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

PI-attenuator

Approximate resistor values for a single pi pad are as follows

Pi_pad

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

250px-T-pad

Approximate resistor values for a single T pad are as follows

T_pad

The H circuit

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

The O circuit

Also known as the O 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:

 

dBAttenuationPower inPower out
3dB0.5100W50W
6dB0.25100W25W
10dB0.1100W10W
20dB0.01100W1W
30dB0.001100W0.1W

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