The Old Power Myth


The Chestnut that won’t go away

By Jeff  , VE1ZAC


My basic license allows me to run 250 Watts!’

‘My basic license allows me to run 560 Watts!’

‘My advanced license lets me run 2kW output!’


You know, there is a good reason why amateur license holders should take a course from a reputable group of instructors. It is necessary to teach prospective hams about what they are legally responsible to control in the operation of their stations. Besides radio content and frequencies, there is a requirement to understand the power limits, and stay within them!


There are, at least, three myths as stated up front about the power output of ham radio transmitters (in Canada). Unfortunately, there is only one way to deal with the truth of the subject: you must become aware of how power is measured, and what those ubiquitous power meters are really telling you. And, yup, that means a little math is needed.


What is Power?


Volts times amps gives us watts. Or power. But that is a little simplistic, isn’t it ? Volts and amps where ? Into what kind of load? To measure it we need to know what kind of load we are looking at. You should realize that power can be measured from DC (no frequency) up to the operating frequency of our transmitter. It is the operating power we are interested mostly, but there are several routes to get there. We use a variety of terms to give us some sense of what power is.  At DC, it is relatively easy to measure voltage across a load and the current flowing into the load.  Normally power supplies try and maintain a constant voltage and allow the current to vary depending on the load. Even though it is DC, it can still vary up and down. So we have need of understanding average DC power or instantaneous DC power.


If we are studying the output of the transmitter, we are obligated to measure what is actually coming out of the rig, which is always AC (unless there is something very wrong with the transmitter!). AC is harder to measure, and requires a little more math to understand what is going on.


Power meters


The ubiquitous little power meter built into most transmitters and most external SWR/Watt meters have the ability to do a pretty good job of measuring what is known as RMS or average power. RMS means “Root mean square”. It is important to realize that these meters look at average power over some time. Why? Because, if you ever look at human speech on an oscilloscope, you will see that it is a very complex waveform. It can be considered to be the sum of many different components of nice sinusoids, but it is still complicated.  Average is the best we can do with out a very sophisticated measuring device. If we are looking at a simple sinusoidal signal, RMS = .707 times the peak voltage. The peak to peak voltage is what you get from both high and low excursions of the signal; therefore RMS also equals (the peak-peak voltage /2) times .707

(.707 is the square root of two, by the way, and is where the Root term comes from).


Example 1: We have a 10 volt peak to peak sinusoidal signal. The RMS voltage is therefore (.707 x 5) = 3.53 volts.   We measure this signal across a 50 ohm load. There fore the current (I) = (3.53/50) = .07 amps, or 70 mA.  The power would be .07 times 3.53 =  .25 watts. This is the RMS power. The peak to peak power would be (10*10/ 50) = 2W, and the peak power would be (5*5/50)= 500 mW.


Simple metering circuits to do this kind of math are fairly easy to build, but work very well only on single frequency sinusoids. A varying frequency sinusoid will throw this accuracy out the window.


There are so called “Peak” reading meters out there, but they are still only capable of accuracy on single frequency sinusoids. They still do time averaging.


The only really accurate (and simple) way to measure power involves the use of a known resistor load and an oscilloscope that can work at the frequency in question. With this device, the true peaks can be seen. However, the “Average “power must still be computed.


Remember, the rigs simple little wattmeter is telling us average power, or RMS power of a single frequency tone. (Yes, like CW!)


History and some transmitter physics


In the early days of ham radio, every one used CW (single frequency sinusoids) and could only accurately measure power with moving coil instruments on AC. RMS power was straight forward to measure. To figure out what was coming from the rig, amateurs looked at what was happening in the final stage of their transmitter. (You should STILL know what is happening there!) Basically, every rig then (and now) has an RF amplifier working there which is capable of a known maximum efficiency.


The basic linear amplifiers are class A, B and A-B. These amps have a maximum practical efficiency of 50% and it is rare to get it to 50%.  You MUST use a linear amplifier for voice


Non linear amplifiers are Class C, D, E and others. These ones can get higher efficiency, sometimes up to 88%. These type amps are OK for CW and FM.


Normally, the transmitter maker can’t put two different amplifier types in their multi mode rigs, so they chose a linear amp to do both jobs.  But, the efficiency stays the same. (That is, less than 50%)


Knowing all this, let’s move on to:


License Class Power Limits


Basic license power is limited to:


a)    DC input power less than 250 watts , or

b)   RF Output power i) of 560 watts peak envelope power for SSB or ii) 190 W carrier power for any other type ( like AM)




The DC input power in a) is the maximum   power as delivered by a DC power supply to the input of the last RF stage in the transmitter.  If we have a “perfect” linear amplifier, that means we might get up to 125 watts on a sinusoid like a CW signal.  Lets assume our instruments can only measure RMS values ( a likely scenario from the old days) so that means our little power meters in the rig might indicate a maximum of 125 watts, key down, continuous transmit into a dummy load.  If we had a class C non linear amplifier with, say, 70 % efficiency, than the output meter would read 175 watts. (Yes, this would be legal)


The other method involves an accurate peak reading of the output power in b).  This can be reasonably assumed to be checked out by an oscilloscope. The 560 watt limit means the max excursion of the SSB signal. (NOT a CW signal!) The instantaneous power if the opposing peaks were sinusoid shaped (a reasonable assumption in a properly adjusted and not over driven transmitter) would be (560 x .707) = 395 watts.  The average power of the transmitter would never be able to reach this value because of the nature of human speech.  This is difficult to measure, but studies show that the peak envelope power of a typical human voice is about 6 times the average power. This varies with everyone’s speech. Remember, this is the actual power limit, not what the little power meter displays.  What would the little power meter on the rig display? Many rig manuals state the transmitter is maximum output when the meter shows an average power of about 1/3 of it’s maximum for human speech.   There fore a 100 watt rig might show 35 watts.  A so called averaging peak meter might still only read a maximum of 100 watts. Remember this is not a true peak, as we can only see those with an oscilloscope.


