Table 1-1.-Radio-Frequency Spectrum
NAVY FREQUENCY BAND USE
Table 1-1 shows the radio-frequency spectrum
broken down into bands that are used by the military.
Each band of frequencies has its own characteristics.
Navy Electricity and Electronics Training Series,
Module 17, Radio-Frequency Communications Principles,
discusses all the bands. This chapter will only discuss
the bands that are of interest to the Aviation Electronic
VLF and LF Band Communications
The very low frequency (VLF) and low frequency
(LF) bands were originally used for radio telegraphy.
Because the wavelengths were in the kilometer range
and higher (30 kHz has a wavelength of 10 kilometers,
or about 6.2 miles), enormous antennas had to been
used. With todays technology, this is no longer a factor.
MF and HF Band Communications
The medium-frequency (MF) and high-frequency
(HF) bands are not only used by the Navy, but portions
are also used by commercial AM broadcasting stations.
These spectrums also include the international distress
frequencies (500 kHz, 2182 kHz, 8364 kHz, 3023.5 kHz
and 5680 kHz).
Signal radiation in these frequency ranges have the
important property of being reflected by the ionosphere.
The ionosphere is a layer of electrically charged
particles at the top of the earths atmosphere. The layer
is caused by the strong solar radiation entering the upper
atmosphere. When a radio wave in the MF or HF range
hits this layer, it is reflected back to earth. Multiple
reflections between this layer and earth are are possible,
allowing great distances to be obtained in these ranges,
particularly the high-frequency band.
The disadvantage of this type of propagation is that
it depends on the characteristics of the ionosphere,
which varies widely, especially during daylight hours.
As a result of this varying, the waves are reflected
differently and take different paths over a period of time.
This causes the signal at the receiver to vary in strength,
which causes the output to fade in and out.
VHF and UHF Band Communications
Signal radiation in these frequency ranges get very
little ionospheric reflection. As a result, communi-
cations in these ranges tend to be line-of-sight and over
a short distance. Line-of-sight means exactly what the
name saysthe transmitter and receiver must be within
a straight visual sighting line from each other. Buildings
and uneven terrain may affect the transmission. The
lower part of the UHF band and the VHF band is also
used for mobile communications and television.