a given coverage area or cell. For example, an IEEE 802.11g network using channel 1 can exist in the same cell as an IEEE 802.11g network using channel 11. This is because these channels use different frequencies that do not cancel or interfere with each other.

FrequencyFrequency

AmplitudeAmplitude

An RF wave with greater amplitude is easier to detect than an RF wave with lesser amplitude. Realize that RF waves travel, theoretically, forever. This being the case, the detectability of the wave is greater at certain distances when the wave starts with a greater amplitude. A wave with a lesser amplitude may not be detectable due to the noise floor. The noise floor can be defined as a measure of the level of background noise.

PhasePhase

Phase is not a characteristic of a single RF wave but is a comparison between two RF waves.

When the waves are in phase, they strengthen each other, and when the waves are out of phase, they sometimes strengthen and sometimes cancel each other. In specific out-of-phase cases, they only cancel each other.

ModulationModulation

Three types of modulations enable carrier signals to carry Three types of modulations enable carrier signals to carry information:information:

Height of signal (amplitude)Height of signal (amplitude)

Frequency of signal (frequency)Frequency of signal (frequency)

Relative starting point (phase)Relative starting point (phase)

Modulation can be done on analog or digital Modulation can be done on analog or digital transmissionstransmissions

Analog ModulationAnalog ModulationAmplitude: Height of carrier waveAmplitude: Height of carrier wave

Amplitude modulation (AM): Changes amplitude so that highest peaks Amplitude modulation (AM): Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bitof carrier wave represent 1 bit while lower waves represent 0 bit

Frequency modulation (FM):Frequency modulation (FM): Changes number of waves representing Changes number of waves representing one cycleone cycle

Number of waves to represent 1 bit more than number of Number of waves to represent 1 bit more than number of waves to represent 0 bitwaves to represent 0 bit

Phase modulation (PM):Phase modulation (PM): Changes starting point of cycle Changes starting point of cycle

When bits change from 1 to 0 bit or vice versaWhen bits change from 1 to 0 bit or vice versa

Digital ModulationDigital Modulation

Advantages over analog modulation:Advantages over analog modulation:

- Better use of bandwidth- Better use of bandwidth

- Requires less power- Requires less power

- Better handling of interference from other signals- Better handling of interference from other signals

- Error-correcting techniques more compatible with - Error-correcting techniques more compatible with other digital systemsother digital systems

Unlike analog modulation, changes occur in discrete Unlike analog modulation, changes occur in discrete steps using binary signalssteps using binary signals

Uses same three basic types of modulation as analogUses same three basic types of modulation as analog

High FrequencyHigh Frequency

Passed along a conductor and then radiated into Passed along a conductor and then radiated into the air via an antennathe air via an antenna

An antenna:

Converts an electrical signal to a Converts an electrical signal to a wirelessly radiated signal (Transmit)wirelessly radiated signal (Transmit)

Converts a wirelessly radiated signal into Converts a wirelessly radiated signal into an electrical signal (Receive)an electrical signal (Receive)

What is RF?What is RF?

RF BehaviorsRF Behaviors

Radio waves move away from the antenna Radio waves move away from the antenna in a straight line in all directionsin a straight line in all directions

Antenna(top view)

RadioWaves

RF BehaviorsRF Behaviors

RF waves that have been modulated to contain information are called RF signals. These RF signals have behaviors that can be predicted and detected

■ Gain■ Loss■ Reflection■ Refraction■ Diffraction■ Scattering■ Absorption■ VSWR■ Return Loss■ Amplification and Attenuation■ Wave Propagation■ Free Space Path Loss■ Delay Spread

GainGain

Gain is defined as the positive relative amplitude difference between two RF wave signals.

Amplification is an active process used to increase an RF signal’s amplitude. = gain.

There are two basic types of gain: active and passive.

Active gain is achieved by placing an amplifier in-line between the RF signal generator and the propagating antenna.

Passive gain is an increase in the amplitude of the signal, in a favored direction, by focusing or directing the output power. Passive gain can be either intentional or unintentional.

GainGain

LossLoss

Loss is defined as the negative relative amplitude difference between two RF signals. Loss can be either intentional or unintentional.

Intentional loss may be necessary to decrease signal strength to comply with standards or to prevent interference by RF attenuator or by mismatch electrical cable .

Unintentional loss can be cause by many factors such as Objects in path, Reflection, Scatter

Loss = Attenuation

RF signal amplitude gain and lossRF signal amplitude gain and loss

ReflectionReflection

When an RF signal bounces off of a smooth, nonabsorptive surface, changing the direction of the signal, it is said to reflect and the process is known as reflection.

