BASIC RADIO THEORY
The study involves application of radio magnetic waves in aid of air navigation. Before we go into various radio aid equipments let us first study the basic properties of electromagnetic waves.
Simplified Transmission and Reception of Radio Waves
Radio waves are the product of the changing fields produced by an alternating current. Alternating current is produced by rotating a loop of wire in a magnetic field (or by rotating the magnetic field itself). This makes electrons flow along the wire in accordance with the alternating voltage produced. Because the voltage and current are alternating, the electrons flow in one direction for half the rotation, and in the reverse direction for the second half. This electron flow, alternately forwards and backwards, means that the current is continually changing. This changing current in turn produces fields along the wire.
If the wire is a closed circuit, then the fields in one part of the wire are generally cancelled out by those in another part. However, an alternating current can be induced in an open circuit with a bare wire at the end. In this case, the fields will propagate outwards from the wire in a normal (at 90° to it) direction. If the wire is of the correct length, the fields will resonate and send continuous alternating waves of energy outwards. This outward propagation of the fields forms the transmitted radio waves.
If a wire of the same length is placed in the same direction in space as the transmitting aerial, the fields will affect the wire and induce an alternating current in it, so a receiver a considerable distance away can receive the transmitted signal exactly. This was basically what Marconi achieved in his experiments, and although modern technology uses sophisticated electronic devices, the effects are the same. In fact, the traditional aerial is called a half wave dipole, and is actually two halves, each the length of one quarter wavelength, fed from the middle.
The electromagnetic waves travel at constant speed through the vacuum of space. The speed varies depending on the density of whatever medium they are travelling through, and we shall see later how this affects the actual propagation around the earth, but in general the gases in our atmosphere change the speed by only a small amount. The speed of propagation of electromagnetic waves (often called the speed of light and incidentally light waves are electromagnetic waves) in air averages approximately 3 x 108 m/s (3 lakh kilometre per second).
Properties of Electromagnetic Waves:
Speed: When alternating current is passed through an aerial, electrical energy is transmitted in the form of waves at a speed of : 186000 sm/sec or 162000 nm/sec or 3×108 m/sec
Electro-magnetic fields: The energy thus transmitted has electrical and magnetic fields. The fields are at right angles to each other. If the aerial is horizontal, the electrical field is horizontal and magnetic field is in vertical plane.
Cycle: One complete oscillation of an AC source i.e. mean value to peak to mean, to peak in other direction to the mean value is called one cycle. In other words A cycle is one complete series of values, or one complete process in a waveform.
Hertz (Hz): Number of cycles per second.
Amplitude: The maximum value attained or displaced on either side of mean position. The amplitude of a radio wave is the maximum strength of the signal during cycle. In any alternating case, the positive amplitude is the same as the negative amplitude. (The part of the curve in Fig 1-2 above the mean or time axis is called positive and the part below is called negative).
Frequency (f): Number of cycles per second expressed in hertz. Frequency of an alternating current or a radio wave is the number of cycles occurring in one second, expressed in hertz (Hz). For example, 100 Hz is 100 cycles per second. Since the number of cycles per second of normal radio waves is very high, it is usual to refer to their frequency in terms of kilohertz, megahertz and gigahertz as follows:
|1 cycle per second||1 Hz (hertz)|
|1000 Hz||1 KHz|
|1000 kHz||1 MHz (megahertz)|
|1000 MHz||1 GHz (gigahertz)|
Wave Length: The physical distance travelled in one cycle.It is defined as the distance between successive crests or the distance between two consecutive points at which the moving particles of the medium have the same displacement from the mean value and are moving in the same direction (two consecutive points in phase).
Wavelength / Frequency Relationship. In one second, a radio wave will cover a geographical distance of 3 X 108 metres. If the wave has a frequency of one hertz, the cycle will take one second to pass a point, and the geographical distance between the start of the cycle and its end will be 3 X 108 metres. (When the end of the cycle reaches the point, the start of the cycle will be 3 lakh km ahead.) That means that the length of the wave, or wavelength, is 3 lakh km. If the frequency of the wave is two hertz, there will be two cycles passing the point in one second, and the wavelength will be half of 3 lakh km, or 1.5 lakh km. Thus as frequency is increased, the wavelength is decreased in the same proportion and vice versa. Putting this in a formula:
|Wavelength = Speed of Radio Wave/ Frequency||λ = C/ f|
Phase : Position of a wave in a cycle at any instant of time is called its phase.
Phase Difference: The angular difference between corresponding points of two wave forms of the same frequency which do not reach the same phase value at the same instant of time.
Polarisation: The plane in which electric vector (field) moves is called its plane of polarization. A vertical aerial transmits electrical vector in vertical plane. Such a transmission is called vertically polarised wave. Receiving aerial should be polarised the same way for optimum reception.
Polar diagram: A locus of relative values of field strength (or power) for transmission or reception of an aerial is called its polar diagram. It may be anywhere between horizontal and vertical direction.In simple words it is graphical representation of relative field strength/power radiated by a transmitting aerial or capacity to receive by a receiving aerial at various points in both horizontal and vertical planes.
Radio frequencies with similar characteristics are divided into following internationally recognised bands.
|Frequency band name||Abbreviation||Frequencies||Wavelength|
|Very low frequency||VLF||3-30 kHz||100-10 km|
|Low frequency||LF||30-300 kHz||10000-1000 m|
|Medium frequency||MF||300-3000 kHz||1000-100 m|
|High frequency||HF||3-30 MHz||100-10 m|
|Very high frequency||VHF||30-300 MHz||10-1 m|
|Ultra high frequency||UHF||300-3000 MHz||100-10 cm|
|Super high frequency||SHF||3-30 GHz||10-1 cm|
|Extremely high frequency||EHF||30-300 GHz||10-1 mm|