How this calculator works
A pi pad has one resistor in series with the signal and one shunt resistor to ground at each port. The values preserve the specified resistive port matches while dissipating power to create attenuation. Unequal impedances impose a minimum loss. This model assumes ideal resistors and matched terminations, without parasitic inductance or capacitance.
Formula
K = 10^(A/20); Rseries = sqrt(Zs Zl)(K² - 1)/(2K); Rinput = Zs(K² - 1)/(K² + 1 - 2K sqrt(Zs/Zl)); Routput exchanges Zs and Zl.
Worked example
At 50 ohms on both ports and K = 2 (6.0206 dB), the series resistor is 37.5 ohms and both shunt resistors are 150 ohms.
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Frequently asked questions
Which resistor is connected in series?
R2 lies between the input and output. R1 and R3 go to the reference ground.
Why can an unequal pad reject a low loss?
Below the minimum attenuation the algebra requires a negative shunt resistance. This passive pad cannot realise that match.
Does a 6 dB pad halve power?
No. About 6.02 dB halves wave voltage and quarters power between matched equal-impedance ports.