This is an old revision of the document!
Build Your Own Attenuator
Discussion
In my experience in professional radio, nobody uses dummy loads. We normally use attenuators. Either we connect transmission equipment to one side of an attenuator and connect measurement equipment (spectrum analysers, protocol analysers, reference receivers, etc.) to the other side; or we connect signal generators to one side and connect receivers to the other side. The attenuator simulates the path loss between transmitter and receiver.
Typically attenuations for this kind of application will be in the range of 60dB to 120dB loss. 80dB loss changes an input of 0.5v RMS into an output of 50μV – S9 signal as received by a HF receiver.
Circuit
The inside of the attenuator is normally a simple L or Pi attenuator constructed of resistors with very little reactance (inductance or capacitance) producing 60-100dB of attenuation between the two ports. At least one port needs to be able to handle high powers.
High power RF resistors generally connect one end to ground (the heatsink) so a Pi attenuator circuit is best. Here is a Pi circuit (source – Wikimedia Commons)
For a 50Ω attenuator, R1 and R3 are slightly more than 50Ω, and R2 is many times 50Ω – such that 1/R1 + 1/(R2+R3) = 1/50