|0...36 dB attenuator built by PH2LB - Click to enlarge the pictures|
The attenuator can handle a maximum power of 20 watt and is used to reduce the power of a 5 watt transceiver.
At first I did not realized why Lex designed the attenuator for 20 W. Like many HAMs I thought, that 20 W is QRO. But please notice that 20 W is the Maximum power. The chosen maximum power of 20 watts for the attenuator is an excellent choice. The attenuator can easily handle a power of 5 watts, without getting hot.
|0...36 dB attenuator by PH2LB|
with 2 W resistors
A resistor that dissipates its maximum power will get very hot.
So it is good practice to choose the maximum power 4 times higher then the continuous power.
Lex choose for a maximum power of 20 watts, to reduce a power of 5 watts.
The resistors have a maximum power of 2 watts. With a power of 1/4 of the maximum power, the resistors will dissipate about 500 mW each and they will not get hot.
|The 20 dB section|
PA1B Power Attenuator Calculator
Lex used the PA1B Power Attenuator Calculator to design his 36 dB attenuator for 20 watts.
This unique spreadsheet in Excel calculates the number of resistors and the values of the resistors from the E12-series for an attenuator section.
In the picture below I show the input values for the section of 20 dB, for a maximum power of 20 watts, that is built with resistors of 2 watts.
|PA1B Power Attenuator Calculator - Input values for the 20 dB section|