(1) The open-wire line shown in A of figure 2-5 is used mainly with
transmitters. The spacing between lines may be varied by changing the
insulators between wires.
Such lines require much time and work to
install. This type of line is not often used with receivers because of
the possibility of noise pickup along the line.
(2) The insulated parallel-wire line (B of fig. 2-5) is widely used for
television reception. The plastic ribbon maintains the spacing between
the conductors. This type of transmission line is flexible and easy to
handle and install, but is subject to noise pickup from external
sources.
(3) The shielded parallel-wire line (C of fig. 2-5) is often used in radio
reception because of reduced noise pickup from external sources along
the line.
(4) The type of transmission line shown in D of figure 2-5 is used for both
transmission and reception.
Receiving-type coaxial cable is usually
small in diameter and very flexible.
The copper-braid shield greatly
reduces noise pickup from external sources.
The transmitting-type
coaxial cable is larger in diameter because a larger center wire is
used to keep resistance losses to a minimum, and a thicker dielectric
material is used to insulate the center conductor from the copper-braid
shield.
Section II.
RADIO SYSTEMS
2-3.
MODULATION
The function of a radio transmitter is to supply power to an antenna at a
definite frequency, and to convey intelligence by means of the radiated wave.
Radio transmitters generate waves which may be either of two types: one is the
continuous wave, the other is the modulated wave.
a. Continuous Wave (CW).
The waveform of the CW or unmodulated wave
resembles the RF current in the tank circuit of an oscillator.
This signal is
called the carrier and conveys no information within itself. The carrier is a sine
wave of alternating current as illustrated in figure 2-3.
At the higher
frequencies, each cycle is so close to the other that the peaks may appear sharp or
pointed, although they are actually rounded.
b. Modulated Wave.
The modulated wave is produced when the
information
signal varies the carrier's amplitude or frequency.
2-4.
TRANSMITTING SYSTEMS
a. Radiotelegraph Transmitter.
(1) The simplest radiotelegraph, or CW, transmitter is shown in figure 2-6.
A key to turn the carrier on and off is connected to the oscillator. A
power supply to furnish power to the oscillator, and an antenna to
radiate the carrier complete the transmitter.
309 L2
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