Attenuators: Everything You Need to Know

What is an attenuator?

An attenuator is a resistive device that reduces the amplitude of a signal without adding distortion to it. The amplitude of a radio signal is the power, so an attenuator is used to reduce the power of a transmission.

When would we use one?

  • When making transmission measurements using highly sensitive equipment. The attenuator reduces power to protect the measuring equipment.
  • To produce low power for QRP transmissions. many modern HF rigs have a minimum power out of around 5 Watts. QRP operators usually use powers well below this.

Attenuator Circuits

The pi circuit

In the circuit below, known as a pi pad;

  • RA = Arm resistor
  • <math>RL_1</math> = leg resistor 1
  • <math>RL_2</math> = leg resistor 2


Approximate resistor values for a single pi pad are as follows


The T circuit

In the circuit below, known as a T pad;

  • RL = Leg resistor
  • <math>RA_1</math> = Arm resistor 1
  • <math>RA_2</math> = Arm resistor 2


Approximate resistor values for a single T pad are as follows


The O circuit

Also known as the O pad.

The H circuit

Also known as the H pad or I circuit(I pad)

Online attenuator calculators

How is attenuation measured?

Attenuation is measured in decibels (dB) of relative power. A guide to the attenuation-dB relationship is:


dB Attenuation Power in Power out
3dB 0.5 100W 50W
6dB 0.25 100W 25W
10dB 0.1 100W 10W
20dB 0.01 100W 1W
30dB 0.001 100W 0.1W


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