Measuring Power Output - Keynote # 5, 1998
This month, some more of your questions. I was asked by KA8EGS, via my web site, about measuring power output. When we say we're running 5 watts output, just exactly what does that mean?
Well, when we are talking about RF power, we are referring to RMS or root mean square power. Without getting overly technical and turning most of my readers off, RMS is simply the best way of fitting the varying potential of an AC signal to a pure DC signal. In other words 5 V RMS AC will produce the same effects (heating, whatever) as 5 V DC. The overly technically minded may find this to be an oversimplification.
OK, let's move on. An AC signal varies from a peak positive voltage to a peak negative voltage regularly in a sine wave form. We can measure an AC signal that varies from +5 to -5 volts in at least 3 ways. We could say it's a 10 V P-P (peak to peak) voltage. Or a 5 V Peak signal. The third way is to say it's a 3.535 V RMS voltage. This is determined by multiplying the peak voltage by .707.
That gives us an idea of what RMS is, and how it is determined. Now let's apply that to actually measuring power. I know we all know Ohm's law and all it's variations. The form we are interested in is P(ower) = E x E (or voltage squared) / R(esistance) since we can easily measure RF voltage and resistance.
Most transmitters have an output impedance (resistance) of 50 ohms so we'll use that for R in our formula. When we feed our transmitter into a 50 ohm dummy load (pure resistance), we can then measure the voltage across that load to get our E value. There are different ways of measuring this voltage, and you must be sure which way your particular device measures it to be accurate. Perhaps the surest way is to use an oscilloscope. Look at the scale on the scope and measure the voltage from the positive peak to the negative. This will give you P-P voltage. Divide by 2 to get peak voltage, then multiply this voltage by .707 to get the RMS voltage to plug into the formula. Let's say you measure 44.7 V P-P on the scope. Dividing by 2 to get peak voltage gives 22.35 V. Multiplying this by .707 gives 15.8 V RMS. Squaring this gives 249.64. Dividing by the 50 ohm resistance then gives us 4.99 watts of RF power.
If you have a peak reading meter, of course you would get the peak voltage, multiply by .707 to get RMS, and proceed from there. Some meters are calibrated or set up to read RMS voltage directly. Then it is even simpler mathematically.
I tried to make this kind of a middle of the road answer, not too overly technical nor too simplistic. To sum up, just be sure you know how you are measuring the RF voltage across the dummy load. Whether it's P-P, peak, or RMS, and proceed from there.
There are also RF output meters that can read your power output directly. Then it is really simple. Again be sure it is RMS power that they are measuring. If it measures peak power, for example, then you are cheating yourself. I am not going to do the math, but your 5 watts measured on a P-P reading device will actually be only around 2.5-3 watts RMS power.
Changing gears now - a couple of articles ago I mentioned that I normally don't call CQ FISTS to get my FISTS QSO's. N6MNZ wrote and asked if we shouldn't call CQ FISTS. There is nothing at all wrong with calling CQ FISTS if you wish to do it. It's simply a personal preference, just like I never use /QRP to denote that I am running QRP. The only time I ever call directional CQ's of any kind or sign /anything after my call is in contests. I find that simply hanging out around the FISTS frequencies and calling a regular CQ nets me lots of FISTS QSO's. Enough, in fact, to have gotten the Century, QRP Century, 2X QRP Century, about 3/5 of the way to the Gold Century, and 1/4 of the way to the Platinum so far.
Perhaps the most commonly asked question is one that concerns the various QRP kits available these days. Which one is best? What do I think of this particular kit? Which kit is good for a beginner to QRP? And so on. A few months ago, we asked for those of you who use one of the kits to write a review of it and submit it for publication. We only covered a couple of kits when that was done, and haven't received anything since.
I am now thinking of adding a page to my web site devoted to the QRP kits with a description of each, where more information can be found about the kit, etc. It will take some time to do that, and I will let you know here in the column when it is done. Or since many of you visit my site often, just check the QRP section for it.
W8GND writes and asks for more information about zero- beating with the various QRP kits. Apparently that is a tricky matter, and I know it varies from kit to kit so it is hard to give a specific answer. However, in next month's column I'll take a shot at it. If you would like to contribute your tips on that matter, Email me or regular mail to 478 E. High St., Kittanning, PA 16201-1304. And be sure to keep visiting my web site at http://home.windstream.net/johnshan/. -30-