3-15
These antenna coupler groups provide a coupling path of prescribed efficiency between each
transmitter and its associated antenna. They also provide isolation between transmitters, tunable bandpass
filters, and matching networks.
TELETYPEWRITER AND FACSIMILE EQUIPMENT
In previous areas we have discussed different methods of voice communications. At times, however,
the message is too long for practical transmission by voice. To get information or an idea across to
another person far away, you may also need a chart, map, or photograph. Teletypewriter (tty) and
facsimile equipment allow us to do just that, with ease. Let's see how this is done.
BASIC PRINCIPLES
To give you an idea of how intelligence is sent via teletypewriter, let's take a look at the manual
telegraph circuit. This circuit, shown in figure 3-16, includes a telegraph key, a source of power (battery),
a sounder, and a movable sounder armature. If the key is closed, current flows through the circuit and the
armature is attracted to the sounder by magnetism. When the key is opened, the armature is retracted by a
spring. With these two electrical conditions of the circuit, intelligence can be transmitted by means of a
teletypewriter code. These two conditions of the circuit are referred to as MARKING and SPACING. The
marking condition occurs when the circuit is closed and a current flows; the spacing condition occurs
when it is open and no current flows.
Figure 3-16.—Manual telegraph circuit.
If the key at station A is replaced by a transmitting teletypewriter and the sounder arrangement at
station B is replaced by a receiving teletypewriter, the basic teletypewriter circuit (loop) shown in figure
3-17 is formed.
