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 at the maximum operating temperature, often 125 °C or 150 °C. At 55 °C inside a closed rack you are already down 20 to 30 percent. At 85 °C, which is a normal interior temperature for equipment in a shelter, a 10 W part may be rated for 4 to 5 W.
VSWR at the load. Reflected power comes back into the pad and is dissipated there too. With a 2:1 mismatch on the output, assume 15 to 20 percent more dissipation than the matched calculation. This is the number that turns a 5 W test into a 6 W thermal load.
Mounting. A 10 W attenuator normally assumes it is bolted to metal with thermal compound. Hanging in free air on a bench, convection carries away a fraction of that, and the case temperature climbs until something gives. If the datasheet does not state the mounting condition for the rating, the rating is a marketing number.
Frequency. Thin-film resistor geometry and skin effect both change with frequency, and the thermal density along the film is not uniform. A part rated for full power at DC is often limited at 18 GHz or 40 GHz. Check the power-versus-frequency curve, not just the headline.
Peak is not average
For pulsed radar and for anything with a duty cycle, the average rating governs heating and the peak rating governs voltage breakdown across the resistor. A part rated 10 W average and 1 kW peak will not survive 1 kW average for a millisecond, and it will flash over at 1 kW peak if the pulse shape is sharper than the test pulse. Ask what pulse width and duty cycle the peak rating assumes.
What the failure looks like
The three signatures are distinguishable, and they tell you which limit you hit:
- Open circuit after months of thermal cycling. The resistor film cracked from repeated expansion, not from one overload event. You were running close to the thermal limit the whole time.
- Discoloured or reflowed centre pin, attenuation still correct. The connector, not the resistor, was the limit. This is common on small packages pushed to their rated power at high frequency.
- Attenuation value drifted up by a few tenths of a dB. Film aging from sustained over-temperature. The part still works, the calibration no longer does.
How to size one properly
- Compute dissipated power: input minus output, in linear terms, not dB.
- Derate from the vendor curve for your actual ambient inside the enclosure.
- Add 20 percent if the load VSWR can reach 2:1.
- Apply 1.5 to 2x margin for a part that will sit in a rack for ten years.
- Measure the case temperature after 30 minutes at the intended power. If you are within 25 °C of the maximum case temperature, you have no margin and should go up a size.
What to ask for before you buy
- Derating curve, with the mounting condition explicitly stated
- Thermal resistance, case to ambient
- Maximum case temperature
- Power rating versus frequency, not just at the band edge
- Connector power limit, which is often lower than the resistor limit
- Whether the part is intended for conduction cooling or convection
Where I sit
I work on these parts at DB Design / MeiXun in Wuxi, so read this as notes from the manufacturing side rather than a neutral survey. We build fixed and variable attenuators alongside coaxial loads and terminations and the coaxial switches that usually sit next to them in the same rack.
If an attenuator in your system keeps dying, the derating curve and the mounting are where the answer usually is, not the wattage on the label.
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