Digital QRP SWR & Power meter
Build a compact QRP SWR and power meter for portable and field use, covering 160m to 6m.
Description
The KD1JV "Melt Solder" Digital QRP SWR/Power Meter operates across the 160m to 6m bands, featuring 9.9-watt and 990-milliwatt power scales, a peak hold mode for SSB, and a VSWR scale from 1:1 to 9.9:1. It utilizes a Stockton directional coupler wound on a single binocular core and an Atmel AVR ATtiny26L microcontroller for processing. The design incorporates W7EL diode compensation in a non-inverting amplifier feedback loop to address diode voltage drop and non-linear behavior at low power levels, with software corrections applied for improved accuracy, particularly at QRP power levels up to 5 watts.
Construction details include parts placement, transformer winding instructions for #26 magnet wire, and calibration procedures using a 1.5V alkaline battery or an oscilloscope with a 50-ohm dummy load. Packaging options are discussed for fitting the board into an Altoids tin or a deeper Whitman’s Sampler tin, with considerations for battery life extension by adding a third or fourth battery. Accuracy tests performed at 7 MHz compare readings against a True RMS power meter based on an Analog Devices AD8361 chip, showing typical deviations of 0-5% across various power levels.