Modulation – what is it and how much is too much?
Youll often hear on the CB radio one operator asking another how is my modulation?. The question is actually something that everyone who uses the CB radio should be asking themselves.
WHAT IS MODULATION?
Well the actual definition of modulation as it refers to CB radios involves a fairly technical explanation, but well try to keep it simple. Basically an audio-frequency wave must be combined with a carrier wave in order to be transmitted over the radio. Either the frequency or the amplitude of the radio wave may be modified. Amplitude Modulation or AM is the mode used by CB radios.
When you use your CB radio and your voice enters the microphone it then goes into the modulating circuit of the radio which creates the modulation that is added to your carrier.
A modulating radio wave looks very similar to a wave youd see in the ocean, except that unlike an ocean wave there is a maximum peak for a AM radio wave. This maximum peak could be explained as maximum modulation or 100% modulation.
As your voice hits its loudest point the modulation peaks as well. If you are talking softly your modulation peaks will be smaller and your modulation may be less than 100%. High modulation nearing 100% in very simple terms means that you are getting the maximum out of your radio and the guy on the other end receiving you will hear your voice without any difficulty.
A majority of the CB radios produced have a special adjustment potentiometer mounted internally that is used specifically to adjust the level of modulation of the radio. The factories install this so that during the testing stage of production the radios modulation level can be set to a factory specification very easily.
Modulation affects the output power of a CB radio. As modulation increases the peak output increases. The FCC has strict rules regarding the output of CB radios and therefore most CB manufacturers set their radio modulation levels around 80% or lower to make sure that they are well within the FCC regulations.
One of the most common modifications that a CB operator will make to their equipment is to internally turn up the modulation adjustment so that the CB radio is producing 100% modulation. Not all radios have this modulation potentiometer though, some merely have a set resistor in place and the only way to increase the modulation internally is to change the value of the resistor or to remove it completely.
Most people dont talk at their loudest all the time and certain vowels or words often are spoken in lower tones which normally would result in less than 100% modulation. To counteract this problem many radio operators use power microphones with high gain or high end radios which have special circuits that help make sure all the highs and lows of your voice are turned into a 100% level of modulation.
If you have a CB radio that has a 4 watt carrier when you apply modulation the output will increase upwards. With 50% modulation your output might increase to a peak output of 7 watts. With 100% modulation your peak output might increase to 12 watts. Since output power also affects the distance and strength of your transmission its easy to see the connection and importance of having a CB that is maintaining near 100% modulation.
OVERMODULATION THE DANGER ZONE
While everyone using a CB should be trying to achieve 100% modulation to make sure they are getting the maximum audio and output out of their radio, there is such a thing as too much modulation.
Since 100% modulation represents a maximum height of the amplitude modulation wave, if someone tries to create modulation greater than 100% a number of bad effects can occur, such as spurious emissions and distortion.
So exactly what causes overmodulation to occur in a CB radio?
Often people will adjust the internal modulation potentiometer in their radio to its absolute maximum. Then, in addition to that modification they will add a very high gain microphone that amplifies their voice to very high levels creating overly strong input into the modulation circuit of their radio. These two things together will produce more modulation than necessary to reach 100%.
Overmodulation will make the CB operator sounds scratchy or may add static type sounds to their transmission. In addition the operators signal may begin clipping, this is when the modulation wave becomes distorted due to the overmodulation and creates what are called harmonics.
When using the CB these harmonics mean that the operators signal has emissions that are not on the same frequency as their transmission. Also referred to as bleedover.
Im sure everyone has heard a station that is overmodulated, or has experienced the harmonics of overmodulation when you are on one channel but can hear someone from another channel bleeding onto your channel.
While everyone wants to sound loud and clear when talking on the CB radio, there is a clear maximum level for modulation. Going over this level will result in overmodulation and will have multiple negative and unwanted effects on your transmission.