Lefty, I have a couple of questions on your RF load. You have folks adjust the voltage up by .4 to allow for the diode, doesn't it have a .7V forward drop? Also, you show two diodes in the picture and only talk about one in the description. I realize that .3V or even 1V with two diodes isn't statistically significant in the scheme of things, but it might confuse someone (besides me).

The author of the article told that he used two series diode because that is what he had on hand and needed the PIV value that two series diodes provides. I assume if you were to use the one in the drawing it has a high enough PIV rating. And yes the diode drop will limit both the total accuracy possible and the minimum amount of RF power that can be measured. But for the purpose of measuring the nominal RF output power of a typical 100 watt ham transmitter, it is simple, cost effective, and that method has been used for many many decades.

I also wonder about the voltage reading since you only use a half wave to charge the capacitor and RMS calculations usually use the entire wave form, but then again this would only be a small error at HF and could probably be ignored. The other question is the value of the capacitor. At 3MHz it's impedance should be (if my math isn't too rusty) less than an ohm rising to around 5ohms at 50Mhz. What am I missing?

Well to review, a half wave filted rectifier with charge up to the peak value (not peak to peak) of the RF signal (ignoring the diode voltage drop), the fact that it's half wave Vs full wave does not effect the measurement value as the meter used to read the value is required to have high (10megohms+) input impedance, so as to not 'load down' the measured value. The value of the cap has no bearing on accuracy as it will charge to the peak value of the RF voltage. Power can be calculated with this by multiplying the peak value by .707 to give the RMS voltage value. Power is then calculated by: (Vrms X Vrms) / 50 ohms. This assumes that the RF being measured is a sine-wave. Keep in mind this kind of measurement circuit is mostly used to just allow a visual reference to be watched as one makes tuning adjustments on a transmitter looking to adjust things for a maximum value. The fact that one can calculate the approximate power is usually just a secondary purpose or goal. Many ham operators certainly use other more accurate watt meters, but it's hard to beat the bang for the buck that this simple diode method allows. I personally didn't build that measurement circuit into my version of the dummy load. I rather just hook my scope to the power input and ground via a BNC T adapter and measure the RF signal directly on my analog oscilloscope as a AC signal.

Lefty