CW is a special case for output measurement, as “Carrier power” is basically a CW tone. So, and intermittent carrier is the same as CW. In this case, 190 watts of output “Key down” power is OK.


So, Myth one and two are dealt with comfortably.  Most rig manufacturers understand these limits and provide a rig which keeps the amateur within the class power limits. If the amateur has a higher powered rig, however, the onus is on them to only operate within their license class. Also remember, that most commercial rigs have linear RF amplifiers as a final stage, thus the 50% efficiency limit.


If a basic licensee wants more power on CW or FM, they can look for a class C amplifier. These are simpler to build by the way, but like more power, you need an advanced license for that.


Advanced class power limits

a)    A DC input power of 1000 watts (RMS) with a linear amplifier means the out put is only going to be 500 watts on our CW signal, key down, RMS type averaging meter, or

b)   2250 Watts Peak envelope on an SSB signal, with a maximum signal power limit of (2250 x .707) = 1590 watts and our typical human speech characteristic would produce about 265 watts on an averaging meter.

c)    Or 750 watts of output carrier power. ( Same as CW example above for basic license)


That takes care of the third Myth.


Question 1 ? 


I bought a linear amplifier that claims it can deliver 2500 watts output peak on SSB. Is it legal?  ……………….Only if we check to see that we are operating within our 2250 watts peak envelope power, and we have an advanced license, of course. That shouldn’t be hard so long as we carefully avoid driving this amplifier near it’s limit.


What about CW, you might ask? If we run it on CW, with full up drive, and the rig didn’t blow up, we would be putting out 1767 watts.  We can not use the b) method for calculating ( SSB only) , and we know we are a max 50% efficiency, so that means we are putting in over 3400 watts input power. Definitely NOT legal. ( Also highly likely to fry this amplifier !)


Question 2 ?


I want more power than 500 watts on CW with an amplifier. How do I get it?.......... Use the a) method and buy or build a class C or higher amplifier.  (Have or get an advanced license first) Aim for 80% efficiency (It IS possible) and you can then get output power of ( .8 x 1000) = 800 watts.


Question 3 ?


What amplifier CAN a basic licensee buy for the  basic 100 watt rig?    …………… None. (Sorry) You are not allowed to build or modify a transmitter or posess and operate a linear amplifier outboard of a commercial prepared unit without an advanced license. There are some cases where you will be OK, however. Notably the Yaesu line of rigs that produce 200 watts ( very close to the limit) and the Elecraft K2 rigs that have an optional module to take the rig from 10 watts to 100 watts. (Technically, the basic licensee is not allowed to build and operate a kit like the K2, but a way around that is to have an advanced ham perform the  final tests and adjustments.)


Question 4 ?


How do I know what the amplifier manufacturer is really delivering for power?   ………… Ah yes, the in-depth question.  They should provide a variety of ratings specifying what they are measuring under what conditions and into what loads. You will likely need to ask questions and sort this out yourself, or get advice from knowledgeable hams, technicians or engineers in this area. Look for test reports from reputable sources.  Like antennas, there is a tendency to over specify a products ratings. IE, tell you the peak to peak instant values, and let you figure the rest out. 


Remember, you are responsible to stay within the license class power limits.


Question 5?


How important is power anyway ? ………   VE1Joe transmits 100 watts into a dipole and the ham on the other end sees an average S Meter reading of S5. It is a good signal and noise level by itself is about S2.  VE1xxx  decides to flip on his 2250 W ( peak envelope power) amplifier. What would happen to the signal at the other ham’s receiver.  Xxx’s basic rig is 100 watts which under certain conditions could be seen as delivering 500 watts peak envelope power on his SSB signal. Therefore the 2250 watt output is  about 4 times  as large, or  6 dB. S meters  are loosely calibrated between 4 and 6 dB per S unit. That means the S meter might go up 1 or 2 whole S units ! Wow, was that ever worth all the hassle of getting that amp….. right?


Question 6?


I understand the transmitter uses an ALC ( automatic level control) to limit power. How does this work?  Most transmitters and amplifiers have the ability to apply a feedback signal to the final amplifier stage which arbitrarily limits the output power of the unit. These things can be tricky. There are several different ways to derive and adjust the ALC before it is applied.  The operator must learn to adjust this properly, if used. It can be a problem in that it just clamps down hard on the output, even if the stage is being driven very hard. This can cause the creation of square wave components in our output… a big no-no !  ALC is good to limit the output, but linearity and signal quality is preserved by adjusting the driving signal down.


 “ A dollar in the antenna is worth 100 dollars in the rig”.


An example of this can be seen if you read the “Quick and dirty 6M antenna” on this website. That little collinear beam boosted the gain of the signal by 6 times in certain directions. That means, that your typical 50 watts delivered to the antenna has an ERP, or effective radiated power of 300 watts !  No linear amp needed. This is a true story… when that antenna was built, the author hooked up with a mega antenna station in Puerto Rico. The other operator had 4 stacked yagis on a 200 foot tower. Huge dollars spent on that setup! He was flabbergasted when he found out that the “Strongest monster signal on the band” ( his words) were coming from a  30 foot wire assembly tossed out from a second story window to a nearby tree and hooked to a barefoot IC706.  (We emailed about that memorable QSO for months!)


The reader might want to look at the “Practical Gain” article on this web site to get an idea on the effect power has on ham signals.