a. High-Frequency Carriers. Radio-frequency carriers occupy the frequency spectrum above approximately
20,000 Hz and are therefore classed as high-frequency carriers.
b. Low-Frequency Carriers. Low-frequency carriers (below 20,000 Hz) will not radiate and are therefore
usually transmitted over wire or cable facilities. They can be transmitted over radio systems by modulating the
radio-frequency carrier.
3-2. COMPARISON OF RADIO AND CARRIER SYSTEMS
a. Radio System. A radio system is a communication system in which the carriers or their modulation
products, or both the carriers and modulation products are transmitted through space by electromagnetic radiation.
(1) Power. Much of the transmitted power in a radio signal is not usable because it is radiated in many
directions in addition to the intended direction toward the receiver site. The signal loss is smaller in a
radio relay system because directional antennas restrict the radiated signal to a narrow transmission
path. There is always a loss caused by signal power absorption in the radio transmission path.
Likewise, landline carrier is still more efficient because the signal path is enclosed within the cable.
(2) Capacity. A radio system is capable of transmitting one signal at a time. If we were to direct more
than one telephone or switchboard line toward a distant location, a separate radio frequency (RF)
would be needed for each channel of communication. A discrete RF would have to be assigned for
each RF carrier. Individual radio transmitters and receivers for each channel of communication
would also have to be provided.
b. Carrier System. By popular usage, "carrier system" designates a communication system in which the
carriers or their modulation products, or both carrier and modulation products are transmitted over wires or
cables. It is in this sense that the term will be used throughout this course.
(1) Power. Loss of signal power in a carrier system is due primarily to line constants of induction,
capacitance, and resistance. It is convenient to lump these values into one quantity called line
attenuation. Line attenuation plus losses induced by reflection of the signal are categorically labeled
line characteristics. These components that make up line characteristics therefore determine to a large
extent the efficiency of carrier communication systems. Since the line characteristics can be
engineered out of the system, the efficiency of power transfer is consequently high.
(2) Capacity. The traffic-handling capacity of a carrier system is primarily limited by the interconnecting
line or cable and the facilities. Each signal modulates a different frequency within a group of
frequencies provided by the carrier system. The number of frequencies that can be modulated
depends on the signal bandwidth and the frequency bandpass of the interconnecting line or cable.
Telephone carrier systems usually handle up to 16 telegraph channels on one voice channel.
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