RF Wireless Characteristics · Topic 1 of 4
RF Spectrum and Decibels
Understand the radio spectrum used by Wi-Fi, channels and channel widths, and how to use dB, dBm and dBi to calculate power, gain and path loss.
- 5 min
- Wi-Fi uses the 2.4 GHz and 5 GHz bands, divided into channels.
- Channels 1, 6 and 11 are the usual non overlapping 2.4 GHz set at 20 MHz.
- Higher frequencies lose more signal and travel a shorter distance.
What it is
Radio frequency (RF) energy is the signal that carries wireless data, and decibels are the units used to measure it.
Wi-Fi works in the 2.4 GHz and 5 GHz bands, and newer devices may also use 6 GHz where it is allowed. Each band is divided into channels.
Decibels let you add and subtract gains and losses instead of multiplying large ratios. Every link budget and every power setting uses them.
How it works
Frequency sets the channel, and the decibel scale sets how strong the signal is.
Frequency and wavelength
Frequency is how many wave cycles happen each second. Wavelength is the distance one cycle covers. The higher the frequency, the shorter the wavelength.
Lower frequencies travel further and pass through walls better. Higher frequencies carry more data but lose more signal over the same distance.
Channels and channel width
A channel is a slice of the band. Its width is how much spectrum it uses, commonly 20 MHz, and wider on newer standards.
| Band | Channel behaviour |
|---|---|
| 2.4 GHz | Few channels, and neighbouring channels overlap. Channels 1, 6 and 11 are the usual non overlapping set at 20 MHz. |
| 5 GHz | Many more channels, and several non overlapping ones. Wider channels use up the available channels quickly. |
A wider channel gives a higher top data rate, but it needs a stronger signal and raises the noise it collects. In busy places a narrower channel is often the better choice.
The decibel scale
A decibel (dB) is a ratio between two power levels. It is not an absolute value.
- An increase of 3 dB roughly doubles the power.
- A decrease of 3 dB roughly halves it.
- An increase of 10 dB multiplies the power by ten.
- A decrease of 10 dB divides it by ten.
These steps add together. A gain of 13 dB is a gain of 10 dB and a gain of 3 dB, so the power rises about twenty times.
dBm and dBi
Two units give the decibel an absolute meaning.
| Unit | Reference | Used for |
|---|---|---|
| dBm | 1 milliwatt | Transmit power and received signal |
| dBi | An ideal antenna that radiates equally in all directions | Antenna gain |
0 dBm is 1 mW. 20 dBm is 100 mW. 30 dBm is 1 W. Received signals are negative numbers, because they are far weaker than 1 mW. A reading of minus 50 dBm is much stronger than minus 80 dBm.
When a question adds or subtracts dB values, do the arithmetic in dB. Only convert to mW if the question asks for it.
Path loss
Signal weakens as it travels. In free space, doubling the distance adds about 6 dB of loss.
Loss is also higher at a higher frequency. A 5 GHz link loses more signal than a 2.4 GHz link over the same distance.
A simple link budget adds the gains and subtracts the losses:
received signal = tx power + tx antenna gain - cable loss - path loss + rx antenna gainSetting transmit power
The transmit power on the legacy wireless package is set on the interface in dBm.
/interface wireless set wlan1 tx-power-mode=all-rates-fixed tx-power=17
/interface wireless print detailtx-power-modedecides how the power value is applied.tx-poweris the power in dBm.
On the wifi driver the same setting is a tx-power value in the configuration. Remember that more power does not fix a weak client, because the client must also be able to transmit back to the access point.
What to remember for the exam
Seven facts cover this topic.
- Wi-Fi uses the 2.4 GHz and 5 GHz bands, divided into channels.
- Channels 1, 6 and 11 are the usual non overlapping 2.4 GHz set at 20 MHz.
- Higher frequencies lose more signal and travel a shorter distance.
- 3 dB doubles or halves power, and 10 dB multiplies or divides it by ten.
- dBm is power against 1 mW, and dBi is antenna gain against an ideal antenna.
- Doubling the distance in free space adds about 6 dB of path loss.
- Gains and losses in dB are added and subtracted.
dBm and dB are not the same. A power in dBm plus a gain in dB gives a power in dBm.
Common mistakes
These four errors cost marks in calculations.
Multiplying dB values. Decibels add and subtract. Only the raw power ratios multiply.
Confusing dBi with dBm. One is antenna gain and the other is power.
Reading negative dBm the wrong way. A value closer to zero is stronger, so minus 55 is better than minus 75.
Assuming wider is always better. A wide channel raises the top rate but also the noise, and it leaves fewer free channels.
Check yourself
Work through these three questions.
How much does a 3 dB increase change the power?
It roughly doubles it. A 10 dB increase multiplies the power by ten.
What is the difference between dBm and dBi?
dBm measures power against 1 milliwatt. dBi measures antenna gain against an ideal antenna that radiates equally in all directions.
What happens to path loss in free space when the distance doubles?
It rises by about 6 dB. A higher frequency also loses more signal over the same distance.
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