Digital display for DIY 25W dummy load – part 1

Digital display for QRP labs 20W dummy load – part 1 and following articles laid out a initial study into the feasibility of an approach of a similar project. This project uses the same display solution for a DIY 25W dummy load / digital wattmeter with very good performance.

This article describes Bruce, VK4MQ's, build.


Bruce built the dummy load wattmeter into a small die cast box.

Above, the front panel view, the OLED display shows power in watts and dBm, and a bar chart display. The unit is battery powered, and has a on/off switch on the front panel.

Courtesy of Bruce, a schematic diagram.

Above is a view into the box. Note the use of the molded in slots to hold the assemblies.

RF input is conducted to an Anaren RFP-150-50TG high grade thick film 50Ω termination screwed to the box bottom (drilled and tapped) with some heat compound to assist heat flow.

Above is an extract from the datasheet, ReturnLoss (-|s11|dB) of the termination in a model test jig is more than 35dB up to 200MHz. Note that not all resistors of this form have very good ReturnLoss. The detector is going to degrade this a little.

Built right onto the load resistor is the detector circuit and its connections lead away to the Arduino Nano microcontroller module at left. The 0.96″ SSD1306 OLED display on the lid is connected by an I2C bus (four wires).

The device is powered by a 9V alkaline battery to the vin pin of the Nano (which contains an on-board voltage regulator), current consumption is about 26mA.

Above is a close up of the load resistor and detector circuit. The BAT46 diode connects from the RF line to a surface mount 0.1µF capacitor soldered to a tab under the fixing screw, and the voltage divider (56kΩ and 1kΩ) to reduce the voltage to 0-1V to suit the ADC on the Nano.

Having built the unit, measurements were made to calibrate it with a polynomial curve fit to suit the detector circuit.

RF performance

The load was swept using a HP VNA. We should not expect this to go to gigahertz since the detector diode connects directly to the 50Ω line.

Above, the ReturnLoss is quit good to 30MHz, acceptable to 55MHz.

Above, InsertionVSWR is better than 1.1 on HF, and about 1.2 on the 6m band.

Meter calibration

A set of measurements were made of Prf vs Vdc at the ADC pin for power from 1 to 25W. One outlier was discarded.

The tabulated data which was imported to Veusz to perform a second order polynomial curve fit.

Above is the curve fit.

The curve fit coefficients and other parameters were coded into the structured EEPROM image above. The minimum displayed power has been set to 4mW, the ADC is close to the noise floor.

So, we have a load + wattmeter with digital display with good accuracy and resolution from 0.004 to 40W (limited in this case to 25W by the diode PIV), a range of 40dB.


In summary, a HF DIY 25W dummy load and wattmeter with accurate digital display from 0.005 to 40W.

Well done Bruce!