Friday, 30 April 2010

CWNA Study Guide Chapter 2 Radio Frequency Fundamentals Key Learning Points - Part 1

I thought this chapter was probably the most important as a fundamental target to knowing how wireless interacts with its environment, what may affect a radio wave and how it may perform.

Understand what a radio wave is including the electromahnetic spectrum and radiation types such as radio, light, x-ray etc

To produce a radio wave an ac current (alternating current passes through an antenna producing a signal. ac current has a  typically sinusoidal waveform as oposed to dc current which is linear at a fixed amplitude.

The signal generated by the antenna is dependent on the changes in the ac current properties.

Properties of the ac current that may vary are wavelength, frequency, amplitude, phase and polarity.

Electromagnetic waves can travel through objects.

In a vaccuum RF travels a the speed of light which is approximately 300,000 km per second or 186,000 miles per second.

When a wave is generated it moves away from the antenna, it is said to propogate.

When a wave propogates and comes into contact with objects it can change, these changes are propogation  behaviours, these include absorption, reflection, refraction, scattering, diffraction, loss, free space path loss and multipath,

Understand RF characteristics and the relationship between wavelength and frequency. There is an inverse relationship between wavelength and frequency.


where λ=wavelenth, ƒ=frequency, c=speed of light

Simple transposition of the above formula can prove the following

The higher the frequency the shorter the wavelength.

The longer the wavelength the lower the frequency.

Typical wavelengths in the 802.11 RF spectrum are

2.4 GHz = 12.5cm or 4.8"

5.0 GHz = 6.25 cm or 2.46"

A wave will attenuate or lose signal strength as it apsses through a medium, a signal with a higher wavelength and therefore a higher frequency will attenuate more for a given medium. An RF wave may attanuate to such a degree that it is below the receive sensitivity of the receiving radio and cannot be demodulated successfully and has become unuseable.

Theoretically a radio wave will travel forever in a vaccuuum.

Coverage distance of a radio wave is dependent on attenuation, brick, glass, water and even air will attanuate an RF signal.

Attenuation in air is called free space path loss.

2.4 GHz radio waves will propogate better than 5.0 GHz radio waves.

Frequenccy is the number of times an event occurs in specified time, in RF it is measured in Hertz which is cycles per second. 2.4GHz is 2.4 billion cycles per second.

Amplitude can be seen as the power or strength of a wave and when you look at an oscilloscope you would see the height of the wave vary with a change in amplitude.

Phase is the reltionship between two or more waves that share the same frequency. Phase can be measured in distance, time or more usually degrees. If two waves are in alignment they are said to be in phase. Phase is imprtant as signal amplitude may increase or decrease dependent on the phase relationship and is known as multipath

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