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Attenuator pad

Pi/T Attenuator pad calculator

Compute Resistors values for symmetrical Pi and T Attenuator pads at a chosen Impedance.

Real-world Attenuator pads need non-inductive Resistors and adequate cooling for high power.

Symmetric Pi and T Attenuator pads tame RF level differences by presenting matched Impedances while dropping the signal by the requested Attenuation. The calculator applies the standard Voltage ratio equations:

A = 10^{A_{\text{dB}} / 20}

Pi network Resistors

R_{\text{series}} = Z_0 \cdot \frac{A^2 - 1}{2A} R_{\text{shunt}} = Z_0 \cdot \frac{A + 1}{A - 1}

T network Resistors

R_{\text{series}} = Z_0 \cdot \frac{A - 1 / A}{2} R_{\text{shunt}} = \frac{2 Z_0 A}{A^2 - 1}

to keep both ports matched.

Use the results to assemble a measurement pad or to protect a receiver front end, e.g. a 6 dB Attenuator pad on a 50 ohm line lowers the voltage by about half while preserving SWR. Choose Resistors with adequate power ratings and keep lead lengths short to avoid stray Inductance at VHF and above.

Ham Calculator

Three practical tools for day-to-day RF work: attenuator pads, balun design, and LC resonance.

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