Antennas and Antenna Arrays - Term Paper Example

ANTENNAS AND ANTENNA ARRAYS The antenna is an aerial device that is designed to transmit a guided electromagnetic wave efficiently through the space or through the air. The waves usually go to an unbound medium (or from a medium that is also unbound). It is made in different shapes and sizes which vary in accordance with the wavelength and the frequency of the desired signal (thinkquest.com n.d.). There exists a wide variety of antenna which is used in a wider variety of industries: radio and television, cellular phone and other wireless communication like the wireless internet. The antenna had been the major element in the construction of wireless communication system and apparatus. It has played such an essential role ever since the first exhibition of wireless technology in 1986, when Heinrich Hertz demonstrated how waves can be transmitted from a device to another. It was further used by Guglielmo Marconi in 1901, when he made the first radio application. The device had been considered a "building block" in terms of wireless communication systems' construction (cushcraft 1998). Before, the device wasn't considered very important in terms of initial system design (cushcraft 1998). However, it became a very crucial structure in the efficient design of any radio system (Kolias 2000). Because of its ability to transmit RF energy between wires, it was hailed as the single device that allows such transition with remarkable speed and efficiency. Antennas have undergone many changes through time and have evolved according to the needs of man. From a simple wire dipole, antennas had more complex structures such as Yagis, parabolic reflectors, microstrip and patch arrays. Concepts Associated with Antennas An antenna converts waves that are generated by electrical signals from devices such as radio into waves that can travel in open space. Transmitting antennas convert such waves, which are also called "guided waves" (since these waves travel through guides like wires/cables), so that it can be passed on to the space an eventually be received by other transmitting devices around. The converted waves which will now travel in open space are called "free-space waves" (waves that travel freely through the air without transmission lines). Receiving antennas take these free-floating waves and convert them into guided waves (thinkquest.com n.d.). Radio waves, on the other hand, are those which change and oscillate rapidly. These waves are actually a type of electromagnetic radiation, and should therefore be considered energy. Radio waves have two closely related properties: wavelength and frequency. Frequency refers to the number of times per second that a wave varies in terms of strength, in other words, how often the wave oscillates. On the other hand, wavelength is defined as the speed of the wave, divided by its frequency. Low frequency waves are those that usually have long wavelengths (which may measure up to hundreds of meters), while radio waves with high frequency have very short wavelengths (which may measure only in centimeters) (thinkquest.com n.d.). Radio waves can be radiated into the open space by an antenna from a transmitting device. It can also take or receive radio waves and guide them to a receiver, which will then reconstruct the waves to relay the original message. The following examples are taken from thinkquest.com (copied verbatim): " …in sending an AM radio transmission, the radio first generates a carrier wave of energy at a particular frequency. The carrier wave is modified to carry a message, such as music or a person's voice. The modified radio waves then travel along a transmission line within the radio, such as a wire or cable to the antenna. The transmission line is often known as a "feed element". When the waves reach the antenna, they oscillate along the antenna and back. Each oscillation pushes electromagnetic energy from the antenna, emitting the energy through free space as radio waves. The antenna on a radio receiver behaves in much the same way. As radio waves traveling through free space reach the receiver's antenna, they set up, or induce, a weak electric current within the antenna. The current pushes the oscillating energy of the radio waves along the antenna which is connected to the radio receiver by a transmission line. The radio receiver amplifies the radio waves and sends them to a loud speaker, reproducing the original message" (thinkquest.com n.d.). Such mechanism happens very fast, in the speed of light. The previous passage explains how fast an antenna transmission and reception should be in order to relay messages from live radio and television shows. Properties of Antenna There are several characteristics of antennas which should be discussed in order to understand which works efficiently given the type of message to be transmitted. It is important for system design engineers, installers and operators to have knowledge of antenna properties because it will be very important in the design and installation of various wireless communication systems. Such knowledge will help these mentioned professionals with the proper selection of transmitting and receiving antennas as well as their right mounting location and orientation. Optimum system coverage will be determined by how efficient waves are transmitted (cushcraft 1998). Aside from the ability to improve overall system