### PA1B accurate 43 dB Attenuator

This accurate attenuator with 1 dB steps is ideal for accurate measurements.

This easy to build power attenuator is built with good available 2 watt resistors from the E12-series. By taking two resistors in parallel, the “strange resistor values” that are needed for an attenuator are made.

The attenuator is fast to operate and will bring you a lot of fun in contest QSO’s with very low power.

#### Resistors of 2 Watts

This attenuator is designed to be built with 18 metal film resistors of **2 Watt** and 18 metal film resistors of **400 mW**. In the schematic the resistors of 400 mW are designated with a star. All resistors are resistors of the E12-series.

Notice that metal film resistors will loose the heat via their wires. Carbon film resistors will loose the heat by radiating from their body.

#### Derating

The power above each section in the schematic is the **maximum power** of that section. This power can be “unleashed” only for a short while, say 10 to 20 seconds.

When you choose a nominal power of 25% of the maximum power (D=0.25), the section of 1 dB and of 2 dB have a **nominal power** of 2 W. The nominal power is 0.25 x 8 = 2 watt. The section of 3 dB and 7 dB have a nominal power of 1.5 W. The section of 10 dB has a nominal power of 1 W and the section of 20 dB has a nominal power of 500 mW.

#### E12-series

The value of the resistors of an accurate attenuator must have a very specific value that is not available as an E12-series value. By placing two resistors of the E12-series in parallel we can make any value.

For example: in the 10 dB attenuator section the resistors of 100 ohm in parallel with 2700 ohm form a resistor with the wanted value of 96.25 ohm with an accuracy that is better than 0.5%

#### Accuracy

Accurate attenuators are build with resistors that are not in available in the E12-series. But by using two resistors of the E12-series in parallel, **any** value can be made.

This design with 6 resistors per section will show an very good accuracy.

In attenuators that I built I noticed that not selected metal film resistors showed an accuracy of 1.5% or better. The accuracy of the first 3 section is about 0.03 dB. The accuracy of the sections of 7 dB and 10 dB will be about 0.05 dB. The accuracy of the 20 dB section will be about 0.1 dB.

#### The section of 7 dB

Unlike other designs I chose a value of **7 dB** for the attenuation of the fourth section.

I use an attenuator with a section of 3 dB and one of 7 dB in CW contest QSO’s with QRPp. The combination of **3 dB**, **7 dB** and **10 dB** creates the possibility to **instantly** change the power in step of 3 dB or 4 dB, by switching one section off and an other section on.

#### More power

The sections of 3 dB and 7 dB can be both switch ON to get an attenuation of 10 dB. The power is then distributed over 2 sections. Together they can handle a maximum power of **6 watts**. This is more than the single 10 dB section.

The same applies to the section of 1 dB and 2 dB, that will give 3 dB when switched ON together.

#### WSPR with low power

For WSPR with low power, I advise to use an attenuation of 10 dB or 20 dB. An Attenuation of 3 dB is too small with WSPR. With 3 dB you will not notice any difference.

Because WSPR transmits for 2 minutes, I advise to use a a derating of 25%.

Tip: Switch ON the sections of 1 dB, 2 dB and 7 dB to get an attenuation of 10 dB.

The sections of 1 dB, 2 dB, 7 dB and 10 dB will give 20 dB.

With the sections of 1 dB and 2 dB in front, the **nominal power** will be 2 watts, because the maximum power will be 8 watts, with a derating of 25%.

#### CW QSO’s with low power

In contest QSO’s most of the time I use the sections of 3 dB, 7 dB and 10 dB, to reduce my power. When I use very low power, I always answer a CQ. Only when my signal is not heard, I increase my power with a step of 3 dB or 4 dB. E.g. by switching from 17 dB to 13 dB. I always listen and transmit through the attenuator. I tune with the attenuator set to 0 dB.

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