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.
It takes 6 resistors in each attenuator sectie, to build an attenuator with an excellent accuracy.
This very accurate attenuator is 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 good 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 lose the heat via the wires. Carbon film resistors will lose the heat by radiating from their body.
PA1B 43 dB Accurate Step Attenuator with steps of 1 dB
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.
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 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.
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.