Radio Frequency Fndamentals

8/13/2019 Radio Frequency Fndamentals

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Radio

Frequency Fundamentals


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What is Radio Wave

Radio wave is a little part of Electro Magnetic

spectrum.

Electro Magnetic waves are made of two parts.

The first part is an electric field

The second part is a magnetic field

The two fields are at right angles to each other

High frequency AC current in copper cangenerate radio waves or radio waves can

generate AC currents


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Electro magnetic Spectrum

Radio Waves

Microwaves

Infra Red

The visible spectrum

Ultra Violet

Gamma Rays

X-Rays


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Approximate RF range assigned to major

categories

1,850 MHz - 1,990 MHzCell phones PCS

1,227 MHz - 1,575 MHzGPS

960 MHz - 1,215 MHzAir Traffic Control

869 MHz - 894 MHzCell phones GSM

824 MHz - 849 MHzCell phones CDMA

174 MHz - 220 MHzTV stations 7-13

88 MHz - 108 MHzFM radio

54 MHz - 88 MHzTV stations 2-6

26.96 MHz - 27.41 MHzCitizen's Band (CB)

5.9 MHz - 26.9 MHzShortwave radio

535 KHz - 1,700 KHzAM radio

9 KHz - 535 KHzAeronautical/Maritime


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Basic concepts of RF behavior

Gain (Amplification)

Loss (Attenuation)

Reflection Refraction

Diffraction

Scattering

VSWR

Return Loss Absorption

Wave Propagation


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Wireless Propagation

Mental picture

Wave is not a spot or a line, but a moving wave.

Like dropping a rock into a pond.

Wireless waves spread out from the antenna.

Wireless waves pass through air, space,

people, objects,


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RF Properties - Amplitude

Amplitude of an RF Signal is analogous to a

voltage level in an electrical signal.

Higher the transmit power, higher the amplitude

(signal strength)

Higher the amplitude farther the wave can travel


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RF Properties Amplitude (Cont)

Example power levels

FM Radio Stations 6000 to 100,000 watts

Microwave ovens 700 to 1000 watts

Cellular phones tenths of a watt to 1 watt

802.11 NICs 1 to 200 milliwatts (milliwatt =

1/1000 of a watt)


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Frequency

The number of times per second that the

signals amplitude peaks is the frequency of the

signal

Frequency is measured in Hertz (Hz)

The slight modulations around the central

frequency are interpreted as ones and zeros.

Signals at different frequencies do not interfere

each other


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Wavelength

The distance between one peak or crest of a wave of

light, heat, or other energy and the next

corresponding peak or crest.

Wavelength (m) = 300,000,000 m/s /frequency (Hz)

An 802.11 signal with a frequency of 2.4 GHz has a

wavelength of :

Wavelength = 300,000,000 m/s / 2,400,000,000 Hz

Wavelength = 0.125 m

Wavelength = 12.5 cm


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Wavelength and Antennae

Antennae are most receptive to signals that

have a wavelength equal to the length of the

antennas element


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Phase

Phase is a method of expressing the

relationship between the amplitudes of two RF

signals that have the same frequency.

If two signals reach their peak at exact same

time, they are said to be in phase

Two signals in phase add their energytogether, resulting in stronger signal

If one signal reaches its peak the same time the

other reaches its trough, they are said to becompletely out of phase.

Two signals completely out of phase cancels

each other, resulting in null


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Polarization

Polarization is the physical orientation of the

antenna in a horizontal or vertical position.

If the antenna is horizontal, then the

polarization is horizontal

If the antenna is vertical, then the polarization

is vertical

Radio wave is made up of two fields

Electric

Magnetic


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Polarization (Cont)

These two fields are plane perpendicular to

each other

The sum of the two fields is called the

electromagnetic field

Plane parallel with antenna element is referred

as E-Plane

Plane perpendicular to antenna element isreferred as H-Plane


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Polarization (Cont)

Horizontal polarization the electric field is

parallel to ground

Vertical polarization the electric field is

perpendicular to the ground

Transmitting and receiving antennas must be

using same polarization to effectively receive

signals. Polarization has little or no effect in enclosed

areas


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Gain

Antenna gain is used to describe an increase

in an RF signals amplitude.

