Radio Wave Propagation
EM waves are cobination of Electric and Magnetic waves. Propagation means how radio waves travel from one point to another point on the earth through free-space. Free space can refer to atmosphere or the air. Electromagnetic Waves (EM waves) are produced when the electrons in a conductor (i.e. antenna wire) are made to oscillate back and forth. These waves radiate outwards from the source at the speed of light. Electromagnetic Waves are collection of Light waves, Radio waves, UV rays and X-rays etc. All these EM Waves differ in frequency and wavelength. They travel faster through a vacuum than through any other medium. EM waves will propagate outwards in all directions. If the medium in which it is propagating (ex: air) is the same everywhere the wave will spread-out uniformly in all directions.
Wherever there is electricity, there is magnetism and vice versa. E & M fields always exist together. These two fields (E & M) are at right angles to each other and the direction of propagation. The angle between any two of these 3 parameters: E, M and the direction of propagation is always 900. Note that the plane of Electric field defines the polarization of the wave.
Our system of global communication depends on radio waves. There are three different types of radio wave propagation – ground, sky and space waves. Radio signal is called EM wave because it is made up of electric and magnetic fields.
- Ground wave
- Sky waves (Ionosphere)
- Space waves (Line of sight)
Before discussing different types of radio propagation, we see the details of Earth atmosphere and surrounding layers as given below:
Layers of Earth atmosphere
Troposphere:
- Height: 10 – 15 km
- Contains 75% of atmosphere mass
- Temperature and pressure decrease as you go higher up the troposphere. Near the top of troposphere, temperature is: -73o C
Stratosphere:
- Height: 35 Km deep i.e., 15 Km – 50 Km above earth
- Gets warmer up as you go higher because of absorption of sun light by ozone
Mesosphere:
- Burning of Meteors takes here
- Height: 50 Km – 80 Km
- -90oC at the top of this layer
Thermosphere:
- Height: 80 Km – outer space
- Colourful AURORAS known as Northern and Southern lights occur in this layer
- Satellites, space shuttles
Ionosphere:
- Existence is due to UV radiation from sun. The air is ionised by the sun’s UV rays. These ionised layers affect the transmission and reflection of radio waves. Ionosphere is the collection of electrically charged ions.
- Ionosphere has important quality of bouncing radio signals transmitted from the earth.
- Due to this layer, all over the world can be reached via radio.
- As the radio signal is transmitted, some of the signal will escape the earth through the ionosphere.
- Overlaps the other atmospheric layers from above the earth
- Located within the thermosphere & height: Up to 350 Km
Exosphere:
- It is the region beyond the thermosphere
GROUND WAVE
Ground waves are also known as surface waves. Ground waves travel along the surface of the earth. Note that these waves are strongest at medium and low frequencies. They can travel at distances beyond the horizon and are always vertically polarised. Horizontal polarised waves are absorbed by the Earth. Along the ground very long-range propagation is possible using ground waves. This kind of wave propagation is strongest at the LF and MF ranges. So, ground waves are the main signal path for radio signals in the 30 kHz – 3 MHz range. The signals can propagate for 100s and sometimes 1000s of miles at these low frequencies. AM broadcast (540 kHz – 1650 kHz) signals are propagated primarily by the ground waves during the day and by sky waves at night.
The conductivity of the earth determines how well ground waves are propagated. The better the conductivity less the attenuation and greater the distance the wave can travel. Propagation is the best over salt water due to its high conductivity. Over deserts, where conductivity is less, propagation is not good. Note that at frequencies beyond 3 MHz the earth begins to attenuate radio signals. So, frequencies greater than 3 MHz range, ground wave propagation is not significant or used.
SKY WAVE (or IONOSPHERE) Propagation
Sky waves are reflections from the ionosphere also known as ionospheric propagation. Radio signals are affected in many ways through which they travel. Reflection, refraction and diffraction may occur. Sky wave signals are radiated by the antenna into the upper atmosphere, where they are reflected in the ionosphere and sent back to earth. Ionosphere has important quality of bouncing back the radio signals transmitted from the earth. Due to this layer, all over the world can be reached via radio. Ionosphere exists between 50 km above the earth and extends to 400 km from the earth.
The ionosphere is the uppermost part of the atmosphere and is ionized by solar radiation. UV radiation from the Sun causes the upper atmosphere to ionize i.e., to become electrically charged. The atoms take on or loose electrons – become positive or negative ions. Free electrons are also present. The ionosphere is divided into layers shown below:
D layer: 75 -95 km, relatively weak ionisation. It is responsible for strong absorption of MF (300 kHz – 3 MHz) during day light and affects AM broadcasting during daytime.
E region: 95 – 150 km. D, E layers are farthest from the sun, so weakly ionized. Exist only during day light hours. During day time, D, E layers absorb radio signals in the MF range from 300 kHz – 3 MHz
F region: About 150 km to 400 km above the surface of the Earth is the F layer. It is the top most layer of the ionosphere. Here extreme ionization takes place. The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F layers are responsible for most sky wave propagation of radio waves, and are responsible for reflection of HF radio waves. VHF/UHF frequecies escape from the ionosphere to form space waves as shown in above figure.
SPACE WAVE
Space waves travel directly from transmitting antenna to receiving antenna without reflection on the ground. These waves neither follow the curvature of the earth nor bend. Space wave communication occurs when both antennas are within LoS (Line of Sight) of each other. LoS means two antennas must be able to see each other. Note that space waves are also known as direct waves. Frequencies in UHF range (TV signals) propagated by means of space waves. LoS range can be found from the height of the transmitting and receiving antennas as follows:
Space wave is also known as direct waves because, waves travel directly from the transmitting antenna to receiving antenna. Here two antennas must be able to see each other. i.e., there must be a line of sight path between them.
Radio Frequency (RF) Spectrum
| S.No. | Frequency band Name | Frequency Range (Hz) | Wave length | Applications |
| 1 | ELF (Extremely Low Frequencies) | 30 – 300 | 104 km to 103km | Power applications |
| 2 | VF (Voice Frequencies) | 300 – 3K | 103km to 100 km | Audio applications |
| 3 | VLF (Very Low Frequencies) | 3K – 30 K | 100 km – 10 km | Submarine communications, Navy and Military communications |
| 4 | LF (Low Frequencies) | 30K – 300K | 10km – 1 km (Long waves) | Marine and Aeronautical communications |
| 5 | MF (Medium Frequencies) | 300K – 3M | 1km to 100 m (Medium waves) | AM broadcast, Marine and Aeronautical communications |
| 6 | HF (High Frequencies) | 3M – 30M | 100m to 10m (Short waves) | Amateur and CB communication |
| 7 | VHF (Very High Frequencies) | 30M – 300M | 10m to 1m | FM and TV broadcasting |
| 8 | UHF (Ultra High Frequencies) | 300M – 3G | 1m to 10cm (Microwaves) | Cellular phones
UHF TV channels |
| 9 | SHF (Super High Frequencies) | 3G – 30G | 10-1m to 10-2m | Satellite communications & RADAR |
| 10 | EHF (Extremly High Frequencies) | 30G-300G | 10-2m to 10-3m | Satellite communications & RADAR |
1K = Kilo = 103
1M = Mega = 106
1G = Giga = 109
1T = Tera = 1012
