An antenna (or aerial) is a transducer that is designed to transmit or receive electromagnetic waves. It is commonly used in radio, cell phones, radar, television broadcasting and other systems that make use of electromagnetic waves. It is a conductor that can transmit, send and receive signals by converting the electromagnetic waves into electrical currents and vice versa. A high gain antenna increases signal strength, where a low-gain antenna receives or transmits over a wide angle. It also has a characteristic known as reciprocity, which means that an antenna will maintain the same characteristics regardless if it is transmitting or receiving. 
There are many types of antennas such as dipole, monopole, yagi, loop and many others types of antennas. A dipole antenna has an aerial half a wavelength long consisting of two rods connected to a transmission line at the center. It is a basic (half-wavelength long) "building block" antenna element. It may be used by itself' however it is usually used in combination with "director" elements and "reflector" elements to make an antenna system to have more gain and directivity than the dipole by itself. 
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The dipole antenna is also an important form of RF antenna which is widely used for radio transmitting and receiving applications. Generally an antenna will release RF energy to be sent to a distant receiver which will then be picked up by the receiving antenna. When the receiving antenna received the RF energy, a voltage will be induced into the antenna that serves as a conductor and used to recover the transmitted RF information. The dipole antenna is often used on its own as an RF antenna, but it also forms the essential element in many other types of RF antenna. As such it is the possibly the most important form of RF antenna. 
1.2 Literature review
The antennas are first designed based on classical electromagnetic field theory. It is established during the second half of the 19th and 20th century where the first radio experiments were being performed. Since that time of experiment, the applications of antenna design have continued to have a steady growth in communication, radar, remote sensing and broadcasting. Antennas are necessary and often a critical part of any system which uses radio propagation as the way of transmitting information. It has been recognized in a very considerable effort spend for the study of antenna properties and the engineering of practical radiating systems.
Design process of an antenna involved a lot of specifications such as radiation pattern, bandwidth, impedance, frequency of operation, gain and efficiency, polarization and noise temperature. There may be also major practical limitations such as dimensional and structural constraints, weight, material, environment factors and costs as theory and practical always have different conditions and not ideal practically. 
An antenna acts to convert guided waves on transmission structure into free space waves. The official IEEE definition of an antenna follows this concept: "That part of a transmitting or receiving system is designed to radiate or receive electromagnetic waves." Most antennas are reciprocal devices and behave the same on transmitting and on receiving.  It is also defines by Webster's Dictionary as "A usually metallic device (as a rod or wire) for radiating or receiving radio waves." 
Antennas reciprocity principle is a fundamental importance in antenna theory and practice as for an antenna that has this characteristic; the properties may be explained (or determined) by analysis (or measurement) with the antenna acting as a transmitter or a receiver. An antenna is said to have reciprocal characteristic if the parameters of the transmitting waves through the antenna can be characterized by symmetric vectors. Generally it means no ferrite or plasma devices within the antenna or in the transmission waves and that it is passive, isotropic and linear. Isotropic means an antenna that radiates its signal 360 degrees both vertically and horizontally-a perfect sphere. 
The reciprocity theory for antennas can be expressed like the following: if a voltage source (zero impedance) is applied to the terminals of an antenna A and the current at the terminals of antenna B is measured, it will then show an equal current (in both amplitude and phase) at the terminals of antenna A if the same voltage source is applied to the terminals of antenna B. The assumptions undertake for this theory are that the voltage source is at the same frequency and that the medium is passive, isotropic and linear. The transmitting and receiving patterns of the antenna are the same under these conditions. 
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Figure 1 : Reciprocity theory: The antenna a and antenna b
Antennas theoretical fundamentals uses Maxwell's equations, which James Clerk Maxwell (1831-1879) presented before the Royal Society in 1864, that combines electric and magnetic forces into a single theory of electromagnetism. James Clerk Maxwell also predicted that the light is explained by electromagnetic and both light and electromagnetic disturbances travel at the same speed. In the year 1887 the German physic's Heinrich Hertz (1857-1894) verified experimentally the claim of James Clerk Maxwell that electromagnetic actions travel through air.
Antennas are treated as transmitting or receiving accordingly for that particular situation. A dipole antenna is an antenna that has omnidirectional coverage. When the antenna is in receiving mode, it acts to collect incoming waves and direct them to a common feed point where a transmission line is attached. In certain cases, antennas focus radio waves just like lenses focusing optical waves. Antennas that have directional characteristics will have its transmitting antenna radiating the electromagnetic power density with intensity that varies with angle around the antenna. 
The sources of the electromagnetic field are the electric charges. A conducting or dielectric structures called antennas or radiators can effectively radiate the electromagnetic waves into free space. When the sources are time varying, the electromagnetic waves propagate away from the sources and that is when the radiation takes place. Thus antenna as a matching device is taken as a transition between the free space and a system used for launching the electromagnetic waves. The system used for launching the electromagnetic waves is either transmission line or waveguide. 
