draythomp:
But, the capacitor is directly across the terminals from center through the diodes to ground, so it's in parallel with the resistors.Measuring across it would see the combination of the impedance of the capacitor (around 1 to 5 ohms depending on frequency) and the reisistance of the load resistors. Something a lot lower than 50 ohms.
No, the resistors (the 50 ohm load) wire to the BNC connector's center pin and ground. The cap is wired to the output side of the diode to ground. The cap just filters the halfwave rectified signal so as to hold a charge equal to the peak of the RF signal voltage. The diode effectivly isolates the cap from the load resistor(s).
I now realize that you didn't write the article, so you can't speak for the author, and I understand that you don't use his measurement techniques; just the load itself. I started wondering when he missed on the diode forward bias drop value and cited how accurate it was. Which it could be at higher wattage levels where a lower value won't matter, but not for a 1W handheld. Now, I'm trying to understand how it can work. Every calculation I do show essentially a dead short through the .01 microfarad capacitor. Even when I consider that the frequency is cut in half by the diode in series.
I bet he meant to type .01pF or something similar, which would make more sense. Or maybe I'm missing something.
Why don't you check out this link about building and using simple low power measurement circuits using diodes. It's better written then that first link I used. The circuit there called 'peak voltage detector' is the exact equivalent of the first one I linked. http://epic.mcmaster.ca/~elmer101/rfpower/rfpower.html and a construction page at http://epic.mcmaster.ca/~elmer101/rfpower/projvi.html
Now if your only interested in measuring low power RF signals then you might look into the chip I built a small RF milliwatt meter with. It has a frequency range of DC to 500Mhz and a power measurement range from -75dbm to +15dbm and is very accurate, which is total of 90db of range, a feat just short of amazing. It's based on the Analog Devices AD1307 logrithmic amp in a 8 pin DIP package and is powerd with just +5vdc. I paid about $15 maybe 10 years ago for the chip, but in my opinion worth every cent as accurate RF power levels to 500Mhz is not a trivial thing. It's output analog DC voltage would be perfect for wiring to a arduino analog input pin for display or logging functions.
http://www.analog.com/static/imported-files/data_sheets/AD8307.pdfLefty
You're right about how simple the load portion of this project is though.