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. ١ 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 ٥