Two Scales RF Wattmeter And Dummy Load

Simple Wattmeter and dummy load that allows you to measure the RF output power. Designed for QRP with 1W / 10W or or 2W / 20W scales.
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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 ) / 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. 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.

RF WATTMETER AND DUMMY LOAD SCHEMATIC

PCB BOARD

I designed the PCB board 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

FRONT PANEL SCALES

I designed the scales for the two options: 1W / 10W scale, or 2W / 20W scale. 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 out.

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

DUMMY LOAD

As a dummy load I used a RFR 50-250 power resistor. 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.

AMMETER

I used a cheap 500mA panel mount ammeter. 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.

WATTMETER CALIBRATION

After building the wattmeter, first calibration step is to use the screw from the front panel 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.

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. 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 DE YO6DXE

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4 Comments

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  1. Regards the equation to convert voltage to power. I believe your calculation may be incorrect, I have always used (V+Vdiode) squared, divided by 2 x R (dummy load).

    The formula takes into account converting peak voltage from the diode detector to RMS, and then using ohms law: V squared / R to calculate power.

    • Hi Al… yes your formula is good if you want the RMS. I was interested in the peak to peak power mostly. But you can set the wattmeter to show you whatever power you need. So you can make the voltage conversion and adjust the meter accordingly depending on your needs. But is good you reminded me as I always wanted to place a note next to the formula so it doesn’t create confusion ( ha ). Thanks buddy.
      73, YO6DXE

    • Hi Paul and thank you. I will also place an order for the boards this coming week. At the moment I am waiting for the ones of the Sputnik Regen Receiver to arrive. I hope it turns out well for you and works great… let me know. I am working on mine also as we speak, as I just printed the new graphics for the meter and want to glue them on.
      73, YO6DXE