Gain can be active or passive

Active gain when a powered amplifier is used

to add energy to a signal before transmission

Passive gain when a high gain antenna is

used to focus the energy of a signal duringtransmission to increase the power within the

beam


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Loss

Loss describe decrease in signal strength

Loss can occur while the signal still in the

cable or in the air.

Impedance mismatches in the cables and

connectors can cause loss


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Reflection

Reflection is the light bouncing back in the

general direction from which it came. Consider a smooth metallic surface as an

interface.

As waves hit this surface, much of their energywill be bounced or reflected.


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Reflected Waves

Reflection The shorter the wavelength of the

signal relative to the size of the obstruction, the

more likely it is that some of the signal will be

reflected off the obstruction.


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Microwave Reflections


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Microwave Reflections (Cont)

Microwave signals:

Frequencies between 1 GHz 30 GHz (this

can vary among experts). Wavelength between 12 inches down to less

than 1 inch.

Microwave signals reflect off objects that arelarger than their wavelength, such as buildings,

cars, flat stretches of ground, and bodes of

water. Each time the signal is reflected, the amplitude

is reduced.


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Reflections


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Multipath Reflection


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Advantage: Can use reflection to go around

obstruction.

Disadvantage: Multipath reflection occurs

when reflections cause more than one copy of

the same transmission to arrive at the receiver

at slightly different times.


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Multipath Reflection

Reflected signals 1 and 2 take slightly longer paths than direct signal, arriving

slightly later.

These reflected signals sometimes cause problems at the receiver by partially

canceling the direct signal, effectively reducing the amplitude.

The link throughput slows down because the receiver needs more time to

either separate the real signal from the reflected echoes or to wait for missed

frames to be retransmitted.

Solution discussed later.


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Refraction

Refraction (or bending) of signals is due to temperature, pressure, and water

vapor content in the atmosphere.

Amount of refractivity depends on the height above ground.

Refractivity is usually largest at low elevations.

The refractivity gradient (k-factor) usually causes microwave signals to curve

slightly downward toward the earth, making the radio horizon father away

than the visual horizon.

This can increase the microwave path by about 15%,

NormalRefraction

Refraction (straight line)

Sub-Refraction

Earth


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Refraction (Cont)

Radio waves also bend when entering different materials.

This can be very important when analyzing propagation in the atmosphere.

It is not very significant in WLANs, but it is included here, as part of a general background

for the behavior of electromagnetic waves.


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Diffraction

Diffraction of a wireless signal occurs when the signal is partially blocked or

obstructed by a large object in the signals path. A diffracted signal is usually attenuated so much it is too weak to provide a

reliable microwave connection.

Do not plan to use a diffracted signal, and always try to obtain an

unobstructed path between microwave antennas.

Diffracted

Signal


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Attenuation

Attenuation is the loss in amplitude that occurs whenever a signal travels

through wire, free space, or an obstruction.

At times, after colliding with an object the signal strength remaining is too

small to make a reliable wireless link.

Same wavelength

(frequency), less amplitude.


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Scattering

Scattered waves are produced by rough surfaces, small

objects or by other irregularities in the signal path

including heavy dust content.


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Absorption

Absorption is the phenomenon where 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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Voltage Standing Wave Ratio (VSWR)

VSWR is a result of mismatched impedance

between RF equipments ( radiator, cables,

connectors etc.)

VSWR causes return loss

VSWR is measured as ratio of forwarded and

reflected power

A VSWR measurement of 1:1 would denote a

perfect impedance match, hence no VSWR in the

signal path


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Effect of VSWR

Excessive VSWR can reduce the amplitude of

transmitted signal

It can burn the radio circuitry if not protected.

VSWR can be measured by SWR meters (not

part of CWNA)


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Intentional Radiator

According to FCC an intentional radiator is an

RF device specifically designed to generate and

radiate RF signals.

In terms of hardware, intentional radiator will

include the RF device and all cabling andconnectors up to but not including the antenna.

Power out of intentional radiator means poweroutput at the end of the last cable or connector

before the antenna


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Equivalent Isotropically Radiated Power (EIRP)

EIRP is the power actually radiated by the

antenna element EIRP takes into account the gain of the antenna

plus power fed by the intentional radiator.

EIRP = power of intentional radiator + gain of

antenna

Radio Frequency Fndamentals

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