performance and coverage, a properly selected antenna may also cause cost-reduction if all access points can be integrated. The reverse effect will happen if an antenna type, which is incompatible with the system, is chosen: it may degrade the coverage and performance of the system and may also lead to higher system cost. The following fundamental properties of antennas have impact on system performance and should thus be considered in antenna selection (cushcraft 1998). Reciprocity This may be considered the basic property of antenna, which can be seen across different types. This simply means that antennas exhibit the same radiation pattern for transmission as well as for reception. This property is usually seen in linear antennas (cushcraft 1998). Polarization It is defined as the “orientation of electromagnetic waves far from the source” (Kolias 2000). Antenna polarization describes the path of E — or H- radiation field that are being transmitted by the antenna. Polarization of the receiving and transmitting antennas should be matched up or paired accordingly (Kolias 2000). Generally, radiated waves are elliptically polarized, meaning, the antenna’s total electric field, or E-field’s components lie in the same path. These components may vary in strength and orientation. The most common cases of elliptically polarized waves are circular and linear. In circular polarization, the two e-field components are the same in magnitude and are oriented 90 degrees out of phase. In linear polarization the wave has only one E-field component. The axial ratio describes the relative strength of these two E-field components in a wave that is polarized elliptically. For purely circular polarization, axial ratio is 0 dB and  for linear polarization. Graphical Description of Polarization Orientation (cushcraft 1998) It is very important that transmitting and receiving antennas have the same axial ratio. The greatest energy will only be transferred if both of these antennas have the same axial ratio, polarization sense, and angle of orientation. It is assumed that distortion will not be made in the path of propagation (cushcraft 1998). Antennas that are linearly polarized must be of exactly the same orientation so as to achieve maximum energy transfer. Reduction in energy transfer happens if there is a mismatch—the transmit and receive antennas are not the same in orientation. The following table is a summary of polarization mismatch between two linearly polarized waves (cushcraft 1998) Polarization Mismatch Between Two Linearly Polarized Waves as a Function of Angular Orientation. (cushcraft 1998) It has always been a misconception in the wireless communication industry that linearly and circularly polarized antennas are always mismatched with 3 dB gap. However, this could only happen if one of these antennas is purely circularly polarized and the other is linearly polarized. In reality, it is not likely to happen that a circularly polarized antenna will actuality have a 0 dB axial ratio with its field components completely out of phase (90 degrees), in the same way that its linearly polarized counterpart may actually have another field component. The following table is a summary of the polarization mismatch that can occur between a circularly polarized and a linearly polarized waves as a function of the former’ s axial ratio. There is an assumption that the field components of circularly polarized waves are orthogonal (cushcraft 1998) Polarization Mismatch between a Linearly and Circularly Polarized Wave as a Function of the Circularly Polarized Wave’s Axial Ratio. (cushcraft 1998) Impedance Antenna impedance refers to the transfer of power from a certain generator to the antenna (if it is used as a transmitter) and the transfer of power from the antenna to a load (if it is used as a receiver). The input impedance of the antenna should perfectly match the impedance of the radiation transmission line. An incompatibility between these two impedances may reduce overall system efficiency. There is a possibility that a reflected wave will be generated at the antenna terminal and may travel back to its source. Voltage Standing Wave Ratio (VSWR) According to cushcraft (1998), the resultant voltage on the transmission line is a combination of both the incident (source) and reflected waves. VSWR is the ratio between the maximum voltage and the minimum voltage along the transmission line. It can be derived from the level of reflected and incident waves. It is an indication of how efficiently an antenna’s terminal input impedance is matched to the characteristic impedance of the transmission line. An incompatibility of impedance will cause an increase in VSWR (cushcraft 1998). Radiation Patterns Antenna radiation pattern is a three-dimensional representation of the antenna radiation far from the source. It provides adequate information on how the device directs the energy it radiates (cushcraft 1998). These usually takes two major forms: the azimuth pattern and the elevation pattern. The azimuth pattern is a graphical representation of energy radiated from the antenna looking at it from above of the antenna, or what is usually called top view (figure b) while the elevation pattern graphs the energy radiated as can be seen from the side, or side view (figure a). The combination of these two graphs will give a three dimensional representation of how the energy is radiated from the antenna (figure c) (Kolias 2000). Radiation Patterns: a) Generic Dipole Elevation Pattern b) Generic Dipole Azimuth Pattern c) 3-D Radiation Pattern (Kolias 2000). It many circumstances, the convention of E-plane and H-plane sweep or pattern is used when representing the antenna pattern. The plane that contains the antenna’s radiated electric field potential is the E-plane and the plane that bears the antenna’s radiated direction field potential is the H-plane. Once the antenna radiation pattern is made with accurate details in a polar plot, several quantitative aspects of the antenna can also be accurately described. The aspects usually include the 3 dB beamwidth, directivity, side lobe level and front to back ratio. These concepts will only be fully grasped if we discuss some important concepts first, like the point source (cushcraft 1998). A point source is the imaginary antenna that direct energy in all directions, as was shown in the following figure. This kind of antenna is called an omnidirectional isotropic radiator which has 0 dB directivity. In reality, an omnidirectional antenna is inferred to be referenced only to the azimuth plane (cushcraft 1998). Spherical Radiation Pattern of a Point Source Antenna. (cushcraft 1998) Directivity This property of antennas may be defined as the radiation power per angle, where “angle” is a two-dimensional solid-angle. It is an important property of an antenna because it describes how efficiently the antenna concentrates radio waves in a given direction (thinkquest.com n.d.). It is usually a ratio of radiation intensity in a particular direction to the average radiation intensity (Kolias 2000). Directivity plays a major role in plotting the radiation pattern of an antenna. Since it is a relative measure of the efficiency of the antenna in terms of focusing the energy it radiates, it can be a basis for representation. The higher the directivity of the antenna, the more focused its radiation pattern will be. It is important to note that it is impossible for an antenna to have a directivity that is less than 0 dB (Cushcraft 1998). Gain Gain refers to the measure of an antenna’s overall efficiency. A very efficient antenna will have a gain that is equal to its directivity. The antenna’s gain is influenced by the efficiency of other antenna properties. Aside from Impedance and VSWR, the following factors may cause a reduction in the antenna’s gain (Cushcraft 1998): Matching Network Losses- According to Cushcraft (1998), “In general, the terminal impedance of an antenna will not exactly match the characteristic impedance of the connecting lines to the required VSWR level. In order to align or match these impedances, a matching circuit or network s constructed at the antenna terminals”. Such matching network may be composed of lumped circuit elements such as inductors and capacitors, transformers which may be coaxial or microstrip, and microstrip circuitry. In these cases, energy is delivered to both the antenna and the matching components. Some of these components may be inherently lost and will dissipate energy delivered to the antenna. Losses in these matching components are minimal but not negligible and will certainly reduce the effective gain of the antenna (Cushcraft 1998). Material Losses- this refers to the metal or dielectric losses of the device. Antennas are made up of discrete materials which are both metallic and non metallic. There is an unavoidable risk of transfer of some energy as heat, and not as a radiation. This energy loss reduces the effective gain of the antenna (Cushcraft 1998). Radome Losses- It is the usual case that the radiating structure of the antenna is housed inside what is called a radome, to protect it from other electric processes from the operating environment. In this case, the energy radiated by the device must first pass through the radome, and will most likely cause some dissipation. The dissipated energy reduces the effective gain of the antenna (Cushcraft 1998). With all of the mentioned factors to be considered, it is apparently essential for the antnna o overcome a lot of instances in order to achieve perfect, or even just acceptable, gain. Antenna design engineers are very much aware of this and thy have already found ways to minimize the losses with a proper design (Cushcraft 1998). Types of Antenna There are many different types of antennas that are currently used in different fields. The following is a description of these types. Scanner Antennas 1/4 Wave Ground Plane Antenna-Wave Ground Plane antennas are single band and vertically polarized antennas. These offer about 3dB of gain in a narrow frequency range. It is very beneficial in terms of its affordability and small size. The ground plane isolates an antenna from having to be tied to earth ground at a specific multiple of the wavelength. It does so by simulating the ground with the elements radially mounted at the bottom. One example of a 1/4 wave is a car mounted antenna. It uses the body of the car as its ground plane (Diaz n.d.) . Discone Antenna-This antenna is similar to the 1/4 wave ground plane antenna optimized--both are optimized for wide frequency bandwidth. On frequencies from about 120-1300 MHz, it offers 0dB of gain. And, if paired with a vertical element on top, it may be usable down to about 30MHz. This type is called a discone because it is composed of a couple of parts. The first part is the disc, a group of