Figure 2: Wave launching system
Besides, receiving or transmitting energy, an antenna is also an advanced wireless system that is usually required to optimize or emphasize the radiation energy in some directions and suppress it in others. Thus the antenna must also serve as a directional device in addition to a probing device. It must then take various forms to meet the particular need and it maybe is a piece of conducting wire, an aperture, a patch, an assembly of elements (array), a reflector, a lens and so forth. 
1.3 Technical objectives
To know more on all types of dipole antenna
To build strong understanding on dipole antenna
To build strong understanding on dipole antenna applications in daily life
To understand the antenna electromagnetic characteristics, radiation pattern, bandwidth, impedance, frequency of operation, directive gain and polarization.
To learn how to use 3D electromagnetic simulation tools
To validate the published dipole antenna design and results using available simulation software
2.1 Radiation pattern
The radiation pattern of an antenna can be defined as the variation in field intensity as a function of position or angle. It is a three dimensional plot of its radiation at far field. A transmitting antenna does not radiate uniformly in all angular directions nor does a receiving antenna detect energy uniformly from all directions. This directional selectivity of an antenna is characterized in terms of its radiation pattern. (9) It is called power pattern when the square of the amplitude of E field is plotted and it is called field pattern or voltage pattern when the amplitude of a specified component of the E field is plotted. (10)
Antennas that have small dimensions compared to the wavelength, a polar plot is often used. As for antennas that have bigger dimensions, a more detailed plot of the radiation pattern is analyzed using the Cartesian plot. (9) An isotropic antenna is the only antenna that will show the same radiation pattern in every spatial direction. It is mainly suitable as a model and comparison standard only as it cannot be implemented for any specified polarization. Dipole is an antenna that possesses directivity.
2.2 Directive Gain
Directive gain can be defined as the ratio of the power density in a particular direction of one antenna to the power density that would be radiated by an omnidirectional antenna
(isotropic antenna). The power density of both types of antenna is measured at a specified distance and a comparative ratio is established. (10) In a static situation, it is possible to use the antenna directivity to concentrate the radiation beam in the wanted direction.
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An isotropic antenna favors no direction which means it has no directivity. It is a point source that radiates equally in all directions. Directivity is depends on the shape of the directive pattern, it will do not take into account any power losses that could occur in an actual antenna model. A practical antenna never has the same intensity in all directions and might even have zero radiation in some directions. (11)
The gain of an antenna in a given direction is the amount of energy radiated in that direction compared to the energy an isotropic antenna would radiate in the same direction when driven with the same input power. Usually we are only interested in the maximum gain, which is the gain in the direction in which the antenna is radiating most of the power. (1)
With reference to an isotropic radiator, n is the efficiency of the antenna
2.3 Frequency and size
Antennas used for HF are different from the ones used for VHF, which in turn are different from antennas for microwave. The wavelength is different at different frequencies, so the antennas must be different in size to radiate signals at the correct wavelength. The antenna designed for this paper is around 2.5GHz to 8GHz onwards.
The bandwidths of an antenna are the range of frequencies over which it can operate correctly and important performance parameters are acceptable. The antenna's bandwidth is the number of Hz for which the antenna will exhibit an SWR less than 2:1.The bandwidth can also be described in terms of percentage of the center frequency of the band.
where FH is the highest frequency in the band, FL is the lowest frequency in the band, and FC is the center frequency in the band. If bandwidth was expressed in absolute units of frequency, it would be different depending upon the center frequency. In this way, bandwidth is constant relative to frequency. Different types of antennas have different bandwidth limitations. (12)
Polarization is the figure traced out with time by the instantaneous electric field vector associated with the radiation from an antenna when transmitting. It can also be defined as the orientation of the electric field of an electromagnetic wave.
The antenna initiallywill determined the polarization of a radio wave. Linear polarization the electric field vector will make it stays in the same plane all the time. Vertically polarized radiation is less affected by reflections over the transmission path. Omnidirectional antennas always have vertical polarization. With horizontal polarization, such reflections cause variations in received signal strength.
Horizontal antennas are less likely to pick up interference made by man, which ordinarily is vertically polarized. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. This rotation may be right or left.
An antenna is basically propagation medium and also a transducer between the radio or radar system. Therefore, an antenna designer must be more particular on the electromagnetic fields characteristics where it transmits or receives from a medium and also the characteristics of the load where the antenna presents to the system (4).
Impedance exists in a transmission line feeding the antenna (7). Normally, an antenna will be coupled to the receiver or transmitter by a transmission line. It may be in the form of coaxial cable, wire, dielectric or metallic waveguide or strip line which is one of the newer forms of transmission line. It is generally to avoid distortion of the information conveyed by achieving the maximum power transfer from the transmission line to the antenna and vice versa (4).
