HF Switched Attenuator

Everyone should have one! A switched attenuator is incredibly useful when making RF measurements, and they are very easy to make. I have used 7 sections switched as 1dB-2dB-2dB-5dB-10dB-20dB-20dB allowing up to 60dB attenuation.

The component values aren't exact "E12" resistor values, so I've used two resistors in most places and used "pi" attenuator elements. A calculator for resistor values can be found on the RF Cafe website. For some reason I couldn't execute their calculator, but the equations are also available there and I put them into a spreadsheet instead. Download the complete circuit diagram: {phocadownload view=file|id=12|target=s}

At HF, this can be constructed simply using DPDT toggle switches and 1/4W leaded resistors. Mine is assembled in a small diecast box approx 100mm long; the inside view is shown. It's hard to go too badly wrong with this!

Performance at HF is excellent: much better than I expected. Here are some measured results. All of the attenuators are within 0.2dB of their intended values. There is a slight rolloff with frequency, reavhing 0.2dB at 30MHz and 0.6dB at 80MHz. The attenuations are all essentially flat to at least 80MHz, although the input VSWR edges up abpove 30MHz.

If I ever need to do VHF measurements, I'll make another one with the same resistor values on a double sided PCB with PCB mounted switches and the "through" path on a 50Ω microstrip line. Until then this one is fine at HF.