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 20 dB, 10 dB, 7 dB and 3 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.
PA1B Fast and Accuarate 40 dB Power Attenuator for QSO's
|PA1B Fast and accurate 40 dB Power Attenuator for QSO's|
I choose to design this attenuator asymmetrically, to reduce the number of resistors.
All attenuator sections are designed for an continuous input power of 5 watts.
This is 10 watts in CW.
The attenuator sections consist each of 3 compound resistors, which each are formed by a number of resistors in parallel. These resistors are a power resistor of 1 watts and a some resistor of a modest power (1/4 watt). The resistors with a star * can be 1/4 watt.
Or you can chose 1 watt resistors for all resistors.
The power resistors, with a lower ohmic value, dissipate most of the power and an other resistor is added in parallel, to arrive accurately on the wanted ohmic value. The power attenuator sections of 20 dB and 10 dB are build with 9 resistors, since they must dissipate much more power than the other sections.
With two or more resistors from the E12-series in parallel,
you can accurately make any value you want.
The resistor value of the combination is most of the time within 1% of the theoretical value.
This is why an accurate attenuator section, always consists of 6 resistors or more.
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.
|Table for the PA1B Fast and accurate 40 dB Power Attenuator|
for a symmetrical attenuator built with resistors of 2 watts.