__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

__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

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 *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.