The Myth of Peaking             


It doesn't take too long before a new CB'er begins to yearn for more. More power, louder modulation, stronger signal, greater distance. The reasons for this are as much psychological as they are technical.  Practically speaking, more power allows you to work the more distant stations, which is an advantage if DX'ing is in your blood.  Also, when playing in that proverbial sandbox that is the CB experience, like in many other competitions in life, people tend to look up to and respect those who excel at their vocations, and display traits which are head and shoulders above their peers.  Likewise then, those same people feel that the stronger they become, the more they will be respected. Power as a sign of status as it were.  

Whatever the underlying reasons, one of the most sought after modifications that CB'ers look for, in their grand quest to be king of the block, is the "peak job".  On the surface, this seems straightforward enough. Take a stock FCC legal 4 watt radio, and "boost" the power output to some higher value.  After all, more power output translates to the transmission of a stronger signal and more range right?  To a point this is true, but it doesn't happen without some tradeoffs. These ramifications of the peak job are not always fully understood by the technically challenged CB'er.  Many of these people become confused and can be easily misled by the many inflated claims made by those unscrupulous "Screwdriver Technicians", who are looking only to make a fast buck, and who work hard to promote radio myths in order to separate the unsuspecting CB'er from his disposable income. 

In this article, I will attempt to explain why simply peaking a CB radio is not really a good value for the money.

Let's first look at signal strength and how it relates to actual transmitter power.

It would be prudent at this point, to mention a unit of relative measurement which relates to this subject.  Many people have heard of the term "db". db is short for "decibel", which is a unit of power relative to a standard reference.  Radio people refer to "db's" fairly regularly. The common "S" (for signal) meter, which is standard equipment on most radios, indicates numeric values which are comprised of 6 db increments until "S9", and then read in straight db afterward. The difference between any 2 "S" units is therefore a difference of 6 db, for any signal under "S9". A station giving you an "S8" reading on the S-meter is 6 db stronger than someone giving you an "S7". This, of course, assumes an accurate and linear "S" meter, which most CB's do not have.  But for the sake of a discussion, lets assume that they do for now. 

So what does this all, mean? Not much yet until you understand the other side of the equation. That is, what it will take in transmit power to raise that all-important signal reading.  On the transmitter side of the equation, every time that you double your power, you increase your output signal by 3 db, or 1/2 of an "S" unit more on another station's "S" meter.  That means that if you want to go from say, an "S7" to an "S8", you would have to double your power twice.  For someone running a 4 watt CB, that equates to raising the carrier power to 16 watts.  And that's just to move one "S" unit.  Expanding on this further, lets look at another example.  Say that you want to go from an "S7" to an "S9".  You would now have to increase 12db (6db per "S" unit), which is the equivalent of going from a stock 4 watt CB, and then turning on a 65 watt amplifier. The steps in wattage get even larger from there.  If you are already running 100 watts, and you want to jump 2 "S" units, you'd have to jump up to over 1600 watts (average NOT peak) to achieve it. You're into serious power at this point.

Ok, now you hopefully get an idea about what is required in the process of raising your signal level.  So now let's look at what a typical FCC legal CB is capable of producing in both power and modulation.  Power and modulation are in a balanced relationship in an AM radio.  If you raise the carrier power, without a proportionate increase in modulator power, you lose modulation percentage, and vice versa.

A stock, out of the box CB is normally set to run at around 4 watts of carrier power (per FCC rules).  Experience has shown me that the most carrier power that a typical radio can be made to produce by alignment only, in most cases, is in the range of 6 to 10 watts.  Since it takes an increase to at least 16 watts of power to jump the 6 db necessary to gain just 1 "S" unit increase, you can see that typical CB radios fall far short in "peaking" potential. Increasing the power from 4 to 6 watts will hardly be noticed on the other end

The other (and very important) factor to consider is modulation.  Modulation is what actually carries your voice. Modulation is rated in terms of percentage, and can also be be referred to as "peak" power.  A 100% modulated AM signal will have a peak power of 4 times the carrier power in watts. This means that a stock CB running 100% modulation on a 4 watt carrier will have a peak power of 16 watts.  CB transmitters are designed to be able to handle the extra peak power contained in the modulation.  But when you "peak tune" a radio to put out more carrier power, there is often not enough extra "headroom" left to fully modulate the new power level to 100%.  The result is a lower peak modulation level along with the accompanied clipping distortion.  Many so-called "CB doctors" try to compensate for the lower modulation level by removing  the modulation limiter.  By doing this, you allow the modulator to increase its level of audio drive up to the point of modulator clipping and saturation.  While this can make you sound louder to a point, this capacity is not infinite and when the modulator reaches the saturation point, the audio wave flat tops and produces audio harmonics and other distortion. This is what is commonly called "splatter", and is responsible for much of the bleed over that is heard on the channels.

Also, for a high level modulation scheme, (Most AM-only radios) you need to produce an equivalent of 50% of your R.F. carrier power, in audio watts for the modulator to 100% modulate the R.F. carrier. This means that for a 4 watt R.F. carrier, you need 2 watts (1 watt per sideband) of clean audio power.  Most CB radios employ an audio amp chip which produces no more than about 3 watts of audio before distortion starts to set in.  So you can't properly 100% modulate an R.F. carrier over about 6 watts without incurring distortion.

For a regulator modulated radio (Most newer AM/SSB radios), the peak power that the modulation can achieve has to be less than or equal to the maximum peak power that the transmitter strip can produce, at the full operating voltage, in order to maintain 100%.  Most modern SSB radios can produce SSB and peak power levels in the 18 to 21 watt range.  Remember that in order to 100% modulate a 4 watt carrier, you need to be able to reach 16 watts (4X4=16). For a 5 watt carrier, that means you need to peak at 20 watts (5X4=20).  For 6 watts of carrier, you need to be able to reach 24 watts peak (6X4=24), and so on.  Since most radios are limited to around 20 watts peak, you can not support more than about 5 watts of carrier, before you start losing peak modulation percentage.

Another factor to consider is the future. When the result of the "peaking" job fails to produce the results which were originally anticipated (and it will!), the owner will then seek out additional power enhancements. This usually means springing for a power amplifier.  Most modern transistorized amplifiers can be driven to their full rated carrier output (peak rated power divided by 4) with as little as 1.5 watts of drive power.  Driving an amplifier like this with a "peaked" and "clipped" radio running a distorted 6 watts output is a recipe ripe for disaster. It will not sound good, and eventually the owner will have to get the radio "detuned" again, so that it will work properly with the amplifier.

So you can see that there is very little benefit to be gained by peaking, and a lot to lose.  Other than falling short of the high signal goal, peaking a radio can also place more strain on the transmit final transistor which can lead to premature failure.  My advice; don't do it!  Save the money and buy a better antenna system. A good quality antenna can achieve signal improvements which will be at least as effective as those from a "peak" job, and usually quite a bit better.  A better antenna will also improve your receive performance as well as your transmit signal.