Steampunk RF Milliammeter

A vintage 1Ω, 200mA FSD ammeter was there, so this steampunk project was taken up.

To start with, the 1Ω shunt and 30Ω series resistors were removed to restore the meter movement's original characteristics (10Ω, 5mA FSD).

The enclosure emerged from a scrapped multimeter housing and a sheet of acrylic. It was easy to work on the acrylic for the required cutouts.

All parts were available in the junk box.

RF Milliammeter

The circuit was wired up 'dead-bug' style.

RF Milliammeter - Schematic

The meter was then roughly calibrated to read around 500 RF milliamperes maximum.

Output of a vintage solid state 15W CW/AM/SSB rig was successfully peaked up using this meter.

Related post: Homebrew RF Milliammeter

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Homebrew RF Milliammeter

The need for a QRP output meter was felt when the homebrew SWR meters / RF ammeters in the shack proved useless for peaking the output of a homebrew 7MHz 5W solid state CW transmitter. For their pointers barely moved.

Hence the decision to build a RF milliammeter on the same lines as my homebrew RF ammeter.

RF Milliammeter

All parts were sourced from the junk box. BNC sockets were used. The primary of the toroidal transformer is a piece of the inner conductor of RG-58/U coax. The secondary is wound with a length of solid hookup wire. A rubber grommet ensures positioning of the toroid.

Inside the RF Milliammeter

The small 1kΩ, 200μA FSD meter makes the unit quite compact. The enclosure of a defective 230V - 110V autotransformer came in handy, with the meter taking the place of the 110V socket.

RF Milliammeter - Schematic

It was roughly calibrated to read around 250 RF milliamperes maximum. The CW transmitter was then easily adjusted.

Related post: Homebrew RF Ammeter
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