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Self impedance of an antenna is defined as its input impedance with all other antennas are COMPLETELY REMOVED i.e away from it.

The presence of near by antenna no.2 induces a current in the antenna no.1 indicates that presence of antenna no.2 changes the impedance of the antenna no.1. This effect is called mutual coupling and RESULTS in mutual impedance.

Self impedance of an antenna is defined as its input impedance with all other antennas are completely removed i.e away from it.

The presence of near by antenna no.2 induces a current in the antenna no.1 indicates that presence of antenna no.2 changes the impedance of the antenna no.1. This effect is called mutual coupling and results in mutual impedance.

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.

• Near field ZONE (Fresnel zone).
• Far field zone (Fraunhofer zone).

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 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.

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.

A SHORT dipole is ONE in which the field is oscillating because of the oscillating voltage and current. It is CALLED so, because the LENGTH of the dipole is short and the current is almost constant throughtout the ENTIRE length of the dipole.

A short dipole is one in which the field is oscillating because of the oscillating voltage and current. It is called so, because the length of the dipole is short and the current is almost constant throughtout the entire length of the dipole.

When the antenna is RECEIVING with a load resistance MATCHED to the antenna radiation resistance, maximum power is TRANSFERRED to the load and the power is also re-radiated from the DIPOLE. This is called antenna matching.

When the antenna is receiving with a load resistance matched to the antenna radiation resistance, maximum power is transferred to the load and the power is also re-radiated from the dipole. This is called antenna matching.

Optimum FREQUENCY for TRANSMITTING between any two points is therefore selected as some frequency lying between about 50 and 85 percent of the predictedmaximum USABLE frequency between those points.

Optimum frequency for transmitting between any two points is therefore selected as some frequency lying between about 50 and 85 percent of the predictedmaximum usable frequency between those points.

The DISTANCE with in which a SIGNAL of GIVEN frequency fails to be reflected BACK is the skip distance for that frequency. The higher the frequency the GREATER the skip distance.

The distance with in which a signal of given frequency fails to be reflected back is the skip distance for that frequency. The higher the frequency the greater the skip distance.

The maximum Frequency that can be REFLECTED BACK for a given distance of transmission is CALLED the maximum usable frequency (MUF) for that distance.

MUF = fcr SEC φi

The maximum Frequency that can be reflected back for a given distance of transmission is called the maximum usable frequency (MUF) for that distance.

MUF = fcr sec φi

It is DEFINED as N = C / VP

where n = √εr

It is defined as n = c / Vp

where n = √εr

The lowest useful HF for a GIVEN distance and transmitter power is DEFINED as the lowest frequency that will give satisfactory reception for that distance and power.

It depends on:

• The effective radiated power
• Absorption character of ionosphere for the paths between transmitter and receiver.
• The required field strength which in turn depends upon the radio NOISE at the receiving LOCATION and type of service INVOLVED.

The lowest useful HF for a given distance and transmitter power is defined as the lowest frequency that will give satisfactory reception for that distance and power.

It depends on:

The phenomenon of SPLITTING the wave into two different components (ORDINARY and extra-ordinary) by the EARTHS MAGNETIC field is called Magneto-Ions Splitting.

The phenomenon of splitting the wave into two different components (ordinary and extra-ordinary) by the earths magnetic field is called Magneto-Ions Splitting.

For any layer, the HIGHEST frequency that will be REFLECTED back for VERTICAL incidence is

fcr = 9 √Nmax

For any layer, the highest frequency that will be reflected back for vertical incidence is

fcr = 9 √Nmax

Frequency whose period is EQUAL to the period of an electron in its orbit under the INFLUENCE of the EARTHS magnetic FLUX density B.

Frequency whose period is equal to the period of an electron in its orbit under the influence of the earths magnetic flux density B.

• Curvature of EARTH.
• Earth’ s magnetic field.
• FREQUENCY of the signal.
• Plane earth REFLECTION.

Due to the EARTH’ s magnetic fields, the ionosheric MEDIUM becomes anisotropic and the INCIDENT plane wave entering the ionosphere will split into ordinary and extra ordinary waves/modes.

When these modes re-emerge from the ionosphere they recombine into a single plane wave again. Finally the plane of polarization will usually have changed, this phenomenon is known as FARADAY's rotation.