This transmission line operation where the impedance exists is called the terminal or driving point impedance (7).Other antenna or objects that are nearby will affect the input impedance, thus suitable terminals for the antenna must be defined (5). If the impedance of the antenna is failed to be matched to its input transmission line leading from the transmitter or to the receiver, the system will degrades because of the reflected power (18).
Figure above shows an antenna that is connected to a transmission line and its equivalent impedance. The reciprocity theorem can be quite useful to reflect the antenna impedance concept.
As a folded dipole
A dipole antenna can be made into a folded dipole as an antenna for the use of FM band. In order for the antenna to have one entire wavelength, the antenna is folded at the tips till it almost meets at the feed point. At resonance, the input impedance is found to be four times increase than of a half wave dipole if it has a constant radius and cross section of the conductor. The design is such that it will have a bigger bandwidth than a normal half wave dipole antenna (19).
As a set-top TV antenna
Dipole antenna is one of the antenna most commonly used for televisions that normally found in household. It is usually called as 'rabbit ears' or 'bunny ears'. Normally the dipole elements should be arranged within the same line. The length and angle is adjustable. Larger size of dipoles can be hung in a V shape position. The radio equipment will be near the center or the end of the dipole on the ground with support for the center. As for dipoles which are shoter, it can be hung vertically.
The figure above shows a rabbit ear that has a UHF loop antenna. Some antennas have extra elements such as the UHF loop antenna to get better reception. It works by a dial with each dial position, the antenna electrical properties are changed or it can also be turned on a vertical axis (19).
As a whip antenna
A dipole antenna can also be a whip antenna. Whip antenna are actually monopoles that can be considered half of a dipole that uses the ground plane as the image of the other half for a quarter wave monopole. Generally dipole antennas are more efficient as the power radiated and radiation resistance is double of a whip antenna. A whip antenna can be as efficient as a dipole if it is used with an infinite conducting ground plane such as the earth surface which means infinite distance from any conductive surfaces (19).
As dipole towers
Basic dipole antenna can be constructed into a large half wavelength towers. The only half-wave dipole towers for long wave ever built is the Warsaw radio mast located at Poland. The tower works a half wave radiator, thus in order for it to work as a half wavelength antenna at its broadcasting frequency, the height was chosen. It was the world tallest structure before it collapsed on 8 August 1991. Countries all over Europe, North Africa and also Notrh America could receive signals from the 2 Megawatt transmitters. There is also the Blaw-Knox Towers. It has a unique design that looks like a 'diamond cantilever' (19).
Figure 7: the Blaw-Knox tower
As a collinear antenna systems
Collinear antennas can be designed by stacking end to end dipole antennas in phase arrays. This system will show more gain in certain directions and the toroidal radiation pattern is also flattened out. At right angles, this gives maximum gain to the axis of the collinear array (19).
Chapter 3 - Dipole design technique
Lately, there is attention focused on UWB (ultra wideband) technology which is suitable for fast and large capacity near distance wireless communication systems. (13) As UWB uses 3.1 to 10.6GHz bands, research and development into antennas capable of obtaining wider band features has become active. There are also studies accelerated on antennas that can be shared with other wireless communication systems such as wireless LAN. (2) In this paper, a wideband planar dipole antenna is proposed as it has a simple structure, compact size and wideband impedance matching. Analysis and measurement of impedance characteristics and of directionality in each frequency band and confirmed that the analysis results and measurement results virtually match are performed. (14)
3.2 Antenna configuration
The wideband planar dipole antenna geometry, parameters, top and side view for a prototype of the proposed planar dipole antenna is shown in figure "9, 10 and 11". The antenna designed for this research project consists of two identical printed circular arms, a 50 ohm microstrip line, a probe connector and a ground plane. The planar arms are parallel to x-y plane and as for microstrip it is along the y-axis. Both arms of the dipole antenna are printed on the opposite sides of the substrate.
The microstrip connects one arm to the inner conductor of the coaxial cable. As for the outer conductor of coaxial cable, it connects to another arm. The arm is also functions as the ground of the microstrip line. The antenna side view looks like the shape of capital letter "T" or also inverse of "L".
The planar dipole antenna arms are in circular shape. The arms can also be designed using different shapes such as square, elliptical, triangular shapes, etc. These different shapes of antenna are studied and found to have wider VSWR bandwidth if the shape of circular, elliptical, hybrid elliptical, rectangular shape is used. This research paper selects circular shape antenna for demonstrating the proposed planar dipole antenna.
The radius of the antenna circular arm is labeled as R. Both arms have small circular cuts with radius r. There is also gap between the two arms with a distance g from top view. The microstrip line width is w for 50 ohm. The top of the substrate to the bottom ground is the distance H and the length of its feed line is l. The feed line is the component that connects to the upper arm. The substrate has a depth of h and permittivity Îµr.
