RF Fundamentals

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# RF Fundamentals - PowerPoint PPT Presentation

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.

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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 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
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
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

Antenna Concepts

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)
• 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
Characteristics of RF Antenna Transmissions (continued)
• Determining extent of “late” multipath signals can be done by calculating Fresnel zone

Figure 3-28: Fresnel zone

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