Due to the earth’ s magnetic fields, the ionosheric medium becomes anisotropic and the incident plane wave entering the ionosphere will split into ordinary and extra ordinary waves/modes.

When these modes re-emerge from the ionosphere they recombine into a single plane wave again. Finally the plane of polarization will usually have changed, this phenomenon is known as Faraday's rotation.

It is used in NORMALLY in MICROWAVE links, and it is FOUND that signal transmitted over the same path in two polarization have independent fading PATTERNS. In broadband dish antenna SYSTEM, Polarization diversity combined with frequency diversity reception achieve excellent results.

It is used in normally in microwave links, and it is found that signal transmitted over the same path in two polarization have independent fading patterns. In broadband dish antenna system, Polarization diversity combined with frequency diversity reception achieve excellent results.

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.

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.

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.

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.

To MINIMIZE the FADING and to avoid the MULTI path interference the technique used are diversity reception. It is obtained by three ways:

• SPACE diversity reception.
• Frequency diversity reception.
• POLARIZATION diversity.

To minimize the fading and to avoid the multi path interference the technique used are diversity reception. It is obtained by three ways:

Inverse bending MAY TRANSFORM LINE of SIGHT path into an obstructed one.
Multi path fading is caused by interference between the direct and ground reflected waves as WELL as interference between two are more paths in the atmosphere.

Inverse bending may transform line of sight path into an obstructed one.
Multi path fading is caused by interference between the direct and ground reflected waves as well as interference between two are more paths in the atmosphere.

TWO types:

1. INVERSE FADING.
2. MULTI path fading.

Two types:

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.

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.

WAVE that is guided along the earth’ s SURFACE like an EM wave is guided by a transmission is called surface wave. Attenuation of this wave is directly affected by the constant of earth along which it TRAVELS.

Wave that is guided along the earth’ s surface like an EM wave is guided by a transmission is called surface wave. Attenuation of this wave is directly affected by the constant of earth along which it travels.

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 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.

GROUND wave CLASSIFIED into two TYPES.

• SPACE wave.
• Surface wave.

Ground wave classified into two types.

WAVES PROPAGATED over other PATHS NEAR the earth surface is CALLED ground wave propagation.

Waves propagated over other paths near the earth surface is called ground wave propagation.

Waves that arrive at the RECEIVER after REFLECTION from the troposphere region is called Tropospheric wave (i.e. 10 KM from Earth SURFACE).

Waves that arrive at the receiver after reflection from the troposphere region is called Tropospheric wave (i.e. 10 Km from Earth surface).

WAVES that ARRIVE at the receiver after REFLECTION in the IONOSPHERE is CALLED sky wave.

Waves that arrive at the receiver after reflection in the ionosphere is called sky wave.

The applications of helical antenna are:

• It became the workhouse of space COMMUNICATIONS for telephone, television and DATA, being employed both on satellites and at ground STATIONS.
• Many satellites including weather satellites, data relay satellites all have helical antennas.
• It is on many other probes of planets and comets, including moon and mars, being USED alone, in arrays or as feeds for parabolic reflectors, its circular polarization and high gain and simplicity making it effective for space application.

The applications of helical antenna are:

ELECTRICALLY Small LOOP antennas is ONE in which the overall LENGTH of the loop is less than one-tenth of the wavelength. Electrically Small loop antennas have small radiation resistances that are usually smaller than their loop resistances. They are very POOR radiators and seldom employed for transmission in radio communication.

Electrically Small loop antennas is one in which the overall length of the loop is less than one-tenth of the wavelength. Electrically Small loop antennas have small radiation resistances that are usually smaller than their loop resistances. They are very poor radiators and seldom employed for transmission in radio communication.

Loop antennas are classified into:

• Electrically Small (CIRCUMFERENCE < λ / 10 )
• Electrically Small (DIMENSION COMPARABLE to λ )

Loop antennas are classified into:

The radiation resistance of a loop antenna can be INCREASED by:

• INCREASING the number of turns.
• INSERTING a ferrite core of very high PERMEABILITY with loop antenna’ s circumference which will rise the magnetic field intensity CALLED ferrite loop.

