Surface-mount circuit board NC2030.

How to Build a QRP RF 50Ω Load and Power Meter

50Ω Load and Power Meter

RF QRP power meter, milliampere DC dial

A 50Ω dummy load is a simple but crucial piece of test equipment. Not to perform tests on its own, but to provide a good testing environment.

A common test is output power of a transmitter, or a stage within a transmitter. At QRP power levels of 5 watts or less, it becomes very practical to combine a dummy load and power meter into a terminating power meter.

Here is how I built one.

The circuit itself is very simple, see the ASCII art diagram below. You will need something like the following:

  • 4  220Ω 1 watt resistors, carbon composition or otherwise non-inductive
  • 22kΩ 0.25 watt resistor
  • 6800Ω 0.25 watt resistor
  • 10kΩ potentiometer
  • 1 mA meter
Circuit diagram of RF QRP power meter.

The power rating of the 220Ω resistors defines the power rating of the entire unit, change that as needed. The remaining resistors, capacitor, and milliammeter can be junk box items. The diode is non-critical, a 1N914 / 1N4148 type should be fine. A 1N34 would be better in theory, but most any small signal diode should work.

1N914 datasheet

If you want to use this with higher power, watch the voltage ratings of the diode and capacitor. A 1N914 / 1N4148 diode is only rated to 75 V reverse voltage, pretty close to what you would have at 100 watt input.

The circuit is built on the back of the panel of a small box.

Why brass instead of aluminum? Brass looks classier.

Interior of RF power meter.

The BNC input is at far right in the views of the interior of the meter face, then the 50Ω load and diode rectifier. Above that is the potentiometer. The meter is at far left.

The close up view of the input circuit shows that the crude construction and junk box parts.

Interior of RF power meter: circuitry.
Exterior of RF power meter: meter, sensitivity control, input.

And here is the finished exterior!

The right side of the box holds a calibration table.

Using an adjustable DC power supply and a bit of math, I measured the input DC voltage required to drive the meter to the indicated dial points, with the potentiometer at both ends of its range.

Again, for higher power levels, you will need to input higher DC voltages to simulate higher powers.

Exterior of RF power meter.
Exterior of RF power meter.

For this collection of junk parts that calibration table worked out as follows:

mA mW, low mW, high
1.00 3480 828
0.95 3090 736
0.90 2770 658
0.85 2430 580
0.80 2140 507
0.75 1900 445
0.70 1660 393
0.65 1420 341
0.60 1210 290
0.55 1030 244
mA mW, low mW, high
0.50 860 202
0.45 695 165
0.40 542 128
0.35 422 99
0.30 320 76
0.25 223 54
0.20 144 34
0.15 86 20
0.10 36 9
0.05 9 2