## PA1B Accurate 43 dB Step Attenuator with steps of 1 dB

### PA1B Accurate 43 dB Attenuator

The **PA1B 43 dB attenuator **has a **excellent **accuracy, because each section is built with 6 resistors.

This very accurate attenuator is be very useful in **accurate **measurements in **dB** in the shack.

The line up of the attenuator sections, from 1 dB at the input, to 20 dB at the output gives an **excellent **power distribution.

The attenuator is easily built, with good **available** resistors from the E12-series.

The attenuator is **fast **to operate in CW contest QSO’s with low power, because of the combination of the sections of 3 dB, **7 dB** and 10 dB

#### 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 from 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.

### PA1B 43 dB Accurate Step Attenuator with steps of 1 dB

#### Nominal power

The power above each section in the schematic, is the **nominal power** of that section.

The nominal input power can be used for a long time, without overheating, damaging or reducing the accuracy of the attenuator. The nominal power is much lower than the maximum power.

I chose the power of each section, in such a way that the next section can handle the power of the previous section.

#### 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 **excellent** accuracy.

In attenuators that I have built and use, I noticed that not selected metal film resistors showed an accuracy of 1.5% or better. The accuracy of the first 3 section is 0.03 dB or better . 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.

Because the sections of 3 dB and 7 dB are consecutive sections, together they have the nominal power of the first section, which is **2 watt**. This is more than the single 10 dB section.

If you want to use the attenuator for 3 dB, he same applies to the section of 1 dB and 2 dB, that will give 3 dB when switched ON together. The nominal power is 3 W.

#### 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 not to use more input power, than the **nominal** power of the first section that is in use.

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 watt. (Not 3 watt because the sections are consecutive.)

#### 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.

### Inspired by. . .

I was inspired to design the **PA1B 43 dB Accurate Attenuator**, by the 81 dB attenuator of the **ARRL** and the **Hendriks** 41 dB Attenuator. The Hendriks 41 dB Attenuator follows the same design as the ARRL attenuator.

**But**. . . the ARRL attenuator and the Hendriks attenuator use the minimal number of 3 resistors in each attenuator section.

It takes 6 resistors to for an attenuator with an **excellent** accuracy.

That is why, a number of sections of these attenuators are not as **accurate** as an attenuator can be, with just a “few” resistors more.