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RF Fundamentals
Lecture 3
2
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
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RF Components
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Units
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Free space path loss calculation
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Units
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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
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Understanding DBs
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RF Measurement: RF Math (continued)
Table 3-3: The 10’s and 3’s Rules of RF Math
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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
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Understanding Dbms
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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
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Understanding Dbs and mWs
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Dbms and mW
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RSSI and SNR
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EIR
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Rules 10 and 3s
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Rules 10s and 3s
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Rules of 10s and 3s
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Example
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Example
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Example
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Example
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Example 2
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Example 2
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Example 2
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Example 2
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Antenna Concepts
• Radio waves transmitted/received using antennas
Figure 3-24: Antennas are required for sending and receiving radio signals
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Characteristics of RF Antenna Transmissions
• Polarization: Orientation of radio waves as they leave the antenna
Figure 3-25: Vertical polarization
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Characteristics of RF Antenna Transmissions (continued)
• Wave propagation: Pattern of wave dispersal
Figure 3-26: Sky wave propagation
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Characteristics of RF Antenna Transmissions (continued)
Figure 3-27: RF LOS propagation
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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
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RF line of sight
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RF Line of sight
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Line of sight
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Line of sight
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Line of sight
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Fresnel Zone
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Fresnel Zone
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Fresnel Zone
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Characteristics of RF Antenna Transmissions (continued)
• Determining extent of “late” multipath signals can be done by calculating Fresnel zone
Figure 3-28: Fresnel zone
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Fresnel zone
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Terrain effects on RF
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Weather effects on RF
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Rain effects in RF
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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
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Characteristics of RF Antenna Transmissions (continued)
Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs
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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
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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
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Antenna Types and Their Installations (continued)
Figure 3-29: Omni-directional antenna
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Antenna Types and Their Installations (continued)
Figure 3-30: Semi-directional antenna
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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
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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
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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
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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
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Lab 2
• LAB A
