RF attenuator calculator

Pi and T attenuators

A resistive attenuator reduces a signal by a fixed number of decibels while keeping the impedance the same at both ends, so it doesn't upset the SWR. The pi pad has two resistors to ground with one in series between them. The T pad has two series resistors with one to ground in the middle. Both give the same attenuation; the pi usually uses more practical values for high attenuation, and the T for low attenuation.

Power handling

In a matched attenuator, the input power minus the output power is turned into heat, and it isn't shared equally. In a pi pad the input shunt resistor and the series resistor take most of it. In a T pad it's the input series resistor. Rate each resistor for at least twice the power shown, and allow more for continuous modes such as FM and FT8. Use non-inductive resistors: carbon film or metal film for low power, thick film power resistors on a heatsink for more. Wirewound resistors are inductive and unsuitable at RF.

A 10 W, 10 dB attenuator is a good addition to a station: it lets a transverter or test receiver be driven safely from a 10 W radio, and a 20 dB pad protects a NanoVNA or spectrum analyser input.

Building for RF

Keep leads as short as possible and build on a ground plane, such as copper-clad board, with a shield between input and output for attenuations above about 30 dB. Several smaller resistors in parallel have less inductance than one large one, and share the heat. For 50 Ω systems, the standard E24 values are usually close enough, but the table shows how far off they are.

Questions