Attenuators

Related wiki pages; Apparatus, Dummy Load

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.

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

Attenuation dB	Arm resistor	Leg resistors
3dB	17.6	292.4
6dB	37.4	150.5
10dB	71.2	96.2
20dB	247.5	61.1

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.

How is attenuation measured?

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

Attenuation dB	Arm resistors	Leg resistor
3dB	8.5	141.9
6dB	16.6	66.9
10dB	26	35.1
20dB	40.9	10.1

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