ANTENNA PARAMETERS

1.    RADIATION PATTERN:

·        An antenna radiation pattern or antenna pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates .

·        It is found in the practice that radiation energy from antenna is not same in all directions. Instead ,it is more in one direction and less or zero in other direction.

·        The energy radiated in a particular direction by an antenna is measured in  the terms of field strength at a point which is at a particular distance from antenna.

·        Radiation pattern of an antenna is nothing but a graph which shows the variation in the actual field of the electromagnetic  field at all points which are at equal distance from antenna.

·        A radiation lobe is a portion of the radiation pattern bounded by regions of

             relatively weak radiation intensity.

v Main lobe

v Minor lobes

v Side lobes

v Back lobes

2.    ANTENNA BEAMWIDTH:

·        The beam width of an antenna is a very important figure of merit and often is used as a trade-off between it and the side lobe level; that is, as the beam width decreases, the side lobe increases and vice versa .

·        The beam width of the antenna is also used to describe the resolution capabilities  of the antenna to distinguish between two adjacent radiating sources or radar targets.

·        Antenna beamwidth is a measure of directivity of an antenna. It is  an angular width in degrees measured on the major lobe. This is called beamwidth because between the half power because the power at the half power is just half.

·        Beam width is always inversely proportional to the Directivity.

                                                             i.            Half power beam width:

                          It is a plane towards maximum radiation of a major lobe is defined as the angle between two directions where the radiation intensity is half off to the maximum value.

                                                           ii.            First null beamwidth:

                         It is an angular beam width subtended on the major loop originating from    origin

3.    FRONT TO BACK RATIO:     

·        The front-to-back ratio (F/B) is used as a figure of merit that attempts to describe the level of radiation from the back of a directional antenna.

·         Basically, the front-to-back ratio is the ratio of the peak gain in the forward direction to the gain 180-degrees behind the peak. Of course on a dB scale, the front-to-back ratio is just the difference between the peak gain in the forward direction and the gain 180-degrees behind the peak. 

·        Higher the front to back ratio the betterment is increased. Generally, front to back ratio can be varied by tuning the parasitic elements of Yagi-Uda antenna.

   

        4.    FIELD REGIONS:

     The  space surrounding an antenna are divided into three regions:

                   i.            Reactive-near region

                 ii.            Radiating –near field region(Fresnel  region)

              iii.            Far field regions(Fraunhofer region)

 Reactive Near-Field Region.

                The portion of the near-field region immediately surrounding the antenna wherein the reactive field (non-radiating field) predominates.

Radiating Near-Field (Fresnel) Region.

                 The region of the field of an antenna between the reactive near-field region and

the far-field region wherein radiation fields predominate and wherein the angular

field distribution is dependent upon the distance from the antenna. If the

antenna has a maximum dimension that is not large compared to the wavelength,

this region may not exist.

 Far-Field (Fraunhofer) Region.

                           The region of the field of an antenna where the angular field distribution is essentially independent of the distance from the antenna.

                         

         

 5.    RADIAN AND STERADIAN:

·        The steradian (symbol: sr) is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane.

A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface.

·        Analogue to radians: In two dimensions, the angle in radians is related to the arc length it cuts out.

6.    RADIATION INTENSITY:      

·        Radiation intensity in a given direction is defined as the power radiated from an antenna per unit solid angle .The radiation intensity is a far-field parameter.  

Total power radiation is given by: 

7.    ANTENNA GAIN:

·        Antenna is a passive element that absorbs or take input power and radiates. The gain of the antenna is always measured with the reference antenna like

v Isotropic

v Horn antenna /Parabolic antenna

·        The antenna’s directivity does not provide us with any information about the antenna’s efficiency, but merely on its radiation pattern’s directive properties. This is the main reason for introducing a new concept called antenna Gain. The antenna Gain is defined as:

·        As one may observe, the definition is similar to that of directivity, but rather then considering the radiated power, the input power is considered.

·        The antenna gain takes into account the antenna efficiency since it is a measure of how much power the antenna radiates in a certain direction, relative to how much power was incident upon the antenna.

 ·        Radiation intensity in a given direction is defined as the power radiated from an antenna per unit solid angle .The radiation intensity is a far-field parameter.  

·        In order to the fully appreciate the meaning of this concept, it may helpful to think of the antenna as an input/output (I/O) system.

·        In the discussed system, the input is represented by the antenna’s input power and the output is represented by the radiated power in a certain direction (which is available for reception by other antennas).

·        The system’s output is nothing but its input multiplied by some constant number. This constant number is proportional to the antenna gain.

·        In that sense, the term gain fits with the terminology used for amplifiers or attenuators.

Antenna gain is specified by three terms

v Directive gain(G_D)

v Power gain(G_P)

v Directivity(D)

Directive gain(G_D):  

·        Directive gain in a given direction is defined as the ratio of the radiation intensity in that direction to the average radiation intensity .

·         Directivity gain depends on  radiation pattern in the space or radiated power in a space. There is no losses  considerations in directive gain.

