Radio Frequency Fundamentals (1 September 2006)

Radio Frequency Fundamentals (1 September 2006)

Copyright 2005 All Rights Reserved
Objectives Upon completion of this lecture you will be able to: Describe the behavior of RF Explain the properties of an RF signal Understand the nature of the intentional Radiator and EIRP February 2005 Copyright 2005 All Rights Reserved

Radio Frequency 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. February 2005 Copyright 2005 All Rights Reserved

Alternating Sine Wave Fundamental Characteristics 1. Amplitude
Explain Amplitude. Explain Frequency Explain Phase. Fundamental Characteristics 1. Amplitude 2. Frequency 3. Phase February 2005 Copyright 2005 All Rights Reserved

Wavelength vs Frequency 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 February 2005 Copyright 2005 All Rights Reserved

Electromagnetic Spectrum Micro = 1 x 10-6 Nano = 1 x 10-9 Pico = 1 x 10-12 Visible Light 800 850 1300 1500 Fiber Optic Wavelength = Speed of Light/Frequency February 2005 Copyright 2005 All Rights Reserved

RF Behavior 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. February 2005 Copyright 2005 All Rights Reserved

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. February 2005 Copyright 2005 All Rights Reserved

RF Behavior Contd 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. February 2005 Copyright 2005 All Rights Reserved

LIGHT 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). AIR WATER REFRACTION APPARENT POSITION 1. The movement of light through fiber or any other medium displays characteristics such as Refraction and Reflection. Refraction: a. The speed of light is the velocity of electromagnetic energy in a vacumn. b. Light traveling from one material to another changes speed which results in a change of direction. c. This change of direction is called refraction. 2. Different frequencies travel at different speeds in the same material - this facto plays a major role in F/O. REAL POSITION February 2005 Copyright 2005 All Rights Reserved

PRISM REFRACTION REFRACTION NOTE: Different frequencies travel at different speeds through the same media. RED ORANGE YELLOW GREEN White Light BLUE VIOLET MATERIAL INDEX (n) LIGHT VELOCITY (km/s) 1. Different frequencies trave la different speeds therefor they dhange direction as different amount (refraction). Refractive Index is a measure of that refraction (change of direction). 2. Index is the ration of light in space to its velocity in a specific medium. N = speed of light/velocity of light in a specific material 3. The refractive index of glass is 1.5 1.5 = 300,000/200,00 The index of glass can be changed by changing its composition VACUMN ,000 AIR ,000 WATER ,000 FUSED QUARTZ ,000 GLASS ,000 DIAMOND ,000 SILICON ,000 GALLIUM ARSINIDE ,000 February 2005 Copyright 2005 All Rights Reserved

RF Behavior Contd 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. Scattering occurs when the medium through which the wave travels: 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. February 2005 Copyright 2005 All Rights Reserved

Voltage Standing Wave Ratio (VSWR)
What is VSWR? Causes of VSWR Effects of VSWR Decreased signal amplitude Transmitter and amplifier failure February 2005 Copyright 2005 All Rights Reserved

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. February 2005 Copyright 2005 All Rights Reserved

VSWR Contd VSWR can cause: A decrease in amplitude of the transmitted signal and Electronics circuitry burning out due to reflected power. VSWR can be prevented by: Making sure there is no impedance mismatch. Using only 50 Ohm devices in a wireless system. Tight connections on all equipment High-quality equipment February 2005 Copyright 2005 All Rights Reserved

Antenna Principles A Transmitting antenna converts electrical energy into RF waves Receiving antenna convert RF waves into electrical energy The physical Dimension of an Antenna are directly related to the transmitting or receiving frequency. The higher the frequency the smaller the transmitter/receiver. February 2005 Copyright 2005 All Rights Reserved

External Antenna Classifications
Omnidirectional Antenna Mast mount omni Pillar mount omni Ground plane omni Ceiling mount omni Semidirectional Antenna Patch Antenna Panel Antenna Sectorized antenna Yagi Antenna Highly Directional Antenna Parabolic dish Grid antenna 1. The first mobile telephone systems began n 1946 in St Louis Mo. The cost was approximately $2500. 2. The first Mobile radio was in 1921and was employed by the Detroit police department. February 2005 Copyright 2005 All Rights Reserved

External Antenna Contd
2.4 Gz 6 dB Log Periodic Antenna 2.4 Gz 5 dB Magnetic Mount Omni Antenna 1. The first mobile telephone systems began n 1946 in St Louis Mo. The cost was approximately $2500. 2. The first Mobile radio was in 1921and was employed by the Detroit police department. February 2005 Copyright 2005 All Rights Reserved

External Antennas 2.4 Gz 14 dBi Radome Yagi Antenna 2.4 Gz 19 dBi Reflector Grid Antenna 1. The first mobile telephone systems began n 1946 in St Louis Mo. The cost was approximately $2500. 2. The first Mobile radio was in 1921and was employed by the Detroit police department. February 2005 Copyright 2005 All Rights Reserved

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 beam. The radiation is measured by the beamwidth in terms of horizontal or vertical degrees. An OmniDirectional has a beamwidth of 360 degrees. Whereas The Yagi may have a beamwidth of 30 degrees. A tightly focused beam (highly directional antenna) will maximize the propagated wave at low power. February 2005 Copyright 2005 All Rights Reserved

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. February 2005 Copyright 2005 All Rights Reserved

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. February 2005 Copyright 2005 All Rights Reserved

Radio Frequency Fundamentals (1 September 2006)

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