Antenna Design, Other Style

Table of Contents I. Introduction Overview How Antennas Work Properties of Antenna A. Size B. Shape C. Directivity Current and Voltage Distribution Antenna Parameters A. Resonant Frequency B. Gain C. Bandwidth D. Impedance E. Polarization F. Efficiency II. Types of Antenna III. Sample Problems IV. Design Proper Theory of Operation Design Computation Construction Procedure 1

Applications Cost Analysis V. Conclusion / Recommendation

Introducti on 2

Antenna Basics Principles

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An antenna or aerial is an electronic component designed to transmit or receive radio waves. The words "antenna" (plural: antennas) and "aerial" are used interchangeably throughout this article. Physically, an antenna is an arrangement of conductors designed to radiate (transmit) an electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or to be placed into an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Antenna radiates and receives radio waves through the air or through space. Antennas are used to send radio waves to distant sites and to receive radio waves from distant sources. Many wireless communications devices, such as radios, broadcast television sets, radar, and cellular radio telephones, use antennas.

OVERVIEW There are two fundamental types of antennas, which, with reference to a specific three dimensional (usually horizontal or

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vertical) plane, are either omni-directional (radiate equally in the plane) or directional (radiates more in one direction than in the other). All antennas radiate some energy in all directions but careful construction results in large directivity in certain directions and negligible power radiated in other directions. By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation, an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). Typically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally well. This property is called reciprocity. The vast majority of antennas are simple vertical rods a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive, and both radiate in and receive from all horizontal directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive in the direction in which the rod points. This region is called the antenna blind cone or null. Antennas have practical use for the transmission and reception of radio frequency signals (radio, TV, etc.), which can travel over great distances at the speed of light, and pass through nonconducting walls (although often there is a variable signal reduction depending on the type of wall, and natural rock can be very reflective to radio signals).

HOW ANTENNAS WORK

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A transmitting antenna takes waves that are generated by electrical signals inside a device such as a radio and converts them to waves that travel in an open space. The waves that are generated by the electrical signals inside radios and other devices are known as guided waves, since they travel through transmission lines such as wires or cables. The waves that travel in an open space are usually referred to as free-space waves, since they travel through the air or outer space without the need for a transmission line. A receiving antenna takes free-space waves and converts them to guided waves. Radio waves are a type of electromagnetic radiation, a form of rapidly changing, or oscillating, energy. Radio waves have two related properties known as frequency and wavelength. Frequency refers to the number of times per second that a wave oscillates, or varies in strength. The wavelength is equal to the speed of a wave (the speed of light, or 300 million m/sec) divided by the frequency. Low-frequency radio waves have long wavelengths (measured in hundreds of meters), whereas high-

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frequency radio waves have short wavelengths (measured in centimeters). An antenna can radiate radio waves into free space from a transmitter, or it can receive radio waves and guide them to a receiver, where they are reconstructed into the original message. For example, 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 length of 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 loudspeaker, reproducing the original message.

PROPERTIES OF ANTENNA An antenna’s size and shape depend on the intended frequency or wavelength of the radio waves being sent or received. The design of a transmitting antenna is usually not different from that of a receiving antenna. Some devices use the same antenna for both purposes.

A. Size 6

An antenna works best when its physical size corresponds to a quantity known as the antenna’s electrical size. The electrical size of an antenna depends on the wavelength of the radio waves being sent or received. An antenna radiates energy most efficiently when its length is a particular fraction of the intended wavelength. When the length of an antenna is a major fraction of the corresponding wavelength (a quarter-wavelength or halfwavelength is often used), the radio waves oscillating back and forth along the antenna will encounter each other in such a way that the wave crests do not interfere with one another. The waves will resonate, or be in harmony, and will then radiate from the antenna with the greatest efficiency. If an antenna is not long enough or is too long for the intended radio frequency, the wave crests will encounter and interfere with one another as they travel back and forth along the antenna, thus reducing the efficiency. The antenna then acts like a capacitor or an inductor (depending on the shape of the antenna) and stores, rather than radiates, energy. The electrical length of an antenna can be altered by adding a metal loop of wire known as a loading coil to one end of the antenna, thus increasing the amount of wire in the antenna. Loading coils are used when the practical length of an antenna would be too long. Adding a coil to a short antenna increases the antenna’s electrical length, improves its resonance at the desired frequency, and increases the antenna’s efficiency. The radio waves used by AM radio have wavelengths of about 300 m (about 1,000 ft). Most AM transmitter antennas are built to a height of about 75 m (about 250 ft), which, in this case, is the length of a quarter-wavelength. With a tower of this height, an AM radio antenna will radiate radio waves most efficiently. Since an antenna that is 75 meters tall would be impractical for a portable AM radio receiver, AM radios use a special coil of wire inside the radio for an antenna. The coil of wire is wrapped around an iron-like magnetic material called a ferrite. When radio waves come into contact with the coil of wire, they induce an electric charge within the coil. The magnetic ferrite helps confine and concentrate the electrical energy in the coil and aids in reception.

