RF Attenuator Resistor Values

- a table or chart of attenuator resistor values for use in Pi and T format resistor RF attenuator pads.

The calculations for the "Pi" and "T" RF attenuator resistor values are relatively straightforward. However it is often convenient to have a chart that provides the attenuator resistor values in a tabular format.

The most common format for RF attenuators is in a 50 ohm system, i.e. one with a characteristic impedance of 50 ohms. Accordingly the table given below is calculated for a system with an impedance of 50 ohms.

Attenuator resistor definitions

The diagrams below show the different attenuator resistor definitions that relate to the attenuator resistor values in the table.

One of the most popular forms of resistor attenuator pad, is the T section pad. It gains its name from the topology of the attenuator pad. The attenuator resistor values are given for this format - the resistor identification in the table relating to the numbers in the diagram.

The basic attenuator T pad showing two series resistors and one to ground
T resistor attenuator pad

The attenuator resistor values are also given for the Pi section attenuator pads. Often there is little to choose between the Pi and T section attenuator pads - often it is the personal preference of the designer.

The basic attenuator Pi pad showing one series resistors and two to ground, one at either end.
Pi resistor attenuator pad

The bridged T attenuator shown below is often used, although it has four resistors in each section rather than the three used in other attenuator pad formats. It is often used in variable attenuators because of the fact that only two resistor elements need to be varied.

The bridged T attenuator pad showing how the circuit arrangement of the basic T attenuator pad has been altered.
Bridged T Attenuator Pad

RF attenuator resistor values chart

The table given below provides the resistor values for Pi and T pad RF resistor attenuator circuits. The values in this table have been calculated for a characteristic impedance of 50 ohms.

RF Attenuator Pad Resistor Values Table
Loss in dB R1 R2 R3 R4 R5 R6
1 2.9 433 870 5.8 6.1 410
2 5.7 215 436 11.6 12.9 193
3 8.5 142 292 17.6 20.6 121
4 11.3 105 221 23.8 29.3 85.4
5 14.0 82.2 179 30.4 38.9 64.3
6 16.6 66.9 151 37.3 48.9 50.3
7 19.1 55.8 131 44.8 61.9 40.4
8 21.5 47.3 116 52.8 75.6 33.0
9 23.8 40.6 105 61.6 90.9 27.5
10 26.0 35.1 96.2 71.2 108 23.2
11 28.0 30.6 89.2 81.7 128 19.6
12 29.9 26.8 83.5 93.2 149 16.8
13 31.7 23.6 78.8 106 173 14.4
14 33.4 20.8 74.9 120 201 12.4
15 34.9 18.4 71.6 136 231 10.8
16 36.3 16.3 68.8 154 265 9.4
17 37.6 14.4 66.5 173 304 8.2
18 38.8 12.8 64.4 195 347 7.2
19 39.9 11.4 62.6 220 396 6.3
20 40.9 10.1 61.1 248 450 5.6

Table of resistor values for 50 ohm attenuators
Resistor designations refer to diagrams above

NB:Attenuator resistor values in the table are for a 50 ohm system.

Attenuator resistor values for other impedance systems

The attenuator resistor values in the table are given for a 50 ohm system as this is the most likely impedance system required. However it is recognised that other impedance systems may also be used.

To convert the values in the table to another value of impedance, they should be multiplied by the factor Z / 50, where Z is the characteristic impedance of the required system.

The RF attenuator resistor values chart given above enables resistors to be chosen for popular values of attenuation more easily than having to calculate each one individually. It provides a quick, at-a-glance reference for the attenuator resistor values. While only covering integer dB steps, it is unlikely that a any intermediate values would be needed. Also any attenuators providing more than 20 dB are likely to be made up from several stages each having a maximum of 20 dB.

By Ian Poole

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