d. Characteristics of frequency bands. The data in the chart below, which are approximate
under normal operating conditions, indicate that each frequency band has certain transmission
characteristics. The exact characteristic depends upon the condition of the propagation medium, the
transmitter power output, and many other factors.
* Troposcatter or ionospheric scatter provides this range.
** Troposcatter or ionospheric scatter requires this power range.
Learning Event 3:
METHODS OF TRANSMISSION
General. The radio communications equipment in lower echelon units is used primarily to
transmit intelligence in the form of speech or telegraphic code. When audio-frequency vibrations
(speech or telegraphic code) activate the ear drum, the effect on the human nervous system is called
sound. This form of acoustical energy travels through the air at a velocity of approximately 1,100 feet
per second. Although sound can be converted to audio-frequency electrical energy, it is not practical to
transmit it in this energy form through the earth's atmosphere by electromagnetic radiation. For
example, efficient transmission of a 20-cycle audio signal would require an antenna almost 5,000 miles
long. None of the above limitations apply when radio-frequency electrical energy is used to carry the
intelligence. Tremendous distances can be covered; efficient antennas for radio frequencies are of
practical lengths; antenna power losses are at reasonable levels; many channels, each carrying
information, can be used; and selectivity or information is possible.
a. Modulation.
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