I have always wanted to have an RF Probe to go along with my multimeter. But each one I built and used seemed to lack the accuracy I was hoping for. After several evening of workbench playing, I finally got it "down to a science" that even I could understand. The big problem for me was trying to figure out why the 4.7 Meg Ohm resistor wasn't dividing the voltage properly so I could just read RMS voltage straight from my multimeter.

The first step is determine your multimeter's input resistance/impdeance (Rm). If the price of your meter is under twenty dollars, chances are it is not a 10 Meg Ohm input impedance meter. To figure out my meter's "real input impedance", you can use Ohm's Law along with a large resistor in series with the meter and power supply.  I set-up a variable DC power supply for +10vdc using the multimeter(DMM) that will be used for the RF Probe. I then connected a variable pot in series with the DMM on the positive lead. I adjusted the pot till I had half of the initial DC power supply's voltage (which would be +5vdc in this case). I disconnected the power supply and using the DMM measured the pot's resistance, which was 9.98M Ohms. So, now I knew that my DMM's Rm was 9.98M Ohm and not 10M Ohm as stated in the manual.

With a known Rm for my DMM, I then needed to determine what the scaling resistor's (Rs) value would be for the RF Probe. The RF Probe consists of one cap, one resistor, and one diode. I used a .01uf cap, a 1N34A diode and a scaling resistor which we are about to determine the value of. In case you haven't seen one, here is the schematic:

To figure out the scaling resistor (Rs) value, multiple the multimeter's input resistance Rm by 0.414. In my case the math went like this:

[tex] Rs = 9.98 x 0.414 = 4.13Meg Ohm [/tex]

I connected two ressitors in series (3.65M + 470K) and is represented by the single resistor symbol in the schematic above. The photo below (thanks to Chuck Carpenter, W5USJ for the image) shows how to layout the probe...mine is on a thin piece of PCB and you can't see the components as well as you can in the one below.

[A SIDE NOTE:After finally getting to this point, I realized that only RF levels of +10dBm or higher can be measured with any sense of accuracy. The formula needed to do this right (not a big fan of math) was not so difficult if reduced down to lowest terms first.]

To use the probe, add 0.25 to the value your meter shows, then square it and divide by 50.The "0.25" is the voltage across the diode when it becomes forward biased and the "50" would be the impedance of the load your measuring--if in doubt, put a 51 Ohm resistor across the probe's tip and ground. If your doing the math:

[tex] P{out} = {(V + 0.25)^2 \over R} [/tex]

Hope this helps if your considering building a RF Probe.  I've built several, but never saw the RMS values I was looking for when measuring known RF levels. It turns out that many meters today are not exactly 10 Meg Ohms as advertised. I have now measured one around 1 Meg, two between 1 and 9 Meg and the one I am using at 9.98 Meg.

My thanks to many folks on the qrp-l.org reflector for being patient with me and helping me come to terms with this whole project!