RefractionRefraction

Refraction occurs when an RF signal changes speed and is bent while moving between media of different densities.

DiffractionDiffraction

Diffraction is a change in the direction of a wave as it passes by the edge of an obstacle.

the wave bends around the objectThe effect of waves turning, or bending around an obstacle

ScatteringScattering

Scattering happens when an RF signal strikes an uneven surface causing the signal to be scattered. The resulting signals are less significant than the original signal.Can occur when a wave strikes an uneven surface and is reflected in many directions simultaneously

Yields many small amplitude reflections and destroys the main signal

Scattering = Multiple Reflections

AbsorptionAbsorption

Absorption is the conversion of the RF signal energy into heat.Many materials absorb RF signals in the 2.4 GHz ISM spectrum. These include water, drywall, wood, and even humans.

VSWRVSWR

Voltage standing wave ratio is a measurement of mismatched impedance in an RF system and is stated as an X:1 ratio.

Cables, connectors, and devices have some level of inherent loss.

If all cables, connectors, and devices in the chain from the RF signal generator to the antenna do not have the same impedance rating, there is said to be an impedance mismatch.

Maximum power output and transfer can only be achieved when the impedance of all devices is exactly the same.

Return LossReturn Loss

This energy that is reflected back toward the RF generator or transmitter results in return loss.

Return loss is a measurement, usually expressed in decibels, of the ratio between the forward current (incident wave) and the reflected current (reflected wave).

To minimize VSWR and return loss, we must avoid impedance mismatches.

Amplification - AttenuationAmplification - Attenuation

Amplification is an increase of the amplitude of an RF signal. Amplification is achieved through active gain and is accomplished with an amplifier.

Attenuation is the process of reducing an RF signal’s amplitude. This is occasionally done intentionally with attenuators to reduce a signal’s strength to fall within a regulatory domain’s imposed constraints.

Loss is the result of attenuation.

Gain is the result of amplification.

Wave PropagationWave Propagation

The way RF waves move through an environment is known as wave propagation.

Attenuation occurs as RF signals propagate through an environment.

The signal cannot be detected after a certain distance, and this becomes the usable range of the signal.

Some of the signal strength is lost through absorption by materials encountered by the RF signal. This is due to a phenomenon known as free space path loss.

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Types of WavesTypes of Waves

Transmitter Receiver

Earth

Sky wave

Space wave

Ground waveTroposphere

(0 - 12 km)

Stratosphere (12 - 50 km)

Mesosphere (50 - 80 km)

Ionosphere (80 - 720 km)

Propagation ModesPropagation Modes

Ground-wave propagationGround-wave propagation

Sky-wave propagationSky-wave propagation

Line-of-sight propagationLine-of-sight propagation

Ground Wave PropagationGround Wave Propagation

Ground Wave PropagationGround Wave Propagation

Follows contour of the earthFollows contour of the earth

Can Propagate considerable distancesCan Propagate considerable distances

Frequencies up to 2 MHzFrequencies up to 2 MHz

ExampleExample

AM radioAM radio

Sky Wave PropagationSky Wave Propagation

Sky Wave PropagationSky Wave PropagationSignal reflected from ionized layer of atmosphere back down Signal reflected from ionized layer of atmosphere back down to earthto earth

Signal can travel a number of hops, back and forth between Signal can travel a number of hops, back and forth between ionosphere and earth’s surfaceionosphere and earth’s surface

Reflection effect caused by refractionReflection effect caused by refraction

Can travel thousands of kilometersCan travel thousands of kilometers

Frequency: 2-30MHzFrequency: 2-30MHz

ExamplesExamples

Amateur radioAmateur radio

CB radioCB radio

Line-of-Sight PropagationLine-of-Sight Propagation

Line-of-Sight PropagationLine-of-Sight Propagation

Transmitting and receiving antennas must be within line of sightTransmitting and receiving antennas must be within line of sight

Satellite communication – signal above 30 MHz not reflected Satellite communication – signal above 30 MHz not reflected by ionosphereby ionosphere

Ground communication – antennas within Ground communication – antennas within effectiveeffective line of line of site due to refractionsite due to refraction

Refraction – bending of microwaves by the atmosphereRefraction – bending of microwaves by the atmosphere

Velocity of electromagnetic wave is a function of the density Velocity of electromagnetic wave is a function of the density of the mediumof the medium

When wave changes medium, speed changesWhen wave changes medium, speed changes

Wave bends at the boundary between mediumsWave bends at the boundary between mediums

Line-of-Sight EquationsLine-of-Sight EquationsOptical line of sightOptical line of sight

