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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi8ITNrXxyhFzUThTSEPv2NGKFivLCCKtfF1nBAGDV6dSCTHGZ0nDwRM5DPTwTvHP3ztRsEV9pFEBchW7tYw4sFAXUhFPmH6ql6hPVRFJzhEeejH4DpQ-JhJgOlwb7H0GzIDZ5I3gpD5vE/s200/untitled.JPG) |
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgnFvavPjLeJkUsw8Ei7JmAtpDKABLXqEPh1tcz7y1Ci_NT4dE_tpFKddb8X7ySczZQiC_FQ7A_TcPyqcagorfqEcZGBtukqKbguTgCwJNCG5IYVCXOdwGaouhx_hmOwKdMd8UM8NFz1no/s200/untitled3.JPG) |
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.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjTxtIaPsZzbzajU6P97jXH1Skpe6hOD2niQbH4CV6-1I1VHHjKWYuO9OTYjx3BVhakA1onL1kLZr9qa_lXKBfSiRoqmNigdzRPy2vk97Q_KWDpzd3E98nAqajzrTLbPi0AwgdOxJ8BXgI/s200/untitled.JPG) |
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
_______________________________