Lecture 2 - RF Fundamentals

Lecture 2 - RF Fundamentals

Radio Frequency Fundamentals

(1 September 2006)

 Describe the behavior of RF

Radiator and EIRP

Objectives

Upon completion of this lecture you will be able to:

 Radio Frequencies are high frequency alternating current (AC) signals that pass along a copper

conductor and then radiated into the air via an

antenna.

 The Antenna

 Coverts the wired signal into a wireless signal and

 Converts the wireless signal into a wired signal

 The RF propagates from the antenna in a straight line in all directions

Radio Frequency

Alternating Sine Wave

Wavelength = 300,000,000 meters per sec/Frequency in Hz.

 Length of one wave at a specific frequency

 As the frequency increases the wavelength decreases

A frequency is the number of wavelengths per unit time

1 cycle /second = 1Hz 1,000 cycles/second = 1Khz 1,000,000 cycles /second = 1Mhz 1,000,000,000 cycles/second = 1Ghz

Wavelength vs Frequency

Electromagnetic Frequency Spectrum

Wavelength = Speed of Light/Frequency

Visible Light

Fiber Optic

Electromagnetic Spectrum

800 850 1300 1500

Micro = 1 x 10-6Nano = 1 x 10-9Pico = 1 x 10-12

 Gain describes an increase in the RF

signal's Amplitude An external source is

normally used to amplify the signal.

 RF Amplifier amplifies the signal while a

 Directional High-gain antenna focuses the beamwidth to increase the signal amplitude

 Reflected signals combined with the main signal can

cause gain

RF Behavior

 Loss describes a decrease in signal strength

Loss is caused by such things as:

 Resistance of cables

 Resistance in connectors.

 Impedance mismatch can cause a signal to be reflected back

toward the source.

 Objects such as trees or buildings in the transmission path can

cause the signals to be absorbed or reflected.

 RF Attenuators and accurate resistors convert frequency to heat to

reduce the signal amplitude

RF Behavior

 Reflection occurs when a propagating electromagnetic wave impacts

an object with dimensions larger than the wavelength of the propagating wave

 Reflections can be caused by the earth, buildings, walls, lakes, metal roofs, metal blinds, metal doors, etc.

 Reflections of the main signal from many objects is referred to as multipath.

 Multipath can degrade or cancel a signal at the receiver.

 Antenna diversity is normally used to overcome Multipath.

 Refraction describes the bending of radio waves as they pass through a

medium of different density.

 For example, RF waves passing through different density of the

atmosphere may change direction away from the receiver.

 Absorption occurs when the RF signal strikes an object and is

absorbed rather than being reflected, refracted or bent.

RF Behavior Contd

Refraction

 Light traveling from one medium to another changes speed.

 When it changes speed it changes direction of travel.

 The change in direction is called Refraction.

 Each object has a Refractive Index (RI).

PRISM REFRACTION

RED ORANGE YELLOW GREEN BLUE VIOLET

MATERIAL INDEX (n) LIGHT VELOCITY (km/s)

NOTE: Different frequencies travel at different

speeds through the same media.

 Diffraction describes an RF wave bending around an obstacle whereas Refraction describes an RF wave bending through a medium.

 Diffraction is the slowing of the wave were the wave front strikes the obstacle while

the rest of the wave front maintains the same speed of propagation.

 Have objects with dimensions that are small compared to the wavelength of the signal and

 The number of obstacles per area is large.

 Scattering can occur

 First, when the wave strikes an uneven object such as sand, rocks, etc., and is

reflected in many different directions.

Second, when a signal wave travels through air particles such as dust, smog, rain,

etc.

RF Behavior Contd

Scattering

 What is VSWR?

 Decreased signal amplitude

 Transmitter and amplifier failure

Voltage Standing Wave Ratio (VSWR)

Voltage Standing Wave Ratio (VSWR)

 VSWR occurs when there is a mismatched impedance

between devices in an RF signal

 For example, if the cable leading to the antenna is 50

ohms and the connector to the antenna is 75 ohms then an

impedance mismatch has occurred.

 Some power will be reflected back toward the transmitter by the

mismatched connector

 Maximum power will not be transmitted to the antenna

 VSWR is expressed as a ratio between two numbers, i.e the ratio of the impedance mismatch to a perfect impedance

 A VSWR of 1:1 denotes a perfect match whereas

 A VSWR of 1.5:1 expresses an impedance mismatch.

VSWR Contd

 A decrease in amplitude of the transmitted signal and

 Electronics circuitry burning out due to reflected power.

Making sure there is no impedance mismatch.

 Using only 50 Ohm devices in a wireless system

 Tight connections on all equipment

 Line of sight (LOS)?

 A Transmitting antenna converts electrical energy into RF waves

electrical energy

directly related to the transmitting or

receiving frequency.

 The higher the frequency the smaller the

Antenna Principles

External Antenna Classifications

 Omnidirectional Antenna

Mast mount omni

Pillar mount omni

Ground plane omni

Ceiling mount omni

 Highly Directional Antenna

External Antenna Contd

2.4 Gz 6 dB Log Periodic Antenna

2.4 Gz 5 dB Magnetic Mount Omni Antenna

2.4 Gz 10 dBi Omni-Directional Antenna

15.5 dBi Flat Panel

Panel (6 – 90 degree beamwidth)

Circular Yagi Antenna

Directional Yagi Antenna

External Antennas

2.4 Gz 14 dBi Radome Yagi Antenna

2.4 Gz 19 dBi Reflector Grid Antenna

Antenna Gain

 The Antenna is a passive device that cannot amplify, filter

or shape the signal

 The antenna, however, can create the effect of

amplification by focusing the RF radiation into a small

Intentional Radiator

 The intentional radiator (IR) includes the RF device and all cabling and

connectors up to but not including the antenna.

 Any reference to power output of the IR refers to the power output at the last cable or connector before the antenna.

 For example, a transmitter with 30 mw of power might lose 15 mw in the cable,

5 mw in the connector leaving only 10 mw at the IR.

 It is the responsibility of the WLAN Administrator to manage power output.

Equivalent isotropically Radiated Power (EIRP)

 EIRP is the power radiated by the antenna and takes into account the gain of the antenna.

 For example, if an antenna had a gain of 10 dBi and was fed by 100 mw of power then the EIRP would be 1000 mW or 1 watt of radiated power.

 It is the responsibility of the WLAN Administrator to assure she is in

conformance with FCC power output regulations and that she has properly calculated a viable link

End of Lecture