1. Chapter 2
Radio Frequency Fundamentals

2. Exam Essentials
Understand wavelength, frequency, amplitude, and phase.
Know the definition of each RF characteristic and how it can affect wireless LAN design.
Remember all the RF propagation behaviors.
Be able to explain the differences between each RF behavior (such as refection, diffraction, scattering, and so on) and the various mediums that are associated with each behavior.
Understand what causes attenuation.
Loss can occur either on the wire or in the air. Absorption, free space path loss, and multipath downfade are all causes of attenuation.
Define free space path loss.
Despite the lack of any obstructions, electromagnetic waves attenuate in a logarithmic manner as they travel away from the transmitter.

3. Exam Essentials
Explain the difference between active and passive gain.
RF amplifiers are active devices, whereas antennas are passive devices.
Explain the difference between transmit and received amplitude.
Transmit amplitude is typically defined as the amount of initial amplitude that leaves the radio transmitter. When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude.
Remember the four possible results of multipath and their relationship to phase.
Multipath may cause downfade, upfade, nulling, and data corruption.
Know the results of intersymbol interference and delay spread.
The time differential between a primary signal and reflected signals may cause corrupted bits and affect throughput and latency due to layer 2 retransmissions.

4. Understanding Wireless
Need to understand how wireless works at the physical layer of the OSI model
RF Signals move through the air in an unpredictable manner
Unbounded Medium

5. What is Radio Frequency (RF)
Part of the Electromagnetic Spectrum
Starts as Alternating Current (AC) generated from a transmitter
Radiated out of an antenna element
Changes in current produce changes in Electromagnetic Field
Pg 31

6. Alternating Current
Current where the magnitude and direction varies in a cycle over time
Produces a sine wave
Fluctuation is the oscillation
Movement of the wave through air is the propagation behaviors
Absorption
Reflection
Scattering
Refraction
Diffraction
Amplification
attenuation
hấp thụ
Reflection
tán xạ
Khúc xạ
nhiễu xạ
khuếch đại
suy giảm
Pg 32

7. RF Characteristics
Wavelength
Frequency
Amplitude
Phase
Pg 32

8. Wavelength Distance between the peaks of the waveform
Distance traveled in a single cycle
Pg 32

9. Wavelength Represented by Greek lambda λ
Inverse relationship between wavelength and frequency
Pg 33

10. Wavelength The higher the frequency, the shorter the wavelength.
The longer the wavelength, the shorter the frequency.
Pg 33

11. Wavelength
As RF travels though space, signal attenuate, or lose signal strength
Shorter Wavelengths will attenuate FASTER
Signals keep traveling, but may be below the sensitivity threshold of receiver.
Pg 34

12. Comparing 5 Ghz and 2.4 Ghz
Higher Frequencies (shorter wavelength) attenuate faster
Attenuation through the air is Free Space Path Loss
Higher Frequencies also don’t penetrate objects as well.
Pg 34

13. Frequency Measurement of how many times something happens in a second
1 hertz (Hz) = 1 cycle per second
1 kilohertz (KHz) = 1,000 cycles per second
1 megahertz (MHz) = 1,000,000 (million) cycles per second
1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
Pg 36

14. Amplitude Measure of signal strength or power
λ is for wavelength
y is for amplitude
Loss of amplitude is attenuation or loss
Transmit Amplitude
Initial amplitude at transmitter
Received Amplitude
Received signal strength
Pg 37

15. Amplitude Different RF technologies use different transmit amplitudes
AM Radio may use 50,000 Watts
Access Poitns from 1mW an 100 mW
Pg 38

16. Phase Difference in degrees separating two overlapping sine waves
Out of phase
Measured from 0-360
0-in phase
90-quarter out of phase
180-cancels out original
Etc.
Pg 100

17. Phase
Pg 100

18. Phase Differences in phase are important to understanding multipath
Can cause interference in signals
Pg 38

19. RF Behaviors Wave Propagation
How waves move through the air and obstacles
Absorption
Reflection
Scattering
Refraction
Diffraction
Loss-Attenuation
FSPL
Multipath
Amplification
hấp thụ
Reflection
tán xạ
Khúc xạ
nhiễu xạ
Sự suy giảm thua lỗ
FSPL đa
khuếch đại
Pg 39

20. Absorption
If a signal does not bounce off, move around, or pass through an object, then 100 % absorption has occurred.
Significant cause of Loss
Most materials absorb some level of RF signal
Brick, Concrete, Water all absorb a lot
Even things with lots of water in them
Drywall absorbs less
Pg 40

21. Reflection
If a wave hits a smooth object larger than itself it may bounce off
Sky wave reflection
Lower Frequencies bouncing off charged particles in the ionosphere
Microwave reflection
Higher frequencies (1 Ghz to 300 Ghz) that bounce off smaller objects like a metal door
Important for WLAN
Buildings, roads, water, earth’s surface
Pg 41

22. Reflection
Can be a problem in WLAN as reflected signals will arrive out of phase with original signal
Multipath
Pg 41

23. Scattering Multiple reflections
If the wavelength is longer than the medium that the wave is passing through
Two types
If the particles are smaller than the wavelength, minor scattering of the signal
If the signal encounters an uneven surface and is reflected in multiple directions
Fences, trees, etc
Pg 43

24. Refraction Signals can also be BENT by refraction
Bending of a signal as it passes through a medium with a different density
Causes the direction of the wave to change.
Issue for long distance bridging
Pg 44

25. Refraction
Pg 44

26. Diffraction Bending of a signal AROUND an object
Also spreads the signal
Usually caused by a partial blockage
If you are behind the blockage, you are in the RF shadow!
Pg 44

27. Loss (Attenuation) Decrease of amplitude or signal strength
On a wire due to impedance
Over the air-Free Space Path Loss
Loss and gain are measured in Deibels
Pg 46

28. Multipath
When multiple signals arrive at receiver due to different obstructions/effects on RF signal
Difference in arrival is delay spread.
Pg 49

29. Multipath
Pg 51