Two Way Radio - Time Out Timer

Time Out Timer

Time Out Timer feature that is used to prevent the user from transmitting on the same channel for extended periods of time. This feature also protects the radio from potential damage that could result by long time transmission.

Squelch Control

Squelch circuitry is used to reduce, or eliminate, unwanted signals and noise before they are heard in the speaker. There are two types of squelch... carrier squelch and coded squelch.

Carrier, or noise compensated squelch, is used to prevent noise from being heard in the speaker when there are no transmissions taking place.

Coded squelch allows listeners to hear only those messages intended for them. The two most common types of coded squelch are tone coded squelch, called Private-Line, and digital coded squelch, called Digital Private Line. It is important to remember that while coded squelch systems eliminate the annoyance of listening to other transmissions, they do not provide privacy.

Each radio is supplied with a switch that defeats the coded squelch circuit. This is necessary since users must listen to be sure there is no other voice traffic on the channel before transmitting. Otherwise, a transmission already in progress could be disrupted by someone else transmitting at the same time.

Radio Frequency Bands

The part of the spectrum we are concerned with are the frequency bands used for radio communications. These are:

• VHF Lowband (25 MHz - 50 MHz)
• VHF Highband (136 MHz - 174 MHz)
• UHF Band (403 MHz - 512 MHz)
• 800/900 MHz Bands

The characteristic behavior of frequencies in different bands in the spectrum is important when choosing a frequency for a radio system.

Frequency Band Characteristics

The frequency band characteristics most important to radio communications include: Propagation, Noise Interference and Range

Propagation

The atmosphere that surrounds the earth acts to attenuate and refract radio signals just as it does light. Just how much it is affected depends on the frequency. As a general rule, the lower the frequency, the less the attenuation, or loss of signal.

Lower frequency radio waves travel better through fog or dust than higher frequency waves. Low frequency AM broadcast radio signals will travel far beyond the horizon and can be reflected back to earth for reception at great distances.

Higher frequency television or FM commercial broadcast stations are absorbed by the earth's atmosphere and are therefore limited to line of sight transmission.

Below 300 KiloHertz, the characteristics are just the opposite. Here, radio waves follow the curvature of the earth for great distances. This type of propagation is called a ground wave. Radio communications over distances up to several thousand miles are possible by making use of low frequency ground waves.

Above 300 KHz to about 30 MHz, the ionosphere will sometimes reflect and/or refract the radio signals. when returned to earth, they are received hundreds or even thousands of miles away. This is called skip and these radio signals are called sky waves.

The frequencies from 30 MHz to about 900 MHtz are the most suitable for two-way radio transmission. Generally, this range of radio frequencies is characterized by line-of-sight propagation.

 

 

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