Antennas and wave propagation ppt presentation

Introduction to antennas Antenna is an important tool in communication engineering. An antenna is structure,usually made from a good conducting material, that has been designed to radiate or receive EM energy in an efficient manner.

Antenna as a transition device at transmitting side

Transmission line may take the form of a coaxial line or a hallow pipe(wave guide). The transmission antenna converts circuit currents into EM photons radiated into space. The receiving antenna converts EM photons into circuit currents. Simplest terms, an antenna converts photons to currents or vice-versa.

Thevenin equivalent of antenna in transmitting mode

Definitions of antenna An antenna is “a usually metallic device (as a rod or wire ) for radiating or receiving radio wave”. Antenna radio may be defined as the structure associated with the region of transition between guided wave and a free space wave or vice versa. [ JOHN D.KRAUS] Antenna is the transitional structure between free-space and a guiding device. [prof. Constantine a.balnis]

Functions of an antenna Antenna acts as transducer Antenna acts as a impedance matching device Antenna acts as a remote sensing temperature measuring device

Properties of antenna Equality in impedance Equality in directional pattern Equality in effective lengths

Antenna parameters Beam patterns Beam area Radiation intensity DirectivityGain Antenna aperture Effective height or area

Basic elements of an antenna Alternating current elements (hertzian dipole) Short dipole Short monopole Half wave dipole Quarter wave monopole

ISOTROPIC RADIATOR An radiator which radiates energy in all directions uniformly is an isotropic radiator. total radiated power is given by P rad = where is the average power density ie.. P avg P avg =

Radiation pattern of isotropic antenna This is the radiation pattern of the hypothetical isotropic antenna. (0 dBi gain) The toothpick represents the axis at which the antenna is placed at the middle of. That means that in this case, the antenna is in the middle of the sphere

Radiation patterns of an antenna The radiations from the antenna in any direction can be measured in terms of the field strength at a point located at a particular distance from the antenna. The radiation pattern is nothing but the graph which shows the variation of actual field strength of the EM field at all points equidistant from the antenna.

Types of radiation patterns Field radiation patternPower radiation pattern

Field radiation pattern If the radiation of the antenna represented graphically as a function of direction it is called field radiation pattern. It is 3-D pattern represents the radiation for all angles of θ and φ The field pattern is expressed mathematically interms of relative field pattern which is commonly called normalized field pattern.

Field radiation pattern

Radiation patterns of non isotropic antenna Now lets compare it to a standard omnidirectional 3 dBi rubber duck antenna: This makes the shape of a donut. As long as you are inside of the red lines you have a 3 dBi gain, but if you get outside of the line you have no reception. This means that you can fly twice as far away from your self as long as you don’t fly outside of the reception field. You can’t fly as high as with the 0dBi antenna, and you can’t fly directly above you.

Normalized field pattern The normalized field pattern for θ and φ components of the electric field are given as E θn = similary E φn =

Power radiation pattern When the radiation in a given direction is expressed in terms of power per unit solid angle is called power radiation pattern Power density ( ) : power flow per unit area and is a function of direction (θ,φ) Power density represented in terms of the magnitude of the electric field intensity as = Normalized power density is = When the radiation in a given direction is expressed in terms of power per unit solid angle is called power radiation pattern Power density ( ) : power flow per unit area and is a function of direction (θ,φ) Power density represented in terms of the magnitude of the electric field intensity as = Normalized power density is =

Power radiation pattern

Field and power patterns

Decibel plot of field pattern

Radiation intensity Radiation intensity is defined as the power per unit solid angle denoted by U and expressed as w/sr. Given an antenna's power density( ) radiation density is calculated by multiplying it with the square of the distance ( ) from the antenna to the designated solid angle:power density u (θ,φ)= (θ,φ)(θ,φ)

Radiation intensity Total power is expressed in terms of the radiation intensity as P rad = = where = sinθ dθ dφ Radiation intensity is also defined as time average power per unit solid angle The average value of the radiation intensity is given by U avg =

Relationship between radiation intensity and field radiation intensity Normalized power pattern is the ratio of radiation intensity u(θ,φ) to its maximum value of u(θ,φ) and is given by = Therefore normalized power pattern is equal to the square of the relative total field patter = = =

Beam solid angle or beam area(Ω) The measure of solid angle is defined as steradian. 1s r = 1 = square degree

Beam solid angle or beam area(Ω) It is defined as the integral of normalized power pattern over a sphere. It is denoted by Ω given by Ω= (θ,φ) the beam is described in terms of the angle subtended by half power points of the main lobe. Ω= Θ HP φ HP steradian It is defined as the integral of normalized power pattern over a sphere. It is denoted by Ω given by Ω= (θ,φ) the beam is described in terms of the angle subtended by half power points of the main lobe. Ω= Θ HP φ HP steradian

Beam solid angle or beam area(Ω) Antenna power pattern and its equivalent solid angle or beam area Ω

Beam efficiency

gain The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. An antenna with a gain of 3 dB means that the power received far from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power.

