Page 333 - Physics Coursebook 2015 (A level)

Chapter 20: Communications systems
Surface (ground) waves travel close to the surface of the Earth. Lower frequencies, up to 3 MHz, diffract around the surface of the Earth because of their long wavelengths; this gives them a long range of up to 1000 km. AM broadcasts in the medium-wave (MW) and long-wave (LW) bands travel efficiently as surface waves.
Sky waves are waves above 3 MHz in frequency, which are diffracted only a little by the Earth and travel almost in straight lines. Some waves at these frequencies travel
for a short distance as surface waves, but they can only travel about 100 km in this way. Sky waves that travel in the atmosphere may be reflected from a layer of charged particles known as the ionosphere. When these reflected waves reach the Earth’s surface they may be reflected
back to the ionosphere. Through multiple reflections by the ionosphere and the ground, sky waves can travel for large distances around the Earth. Because the density of charged particles in the ionosphere is constantly changing, the reflection of sky waves is not reliable. Short-wave (SW) radio uses frequencies in the high-frequency (HF) band that reflect from the ionosphere. Your radio set can receive distant SW radio stations at night, when the frequencies reflected by the ionosphere are not absorbed by other regions in the atmosphere.
Space waves are sky waves with a frequency greater than 30 MHz, which pass through the ionosphere. The transmission is line-of-sight, so, if the receiver and transmitter are on the Earth’s surface, there must be
a clear line between the receiver and the transmitter
(not blocked by hills or mountains). Some police and emergency services, as well as television transmissions, use space waves with frequencies above 30 MHz in the very-high-frequency (VHF) and ultra-high-frequency (UHF) bands. The boundary between radio waves and microwaves is not clearly defined, although frequencies above 1 GHz are generally described as microwaves. Microwaves are able to pass through the ionosphere to reach satellites in space. BluetoothR technology and Wi-FiR use microwaves for communication around the home. Using Wi-FiR , microwaves link your laptop to the main computer in your home.
Table 20.4 is a summary of the frequencies used for the different radio transmissions and the distances travelled. The wavelength of any radio wave can be found using the formula c = f λ where c = 3.0×108 ms−1, which is the speed of the radio wave and the speed of light. So, for example, a wave of frequency 1 GHz has a wavelength of:
λ = 3.0 × 108 = 0.3 m 1×109
surface wave
sky wave
up to 3 MHz
3–30 MHz
LW and MW radio in the LF band
SW radio in the HF band
up to 1000 km
worldwide by reflection
Frequency range
Communication method
and waveband
Distance travelled
space wave
30–300+ MHz
FM radio in the VHF band, TV and mobile phones in the UHF band
line-of-sight
microwave
1–300 GHz
microwave, satellite links and Wi-Fi in the super- high-frequency (SHF) and extra- high-frequency (EHF) bands.
line-of-sight except when retransmitted by satellite
Table 20.4
Data for radio and microwaves.
The radio wave given out by a transmitting aerial travels in all directions. The atmosphere absorbs the wave, the amount of absorption depending on the radio frequency. The distance travelled by a radio wave therefore varies with its frequency. In some cases, the aerial focuses the radio waves towards the receiver by using an aerial shaped as a dish, as shown in Figure 20.17. Without such a dish, the strength of the signal decreases strongly with distance.
The microwave tower shown in Figure 20.17 holds a number of parabolic reflectors or dishes. Each dish points towards a dish on another tower some miles away and they transmit microwaves back and forth between them. The transmission is line-of-sight and the height of the tower increases the distance of transmission.
Figure 20.17
A microwave tower.
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