elements arranged parallel to the ground around the top. The other part is the cone, the diagonal radial elements placed around the bottom (Diaz n.d.) . Dipole Antenna Dipole antennas have generalized radiation pattern. According to Kolias (2000), “First, the elevation pattern shows that a dipole antenna is best used to transmit and receive from the broadside of the antenna. It is sensitive to any movement away from a perfectly vertical position. You can move about 54 degrees from perfect verticality before the performance of the antenna degrades by more than half…” Some other dipole antennas may have varying amounts of vertical variation before a performance degradation may be noticed. There are many types of dipole antennas and a couple will be discussed here. They are the short dipole antennas and the multiple element dipole antennas. Short Dipole Antenna This is considered the simplest antenna. Short dipole antennas are also known as Hertzian dipole antennas. Multiple Dipole Antenna Multiple element dipole antennas are almost like the short dipole antennas, because they share some of the general characteristics: similar radiation pattern and azimuth pattern. However, these two types of antennas differ in terms of their directionality. It is common that multiple elements cause increase in effective gain (Kolias 2000). In this type of antenna, multiple elements are used to construct the device, making it able to be configured with varying amounts of gain. Such ability will allow the antenna to radiate equally not only in all directions in the horizontal plane, but also in any horizontal configuration (Kolias 2000). Multiple Element Dipole Elevation Pattern Source: Maxrad, Inc. Yagi Antenna Yagi antennas are characterized by an array of independent antenna elements, with only one element driven to transfer the electromagnetic waves. Yagi Antenna Construction, Yagi-Uda Antenna (Source: Sasaki Printing and Publishing Company, 1954) Yagi Antenna Elevation Radiation Pattern (Source: Maxrad, Inc.) This type of antenna is inferior to parabolic dish antennas in terms of directionality, but superior to flat panes antennas in the same aspect. Flat Panel Antenna Characterized by a flat (square or rectangular shape), flat panel antennas are configured in a patch type format. These are quite directional, as most of its energy radiated in both vertical and horizontal planes. Flat panel antennas can be made to have different amounts of gain based on how it is constructed, and this can provide efficient directivity and greater gain (Kolias 2000). Parabolic Dish Antenna Both physical features and multiple elements are used by this type to achieve extremely high amount of gain and very sharp directivity. Parabolic dish antennas have reflective dishes with a parabolic shape that serves as a focusing structure. All received electromagnetic waves are focused by the dish to a single point, efficiently directing it towards another device. The dish also works to “catch” all the radiated energy from the antenna and concentrate it in a very narrow beam in the process of transmission. High gain is achieved by this type of antenna through its ability to harness all of its power and sending it in a single direction (Kolias 2000). Elevation Pattern of a Parabolic Dish Antenna (Source: Maxrad, Inc.) Slotted Antenna Radiation characteristics of this antenna are very similar to the dipole antennas (azimuth patterns) except for its physical appearance. It is constructed as an antenna consisting only of a narrow slot cut into the ground plane. This type provides little gain, and has a low directivity. However, this type became attractive because of the ease with which it can be constructed, aside from its very low cost. These factors, if taken at face value, usually offset the mediocre performance of this type of antenna (Kolias 2000). Microstrip antenna This type can be made to emulate many of the different styles of all the other types discussed above. However, even though this may sound very promising, the use of microstip antennas also has some disadvantages. These types are manufactured with PCB traces on actual PCB boards, which can be small and lightweight. Due to this, the antenna may not be able to handle as much power as the other antennas, which makes this type not suitable for wideband communication systems (Orban and Moernaut n.d.) . Horn antennas For applications that require compact size, relatively low sidelobes and high front to back ratio, horn antennas would be the perfect choice. Mobile systems such as aerial, roving and portable camera systems use this kind of antennas. Horn antennas are ideal in aerial uplook and downlook use. It is considered wideband and covers 1.7 GHz to 2.7 GHz. This kind is usually used to substitute disk rod or helical antennas. Although horn antennas may appear rugged, it has proven to be weather resistant and endures the rigors of ENG and remote EFP broadcast production (NS Microwave Website n.d.). Horn antennas are versatile. In fact, these may be used as feed element on the Offset Fed antennas. These provide multiple polarity and frequency options for Offset Fed reflectors (NS Microwave Website n.d.). Helical Antennas For a simpler way of achieving high gain as well as a broad band of frequency characteristics, Helical antennas are used. This kind of antenna radiates when the helix circumference is of the order of one wavelength, and the radiation