PA1B Attenuators

Attenuator design

This attenuator is a Pi-type, 50 ohm attenuator for 10 dB.

The resistors R1 at the input and R3 at the output of the attenuator, always have the same value.

In a 10 dB attenuator they both must be 96.25 ohm. The resistor R2 has a value of 71.15 ohm.

The values of 96.25 ohm and 71.15 ohm are not available in the E12-series.

But luckily any resistor value can be made with great accuracy, by placing two resistor of the E12-series in parallel.

10 dB attenuator with 3 resistor values

 Precise resistor values for a very accurate 10 dB attenuator 

Any resistor value

You can make any resistor value you want, with two resistors from the E12-series in parallel,

Numbers of the resistors

The resistors in the PA1B attenuators are numbered from left to right.  Attenuators built with 6 resistors are very accurate.

In the PA1B power attenuators, each number of a resistor, like R1, R2 or R3 refers to two (or more) resistors in parallel.

By placing several resistors in parallel, the attenuator can dissipate more power. The higher the power, the more resistors are used.

PA1B Attenuator

Numbered from left to right

Design table for PA1B attenuators

This table give the precise resistor values, that are needed to design accurate PA1B power attenuators for 1 dB, 2 dB, 3 dB, 7 dB, 10 dB and 20 dB.

Design table for attenuators

Design table for attenuators

Accurate 10 dB attenuator

Very accurate 10 dB attenuator

Very accurate attenuator

A very accurate 10 dB attenuator

Accurate attenuators can be built with good available resistors from the E12-series.

This very accurate 10 dB attenuator is built with 6 resistors. The resistors R1, R2 and R3 are compound resistors and are each formed by two resistors, from the E12-series in parallel.

The resistor R1  and R3 have a wanted value of 96.25 ohm and are both formed by a resistor of 100 ohm and a resistor of 2700 ohm in parallel.

The resistor R2  has a wanted value of 71.15 ohm and is formed by a resistor of 82 ohm and a resistor of 560 ohm in parallel.

Precise value

To make a very precise combination of resistors in parallel, I take the next higher value in the E12-series. The resistor of 100 ohm is chosen, because it is the next value in the E12-series, that is higher than the wanted value of 96.25 ohm. The second resistor in parallel is 2700 ohm. This combination comes very close to the wanted value of 96.25 ohm.

The resistor of 71.15 ohm is formed by a resistor of 82 ohm, parallel to a resistor of 560 ohm.

Accuracy

The accuracy of the individual resistors is always better than 5%. All resistors that I have measured with the ohm meter, while building several attenuators, showed a accuracy of 1.5 % or better.

The combination of each two resistors in parallel, shows an accuracy of 2% or better.

The attenuators built with 6 resistors are very accurate.

6 Resistors

A very accurate attenuator must be built with 6 resistors of the E12-series.

10 dB power attenuators

The 10 dB attenuators shown here, are all built with good available 1/4 W resistors of the E12-series.

Each of the 4 attenuators is designed for a different value of input power.

They cover the power spectrum from 500 mW to 5 watts.
The higher the input power the more resistors are needed.

Power

Please notice that if you unleash the maximum power on an attenuator with no cooling, the resistors will easily become (very) hot in a short while.

This can reduce the accuracy of the attenuator and can can shorten the life span.

Attenuator for 10 dB - 500 mW

With 6 resistors of 1/4 W the attenuator is good for a nominal input power of 125 milliwatt.
The maximum input power is four time as high and is 500 mW.

Nominal input power 125 mW
Nominal input power in CW is 250 mW

Attenuator for 10 dB -1.6 W

With 9 resistors of 1/4 W the attenuator is good for a nominal input power of 400 milliwatt.
The maximum input power is four times as high and is 1.6 W.

Nominal input power 400 mW
Nominal input power in CW is 800 mW

Attenuator for 10 dB - 2.8 W

With 14 resistors of 1/4 W the attenuator is good for a nominal input power of 700 milliwatt.
The maximum input power is four time as high and is 2.8 W.

Nominal input power 700 mW
Nominal input power in CW is 1.4 W

Attenuator for 10 dB - 5 W

With 18 resistors of 1/4 W the attenuator is good for a nominal input power of 1.25 Watt.
The maximum input power is four time as high and is 5 W.

Nominal input power 1.25 W
Nominal input power in CW is 2.5 W

The resistors will get warm

Tjeerd “Gose” PA3GNZ was inspired by this page about attenuators and some additional info about power attenuators, that I sent him.
So he designed a 10 dB attenuator, using 2 watt resistors.

Tjeerd uses the attenuator, to reduce the power of his WSPR signal.

He noticed that during the 2 minute transmission period the attenuator gets warm. You can find the drawing of the symmetrical 10 dB attenuator, on the weblog of Tjeerd >>.