Why Your 10 W Attenuator Failed at 5 W

A 10 W attenuator running at 5 W continuous should be the least interesting part on the bench. It still comes back. When it does, the report usually says "overpowered", and the test log says 5 W, and both are wrong. The rating on the label is measured under conditions that almost never exist in a rack: 25 °C ambient, VSWR 1.0, bolted to a heat sink of a specified size, continuous, at the low end of the band. Move any one of those and the 10 W part is a 6 W part or worse. Here is where the margin goes. An attenuator is a heater with connectors A 10 dB attenuator turns 90 percent of the input into heat. A 20 dB pad turns 99 percent into heat. At 5 W input through a 10 dB pad, 4.5 W is being dissipated inside a package the size of a thimble. That is the whole design problem, and it is why the mechanical drawing matters as much as the electrical one. Four things that eat the margin Ambient temperature. Most parts derate linearly from full power at 25 °C to zero a