along the axis of the helix is found to be the strongest. Helical antennas are generally directional. The electromagnetic field of the helical antenna rotates about the axis of the helix in the direction of the helix turn, thus making the radiation circularly polarized. Such circular polarization of the radiation is either clockwise or counter clockwise (Hasting Wireless Website n.d.) . The strength of the helical antenna will remain the same, even in the direction of maximum radiation with a simple monopole or dipole antenna. This will always be the case as long as the dipole is perpendicular to the axis of the helix. The field is elliptically polarized on the side of the helical antenna, which means that the horizontal and vertical portions of the signal will not be of equal proportions (Hasting Wireless Website n.d.) . Antenna Arrays Since it is not really very realistic for some single-element antennas to both achieve the perfect gain and meet all of the radiation requirements, designers of the device thought of a way to combine several single antenna elements to form antenna arrays (Balanis 1997). An antenna array is formed by antenna elements. There are one and two dimensional antenna arrays (Mailloux 1994). Since one type of antenna exhibit its unique radiation pattern, a combination of several elements of different types will exhibit another radiation pattern, which is different from those single elements. This is due to the array factor. The effect of combining radiating elements in an antenna array is quantified by the array factor (Mailloux 1994). This factor combines these effects while it discriminates the effects of the element specific radiation pattern. Array factor determines the overall radiation pattern of an array by combining the radiation pattern of the antenna elements (Balanis 1997). There are various ways in which antennas may be arranged (MSU Website n.d.) Linear array- antenna elements may be arranged in a straight line. N-Element Linear Array-Assuming all of the antenna elements are identical, the array factor will be independent of the antenna type. Because of this, isotropic radiators may be utilized to derive the array factor, for simpler algebraic calculation (MSU Website n.d.) . Uniform N-element Linear Array- This type of array is characterized by uniformly spaced identical elements of the same magnitude with a lineraly progressive phase from one element to another (MSU Website n.d.) . Broadside and endfire arrays- Broadside means that the radiation is perpendicular to array orientation, while endfire means that radiation is in the same direction as the array orientation (Balanis 1997). Hansen-Woodyard End-fire Array-This is a special array designed for maximum directivity. To have an increase in the directivity in a closely-spaced electrically long end-fire array, Hansen and Woodyard analyzed the patterns and found out that an additional phase shift increased the directivity of the array remarkably, as compared to that of the ordinary end-fire array. For very long arrays, Hansen-Woodyard end-fire array usually approaches one-fourth of a wavelength. The Hansen-Woodyard end-fire design increases the directivity of the array, but it may be at the expense of higher sidelobe levels (MSU Website n.d.) . Binomial Arrays-Equivalent arrays with more elements may also be formed. The current coefficients of the resulting N-element array take the form of a binomial series (MSU Website n.d.) . Circular array- antenna elements may be arranged in a circle, or in a ring-like arrangement (MSU Website n.d.) . Planar array- elements of the antenna may be arranged over a planar surface such as a rectangular or square plane (MSU Website n.d.) . Conformal array- elements of the antenna may be arranged according to the shape of the surface on which it would be mounted, such as aircraft skins (MSU Website n.d.) . Defining the Array Factor The array factor is dependent on a number of things: number of elements, element spacing, phase of the applied signal of the element and amplitude. The surface area of the overall radiating structure is determined by the number of elements and the element spacing. This surface area is called the aperture. The size of the aperture is directly proportional to gain, meaning, the larger the aperture, the higher the gain. A measure of the efficiency of the aperture is called the aperture efficiency (Balanis 1997). The influence of these things can be further explained with the help of isotropic radiating elements. Isotropic radiating elements radiate an amount of power that is equal in all directions. This means that, if it has a directivity of 1 (or 0dB, as was mentioned earlier), and a gain of 1 (also 0 dB), it has a 100% efficiency (Balanis 1997). The influence of the number of elements on the array factor The directivity of an array is directly proportional to the number of elements. Take for example the following figure (Figure from Balanis 1997). Directivity of a 2 (red), 5 (green) and 10 (blue) element array with 0.4 element spacing. What is shown is the directivity of three arrays with red, green and blue elements. For all the arrays, the element spacing is 0.4 times the wavelength. Notice the presence of side lobes which is next to the main lobes--this is not unusual for arrays. The increase in the number of elements means an increase in the number of side lobes and the side lobe level. It is essential to