Power gain(G_P):  

·         It is the ratio of the power density radiated in a particular direction by the subject antenna to the power density radiated in that direction by the reference antenna with same input power.

·         In G_P   losses are considered

                           For example:

v Iron losses in antenna

v Reference due to transmission line and antenna connections

v Dielectric losses

v   Scattering losses.

Directivity(D):

·        Directivity can be defined as the ratio of the total solid angle to the antenna beam solid angle .

·        The ratio between the radiation intensity of the antenna and the radiation intensity, assuming we spread all the of the radiated power isotropically.

·        In directions wherein the directivity is low valued, the radiated power represents a small portion of the total radiated power.

Similarly, in directions wherein the directivity is high valued, the radiated power represents a significant portion of the total radiated power.

8.    ANTENNA EFFICIENCY:

·        In reality, not all of the EM power delivered to the antenna is converted into         radiation, i.e.

        ·        There are several inherent loss mechanisms responsible for the dissipation of                  the incident power. These include: dielectric losses, conduction losses, and                 reflection losses.

·        Conductor losses and dielectric losses are caused due to the finite conductivity of the antenna’s conductors and dielectrics. This means that some power is always dissipated as heat on those materials.

·         Reflection losses are caused due to an impedance mismatch between the antenna and its driving transmission line. This would be discussed later in more detail.

·         The antenna Efficiency is defined as the ratio, in percent, between the radiated power and the incident power:

·        It is clear that the radiated power must be smaller than the incident power, since part of the later is always dissipated or reflected. Therefore, the efficiency will be less than 100%.

·        An efficient antenna will radiate the majority of the incident power upon it, so its efficiency will approach 100% (minor dissipations and reflections).

The antenna efficiency can be further represented as a multiplication of three sub-efficiencies, each accounts for different loss mechanism·     

          ·        Where e_c  is the conduction efficiency, e_d  is the dielectric efficiency ,e_r                      is the radiation efficiency.

9.    EFFECTIVE APERTURE:

·        Effective aperture (A_e )  of an antenna is defined as an ability of an antenna to extract an energy from electromagnetic waves.

·         It is defined as the ratio of the power received at the antenna  load terminal to the pointing vector of an incident wave.

                                 A_e =w/p

                                  Where:     w=power received

                                                     P=ponyting vector

Scattering aperture (A_s):  

·        It is defined as the ratio of the pre-radiated power to the power density of incident wave

            A_s=pre radiated/P=>I² R_r/p

                                             P: ponyting vector

Loss Aperture(A_l):  

·        It is defined as the ratio of the loss in an antenna to the power density in the incident wave      

                            A_l=P_loss/P= >I² R_l/P

Collecting Aperture(A_c):

·        It is the sum of effective aperture (A_e) ,scattering aperture(A_s) and loss aperture(A_l).

                  A_c= A_e + A_s +A_l

                       = I² R_L +I² R_r + I² R_l

                      A_c  =I²(R_L+ R_r+ R_l)

Physical Aperture(A_P):

·         It is defined as a complete physical cross section perpendicular to the direction of the propagation of the incident wave where the antenna is oriented for maximum response. Physical aperture is more meaningful to big antennas.

10.    RECIPROCITY:

·        Reciprocity is one the most useful (and fortunate ) property of antennas. Reciprocity states that the receive and transmit properties of an antenna are identical.

·        Hence, antennas do not have the distinct transmit and receive radiation patterns in the transmit mode then you also know the pattern in the receive mode .This makes much simpler, as you can imagine .

·        The reciprocity theorem is applied to transmitting and receiving antenna systems in order to establish several important relationships.

·         Formulas are deduced which establish a relation between the receiving current and power of any given antenna and the field intensity of the arriving waves, all the parameters entering into the formulas being the parameters of the same antenna when it is used as a transmitting antenna.

·        It is shown that, in the case of strong interference,

§  the highest possible directivity is of importance both in the transmitting and in the receiving antennas and

§   the efficiency and the coefficient of exploitation of the receiving antenna are of no importance.

·        In the case of low interference, it was found that,

§  the directivities of both the receiving and transmitting antennas are of equal importance and

§  the efficiency and coefficient of exploitation of the receiving antenna are just as important as the efficiency of the transmitting antenna.

11.    EFFECTIVE LENGTH:

·        Is a parameter of wire antennas that characterizes the antenna’s efficiency in     transmitting and receiving electromagnetic waves. 

·        The effective length for a receiving antenna is defined as the ratio of the 

electromotive force at the receiver input to the intensity of the electric field incident on the antenna; for a transmitting antenna, it is defined as the length in free space of a conductor with a uniform and in phase current distribution along its entire length that generates the same field intensity in the direction of maximum radiation as a real antenna under conditions of equal current

 amplitudes in the conductor and the real antenna.The effective antenna length is   numerically identical in transmission and reception.

