Ken (and others interested), The DL1 has a formula in its manual that considers the diode forward voltage. You can use that to achieve accuracy if you are using a DL1. You can include similar terms in your measurements and calculations if you are using an alternative. Apply Ohm's law for power in whatever form your measurement technique finds more suitable.
Short tutorial on measurement accuracy follows: If you draw out the measurement circuit then figure out all the voltage drops or current loops, that will provide you with an accurate picture of how to do the math to figure out the total solution. Then one only needs to consider the accuracy of the measurement equipment in recording the values obtained during measurement - i.e. your DMM scale may be accurate to .03% +/- 2 counts in the low order digit, and that must be considered when calculating the accuracy of your measurements and results. It is often informative to calculate the range of error in the result considering the limits of your instrumentation - the specs for your measurement device will tell you what its limits are. Just because you can see a number in a digital display does not mean it is telling you the whole story. Knowing the limits of your measurement equipment goes a long way toward achieving accuracy. Any piece of measurement equipment is not to be trusted until it has been verified. In power measurements, an item that is frequently ignored beyond the diode voltage is the accuracy of the load resistor at the frequency of measurement. A good antenna analyzer can tell you what the real impedance is at the frequency of interest. If the load has some reactance, you can include that in the calculations too - it is a simple application of Ohm's law for power, but if there is any reactive component, one must use it in the calculations too, and including the complex values (R+jx) in the computations can become messy (but it will be accurate). All the efforts to have a non-reactive dummy load is a quest to simplify the calculations (and keep the math in the real plane). There are components readily available today that are useful in calibrating equipment - such as precision voltage references and precision resistors (I have found thick film 1% 50 ohm power resistors by Caddock to be good at RF (values other than 50 ohm have proven to be capacitive). If you want a sanity check on items purchased for use as standards, buy 3 or more of them then meausre and compare them before use - cast out any that do not agree with the rest of the batch (that does not increase the tolerance unless your measurement equipment is sufficiently accurate and calibrated). Care for these 'standards' well - mark them as standards and put them in a safe place and use them only for calibration. Using this technique, even homebrew equipment can be calibrated and that calibration tracable back to your set of standards. Knowing the limits of your measurement equipment is key to achieving measurement accuracy - and yes, you will have to 'do the math'. 73, Don W3FPR > -----Original Message----- > > Suppose you have a good DC standard, such as Don's or Joe > Geller's designs. > Is there anyway to bootstrap accurate RF voltage measurements > from accurate > DC measurements? > > Thanks and 73, > Ken > K3VV _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
