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CHAPTER 4
NOISE AND INTERFERENCE

While listening to the radio during a thunderstorm, youíre sure to have noticed interruptions or static at one time or another. Perhaps you heard the voice of a pilot communicating to a control tower when you were listening to your favorite FM station. These are examples of interference that affect a receiverís performance.

Annoying as this may be while youíre trying to listen to music, noise and
interference can be hazardous in the world of communications, where a
missionís success or failure depends on receiving and understanding the
transmitted message.

Sources of Noise

Receiver noise and interference come from both external and internal sources.

Internal noise is created within the circuits of the receiver itself. Power supplies and frequency synthesizers are prominent sources of noise within the radio.

But some noise comes from thermal agitation of the molecules that comprise
electronic components in the amplifier stage closest to the receiver antenna.

External noise comes to the receiver by way of the antenna from sources
outside the radio and frequently exceed internal receiver noise.

Natural Sources of Noise

In the HF frequency band, lightning is the main atmospheric (natural) source of noise. Atmospheric noise is highest during the summer and greatest at night, especially in the l- to 5- MHz range. One of the advantages of the VHF and UHF bands is that they are above this large source of noise.

Another natural noise source is galactic or cosmic noise, generated in space. At 20 MHz (just below the VHF band), space noise is generally larger than that of internally generated noise. This noise tapers off so that at around 200 MHz, it is about equal to internal noise. At higher frequencies it is insignificant.

Man- Made Noise

Power lines, computer equipment, and industrial and office machinery produce man- made noise, which can reach a receiver through radiation or by conduction through power cables. This type of man- made noise is called electromagnetic interference (EMI) and it is highest in urban areas. Grounding and shielding of the radio equipment and filtering of AC power input lines are techniques used by engineers to suppress EMI.

Unintentional Radio Interference

At any given time, thousands of radio transmitters compete for space on the
radio spectrum above and below the VHF/ UHF range of frequencies. Harmonics
of HF transmitters fall within the VHF band, and commercial FM stations and
other wireless radio emissions fall directly within the VHF and UHF bands.

The radio spectrum is especially congested in Europe due to population density. A major source of unintentional interference is the collocation of transmitters, receivers, and antennas. Itís a problem on ships where space limitations dictate that several radio systems are located together. Ways to reduce collocation interference include locating and carefully orienting
antennas; using receivers that wonít overload on strong, undesired signals; and using transmitters designed to minimize harmonics and other spurious emissions.

Intentional Interference

Deliberate interference, or jamming, results from transmitting on operating
frequencies with the intent to disrupt communications. Jamming can be directed at a single channel or be wideband. It can be continuous (constant transmitting) or look- through (transmitting only when the signal to be jammed is present).

Modern military radio systems use spread- spectrum or frequency- hopping
techniques to overcome jamming and reduce the probability of detection or
interception. Spread- spectrum techniques are techniques in which the
modulated information is transmitted in a bandwidth considerably greater
than the frequency content of the original information.

Multipath Distortion

Signals from a transmitter reach the receiver via multiple paths and arrive at slightly different times (see Chapter 2). These multiple signals are as disruptive to communication as signal interference from other transmitters.

Signal Quality Measurement

Signal quality is indicated by signal- to- noise ratio (SNR), measured in decibels (dB). The higher the SNR, the better the signal quality. Every 3 dB of SNR corresponds to a ratio of two- to- one. Thus a 9 dB SNR means that the signal is eight times greater than the noise. A commonly considered SNR lower limit for adequate reception is 10 dB. This means that the signal has ten times as much power as the noise.

Reducing the Effect of Noise and Interference

Engineers use various techniques to combat noise and interference, including:

(1) boosting the effective radiated power,
(2) providing a means for optimizing operating frequency,
(3) choosing a suitable modulation scheme,
(4) selecting the appropriate antenna system, and
(5) designing receivers that reject interfering signals.

SUMMARY

Natural (atmospheric) and man- made sources cause noise and
interference. Power lines, computer terminals, and industrial
machinery are prominent causes of man- made noise.

Congestion of radio transmitters competing for limited radio
spectrum causes interference.

Collocated transmitters interfere with nearby receivers.

Jamming, or deliberate interference, results from transmitting on
operating frequencies with the intent to disrupt communications.

Multipath interference can be considered another form of noise.

Proper antenna selection and advanced modulation techniques
can reduce the effects of noise and interference.