RF attenuation calculator

Pi attenuator calculator

Design a symmetrical resistive Pi pad for matched source and load impedances. Calculate ideal resistor values, test practical E24 substitutions and estimate the resulting attenuation, match and power budget.

Calculated network

Series resistor

71.15 Ω

Input shunt

96.25 Ω

Output shunt

96.25 Ω

Nearest E24 starting point

Series: 68 Ω
Each shunt: 100 Ω

Estimated attenuation: 9.629 dB
VSWR at design impedance: 1.007:1

E24 values are a procurement starting point. Frequency-dependent parasitics and resistor tolerance are not modeled.

Ideal load power

100 mW

Ideal pad dissipation

900 mW

How to use the calculator

Start with the electrical interface—not the resistor series.

01
Set the reference impedance
Enter the impedance the source and load are designed to present. The equations assume equal terminations.
02
Choose insertion loss
Enter the required attenuation in decibels. The tool converts dB to the linear voltage ratio used by the network equations.
03
Check the realizable network
Compare ideal values with E24 parts, then evaluate the changed attenuation, VSWR, tolerance and power requirements.

Equations and assumptions

The network is a symmetrical Pi pad with one series resistor and two equal shunt resistors.

Voltage ratio

K = 10^(A_dB / 20)

Series resistor

Rₛ = Z₀(K² − 1) / 2K

Each shunt resistor

Rₚ = Z₀(K + 1) / (K − 1)

These equations preserve the specified reference impedance at both ports when the opposite port is terminated in that same impedance. The E24 check uses an ABCD-network calculation to estimate S21, S11 and VSWR after rounding.

Use the result as a design starting point. Verify resistor tolerance, voltage rating, pulse energy, individual dissipation, package parasitics, PCB geometry and measured S-parameters before releasing an RF or high-power design.

Practical design checks

A mathematically correct pad can still perform poorly when the real interface differs from the model.

Termination and reference plane

Confirm where 50 Ω or 75 Ω is defined and whether cables, connectors, fixtures and instruments preserve it. Mismatch changes measured attenuation.

Frequency and layout

At RF, minimize series trace length and shunt-ground inductance. Small resistor geometry and short ground returns normally matter more as frequency rises.

Tolerance and temperature

Resistor tolerance alters loss and return loss; temperature coefficient can move those values during operation. Model the worst combination when match is important.

Power distribution

Total pad loss is not divided equally among the three resistors. Calculate each element's RMS and peak dissipation for the actual source level and mismatch case.

Frequently asked questions

What impedance should I enter for a Pi attenuator?

Use the reference impedance that both the source and load are intended to present. RF systems commonly use 50 Ω, while some video and cable systems use 75 Ω. This calculator assumes the same impedance on both sides.

Why do the E24 resistor values change the attenuation?

The exact equations usually produce non-standard resistance values. Rounding each resistor changes both transmission loss and the input/output match, so the calculator estimates attenuation and VSWR again using the suggested E24 values.

Does the power result give the required wattage for each resistor?

No. It shows ideal total power delivered to the load and total power dissipated by the pad. Individual resistor dissipation depends on circuit voltages, mismatch, peaks and layout, so each resistor must be checked separately with margin.

Will the calculated pad have the same attenuation at every frequency?

Only in an ideal resistive model. At higher frequencies, resistor parasitics, pads, traces, ground inductance and connector transitions change insertion loss and return loss. Validate the physical design across its operating band.