Antenna Defination

ANTENNA DEFINATION An Antenna is defined in the following ways: 1. An antenna is a piece of conducting wire or rod with excitation. 2. An antenna is a source of radiator of Electromagnetic waves. 3. An antenna is a sensor of electromagnetic waves. 4. An antenna is a transducer. 5. An antenna is an impedance matching device.

FUNCTIONS OF AN ANTENNA 1. It is used as a transducer that is, it converts electrical energy into EM energy on the transmitting side and it converts EM energy into electrical energy on the receiving side. 2. It is used as an impedance matching device, that is, it matches the transmitter and free space on the transmitting side and it matches free space and the receiver on the receiving side. 3. It radiates in the desired directions and suppresses in the unwanted directions. 4. It is used as a radiator as well as a sensor of EM waves. 5. It has identical impedance, identical directional characteristics and the same effective length when used for transmitting and receiving purposes. These properties can be proved using reciprocity theorem.

ANTENNA PAPAMETERS 1. Antenna Impedance: It is defined as the ratio of the input voltage to the input current. Za=Vi/Ii Ω. 2. Radiation Resistance (Rr): Rr is defined as the fictitious or hypothetical resistance that would dissipate an amount of power equal to the radiated power. Or, Rr=(Power radiated/Irms^2). 3. Directional Characteristics: These are also called radiation characteristics or radiation pattern. These are of two types: (a)Field Radiation Pattern: It is the variation of the absolute value of field strength as a function of Θ. That is, E vs Θ is called field strength pattern. (b)Power Pattern: It is the variation of radiated power with Θ. That is, P vs Θ is called Power pattern. More generally an antenna radiation pattern is a three dimensional variation of the radiated field.

4. Effective Length of an antenna (ℓeff): It is used to indicate the effectiveness of the antenna as a radiator or receiver of EM energy. ℓeff of transmitting antenna: It is that length of an equivalent linear antenna that has a constant current along its length and which radiates the same field strength as the actual antenna. ℓeff of receiving antenna: It is defined as the ratio of the open circuit voltage developed at the terminals of the antenna under the received field strength. i.e. ℓeff(rec)=Voc/E. Effective length of an antenna is always less than the actual strength. 5. Radiation Intensity(RI): It is defined as the power radiated in a given direction per unit solid angle…i.e. RI=r^2P=(r^2E^2)/ηo watts/unit solid angle where ηo= intrinsic impedence of the medium. r=radius of the sphere. P=power radiated-instantaneous. E=Electric Field Strength(V/m) RI=RI(Θ,Φ) is afunction of Θ and Φ. 6. Directive Gain(Gd): It is defined as the ratio of radiation intensity in that direction to the average radiation intensity…i.e. Gd = RI/(RI)av = RI/(Wr/4Π) i.e. Gd = 4Π(RI)/Wr. Where Wr= Radiated power. 7. Directivity: It is defined as the ratio of maximum radiation intensity to the average radiation intensity. i.e.D=(Gd)max D in dB = 10 log (Gd)max. 8. Power Gain(Gp): It is defined as the ratio of radiated power to the input power i.e. Gp=4Π(RI)/Wt Where Wt=Wr+Wl where Wl=ohmic losses in the antenna. 9. Antenna Efficiency(η): It is defined as the ratio of the radiated power to the input power. i.e. η = Wr / Wt. =Gp / Gd. 10. Effective Area: It is defined as Ae=(λ^2/4Π)Gd or Ae=Wr/P. where Wr=received power. P=power flow per square meter for the incident wave.

11. Antenna Equivalent Circuit: It is a series Ra, La, Ca circuit. The main difference between the antenna equivalent circuit and an RLC circuit is that Ra, La and Ca vary with frequency. As a result, the antenna conductance peak occurs not at the resonant frequency but at a frequency slightly away from Fr.

RADIATION MECHANISM : When a transmitting antenna is excited with an alternating voltage, the initial motion of a wave which is propagated through space is striated by a balance motion of charges in an antenna. The transmitting antenna has characteristics similar to those of a resonant circuit. When energy is supplied to it, resonant oscillations occur in the antenna and violent variation in charge form an electric vector. The same violent motion of charges creates a magnetic field about the antenna in the same manner as a magnetic field. expands and collapses about a resonant circuit tank coil. If energy is continuously applied to the antenna, energy moves away from the antenna into space in the form of EM waves. It may be noted that it is not the original antenna charge themselves that moves through space but rather the motion they create. The charges around an antenna are set into motion first and they in turn, set other charges further separated from the antenna into motion. This disturbance fans out from the antenna into space. When motion of charges forms an electric field and a magnetic field which result from notion of charges. These electric and magnetic fields are perpendicular to each other. The motion of these fields in the form of EM waves has no boundaries and expands spherically. The EM energy density decreases with distance as they propagate.

HERTZIAN DIPOLE Hertzian Dipole is defined as an infinitesimal current element Idℓ which does not exists in real life. It is a short linear antenna which when radiating , is assumed to carry constant current along its length. As Hertzian dipole and alternating current elements are virtually the same, the radiated power and radiation resistance are given by Pr=80Π^2(dℓ/λ)^2*Ieff^2 watts

and Rr=80Π^2(dℓ/λ)^2 Ω.

For a half-wave dipole which resembles in properties the Hertzian dipole antenna, the radiated power is Pt=73*Ieff^2 and the radiation resistance of half-wave dipole is Rr=73 Ω.