NAVY FREQUENCY BAND USE

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 Technician. 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 today’s 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 earth’s 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 says–the 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. 1-2