The radiation resistance of a loop antenna can be increased by:

Radiation RESISTANCE of a small LOOP is GIVEN by

Rr = 31,200 ( A / λ2 )2

Radiation resistance of a small loop is given by

Rr = 31,200 ( A / λ2 )2

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).

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 radiation resistance of a HALF wave DIPOLE is GIVEN by

RR = 73 ohm.

The radiation resistance of a half wave dipole is given by

Rr = 73 ohm.

The EFFECTIVE APERTURE of a half wave DIPOLE is given by

Ae = 0.13 λ2.

The effective aperture of a half wave dipole is given by

Ae = 0.13 λ2.

A half wave antenna is the fundamental RADIO antenna of metal rod or TUBING or THIN wire which has a physical LENGTH of half wavelength in free SPACE at the frequency of operation.

A half wave antenna is the fundamental radio antenna of metal rod or tubing or thin wire which has a physical length of half wavelength in free space at the frequency of operation.

A DIPOLE antenna MAY be defined as a symmetrical antenna in which the two ENDS are at equal POTENTIAL relative to the midpoint.

A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to the midpoint.

The EFFECTIVE aperture of a SHORT DIPOLE is GIVEN by

AE = 0.119 λ2.

The effective aperture of a short dipole is given by

Ae = 0.119 λ2.

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 transmission line as a RESISTIVE COMPONENT and this is known as the radiation resistance.

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 transmission line as a resistive component and this is known as the radiation resistance.

As the distance from the current element or the short dipole increases, both induction and radiation FIELDS emerge and START decreasing. However, a distance reaches from the conductor at which both the induction and radiation field becomes equal and the particular distance depends upon the WAVELENGTH. The TWO fields will THUS have equal amplitude at that particular distance. This distance is given by r = 0.159 λ.

As the distance from the current element or the short dipole increases, both induction and radiation fields emerge and start decreasing. However, a distance reaches from the conductor at which both the induction and radiation field becomes equal and the particular distance depends upon the wavelength. The two fields will thus have equal amplitude at that particular distance. This distance is given by r = 0.159 λ.

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.

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.

The induction field will predominate at points close to the current element, where the distance from the center of the dipole to the particular POINT is less. This field is more effective in the vicinity of the current element only. It REPRESENTS the energy STORED in the magnetic field surrounding the current element or conductor. This field is also known as NEAR field.

The induction field will predominate at points close to the current element, where the distance from the center of the dipole to the particular point is less. This field is more effective in the vicinity of the current element only. It represents the energy stored in the magnetic field surrounding the current element or conductor. This field is also known as near field.

SINCE,the short electric dipole is so short, the current which is flowing through the dipole is assumed to be constant throughout its length. The effect of this current is not felt INSTANTANEOUS at a distance POINT only after an interval equal to the TIME required for the wave to propagate over the distance r is called the retardation time.

The retarded current [I]=Io exp( JW' ( t - r / c ) )

Where w'r / c is the phase retardation.

Since,the short electric dipole is so short, the current which is flowing through the dipole is assumed to be constant throughout its length. The effect of this current is not felt instantaneous at a distance point only after an interval equal to the time required for the wave to propagate over the distance r is called the retardation time.

The retarded current [I]=Io exp( jw' ( t - r / c ) )

Where w'r / c is the phase retardation.

• Binomial ARRAY:TAPERING follows the COEFFICIENT of binomial series.
• Dolph Tchebycheff Array: Tapering follows the coefficient of Tchebycheff polynomial.

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.

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.

• Isotropic SOURCE RADIATES energy in all DIRECTIONS but non-isotropic source radiates energy only in some desired directions.
• Isotropic source is not PHYSICALLY realizable but non-isotropic source is physically realizable.

Advantage:

• No MINOR lobes.

Disadvantages:

• Increased beam WIDTH.
• MAINTAINING the LARGE ratio of current amplitude in large arrays is difficult.

Advantage:

Disadvantages:

It is an array in which the amplitudes of the ANTENNA ELEMENTS in the array are arranged according to the coefficients of the BINOMIAL SERIES.

It is an array in which the amplitudes of the antenna elements in the array are arranged according to the coefficients of the binomial series.

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.

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.