RF Fundamentals Lecture 3

Objectives Describe RF loss and gain, and how it can be measured List some of the characteristics of RF antenna transmissions Describe the different types of antennas

RF Components

Units

Free space path loss calculation

Units

RF Measurement: RF Math RF power measured by two units on two scales: Linear scale: Using milliwatts (mW) Reference point is zero Does not reveal gain or loss in relation to whole Relative scale: Reference point is the measurement itself Often use logarithms Measured in decibels (dB) 10’s and 3’s Rules of RF Math: Basic rule of thumb in dealing with RF power gain and loss

Understanding DBs

RF Measurement: RF Math (continued) Table 3-3: The 10’s and 3’s Rules of RF Math

RF Measurement: RF Math (continued) dBm: Reference point that relates decibel scale to milliwatt scale Equivalent Isotropically Radiated Power (EIRP): Power radiated out of antenna of a wireless system Includes intended power output and antenna gain Uses isotropic decibels (dBi) for units Reference point is theoretical antenna with 100 percent efficiency

Understanding Dbms

RF Measurement: WLAN Measurements In U.S., FCC defines power limitations for WLANs Limit distance that WLAN can transmit Transmitter Power Output (TPO): Measure of power being delivered to transmitting antenna Receive Signal Strength Indicator (RSSI): Used to determine dBm, mW, signal strength percentage Table 3-4: IEEE 802.11b and 802.11g EIRP

Understanding Dbs and mWs

Dbms and mW

RSSI and SNR

EIR

Rules 10 and 3s

Rules 10s and 3s

Rules of 10s and 3s

Example

Example

Example

Example

Example 2

Example 2

Example 2

Example 2

Antenna Concepts Radio waves transmitted/received using antennas Figure 3-24: Antennas are required for sending and receiving radio signals

Characteristics of RF Antenna Transmissions Polarization: Orientation of radio waves as they leave the antenna Figure 3-25: Vertical polarization

Characteristics of RF Antenna Transmissions (continued) Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation

Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation

Characteristics of RF Antenna Transmissions (continued) Because RF LOS propagation requires alignment of sending and receiving antennas, ground-level objects can obstruct signals Can cause refraction or diffraction Multipath distortion: Refracted or diffracted signals reach receiving antenna later than signals that do not encounter obstructions Antenna diversity: Uses multiple antennas, inputs, and receivers to overcome multipath distortion

RF line of sight

RF Line of sight

Line of sight

Line of sight

Line of sight

Fresnel Zone

Fresnel Zone

Fresnel Zone

Characteristics of RF Antenna Transmissions (continued) Determining extent of “late” multipath signals can be done by calculating Fresnel zone Figure 3-28: Fresnel zone

Fresnel zone

Terrain effects on RF

Weather effects on RF

Rain effects in RF

Characteristics of RF Antenna Transmissions (continued) As RF signal propagates, it spreads out Free space path loss: Greatest source of power loss in a wireless system Antenna gain: Only way for an increase in amplification by antenna Alter physical shape of antenna Beamwidth: Measure of focusing of radiation emitted by antenna Measured in horizontal and vertical degrees

Characteristics of RF Antenna Transmissions (continued) Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs

Antenna Types and Their Installations Two fundamental characteristics of antennas: As frequency gets higher, wavelength gets smaller Size of antenna smaller High-gain antennas offer larger coverage areas than low-gain antennas at same input power level Omni-directional antenna: Radiates signal in all directions equally Most common type of antenna

Antenna Types and Their Installations (continued) Semi-directional antenna: Focuses energy in one direction Primarily used for short and medium range remote wireless bridge networks Highly-directional antennas: Send narrowly focused signal beam Generally concave dish-shaped devices Used for long distance, point-to-point wireless links

Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna

Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna

WLAN Antenna Locations and Installation Because WLAN systems use omni-directional antennas to provide broadest area of coverage, APs should be located near middle of coverage area Antenna should be positioned as high as possible If high-gain omni-directional antenna used, must determine that users located below antenna area still have reception

Summary A type of electromagnetic wave that travels through space is called a radiotelephony wave or radio wave An analog signal is a continuous signal with no breaks in it A digital signal consists of data that is discrete or separate, as opposed to continuous The carrier signal sent by radio transmissions is simply a continuous electrical signal and the signal itself carries no information

Summary (continued) Three types of modulations or changes to the signal can be made to enable it to carry information: signal height, signal frequency, or the relative starting point Gain is defined as a positive difference in amplitude between two signals Loss, or attenuation, is a negative difference in amplitude between signals RF power can be measured by two different units on two different scales

Summary (continued) An antenna is a copper wire or similar device that has one end in the air and the other end connected to the ground or a grounded device There are a variety of characteristics of RF antenna transmissions that play a role in properly designing and setting up a WLAN

Lab 2 LAB A