One Watter: Watt's up?

Just so you don't think my workbench is orderly and immaculate all the time..

The workbench in mid-crisis.

I was closing in on the finish and had added the final amp and output bandpass filter (actually completing the build) and was testing/aligning the radio when I hit a bump in the road. Try as I would setting the output power to one watt, I could not goose the little beast over 0.7W out. I peaked the bandpass filter over and over again. I check out the parts, solder joints, etc. but could not budge the output power past that level..

..at least that's what my OHR WM-2 watt meter was telling me.

My OHR telling me it's only 0.5W out.

But my trusty, rusty 7704 sang a different tune:

Consulting with my arch-friend, WB6JDH, and doing a little research, I came across a tour de force slide presentation entitled Math for the General Class Radio Operator that N9XH did for the ARRL and those sources cleared a lot up for me. Here are four salient slides from that presentation, right in the wheelhouse with respect to addressing this situation. They are self-explanatory:

So, extrapolating from the above,

(1) Determine total peak-to-peak volts:

4.8 divisions x 5 volts per division = 24 volts peak-to-peak

(2) Convert from peak-to-peak to RMS:

24 volts peak-to-peak / 2 = 12 volts peak x 0.7070 = 8.484 volts RMS

(3) Now solve I = E/R for I (current in amps):

I = 8.484/ 50 ohms = 0.16968 amps

(4) Now solve for P = I x E (power in watts):

P = 8.484 x 0.16968 = 1.440 watts

But Dick simplified this by using .3535 (half of 0.7070) and skipping the determination of the current and squaring the voltage. I will leave you to derive that, but it works out the same:

(1) 24 volts peak-to-peak x 0.3535 = 8.484 volts RMS
(2) 8.484 RMS volts squared = 71.918
(3) 71.978 / 50 ohms = 1.440 watts

Although it is good to know the derivation, I am going with Dick's method as it is shorter and handheld calculator friendly. Anyway, however you run the numbers, it works out to 4 divisions being damned close to 1 watt. (About 0.996 watts, actually.)

Now here's the rub. I went back and re-ran the calibration on my OHR WM-2 and it *still* came out around 0.7 watts when the 1-Watter output was set to 4 divisions (20 volts peak-to-peak) on the scope and I cannot explain away the difference. Trying a couple of other watt meters I had in on the bench, I found a some were "inaccurate", but a Diamond SX-200 was "dead on".

I guess it's why I call this "the man with two watches" syndrome. You know - a man with one watch knows what time it is; a man with two watches is never quite sure.

Failing a plausible explanation for the differences and trusting in my scope (a calibrated Tektronix 7704) as being the most accurate source -- AND since my other measurements of various circuit points on my 1-Watter substantially agree with those presented in K7QO's Phase 8 and Phase 9 videos, I deemed that the standard. And, actually, went back to calibrate the OHR WM-2 using the 1-Watter as a reference.

Now, here's an interesting factoid in this which lends to the mystery. About four weeks ago -- sometime around the first part of December 2015 -- wanting to give QRPp (QRP operation at one watt or less) a try, I got out my old NorCal 40A and set it to one watt using the OHR WM-2! Going back and measuring the NorCal 40A with the same WM-2 used to set it to one watt out, I got a similar 0.7 watt output reading. Netiher the WM-2 nor the NorCal 40A was touched in the interim.

Strange, huh?

Anyway, my final pronouncement on the 15 Meter 1-Watter is that it is a fun kit, very reasonably priced, and with Chuck and Diz's documentation, and excellent source of knowledge for the builder who wants to be much more than an appliance operator or "Baofeng Tech".

"Who knows what evil lurks in the heart of.."

"I do, old son.."

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