            ·         When a receiving antenna intercepts incident electromagnetic waves, a voltage is             induced across the antenna terminals. The effective length h_e of a receiving                   antenna is defined as the ratio of the open circuit terminal voltage to the incident             electric field strength in the direction of antennas polarization. Effective length               he is also referred to as effective height.

     

12.    ANTENNA BANDWIDTH:

·        The range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard. Strictly speaking , bandwidth is related to antenna can be specified in different ways

v Bandwidth over which we get a good front to back ratio .

v A bandwidth over which we get a higher antenna gain .

v A bandwidth over which we get a low standing wave ratio.

·        In other words, bandwidth depends on the overall effectiveness of the antenna through a range of frequencies, so all of these parameters must be understood to fully characterize the bandwidth capabilities of an antenna.

·        This definition may serve as a practical definition, however, in practice; bandwidth is typically determined by measuring a characteristic such as SWR or radiated power over the frequency range of interest.

·        For example, the SWR bandwidth is typically determined by measuring the frequency range where the SWR is less than 2:1. Another frequently used value for determining bandwidth for resonant antennas is the -3dB Return Loss value.

13.    ANTENNA BEAM EFFICIENCY:

·        Beam efficiency is frequently used parameter to gauge the performance of an antenna.

·        Beam efficiency is the ratio of the power received or transmitted within a cone angle to the power received or transmitted by the whole antenna.

·         Thus, beam efficiency is a measure of the amount of power received or transmitted by the minor lobes relative to the main lobe.

14.    ANTENNA TEMPERATURE:

·        Antenna Temperature ( T_A) is a parameter that describes how much noise an antenna produces in a given environment. This temperature is not the physical temperature of the antenna.

·         Moreover, an antenna does not have an intrinsic "antenna temperature" associated with it; rather the temperature depends on its gain pattern and the thermal environment that it is placed in.

·        Antenna temperature is also sometimes referred to as Antenna Noise Temperature.

·        To define the environment, we'll introduce a temperature distribution - this is the temperature in every direction away from the antenna in spherical coordinates.

·        For instance, the night sky is roughly 4 Kelvin; the value of the temperature pattern in the direction of the Earth's ground is the physical temperature of the Earth's ground.

·        This temperature distribution will be written as T(θ,Ø). Hence, an antenna's temperature will vary depending on whether it is directional and pointed into space or staring into the sun. For an antenna with a radiation pattern given by R(θ,Ø) the noise temperature is mathematically defined as:

   

·        This states that the temperature surrounding the antenna is integrated over the entire sphere, and weighted by the antenna's radiation pattern.

·        Hence, an isotropic antenna would have a noise temperature that is the average of all temperatures around the antenna; for a perfectly directional antenna (with a pencil beam), the antenna temperature will only depend on the temperature in which the antenna is "looking".

·        The noise power received from an antenna at temperature can be expressed in terms of the bandwidth (B) the antenna (and its receiver) are operating over: 

                                               P_N=kT_A B

15.    POLARIZATION:

·        The polarization of an antenna is defined as the polarization of the EM wave it radiates in the far field.

·        The EM wave radiated by the antenna is a mixture of electric and magnetic fields. If we were to track the curve traced by the tip of the electric field vector, in some fixed location in space, we would get, as time varies, a curve referred to as the polarization ellipse .

·        Note, that for each fixed location we would generally get different curves, that is to say : the antenna polarization is dependent upon the direction of observation.

The curve is referred to as the polarization ellipse, since it forms an ellipse for an arbitrarily polarized antenna

·        Polarization may be classified as linear, circular or elliptical depending on the properties of the polarization ellipse.

·        If the ellipse has equal minor and major axis it transforms into a circle. In that case we say that the antenna is circularly polarized.

·        If the ellipse has no minor axis it transforms into a  straight line, In that case we say that the antenna is linearly polarized.

·        The various polarization types are graphically demonstrated in figure

·        Each polarization has a sense. For a linearly polarized antenna it is defined by the tilt angle of  the polarization ellipse, denoted by τ. Linear polarizations are classified by that sense (90º vertical, 0º horizontal,  ± 45º slant).

·        For a circularly polarized antennas the sense is given by the nature of movement of the electric field vector tip: clockwise or counterclockwise (RHCP for clockwise, LHCP for counterclockwise). 

·        An illustration is given in figure

·        Each polarization has en orthogonal counterpart (Vertical and Horizontal, RHCP and LHCP, ± 45º slant, etc). Furthermore, each polarization can be constructed out of any two orthogonal polarizations.

16.    AXIAL RATIO:

·        This parameter is majorly used to describe the polarization nature of circularly polarized antennas.

·        The Axial Ratio (AR)is defined as the ratio between the minor and major axis of the polarization ellipse.

·         Recall that if the ellipse has en equal minor and major axis it transforms into a circle, and we say that the antenna is circularly polarized. In that case the axial ratio is equal to unity (or 0 dB).

·         The axial ratio of a linearly polarized antenna is infinitely big since one of the ellipse axis is equal to zero. For a circularly polarized antenna, the closer the axial ratio is to 0 dB, the better.