Communication Systems 669
- Ground-wave propagation
- Sky-wave propagation
- Line-of-sight (LOS) propagation.
In the frequency bands that are primarily used to provide navigational aids from shore to
ships around the world (VLF to LF to MF), the available channel bandwidths are relatively small,
and hence the information that is transmitted through these channels is relatively slow speed and
generally confined to digital transmission. Noise at these frequencies is caused by thunderstorm
activity around the globe, whereas interference is caused by the many users.
For frequencies of 0.3 to 3 MHz, in the MF band, ground-wave (or surface-wave) propagation,
illustrated in Figure 15.0.2, is the dominant mode used for AM broadcasting and maritime radio
broadcasting. Dominant disturbances include atmospheric noise, human-made noise, and thermal
noise from electronic components. The range is limited to about 100 miles for even the more
powerful radio stations.
In the ionosphere, the rarefied air becomes ionized, mainly due to ultraviolet sunlight. The
D-region, usually falling between 50 and 90 km in altitude, will reflect waves below 300 kHz or
so, and attenuate higher frequency waves (300 kHz<f<30 MHz), especially in the daytime.
The D-region mostly disappears at night. The E-region (about 110 km in altitude) reflects high
frequencies (3 MHz<f<30 MHz) during the daytime, and medium frequencies (300 kHz<f
<3 MHz) at night. The F 1 -region (about 175 to 250 km in altitude) is distinct from the F 2 -region
(250 to 400 km in altitude) only during the day; at night they merge. Waves that penetrate the
E-region usually go through the F 1 -region as well, with some attenuation being the primary effect.
The F 2 -region provides the main means of long-distance, high-frequency (3 MHz<f<30 MHz)
communication by wave reflection. Sky-wave propagation is illustrated in Figure 15.0.3.
Signal multipathoccurs with electromagnetic wave propagation via sky wave in the HF
range. When the transmitted signal reaches the receiver through multiple propagation paths with
different delays,signal fadingmay result. Both atmospheric noise and thermal noise become the
additive noise at high frequencies. It is possible to have ionospheric scatter propagation in the
frequency range of 30 to 60 MHz, and tropospheric scattering in the range of 40 to 300 MHz; but
relatively large antennas are needed with a large amount of transmitted power, because of large
signal propagation losses.
Frequencies above 30 MHz, propagating through the ionosphere with relatively little loss,
make satellite and extraterrestrial communications possible. In the VHF band and higher, the
dominant mode is line-of-sight (LOS) propagation, in which the transmitter and receiver antennas
must be in direct LOS with relatively little or no obstruction. That is why television stations
Ground or surface
wave
Earth
Troposphere
(20 km above
earth surface)
Figure 15.0.2Illustration of ground-wave propagation in MF
band.
Ionosphere (with D, E, F regions)
Sky waves
Earth
(50 to 600 km
above earth
surface)
Figure 15.0.3Illustration of sky-wave propaga-
tion.
