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.
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
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.
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
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
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
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 >>.