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

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
— 22kΩ 0.25 watt resistor
— 6800Ω 0.25 watt resistor
— 10kΩ potentiometer
— 1 mA 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 type should be fine. A 1N34 would be better in theory, but most any small signal diode should work.

 BNC       1N914
input      type         6800 ohm
 (o)--+------|>|----+----/\/\/\/\------+
      |             |                  |
      |             |                  v
    +-+-+-+         |           +--/\/\/\/\/\---+
    > > > >         |           |  10 kohm pot  |
    > > > >       -----         | (sensitivity) +-------+
    > > > >       -----         |               |      |
    > > > >         |0.001uF    +---/\/\/\/\----+   +-----+
    +-+-+-+         |                22 kohm        |meter|      Firefox users may find
      |4x 220 ohm   |                               +-----+      that "monospace" isn't
      |55 ohm net   |                                  |         such a mono-spaced font
    -----         -----                              -----       after all, and Courier
     ---           ---                                ---        may be far better for
      -             -                                  -         ASCII art.

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

Why brass instead of aluminum? Brass looks classier.

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

Here is a close up view of the input circuit.

Crude construction and junk box parts!

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. For this collection of junk parts that happened to be:

Meter reading	Sensitivity
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
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