consider that due to the definition of the array factor, there existed two main lobes (Balanis 1997). The influence of element spacing Like the number of elements, the element spacing also has a great influence on the array factor. A larger element spacing would cause higher directivity. However, the spacing of the elements is usually kept smaller than half of the wavelength to avoid having what is called grating lobes. A grating lobe is an undesirable peak value in the radiation pattern of the array. The following figure shows the array factors of a 5 element array with various element spacing (Balanis 1997). Directivity of a 5 elements array with 0.2 (red), 0.3 (green) and 0.5 (blue) times l element spacing (Figure from Balanis 1997). Directivity of a 5 elements array with 0.5 (red), 0.75 (green) and 1 (blue) times l element spacing (Figure from Balanis 1997). Directivity of a 5 elements array with 1 (red), 1.5 (green) and 2 (blue) times l element spacing (Figure from Balanis 1997). Influence of the radiating element properties on the overall radiation pattern Given below are a number of total radiation patterns to show the effect of the radiating element radiation pattern for the general radiation of the array. The following figure exhibits the radiation pattern of an isotropic element (in red), the array factor, the combined radiation pattern (both are in green). In this particular case, the overall radiation pattern is the same as the radiation factor. This is because an isotropic element radiates the same amount of power in all directions (Balanis 1997). Directivity of an isotropic source (red) in a 5 elements array (green) with 0.4 l element spacing (Figure from Balanis 1997). The following figure shows the radiation pattern of a dipole (which was discussed earlier, in red) the same array factor in the previous figure (in green) and the overall radiation pattern of the array with dipoles (in blue). It can be clearly seen that the overall radiation pattern is distinct from the array factor. In other words, the directivity increased with the dipole's directivity and the overall radiation pattern is slightly modified due to the dipole's radiation pattern (Balanis 1997). Directivity of a dipole in a 5 elements array with 0.4 l element spacing (Figure from Balanis 1997). The following figure shows the radiation pattern of a dipole on an infinite ground plane, which is in red, the same array factor in the previous figure, and the overall radiation pattern of the array with dipoles on an infinite ground plane (in blue). The dipole, as can be seen in the picture has a radiation lobe in the positive z axis alone, and thus, the directivity of the dipole has increased with 3 dB, primarily because of the ground plane (Balanis 1997). Directivity of a dipole on infinite ground in a 5 elements array with 0.4 l elements spacing (Balanis 1997). Feeding of an array In the previous discussion of arrays, the element spacing has been kept constant, and the elements were fed with the same phase as well as amplitude. Because of this the resulting arrays were linear with uniform spacing, amplitude, and equal phase. However, the power doesn't necessarily have to be distributed with equal amplitude and phase. In fact, unequal phase and power distribution to the individual elements can actually be used to modify or change the side lobe level, directivity and direction of the main lobe. However, it should be noted that any modification to an array may have an adverse effect on the performance of the array, thus, a careful; tradeoff is required. An optimization of the power distribution (to reduce the side lobe level) will cause a decrease in the efficiency of the array. An optimization of the phase distribution (to do beam steering), new lobes will show up as the main beam is deflected sideways (Balanis 1997). Feed network of the array Each antenna element in the array is fed using a feed network. The complexity of the feed network is dependent on the amplitude, phase distribution bet when the elements, and the ability to do beam steering. It is essential that the feed network is the most complex part of the array (Balanis 1997). Works Cited Balanis C. A. 1997. Antenna Theory 2nd Edition. Wiley Inc. Cushcraft Corporation. 1998. Antenna Properties and Their Impact on Wireless System Performance. Available at: http://www.cushcraft.com/comm/support/pdf/Antenna-Properties-an-14998.pdf Diaz, Mike (n.d.). Guide to Scanner Antennas. Available at: http://www.fordyce.org/scanning/scanning_info/scanant.html Hastings Wireless Website (n.d.). Helical Antennas. Available at: http://hastingswireless.homeip.net/index.php?page=antennas&type=helical Kolias, N. J. et al. 2000. Antennas. C.R.C. Press L.L.C. Available at: http://www.engnetbase.com Mailloux, R. J. 1994. Phased Array Antenna Handbook. Artech House Mississippi State University College of Engineering (Department of Electrical and Computer Engineering) Website (n.d.). Antenna Arrays. Available at: http://www.ece.msstate.edu/~donohoe/ece4990notes6.pdf NS Microwave Website (n.d.). Horn Antennas. Available at: http://www.microwave.com/pdfs/datasheets/components/horn.pdf Orban, D. & Moernaut G. (n.d.) The basics of patch antennas. Available at: http://www.orbanmicrowave.com/The_Basics_Of_Patch_Antennas.pdf Programming and Electronics Network. No date. Antenna. Available at: http://library.thinkquest.org/C006657/electronics/antenna.htm Read More