Effective, or radio, line of sightEffective, or radio, line of sight

dd = distance between antenna and horizon (km) = distance between antenna and horizon (km)

hh = antenna height (m) = antenna height (m)

K = adjustment factor to account for refraction, K = adjustment factor to account for refraction, rule of thumb K = 4/3rule of thumb K = 4/3

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Line-of-Sight EquationsLine-of-Sight Equations

Maximum distance between two antennas for LOS Maximum distance between two antennas for LOS propagation:propagation:

hh11 = height of antenna one = height of antenna one

hh22 = height of antenna two = height of antenna two

2157.3 hh

LOS Wireless Transmission LOS Wireless Transmission ImpairmentsImpairments

Attenuation and attenuation distortionAttenuation and attenuation distortion

Free space lossFree space loss

NoiseNoise

Atmospheric absorptionAtmospheric absorption

MultipathMultipath

RefractionRefraction

Thermal noiseThermal noise

AttenuationAttenuation

Strength of signal falls off with distance over transmission Strength of signal falls off with distance over transmission mediummedium

Attenuation factors for unguided media:Attenuation factors for unguided media:

Received signal must have sufficient strength so that Received signal must have sufficient strength so that circuitry in the receiver can interpret the signalcircuitry in the receiver can interpret the signal

Signal must maintain a level sufficiently higher than noise Signal must maintain a level sufficiently higher than noise to be received without errorto be received without error

Attenuation is greater at higher frequencies, causing Attenuation is greater at higher frequencies, causing distortiondistortion

Free Space Path LossFree Space Path Loss

Free space path loss is a weakening of the RF signal due to a broadening of the wave front.

A 2.4 GHz signal, such as that used by many IEEE devices, will attenuate by approximately 80 dB in the first 100 meters and then by another 6 dB in the second 100 meters.

Multipath and Delay SpreadMultipath and Delay Spread

When signals bounce around in an environment through reflection, refraction, diffraction, and scattering, they create an effect known as multipath.

Multipath occurs when multiple paths of the signal arrive at the receiving antenna at the same time or within a small fraction of a second (nanoseconds) of each other.

The difference in time between the first and second signals arriving at the receiver in a multipath occurrence is known as the delay spread.

Multipath and Delay SpreadMultipath and Delay Spread

When the delay spread is greater, so that the signals arrive out of phase, the signal will either be downfaded, corrupted, or nullified.

The Effects of Multipath PropagationThe Effects of Multipath Propagation

Multiple copies of a signal may arrive at different Multiple copies of a signal may arrive at different phasesphases

If phases add destructively, the signal level relative If phases add destructively, the signal level relative to noise declines, making detection more difficultto noise declines, making detection more difficult

Intersymbol interference (ISI)Intersymbol interference (ISI)

One or more delayed copies of a pulse may arrive One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a at the same time as the primary pulse for a subsequent bitsubsequent bit

Antennas & Antenna Systems

Omnidirectional/Dipole AntennasOmnidirectional/Dipole Antennas

Omnidirectional antennas, the most popular type being the dipole antenna, are antennas with a 360-degree horizontal propagation pattern.

Omnidirectional Antenna UsageOmnidirectional Antenna Usage

Omnidirectional antennas provide coverage on a horizontal plane with some coverage vertically and outward from the antenna. This means they may provide some coverage to floors above and below.To reach people farther away horizontally: use higher gainTo reach people farther up or down vertically: use lower gain

Semidirectional AntennasSemidirectional Antennas

Semidirectional antennas are antennas that focus most of their energy in a particular direction.

Patch, Panel, and Yagi are semidirectional antennas.

Patch and panel antennas usually focus their energy in a horizontal arc of 180 degrees or less, whereas Yagi antennas usually have a coverage pattern of 90 degrees or less.

Semidirectional AntennasSemidirectional Antennas

Highly Directional AntennasHighly Directional Antennas

Highly directional antennas are antennas that transmit with a very narrow beam. These types of antennas often look like the satellite dish. They are generally called parabolic dish or grid antennas. They are mostly used for PtP or PtMP links.

Sectorized and Phased-Array AntennasSectorized and Phased-Array Antennas

A sectorized antenna is a high-gain antenna that works back-to-back with other sectorized antennas.

A phased-array antenna is a special antenna system that is actually composed of multiple antennas connected to a single processor. The antennas are used to transmit different phases that result in a directed beam of RF energy aimed at client devices.

MIMO Antenna SystemsMIMO Antenna Systems

Multiple-Input Multiple-Output (MIMO) can be described as any RF communications system that has multiple antennas at both ends of the communications link being used concurrently.

The proposed 802.11n standard include MIMO technology.