PA1B accurate 43 dB Attenuator
This accurate attenuator with 1 dB steps is ideal for accurate measurements.
The attenuator is fast to operate and will bring you a lot of fun in contest QSO’s with very low power.
This easy to build power attenuator is built with good available 2 watt resistors from the E12-series.
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.
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.
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%
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 unselected 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.
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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