QRP Two Scales RF Wattmeter And Dummy Load

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I used to have an older video about an RF dummy load on YouTube. I decided to delete that one and replace it with something new. Here is an updated version of that simple project. An RF power meter and 50 Ohm dummy load in one. You also have an output where you can connect a voltmeter for precise measurements.

If you decide to use a voltmeter for precise measurements, here is the formula to calculate the peak to peak power output:

P ( W ) = ( V + 0.25 )^2 / 50, where 0.25 is the voltage drop of the diode. If using another diode check the voltage drop of the particular diode you are using. Do not forget that this is a QRP Wattmeter. Transmitting into the Wattmeter / Dummy Load for too long will damage the diode. Try to keep it short for measurements only.

The Wattmeter is based on another simple QRP Power Meter by VK3YE as I built Peter’s version too in the past. Both seem pretty accurate on HF, as well on VHF or UHF. It all depends on the diode used in the circuit.

THE SCHEMATIC

I wanted something simple, so everything is minimal ( Fig. 1 ). If you do not want to use the ammeter is one thing less. But I found it really useful in doing fast measurements. No need to calculate using formulas, even if it may not be 100% accurate. But if you want you can get very accurate as well. You have two options when it comes to the schematic. The 1W and 10W version: PT1 – 10K and PT2: 47K. Or you can build a 2W and 20W version where PT1 – 22K and PT2 100K. I did not do fancy calculations, I used values by trial on error.

Fig. 1 – Two Scales RF Wattmeter And Dummy Load Schematic

PCB BOARD

I designed the PCB board ( Fig. 2 ) based on the ammeter I purchased. You have details on the part number in the Info File from the download folder. The PCB board is also easy to make using the toner transfer method, if you cannot afford to order the PCB boards.

FIG. 2 – Two Scales RF Wattmeter And Dummy Load PCB Board

FRONT PANEL SCALES

I designed the scales for the two options: 1W / 10W scale ( Fig. 3 ) , or 2W / 20W scale ( Fig. 4 ). Keep in mind that they are not extremely accurate. I tried my best but at the end is homebrew equipment. The 1W / 10W scale I tested a lot and seems to do fine. I am not 100% sure about the 2W / 20W as I do not have any TX or TRX with 20W output power.

No need to save the images. You have the PDF file of the graphics in the download folder.

Fig. 3 – 1W / 10W Scale

Fig. 4 – 2W / 20W Scale

DUMMY LOAD

As a dummy load I used a RFR 50-250 power resistor ( Fig. 5 ). It can take way more power, but is small and fits well on the board. If you want to use another dummy load, just solder the wires of your own dummy load to the PCB board. I used this one for convenience as I do not need to worry that it won’t handle the power or it will overheat. So far up to 10W it doesn’t seem to need cooling. If measuring higher power you can place the resistor on a heatsink instead and solder connecting wires to the PCB board ( as short as possible ).

Fig. 5 – RFR 50-250 Dummy Load Resistor

AMMETER

I used a cheap 500mA panel mount ammeter ( Fig. 6 ). If yours is different and the PCB board doesn’t fit on the back screws, just use two wires from the PCB to the meter. I hope they fit though. I guess it can be replaced with others as well. They seem to have similar sizes usually.

Fig. 6 – 500mA Ammeter

WATTMETER CALIBRATION

After building the wattmeter, first calibration step is to use the screw from the front panel ( Fig. 7 ) and set the meter to zero on the scale.

The second step is to feed a voltage that calculated gives you the maximum power on the scale you are calibrating. Alternately you can transmit a CW signal with a known power. Calibrate the meter for the known power using the trimmer resistor. PT1 is for the low power scale and PT2 for the high power scale. Do NOT transmit 20W when the wattmeter is set on a 2W scale as the meter will burn.

Fig. 7 – Ammeter Calibration

FINAL THOUGHTS

I hope you found it useful and is not so hard to build and calibrate. Remember one more time, it may not be 100% accurate. For very precise measurements, use the voltmeter and measure the voltage from the voltmeter probe. With the help of the formula, you will be able to calculate the exact power. Use the formula of choice in order to calibrate the wattmeter to show you the readings you want. If you have a calibrated wattmeter that you can borrow you can use that one as reference in order to calibrate your handmade one.

Fig. 8 – QRP Two Scales RF Wattmeter And Dummy Load

Mine ( Fig. 8 ) turned out really well and it seems to be acurate enough for me. The only thing that I had no time yet, was to build an enclosure for it. One more tip, try to keep the connection going to the BNC connector as short as possible. In the future I will try to build more measurement equipment. As always, feel free to comment down below if you have any questions.

73, YO6DXE