Once in common use in radio, the passive attenuator is used less frequently today because of active circuitry and bridging impedances instead of matched impedances.

The basic attenuator is a T pad, which provides an unbalanced attenuator path referenced to ground. The balanced version is an H pad.

A pad can have equal or unequal input and output impedance.

To calculate the values for a pad, you must know the desired input impedance, output impedance and loss in dB.

To calculate the values for an H pad, first calculate the values for a T pad, and then divide the values of the input and output resistors (R₁ and R₂) in half to equally divide the load.

The value of loss in dB must be converted to a ratio of current, voltage or power. The formula to determine K is

K = 10^{(desired attenuation in dB/20)}

Note: For all these formulae, Z and R values are expressed in ohms.

Equal Impedance Pad Calculation

The formulae to calculate a pad of equal input and output impedance is simplified because some of the values effectively cancel each other. Use the formulae in the next section to calculate values for pads with unequal impedances.

When Z₁ equals Z₂, R₁ will equal R₂.

Unequal Impedance Pad Calculation

The value of R₃ can be used to determine the values of R₁ and R₂, so it is easier to calculate it first. The equation for R₃ can be substituted in the equations for R₁ and R₂ if you prefer to repeat the work.

T and H pads are bidirectional, so either side can be used as the input. When calculating the values for a pad with unequal impedances, the value of Z₁ must be greater than the value of Z₂.

Javascript Pad Calculator

This calculator requires a Javascript-capable browser.

This Javascript attenuator calculator provides resistor values for unbalanced T-pads and for balanced H-pads. Use resistor values closest to the values shown. Attenuator calculations greater than 40dB may not provide satisfactory results. Enter the values for input impedance, output impedance and the desired attenuation then click the calculate button. The results will show in the diagram below. Do not enter data into the diagram.

Input impedance of attenuator ohms	Output impedance of attenuator ohms	Attenuation required (1-40): dB

Unbalanced T-pad resistor values - results

R1 OHMS		R2 OHMS
Z1 Input Impedance OHMS	R3 OHMS	Z2 Output Impedance OHMS

Balanced H-pad resistor values - results

R1/2 OHMS		R2/2 OHMS
Z1 Input Impedance OHMS	R3 OHMS	Z2 Output Impedance OHMS
R1/2 OHMS		R2/2 OHMS

Note: The Javascript for the calculator appears on many websites without any attribution. It is included here for convenience.