Accuprobe kit is in the can

I finished the Accuprobe kit from the NorTex QRP Club. This is the fourth kit I have built this month. Building the Accuprobe falls squarely into the beginner category for small kits. Assembling the circuit board was dead simple but the project box was a little more complicated. I spent quite a bit of time shaping the probe with a file. Understanding what the device is good for is the hardest part of the kit. The Accuprobe attaches between your digital multi-meter and a circuit you want to test. The frequency limit at which your meter will measure AC is too low to measure RF, unless you have a very expensive meter. Where the Accuprobe plays a part is by converting the RF signal into DC voltage that your DMM can measure. Not that I really understand it yet or anything. N5ESE wrote an easy (heh) reading description of what an RF probe is for along with a much simpler, though less accurate, design. I am not certain his formula for calculating transmitter output power is quite right for the Accuprobe. His formula included a number for the barrier voltage of the diode in the simple circuit. Just learning as I go here and cannot really say for certain until I find someone who can set me strait. Anyway, you should be able to calculate the wattage of a QRP rig with the Accuprobe because the tester will be able to pick up RF voltage. Hook up a dummy load, key the rig, and measure the voltage. Calculate wattage with Joule’s law:

E^2/R

Been a long time since I learned that stuff; 25 years plus. E is the voltage you read through the Accuprobe and R is 50 ohms, your dummy load resistance. Accuracy depends on having a dummy load that is close to 50 ohms.

Play the text of this article in Morse code!

Leave a Reply