04-01-2013, 04:00 PM
Link Budget Calculation
Link Budget.pdf (Size: 1.19 MB / Downloads: 144)
dB math
• decibels are a relative measurement unit unlike the absolute
measurement of milliwatts
• the decibel (dB) is an expression of the relationship between a
variable quantity and a known reference quantity
• the calculation of decibels uses a logarithm to allow very large or
very small relations to be represented with a conveniently small
number
• on the logarithmic scale, the reference cannot be zero because
the log of zero does not exist!
antenna gain
•an antenna element is a passive device
•the antenna can create the effect of
amplification by virtue of its physical shape
•by intentional radiator we mean the RF device
and all cabling and connectors up to, but not
including, the antenna
intentional radiator
•any reference to “power output of the
Intentional Radiator” refers to the power
output at the end of the last cable or connector
before the antenna
•if a 30 mW transmitter loses 15 mW of power in
the cables and another 5 mW from the
connectors, the power of the intentional
radiator is 10 mW
EIRP
•the EIRP is the Equivalent Isotropically Radiated
Power
•EIRP is the power actually radiated by the antenna
element and is important because
•it is regulated by the FCC or other regulatory
agency
•it is used in calculating whether or not a wireless
link is viable
PtMP
•for our example, since the antenna gain is 12dBi,
the power of the intentional radiator must be
reduced by 6dB
•this reduction will result in an intentional radiator
power of 24 dBm, or 36 dM of EIRP which is 4
Watts
•the end result is that the power at the intentional
radiator must never be more than 1 Watt, and the
EIRP must never be above 4 Watts for a PtMP
connection