A loop antenna is a radiating coil of any convenient cross-section of one or more turns carrying radio frequency current. It may assume any shape (e.g. rectangular, square, triangular and hexagonal).
The maximum Frequency that can be reflected back for a given distance of trmission is called the maximum usable frequency (MUF) for that distance.
MUF = fcr sec φi
The polarization of the radio wave can be defined by direction in which the electric vector E is aligned during the passage of atleast one full cycle.Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space. The polarization are three types. They are Elliptical polarization ,circular polarization and linear polarization.
Antenna is a trition device or a trducer between a guided wave and a free space wave or vice-versa. Antenna is also said to be an impedance trforming device.
The radiation resistance of a loop antenna can be increased by:
This method takes advantage of the fact that signals of slightly different frequencies do not fade synchronously. This fact is utilized to minimize fading in radio telegraph circuits.
The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e Aperture efficiency = hap = Ae / Ap (dimensionless).
Ground wave classified into two types.
Frequency whose period is equal to the period of an electron in its orbit under the influence of the earths magnetic flux density B.
A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to the midpoint.
Effective aperture (Ae): It is the area over which the power is extracted from the incident wave and delivered to the load is called effective aperture.
Scattering aperture (As): It is the ratio of the reradiated power to the power density of the incident wave.
Loss aperture (Ae): It is the area of the antenna which dissipates power as heat.
Collecting aperture (Ae): It is the addition of above three apertures.
Physical aperture (Ap): This aperture is a measure of the physical size of the antenna.
Graphical representation of the radial component of the poynting vector Sr at a constant radius as a function of angle is called power density pattern or power pattern.
To minimize the fading and to avoid the multi path interference the technique used are diversity reception. It is obtained by three ways:
The phenomenon of splitting the wave into two different components (ordinary and extra-ordinary) by the earths magnetic field is called Magneto-Ions Splitting.
Normally the electric field E is perpendicular to the direction of wave propagation. In some situation the electric field E is parallel to the wave propagation that condition is called Cross field.
The radiation field will be produced at a larger distance from the the current element, where the distance from the centre of the dipole to the particular point is very large. It is also called as distant field or far field.
This method exploits the fact that signals received at different locations do not fade together. It requires antennas spaced at least 100λ apart are preferred and the antenna which high signal strength at the moment dominates.
Advantage:
Disadvantages:
In this array the antenna elements are arranged coaxially by mounting the elements end to end in straight line or stacking them one over the other with radiation pattern circular symmetry.
Eg. Omnidirectional antenna.
It is defined as the fictitious resistance which when inserted in series with the antenna will consume the same amount of power as it is actually radiated. The antenna appears to the trmission line as a resistive component and this is known as the radiation resistance.
Fading is variation of signal strength occur on line of sight paths as a result of the atmospheric conditions. It can not be predicted properly.
Broad side array is defined as an arrangement in which the principal direction of radiation is perpendicular to the array axis and also the plane containing the array element.
Similar point sources with equal maximum amplitudes are called identical point sources.
It is defined as the ratio of the power radiated in desired direction to the power radiated in the opposite direction. i.e FBR = Power radiated in desired direction / power radiated in the opposite direction.
Waves that arrive at the receiver after reflection from the troposphere region is called Tropospheric wave (i.e. 10 Km from Earth surface).
If an e.m.f is applied to the terminals of an antenna no.1 and the current measured at the terminals of the another antenna no.2, then an equal current both in amplitude and phase will be obtained at the terminal of the antenna no.1 if the same emf is applied to the terminals of antenna no.2.
It is made up of direct wave and ground reflected wave. Also includes the portion of energy received as a result of diffraction around the earth surface and the reflection from the upper atmosphere.
It is the waves originate at a fictitious volume less emitter source at the center ‘O’ of the observation circle.
The beam area or beam solid angle or WA of an antenna is given by the normalized power pattern over a sphere.
WA = ò ò4p Pn ( q,f ) dW
Where dW = Sin q dq.df
The regions containing the radiations that are present around the antenna are called field zones. The fields around an antenna ay be divided into two principal regions.
The power radiated from an antenna per unit solid angle is called the radiation intensity U (watts per steradian or per square degree). The radiation intensity is independent of distance.
The lowest useful HF for a given distance and trmitter power is defined as the lowest frequency that will give satisfactory reception for that distance and power.
It depends on:
Whenever the variation of the amplitude and the phase of the field with respect to the absolute angle for any two sources are same then they are called similar point sources.
The maximum amplitudes of the individual sources may be unequal.
The need for a binomial array is
The effective aperture of a short dipole is given by
Ae = 0.119 λ2.
The dipole has two equal charges of opposite sign oscillating up and down in a harmonic motion. The charges will move towards each other and electric filed lines were created. When the charges meet at the midpoint, the field lines cut each other and new field are created.This process is spontaneous and so more filed are created around the antenna. This is how radiations are obtained from a short dipole.
Self impedance of an antenna is defined as its input impedance with all other antennas are completely removed i.e away from it.
An array is linear when the elements of the array are spaced equally along the straight line. If the elements are fed with currents of equal magnitude and having a uniform progressive phase shift along the line, then it is called uniform linear array.
Two types:
It is defined the angle between the first nulls (or) it is defined as twice the angle between the first null and the major lobe maximum direction.
The fields around an antenna may be divided into two principal regions.
Tapering of array is a technique used for reduction of unwanted side lobes. The amplitude of currents in the linear array source is non-uniform; hence the central source radiates more energy than the ends. Tapering is done from center to end.
Side Lobe Ratio is defined as the ratio of power density in the principal or main lobe to the power density of the longest minor lobe.
Waves propagated over other paths near the earth surface is called ground wave propagation.