B-1. A radio set consists of a transmitter and receiver. Other items necessary for operation include a source of electrical power and an antenna for both radiation and reception of radio waves.
B-2. The transmitter contains an oscillator that generates RF energy in the form of alternating current. A transmission line, or cable, feeds the RF to the antenna. The antenna converts the alternating current into electromagnetic energy that is radiated into space; a keying device is used to control the transmission. B-3. Normally, in SC radio operations, the receiver uses the same antenna as the transmitter to receive electromagnetic energy. The antenna converts the received electromagnetic energy into RF alternating current. The RF is fed to the receiver by a transmission line or cable. In the receiver, the RF is converted to audio frequencies. The audio frequencies are then changed into sound waves by a headset or loudspeaker. B-4. Communications are possible when two radio sets operate on the same frequency, with the same type of modulation, and are within operating range.
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RANSMITTERB-5. The simplest radio transmitter consists of a power supply and an oscillator. The power supply can be batteries, a generator, an alternating current power source with a rectifier and a filter, or a direct current rotating power source. The oscillator, which generates RF energy, must contain a circuit to tune the transmitter to the desired operating frequency. The transmitter must also have a device for controlling the emission of the RF signal. The simplest device is a telegraph key, a type of switch for controlling the flow of electric current. As the key is operated, the oscillator is turned on and off for varying lengths of time. The varying pulses of RF energy produced correspond to dots and dashes. This is a CW operation, and is used when transmitting international Morse code.
B-6. A CW radio transmitter is used to generate RF energy, which is radiated into space. The transmitter may contain only a simple oscillator stage. Usually, the output of the oscillator is applied to a buffer stage to increase oscillator stability, and to a PA that produces greater output. A telegraph key may be used to control the energy waves produced by the transmitter. When the key is closed, the transmitter produces its maximum output; when the key is opened, no output is produced.
B-7. By adding a modulator and a microphone, a radiotelephone transmitter can transmit messages by voice. When the modulating signal causes the amplitude of the radio wave to change, the radio is an AM set. When the modulating signal varies the frequency of the radio wave, the radio is an FM set.
Transmitter Characteristics
B-8. The reliability of radio communications depends on the characteristics of the transmitted signal. The transmitter, and its associated antenna, forms the initial step in the transfer of energy to a distant receiver. B-9. Ground-wave transmission is used for most field radio communications. The range of the ground
the applicable portions of the medium frequency (MF) band (300–3000 kHz) to the HF band (3.0–30 MHz). When the transmitter is operating at frequencies above 30 MHz, its range is generally limited to slightly more than LOS. For circuits using sky wave propagation, the frequency selected depends on the geographic area, season, and time of day.
Note. Frequency selection is the responsibility of the frequency manager not the RTO.
B-10. For maximum transfer of energy, the radiating antenna must be the proper length for the operating frequency. The local terrain determines, in part, the radiation pattern, and therefore affects the directivity of the antenna and the possible range of the set in the desired direction. When possible, several variations in the physical position of the antenna should be tried to determine the best operating position for radiating the greatest amount of energy in the desired direction.
B-11. The range of a transmitter is proportional to the power radiated by its antenna. An increase in the power output of the transmitter results in some increase in range. Under normal operating conditions, the transmitter should feed only enough power into the radiating antenna to establish reliable communications with the receiving station. Transmission of a signal more powerful than required is a breach of signal security, because adversary DF stations may instantly and more easily fix the location of the transmitter. Also, the signal can interfere with friendly stations operating on the same frequency.
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ECEIVERB-12. A radio receiver can receive modulated RF signals that carry speech, music, or other audio energy. It can also receive CW signals that are bursts of RF energy conveying messages by means of coded (dot/dash) signals.
B-13. The process of recovering intelligence from an RF signal is called detection; the circuit in which it occurs is called a detector. The detector recovers the intelligence from the carrier and makes it available for direct use, or for further amplification. In an FM receiver, the detector is usually called a discriminator. B-14. An RF signal rapidly diminishes in strength after it leaves the transmitting antenna. Many RF signals of various frequencies are crowded into the RF spectrum. An RF amplifier selects and amplifies the desired signal; it contains integrated circuits or microprocessors to amplify the signal to a usable level. The RF amplifier is included in the receiver to sharpen the selectivity, and to increase the sensitivity. The RF amplifier normally uses tunable circuits to select the desired signal.
B-15. The signal level of the output of a detector, with or without an RF amplifier, is generally very low. One or more audio frequency amplifiers are used in the receiver, to build up the signal output to a useful level to operate headphones, a loudspeaker, or data devices.
Receiver Characteristics
B-16. When the transmitted signal reaches the receiver location, it arrives at a much lower power level than when it left the transmitter. The receiver must efficiently process this relatively weak signal to provide maximum reliability of communications.
B-17. Sensitivity describes how well a receiver responds to a weak signal at a given frequency. A receiver with high sensitivity is able to accept a very weak signal, and amplify and process it to provide a usable output. The principal factor that limits or lowers the sensitivity of a receiver is the noise generated by its own internal circuits.
B-18. Selectivity describes how well a receiver is able to differentiate between a desired frequency and undesired frequencies.
B-19. In field radio communications, the type, location, and electrical characteristics of the receiving antenna are not as important as they are for the transmitting antenna. The receiving antenna must be of sufficient length, be properly coupled to the input of the receiver circuit, and (except in some cases for HF sky wave propagation) must have the same polarization as the transmitting antenna.