Antenna efficiency The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna's input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna, or reflected away due to impedance mismatch Efficiency is ultimately a ratio, giving a number between 0 and 1 Antenna efficiency is also frequently quoted in decibels (dB); an efficiency of 0.1 is 10% or (-10 dB), and an efficiency of 0.5 or 50% is -3 dB.decibels (dB)

Antenna gain The term Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Antenna gain is more commonly quoted in a real antenna's specification sheet because it takes into account the actual losses that occur. An antenna with a gain of 3 dB means that the power received far from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power.

Directivity gain is defined as the ratio of the power density P D (θ,φ) to the average power radiated. G D (θ,φ) = = = = The directive gain can be defined as the measure of the concentration of the radiated power in a particular direction (θ,φ).

Power gain The power gain of an antenna in a given direction is 4π times the ratio of the radiation intensity in that direction to the total power delivered to the antenna. = where = total input power The power gain of an antenna in a given direction is 4π times the ratio of the radiation intensity in that direction to the total power delivered to the antenna. = where = total input power

Effective aperture In receiving mode, the maximum power received in a receiving antenna is. Consider this power to be that intercepted from the incoming wave by a maximum effective area. If the power density of the incoming wave is S, then = is called maximum effective aperture of the antenna. S. is related to directivity by D=

Effective length or height The term effective height or length represents the effectiveness of an antenna as radiator or collector of an EM wave energy. it indicates how far an antenna is effective in transmitting or receiving the EM wave energy.

Effective length or height The effective length is defined as the length of an equivalent linear antenna which has current I(0) along its length at all points radiating the field strength in direction perpendicular to the length same as actual antenna. For the receiving antenna, the effective length defined as ratio of the open circuit voltage developed at the antenna terminals to the given received field strength.

Radiation resistance

Antenna impedance Antenna impedance relates the voltage to the current at the input to the antenna. suppose the impedance is given by a complex number, say Z=50 + j*50 ohms. The real part of the antenna impedance represents power that is either radiated away or absorbed within the antenna. The imaginary part of the impedance represents power that is stored in the near field of the antenna. This is non-radiated power. An antenna with a real input impedance (zero imaginary part) is said to be resonant. Note that the impedance of an antenna will vary with frequency.

If an alternation current I is flowing in the short element dl,the effect is not realized at a distant point instantaneously but only after an interval equal to the time needed for the disturbance to propagate over the distance r. The expression for magnetic vector potential A in time varying conditions where distance travelled in terms of wavelength is given by The expression for electric scalar potential V in time varying conditions where distance travelled in terms of wavelength is given by If an alternation current I is flowing in the short element dl,the effect is not realized at a distant point instantaneously but only after an interval equal to the time needed for the disturbance to propagate over the distance r. The expression for magnetic vector potential A in time varying conditions where distance travelled in terms of wavelength is given by The expression for electric scalar potential V in time varying conditions where distance travelled in terms of wavelength is given by

Radiations from the alternating current element To calculate the EM field radiated in a space by a short dipole, retarded potential is used. A short dipole is an alternating current element. It is also called an oscillating current element. In general, a current element IdL is nothing but an element of length dl carrying filamentary current I. An antenna is considered to be made up of large numbers of such elements connected end to end. The vector potential is given by, The vector potential in z-direction is given by, To calculate the EM field radiated in a space by a short dipole, retarded potential is used. A short dipole is an alternating current element. It is also called an oscillating current element. In general, a current element IdL is nothing but an element of length dl carrying filamentary current I. An antenna is considered to be made up of large numbers of such elements connected end to end. The vector potential is given by, The vector potential in z-direction is given by,

Radiations from the alternating current element Now the magnetic field is given by As we are using spherical co-ordinate system, to find the curl of A, we must find the components of In r, ѳ and φ directions are

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