## PA1B Attenuators

### Attenuator design

This attenuator is a **Pi-type**, 50 ohm attenuator for 10 dB. It is designed for 50 ohms and must be **terminated** with **50 ohms**, to give the right attenuation. Please notice that 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 attenuators can be switched on or off. With the switch in the lower position the signal will go through the attenuator.

### Numbers of the resistors

The resistors in the **PA1B attenuators** are numbered from **left** to **right**. In the PA1B attenuators and PA1B power attenuators, each number of a resistor, like **R1**, **R2** or **R3** refers to **two** (or more) resistors **in parallel**. Attenuators built with **6 resistors** are very **accurate**.

### A very accurate 10 dB attenuator

The PA1B attenuators are build with good **available **resistors from the E12-series. Use carbon resistors or metal film resistors.

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 two resistors in parallel.

To make a 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%. The resistors that I have measured with the ohm meter, before building several attenuators, showed a accuracy of 1.5 % or better. The combination of each two resistors in **parallel**, showed an accuracy of 2% or better. The attenuators built with **6 resistors** are very **accurate**.

#### It takes **6 resistors** to make a **very accurate** attenuator.

### 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 diferent 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. Please notice that the power of each attenuator is the **maximum power** and with no cooling of the resistors, they will easily become hot in a short while.

The basic 10 dB attenuator consists of 6 resistors and is good for a maximum input power of 500 mW – Nominal 125 mW. Built with 9 resistors the attenuator is suitable for 1.6 W. This is a nominal power of 400 mW.

With 13 resistors the attenuator is good for 2.8 W (Nominalpower 0.7 W or 1.4 W in CW) Built with a total of 20 inexpensive resistors the attenuator can handle a maximum input power of 5 W or a nominal power of 1.25 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**, for an input power of 5 watts, using 1 watt resistors. Tjeerd uses the attenuator, to reduce the power of 5 watts, that is coming from the set, to reduce his WSPR signal to 500 mW. He noticed that during the 2 minute transmission period the attenuator gets **warm**.

You can find the drawing of the **symmetrical 10 dB attenuator**, for an input power of 5 watts, on the **weblog of Tjeerd >>**.