A man with one wristwatch knows what time it is; a man with two watches is never quite sure.I use my scope a Tek 7704A ("Old Betsy") from the 70s and my OHR WM-2 watt meter alternately whichever one is most convenient when I am building and testing radios.
A couple of points here: I am a dilettante and not a wizard like my friend Dick, WB6JDH, who knows everything and can solve any problem you throw at him. (And, believe me, I have thrown a lot of problems at Dick!) Also, I have old-timey test gear that is wa-a-a-a-y complex -- like my 7704A -- and I don't know how to push the envelope on it but I try to learn. That's the fun part of ham radio.
But, some time last year during a spate of building some kits and other projects, I noticed that the WM-2 did not agree with the Tek scope. It got to be annoying so the OHR calibration process was revisited -- but it did not obviate the discrepancy. Some time ago, Marshall Emm, N1FN (SK), and I communicated on the WM-2 re calibration and he assured me that -- in essence -- whatever gets you through the night works just fine for calibration. Obviously, the beauty of the WM-2 was that one could build a reasonably accurate QRP watt meter and calibrate it by looping DC voltage levels through the meter circuit and at each level for the three scales, setting the meter to full scale ensured an accurate RF reading. Marshall's email assured me that using a scope to calibrate the WM-2 was preferable. And that brings me to the nut of the story: how accurate is the stuff on my bench?
The WM-2 watt meter is one of the best little watt meters going because (1) it's a kit and you can build it and (2) it's reasonably accurate and (3) it covers the 0-100 mW range. The downside is that it is a little expensive (around $150) and it is not that accurate.
My Bird wattmeter (a gift from my late father) was the purported gold standard in my younger days and I relied on that.
And I have one or two Diamond SX-200 (and other models) and find them to be surprisingly accurate. But how do I know this? Well, I checked it against "Old Betsy". In fact, I have checked them all against the Tek scope and calibrated them (when I can). I am not a metrician nor do I make any pretense at achieving a heavenly degree of accuracy, but a calibrated scope will get you "close enough for government work". There's a great web page on the subject by Adam Farson, AB4OJ, that can be found here. It begins as follows:
Adam goes into how a scope can be used to measure RF power from the peak-to-peak voltage (Vpp) of a signal displayed on an oscilloscope and his calculation of power from Vpp borders on a derivation so I will confine the formula I discovered in one of Wayne Burdick's Wilderness Radio creations -- I think it was the Sierra. Wayne says, simply:
So, when I found my Tek 7704A annoyingly out of sync with my WM-2, I bailed on the OHR calibration routine (using the test loop voltage values) and set it against what my scope told me. By using an FT-817 at 30 meters (10,100 Kc; middle of 2-30 Mc HF band) as a power source. The 5.0, 2.5, and 1.0 watt levels can be used for the 10-watt range and the 1.0 and 0.5 watt levels can serve to calibrate the 1-watt range. For the 100 mW range, I used an attenuator in series on the 0.5 watt level. See the table below for mine.
The last four were obtained by means of feeding the 0.5 watt output into an attenuator and knocking the signal down to levels that allow the WM-2 to be "spot checked" on the 100 mW range. Note that none are close to the actual scale markings so they have to be "eyeballed". (This is, in fact, true of the 10 watt and 1 watt range as well.) However, an eyeball approximation was far better than the inaccuracy the WM-2 displayed by using the voltage/test loop method. Here's the attenuator setup:
I happened to score that Telonic attenuator at a swap meet for $10 and it is immaculate as well as dead balls on. But you have to be careful with the power input as more than a watt will probably fry the unit.
Incidentally, much earlier on I discovered the dirty little secret that the Bird watt meter slugs can become notoriously inaccurate and are not to be trusted. When it was discovered that my HF 50 watt and 100 watt slugs were all over the map, I followed the procedures outlined in several videos of which WB9FOL's is typical.
I pried open my two slugs and found that the pot had wandered off value bit but could be adjusted and I did so scaling them against the scope for lower power values. It was then that I noticed that the Diamond SX-200s were surprisingly accurate at low power and I used them to scale the two bird slugs up to the 100 watt level on my IC-706MKIIg and IC-7200 rigs.
There are, of course, methods to "reduce" or divide higher levels of power so they can be measured by a scope. These are merely extensions of the attenuator in the scope and the 10:1 probe but huskier. In fact, AB4OJ includes the principles for one in his write-up:
..as I said, close enough for government work.
Post Script: Here is a handy Vpp to Power conversion chart for 2 Vpp intervals up to power in watts.