# CWNA Chapter 3 – Radio Frequency Components, Measurements, and Mathematics

My Notes from chapter 3 of the CWNA study guide

RF Components

• Transmitters
• Initial component in the creation of wireless medium.
• Transmitters job is to begin the RF communications
• Transmitter starts AC signal oscillating  at the RF frequency
• Transmitter takes data provided and modifies the AC signal using a Modulation technique
• Antenna
• Two functions when connected to the transmitter it collect the AC signal received from the Transmitter and directs or radiates it out.
• When connected to the receiver it takes the signal and directs the AC signal to the receiver.
• Two ways to increase the power output of an antenna
• Generate more power at the transmitter
• Direct or focus the RF signal that is radiating from the antenna
• Takes the carrier signal received from the antenna and translate the modulated signals into 1s and 0s
• Then passes the data onto the network/computer/device
• FCC defines and intentional Radiator as “a device that intentionally generates and emits radio frequency energy by radiation or induction”
• Regulatory bodies limit the amount of power that is aloud to be generated by an IR
• Components making up the IR:
• Transmitter
• All cables
• Connectors
• Any other equipment (grounding, lightning arrestors, amplifiers, attenuators, and so forth) between the transmitter and the and antenna
• IR power measured at the connector that provides the input to the antenna
• Power usually measured in Milliwatt or Decibels relative to 1 Milliwatt (dBm)
• Equivalent Isotopically Radiated Power (EIRP)
• Highest RF signal strength that is transmitted from a particular antenna
• Regulatory bodies limit the amount of EIRP of an antenna

Units of Power and Comparison

• Watt ( W )
• Unit of power
• One watt is equal to 1 ampere (A) of current flowing at 1 volt (V).
• Watt ( W ) = Volt (V) x Ampere (A)
• Milliwatt (mW)
• Unit of power
• 1 Milliwatt = 1/1,000 watts
• Decibel (dB)
• Unit of comparison
• Difference between two values. dB is a relative expression used to represent a difference between two values
• In wireless networking dBs are often used to compare the power of two transmitters or more often to compare the difference or loss between the EIRP output of a transmitters antenna and the amount of power received by the receivers antenna
• dBi
• Used to compare the output of one antenna to another
• The gain or increase of power from an antenna when compared to what an isotropic radiator would generate is known as decibels isotropic (dBi)
• dBd
• Relative measurement and not a unit of power
• Antenna industry users two dB scales to describe the gain of antenna
• dBi
• dBd
• dBd decibel gain relative to a dipole antenna
• dBd value is the increase in gain of an antenna when compared to the signal of a dipole antenna
• How to compare two antennas one in dBi and other in dBd
• Standard dipole antenna has dBi of 2.14
• If antenna has value of 3 dBd, this means it is 3 dB greater than a dipole antenna
• Because value of dipole antenna is 2.13dBi all you need to do is add 3 to 2.14
• So a 3dBd antenna is equal to a 5.14dBi antenna
• dBm
• Compares a signal to 1 Milliwatt of power
• Decibels relative to 1 Milliwatt
• 0dBm = 1 Milliwatt
• dBm = 10 x log10(PmW)
• +6dB doubles the distance of the usable signal
• -6dB halves the distance of the usable signal
• dBm makes it easy to calculate the effects of antenna gain on a signal.
• Inverse Square Law
• Originally developed by Isaac Newton
• Law states that the change in power is equal to 1 divided by the square of the change in distance
• As the distance from the source of the signal doubles the energy is spread out over four times the area, resulting in one-fourth of the original intensity of the signal.
• Free space path loss formula:
• FSPL = 36.6 + (20log10(F)) + (20log10 (D))
• FSPL = Free space path loss
• F = Frequency in MHz
• D = Distance in miles between antennas
• FSPL = 32.4 + (20log10 (F)) + (20log10 (D))
• FSPL = Free space path loss
• F = Frequency in MHz
• D = Distance in Kilometres between antennas
• FSPL is based on Newtons inverse square law.

RF Mathematics

• Rules of 10s and 3s
• For ever 3dB of gain (relative), double the absolute power (mW)
• For every dB of loss (relative), halve the absolute power (mW)
• For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10
• For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10
• Noise Floor
• Noise floor is the or background level of radio energy on a specific channel.
• This can include modulated or encoded bits from nearby 802.11 transmitting radios or unmodulated energy coming from non-802.11 devices such as microwave ovens, Bluetooth device
• The Amplitude of the noise floor varies in different environments.
•  2.4GHz will have higher Noise floor than 5GHz as the bands are more crowded.
• Signal-to-Noise Ratio (SNR)
• Is the difference in decibels between the received signal and the background noise level (noise floor), not actually a ratio.
• Example:
• Radio receives a signal of -85dBm and the noise floor is measured at -100dBm the difference is 15bD therefore the SNR is 15dB • The power level of an RF signal required to be successfully received by the receiver radio.
• The lower the power level that a receiver can successfully process the better the receive sensitivity.
• In WLAN equipment the receive sensitivity is usually defined as a function of the network speed.
• Link budget is the sum of all the planned and expected gains and losses from the transmitting radio, through the RF medium, to the receiver radio • Fade Margin / System Operating Margin
• Fade Margin is a level of desired signal above what is required.
• Effectively is a margin added to the required signal level to account for outside factors causing the signal level to fluctuate
• Normally used for outdoor WLAN bridge links.

## 2 thoughts on “CWNA Chapter 3 – Radio Frequency Components, Measurements, and Mathematics”

1. Daniel Ordonez (@WifiguyMX)

Thanks for your notes!! Don’t let it down I know we comment quit but keep it doing 🙂

I kept behind because I’m having some issue with a deployment but I hope take it back

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2. Tamer Elsheikh

Really nice work. thank you

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