# 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
• [itex]RL_1[/itex] = leg resistor 1
• [itex]RL_2[/itex] = 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
• [itex]RA_1[/itex] = Arm resistor 1
• [itex]RA_2[/itex] = Arm resistor 2

Approximate resistor values for a single T pad are as follows

### The O circuit

Also known as the O pad.

## 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