PA1B Power attenuators
A power attenuator is an attenuator that can dissipate a certain power.
Power distribution in power attenuators
The 4 schematics below, show the resistor values and the power distribution in Pi-type, 50 ohm attenuators of 3 dB, 7 dB, 10 dB and 20 dB, for an input power of 5 watts. The resistors R1 to R12 have values that can not found in the E12-series.
But with two or more resistors from the E12-series in parallel, we can make any value we want.
The attenuators have to dissipate all the power that is not delivered to the output. Please notice that if the attenuator has to dissipate a continues power of 5 watts, it is good practice to take a power attenuator that can handle 20 watts. Specially when the power attenuators has no cooling.
The attenuator of 20 dB has to dissipate the highest power of the 4 attenuators, since the power to the output is only 50 mW.
The attenuator of 3 dB has to dissipate the lowest power, since half of the power is delivered to the output.
The 3 dB attenuator reduces the power to 1/2. Half of the power is directly transferred to the output. The other half of the power will be dissipated by the attenuator. R2 dissipates 1/4 of the input power.
The 7 dB attenuator reduces the power to 1/5. R4 and R5 dissipate each 1/3 of the input power.
The 10 dB attenuator reduces the power to 1/10. R7 dissipates half of the input power and R8 dissipates 1/3 of the input power.
The 20 dB attenuator reduces the power to 1/100. R10 dissipates most of the input power and R11 dissipates 1/6 of the input power. With an input power of 5 W, the resistor R10 has to dissipate 4.1 W.
The PA1B 3 dB, 7 dB, 10 dB and 20 dB attenuator
This attenuator is very fast in QSO’s. You never miss an opportunity to answer a CQ.
Fast in QSO’s
A simple way to reduce the power of your QRP set to milliwatt level, without modifying the set, is the use of attenuators. The combination of 3 dB, 7 dB, 10 dB and 20 dB is fast to switch, when you want to increase your power in QSO’s. In one move, one attenuator section is switched OFF and an other is switched ON, to increase the power with a step of 3 db or 4 dB.
Although all sections are designed for a power of 6 watts, this line up is the best to distribute the input power over the sections. To reduce the number of resistors, I chose to use good available (metal film) resistors of 2 watts. Please notice that the resistors with a star can be 250 mW resistors. All attenuator sections are designed for an maximum input power of 6 watts. This power can only be used for about 20 seconds. Without cooling the resistors will get hot in a short while.
With two or more resistors from the E12-series in parallel, any value can be made accurately. In practice the accuracy of the resistors will be about 2%, so the accuracy of the attenuator is very good. The resistor value of the combination is most of the time within 1% of the theoretical value. The measured accuracy of the 20 dB attenuator sections is 0.1 dB. The other sections are even more accurate
The choice of placing resistors in parallel, leads to a very simple layout of the attenuator sections. There’s NO need for a PCB. hi.
Fast and accurate 40 dB Power Attenuators for 1.25 W, 2.5 W or 5 W
Fast and accurate 40 dB Power Attenuators for 1.25 W, 2.5 W or 5 W with resistors of 1/4 W, 1/2 W or 1 W.
The shown Pi-type, 50 ohm attenuators are very accurate and designed for an input power of 1.25 W to 5 watts. They are built with induction free resistors from the E12-series. The attenuators are placed in cascade. The attenuators can be switched on or off. With the switch in the lower position the signal will go through the attenuator.
All four attenuators must be able to dissipate the same power, because they can be switched independently.
Power of the resistors
The table below shows the maximum continuous input power for attenuators built with resistors of 1/4 W, 1/2 W and 1 W. E.g. an attenuator for 20 dB built with resistor of 1/4 W can dissipate 1.25 W. The higher the power of the resistors the higher the maximum input power of the attenuators. If you build these attenuators with resistors of ½ watt, then the maximum continuous input power is 2.5 watt. Or 5 watts with resistors of 1 W.
Power in CW This table gives maximum input power in CW. Since the average power in CW is about half of the continuous power, the power can be twice the continuous power. If you build these attenuators with resistors of ½ watt, then the maximum input power in CW is 5 watt. Or 10 watts in CW with resistors of 1 W.
Please notice that this attenuator is asymmetrical. Input and output can NOT be reversed.