Survival Arts

Tactical Radio Communications, installment 3

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Mode

There are two main methods of sending information (in this case your voice) over radio waves. Amplitude modulation (AM) and frequency modulation (FM). AM is the older of the two methods, and transmits a voice by varying the power output of the transmitter in proportion to the loudness of the audio. Dead silence means the transmitter produces no power. As the audio becomes louder the power output of the transmitter increases. The transmitter and receiver can be fairly simple in design. AM has one major drawback: any interference on the same frequency is simply mixed into the desired signal. This makes AM very susceptible to lightning, atmospheric noise and any other transmitters in the area. Another drawback of AM transmitters is that they require special ‘linear’ amplifiers, which are not very power efficient.

FM varies the frequency of the signal in proportion to the audio. Thus, dead silence produces a constant steady signal with no frequency shift. As the signal of the audio increases the frequency changes more. Most two-way radios shift their signal up to +/- 5kHz. FM has one large advantage over AM. If two or more signals are present on the same frequency, the stronger signal will be the only one heard. This is called ‘FM capture effect’. Thus FM radios produce higher quality signals and have very good immunity to interference.

A third type of modulation method is used in modern radios, called phase modulation. This changes the phase of the signal in relation to the audio input. The effect of phase modulation is to produce a signal that has the characteristics of both AM and FM. The primary practical advantage of phase modulation radios is that they can be used to talk to both AM and FM radios.

There are other modes of transmission, but most are not used in portable equipment so they will not be addressed here.

Posted by Eric Cartman at November 25, 2002 09:19 AM | TrackBack