RF Fundamentals Lecture 3. 2 Objectives Describe RF loss and gain, and how it can be measured List...

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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

Free space path loss calculation

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

Rules 10 and 3s

Rules 10s and 3s

Rules of 10s and 3s

Example

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

Figure 3-27: RF LOS propagation

• 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

Fresnel Zone

• 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

• 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

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

Figure 3-29: Omni-directional antenna

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 A

RF Fundamentals Lecture 3. 2 Objectives Describe RF loss and gain, and how it can be measured List...

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