The proposed antenna was first analyzed by using the electromagnetic full wave simulator CST Studio Suite to find the impedance and bandwidth. It is then fabricated on a Rogers RT/ Duroid 5880 TM substrate of a dielectric constant Îµr of 2.2, conductor loss (tanÎ´) of 0.0009 with the thickness of 0.787mm (20).
3.3 The antenna design
The measurement results from the research paper were taken using the Agilent E8362B network analyzer. The optimized parameters for the printed planar dipole antenna in accordance with the design considerations are given as follows:
Figure 9 Figure 10
3.4 Analysis of antenna
The frequency operation range of the proposed antenna was determined by the radius (R) of circular arm. It can be approximately evaluated using equation below:
The other geometric parameters will affect the antenna's performance over the whole frequency band. H is selected around quarter wavelength of FL. As for parameters r and d, it influences the impedance sensitively over the frequency band. The circular cut structure of the antenna arms is important to adjusting the return loss values (20).
Results and discussion
The analysis of the antenna results is measured and simulated using the CST studio suite. The electromagnetic simulation software CST Studio Suite comprises tools for the design and optimization of devices operating in a wide range of frequencies - static to optical. Analyses may include thermal and mechanical effects, as well as circuit simulation. All programs are accessible through a common interface with facilitates circuit and multi-physics co-simulation
It is a specialist tool for the fast and accurate 3D EM simulation of high frequency problems. It enables the fast and accurate analysis of high frequency (HF) devices such as antennas, filters, couplers, planar and multi-layer structures and SI and EMC effects. Exceptionally user friendly, CST MWS quickly gives you an insight into the EM behavior of high frequency designs. It has shorter development cycles, virtual prototyping before physical trials and optimisation instead of experimentation. (15)
4.2 Results and discussion
4.2.1 Return Loss
The impedance bandwidth of the proposed antenna shows S11 better than -10dB covers a very wide frequency ranging from 2.5Ghz to more than 8.0 GHz. It can be seen from the research paper, the graph simulated, research and measured results are not the same. The reason of the difference between simulated and measured results comes from SMA connector. The metal has zero thickness in the simulated modelling but the fabricated one is not. The graph shows that both measured and the simulated curves have three pits at three different frequencies. The results got from CST which is the research results differs from the article results as the design geometry have a slight difference in terms of the measurement. There were a few measurements that are arbitrarily and simulated over again to get the closest possible results to the article.
It shows that the printed planar dipole antenna has three different modes which is
: One due to inverted L structure (resonance at 1.1GHz),
: The second due to dipole mode (around at 3.2GHz)
: The third due to disk loaded monopole mode (around 7.8 GHz).
The frequency operating range is mainly determined by adjusting the value of R. The low frequency FL is roughly evaluated using the following equation:
The length unit used for all dimension is in mm. The effect of other geometric parameters (such as l and d) on the frequency range is not patency.
4.2.2 Radiation pattern
Figure 13 Figure 14
The E-plane pattern of the antenna is not symmetrical at frequency of 3GHz. It is due to the unbalun current on the circle arms are too strong around the resonant frequency. The shape of E-plane pattern is symmetrical when the frequency is higher than 3.5GHz.
Figure 16 Figure 17
4.2.3 Other parameters
Based on the optimized parameters of circular dipole antenna presented, the effects of H,l,r and d are also studied. Simulated results indicate that H has no severe impact on the frequency response. l mainly controls the upper band cut frequency due to the third mode excitation control. It is also observed that small value of H (17mm) has better radiation pattern at 8GHz but it will bring on difficult matching at the center frequency range from 4GHz to 6GHz.
The results indicate that the return loss varies slowly with d but the curves vary fast with r. The impedance is sensitive to parameters r. Adjusting the value of r will result in better impedance matching over the wideband.
These antennas are with wide frequency bandwidth and compact size. Several simulation results are carried out and the properties of the antennas are presented. Measurement and simulation show that the advantages of easy feed implementation and better impedance matching. It is expected that the antenna has proper properties for wideband applications.
This type antenna has 100% impedance bandwidth. It provides low cross polarization level and relatively high gain in the frequency range from 2.5GHz to 8 GHz. Furthermore the radiation direction is in the vertical plane of planar dipole. The thickness of the antenna is smaller than the radiation antenna.
5.2 Future work
Further studies on dipole antenna using more published journal or books are recommended so that more comparison can be done on certain areas of the antenna. It is also recommended to use another type of electromagnetic simulation software such as the ANSOFT HFSS to simulate out the results and compare it with the published article and the results from the CST Microwave Studio. Published paper used for research should be checked to have all the proper design geometry given as to avoid contradict results in future.