1. Units of Power and Comparison
Key ideas for wireless are coverage and performance
To measure power, we can measure absolute or relative power
Absolute is compared to a known scale
Relative is to another signal
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2. Units of Power and Comparison
Comparative units can help:
compare coverage areas for different signals
Measure gain or loss
Measure the change in power
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3. Units of Power and Comparison
Units of Power (absolute)
Units of Comparison (Relative)
watt (W)
decibel (dB)
milliwatt (mW)
dBi
dBm
dBd
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4. Watt Basic unit of power Volts x Amps Ability to move/push/etc
1 ampere of current at 1 volt
Volts x Amps
Ability to move/push/etc
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5. MilliWatt (mW) 1/1000 th of a watt
Most equipment is measured in milliwatts
Usually 1 to 100 mw
FCC may allow up to 1 W in some cases, but it isn’t usually needed except in point to point.
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6. Decibel (dB) Base unit of comparison, not of power
Represents the difference between two values
Compare the power of two transmitters
Compare the output of a transmitter and received at the receiver
From the term bel
Bell Labs
10 to 1 ratio
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7. Decibel (dB) Bels are logarithmic 101=10 Log10(10)=1 102=100
Use the log10 to calculate
101=10
Log10(10)=1
102=100
Log10(100)=2
103=1000
Log10(1000)=3
104=10000
Log10(10000)=4
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8. Decibel (dB) Decibels are 10 x a bel bel=log10(P1/P2)
decibel= 10Xlog10(P1/P2)
No log math on the test!
We use decibels instead of watts as it is easier to write in many cases.
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9. Decibel (dB)
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10. dBi Antennas are compared to isotropic radiators
The difference between the theoretical isotropic radiator and the actual antenna can be measures in decibels isotropic (dBi)
Relative measurement
Change in power relative to an antenna
Measure of antenna gain
Measured at focus point
Always a gain, not a loss
No-gain or unity gain (0 dBi)
Think antenna Gain
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11. dBd A second relative measure of strength Decibel dipole
Decibel gain relative to a dipole antenna
Can also compare to dBi
Standard dipole is 2.14 dBi
If an antenna is 3 dBd the total is additive
2.14+3=5.14 dBi
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12. dBm Absolute measurement So 100 mW= +20dBm
Decibels relative to 1 mw of power
So 100 mW= +20dBm
Can also calc from a dBm value
PmW=log-1(PdBm/10)
1 mW is reference and 0 dBm is 1 mW
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13. dBm Why use dBm? Also, the 6dB rule Also helps when adding units
Easier to grasp -100dBm than mW
Also, the 6dB rule
If you double the distance between a received and transmitter, the received signal will decrease by 6 dB.
Also, every 6dBi of gain will double the usable distance of the RF signal
Also helps when adding units
If transmitter is +20dBm and the antenna is 5 dBi, the EIRP is 25 dBm
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14. Inverse Square Law
The 6 dB rule is based on Isaac Newton’s inverse square law.
Change in power is equal to the
square of the change in distance
If you double the distance, the power will change by (2xD)2
FSPL = (20log10(f)) + (20log10(D))
FSPL = path loss in dB
f = frequency in MHz
D = distance in miles between antennas
FSPL = (20log10(f)) + (20log10(D))
D = distance in kilometers between antennas
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15. RF Math No need for LOG on test. Rules of 10s and 3s
Provide for approximate values
For every 3 dB of gain (relative), double the absolute power (mW).
For every 3 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.
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16. RF Math
For every 3 dB of gain (relative), double the absolute power (mW).
For example, if your access point is configured to transmit at 100 mW and the antenna is rated for 3 dBi of passive gain, the amount of power that will radiate out of the antenna (EIRP) will be 200 mW
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17. RF Math
For every 3 dB of loss (relative), halve the absolute power (mW).
Conversely, if your access point is configured to transmit at 100 mW and is attached to a cable that introduces 3 dB of loss, the amount of absolute amplitude at the end of the cable will be 50 mW
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18. RF Math
For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10.
In another example, if your access point is configured to transmit at 40 mW and the antenna is rated for 10 dBi of passive gain, the amount of power that radiates out of the antenna (EIRP) will be 400 mW
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19. RF Math
For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10.
Conversely, if your access point is configured to transmit at 40 mW and is attached to a cable that introduces 10 dB of loss, the amount of absolute amplitude at the end of the cable will be 4 mW.
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20. RF Math dBm is a measure of power dB is a unit of change
dB can be applied to dBm
So, if you have +10dBm and increase by 3 dB, you have +13 dBm
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21. Step by Step Exercise 3 + X 2 10 - /
dBm mW 1
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22. RF Math Summary Log Functions Rules of 10 and 3 dBm =10 × log10(mW)
mW = log–1 (dBm ÷ 10) = 10(dBm ÷ 10)
Rules of 10 and 3
3 dB gain = mW × 2
3 dB loss = mW ÷ 2
10 dB gain = mW × 10
10 dB loss = mW ÷ 10
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23. RF Math Summary
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