INSERTION LOSS Û POWER GAIN Û RETURN LOSS
5.1 INTRODUCTION
™ In microwave systems
)
insertion loss Û return loss FF used to
quantify
absorption Û
reflection
of
microwave power.
5.2
INSERTION LOSS AND ATTENUATION
™ At low frequency
) Ideally FF wires and cables have no
resistance;
) Actually FF they have a small resistance
power dissipated as heat.
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™ At high frequency
) Ideally FF cables or waveguide have no
resistance;
) Actually FF they absorb microwave power
due to the skin depth FF absorption power FF
converted to heat
™ Two reason for
the power coming out of the line FF less than that
entering the line.
) attenuation FF absorption FF dissipation
)
reflection FF a component or device reflects
some of the input power without absorbing any
™ Insertion loss
FF IL
FF accounts for
reflection as well as attenuation.
™ IL FF the same as attenuation
) if there is no power reflection.
IL = Pin / Pout
pure number
™ Pin = 10 mW Û Pout = 8 mW
2 mW of Pin FF absorbed or reflected.
٢
IL = Pin / Pout = 10 mW / 8 mW = 1.25 pure number
™ IL FF applies to passive component only
™ active devices FF amplifier FF add power to
the input FF Pout >> Pin.
) IL = 1 FF for truly lossless component
Pin = Pout
) IL = ∞ FF for a component of 100 %
absorption or reflection
) IL (dB) =
Pin(dBm) - Pout (dBm).
Example ( 5. 2. 1 ) Find IL for Pin = 17 dBm (50 mW)
and Pout = 10 dBm (10 mW)
IL (dB) 17 dBm - 10 dBm = 7 dB
OR
IL = 50 mW /10 mW= 5
5 = 7 dB
™ For manufacturers
see chapter (1)
FF to mark the attenuation of
the products in dB
) Cable marked 0.1 dB / ft FF one foot has 0.1
dB attenuation
FF five feet FF 0.5 dB
attenuation.
٣
™ If any two of the three quantities FF IL Û Pin Û Pout
are known the third FF easily determined.
IL = Pin / Pout
Pout = Pin / IL
Pin = Pout x IL
™ In dBm and dB
IL (dB) = Pin (dBm) - Pout (dBm)
Pout (dBm) = Pin (dBm) - IL (dB)
Pin (dBm) = Pout (dBm) + IL (dB)
Example (5.2.2)
™ Find IL in number and in dB for
Pin = 100 mW ( 20 dBm) and Pout = 50 mW (17dBm)
) In number :-
IL = 100 mW/50 mW = 2
) IndB :-
IL (dB) = 20 dBm - 17 dBm = 3 dB
™ Find Pout in number and in dB for
٤
Pin = 5 mW ( 7 dBm) and IL = 5 (7dB)
) In number :-
Pout = 5 mW/5 = 1 mW
) In dB :-
Pout (dBm) = 7 dBm - 7 dB =0 dB
™ Find Pin in number and in dB for
Pout = 3 mW ( 5 dBm) and IL = 2 (3 dB)
) In number :-
Pin = 3 mW x 2 = 6 mW
) In dB :-
Pin (dBm) = 5 dBm + 3 dB = 8 dBm
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