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Back To Basics: Overmodulation

Overmodulation Leads To Interference!

Question B-001-019-003 (A) - From the Canadian Basic Question Bank:

An amateur station using radiotelephony must install a device for indicating or preventing:

a. overmodulation 
b. resonance 
c. antenna power
d. plate voltage

The  key word here is must. All four of the above can be measured in a transmitting station but not all are required to be indicated or prevented. Only one is harmful to the extent that it can seriously affect enjoyment of the airwaves and cause interference to other Amateurs.

Overmodulation is the condition that prevails in telecommunication when the level of the modulating signal [the intelligible portion holding information— such as spoken audio from a mic] exceeds the value necessary to produce 100% modulation of the carrier. A carrier signal is one with a steady waveform, constant height (amplitude) and frequency shown in the diagram as the envelope. Modulation is superimposed on the carrier at the transmitter and recovered at the receiver.

In layman's terms, the signal is going "off the scale". Overmodulation results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal. This means that the envelope of the output waveform is distorted. The usual way of ensuring you are not over-modulating is to use the ALC on your HF radio, which will tell you if the audio level is too high.  It is also good practice to monitor your own audio while transmitting to ensure you are not too close to the mic or speaking too loudly, as the presence of distortion will be a sure sign that your audio is driving the transmitter too hard.

In the diagrams above, A indicates an Amplitude Modulated (AM) signal that is modulated to a low percentage. When modulation is increased to 100%, as in B, we are on the threshold of overmodulation. When increased above 100% as in C, the signal is said to be overmodulated resulting in distortion and spurious emissions.

See a demonstration here on YouTube.

There are several questions on overmodulation in the question bank and one is always on the Basic exam.

Resonance is a desirable state when tuning to a specific frequency but does not have to be measured.  Antenna power and plate voltage may be measured but that is also not mandatory.

The correct answer to this question therefore is ‘A’ Overmodulation.

More information on overmodulation? 

~ John VE7TI

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