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Television and FM radio use tall broadcast towers as well but use much shorter wavelengths, corresponding to much higher frequencies, than AM radio. Therefore, television and FM radio waves have wavelengths of only about 3 m (about 10 ft). As a result, the corresponding antennas are much shorter. Buildings and other obstructions close to the ground can block these highfrequency radio waves. Thus the towers are used to raise the antennas above these obstructions in order to provide a greater broadcasting range. Receiving antennas for television sets and FM radios are small enough to be installed on these devices themselves, but the antennas are often mounted high on rooftops for better reception.

B. Shape Antennas come in a wide variety of shapes. One of the simplest types of antennas is called a dipole. A dipole is made of two lengths of metal, each of which is attached to one of two wires leading to a radio or other communications device. The two lengths of metal are usually arranged end to end, with the cable from the transmitter or receiver feeding each length of the dipole in the middle. The dipoles can be adjusted to form a straight line or a V-shape to enhance reception. Each length of metal in the dipole is usually a quarter-wavelength long, so that the combined length of the dipole from end to end is a half-wavelength. The familiar “rabbit-ear” antenna on top of a television set is a dipole antenna. Another common antenna shape is the half-dipole or monopole antenna, which uses a single quarter-wavelength piece of metal connected to one of the twin wires from the transmitter or receiver. The other wire is connected to a ground, or a point that is not connected to the rest of the circuit. The casing of a radio or cellular telephone is often used as a ground. The telescoping antenna in a portable FM radio is a monopole. This arrangement is not as efficient as using both ends of a dipole, but a monopole is usually sufficient to pick up nearby FM signals.

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Satellites and radar telescopes use microwave signals. Microwaves have extremely high frequencies and, thus, very short wavelengths (less than 30 cm). Microwaves travel in straight lines, much like light waves do. Dish antennas are often used to collect and focus microwave signals. The dish focuses the microwaves and aims them at a receiver antenna in the middle of the dish. Horn antennas are also used to focus microwaves for transmission and reception.

C. Directivity Directivity is an important quality of an antenna. It describes how well an antenna concentrates, or bunches, radio waves in a given direction. A dipole transmits or receives most of its energy at right angles to the lengths of metal, while little energy is transferred along them. If the dipole is mounted vertically, as is common, it will radiate waves away from the center of the antenna in all directions. However, for a commercial radio or television station, a transmitting antenna is often designed to concentrate the radiated energy in certain directions and suppress it in others. For instance, several dipoles can be used together if placed close to one another. Such an arrangement is called a multiple-element antenna, which is also known as an array. By properly arranging the separate elements and by properly feeding signals to the elements, the broadcast waves can be more efficiently concentrated toward an intended audience, without, for example, wasting broadcast signals over uninhabited areas. The elements used in an array are usually all of the same type. Some arrays have the ability to move, or scan, the main beam in different directions. Such arrays are usually referred to as scanning arrays. Arrays are usually electrically large and have better directivity than single element antennas. Since their directivity is large, arrays can capture and deliver to the receiver a larger

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amount of power. Two common arrays used for rooftop television reception are the Yagi-Uda array and the log-periodic array. A Yagi-Uda consists of one or more dipoles mounted on a crossbar. The dipoles are of different lengths, corresponding to the different frequencies used in broadcast television transmission. Additional pieces of metal, which are called directors and reflectors, are placed on the crossbar in front of and behind the dipoles. Directors and reflectors are not wired into the feed element of the antenna at all but merely reflect and concentrate radio waves toward the directors. Yagi-Uda antennas are highly directive, and receiving antennas of this type are often mounted on rotating towers or bases, so that these antennas can be turned toward the source of the desired transmission. Logperiodic arrays look similar to Yagi-Uda arrays, but all of the elements in a log-periodic array are active dipole elements of different lengths. The dipoles are carefully spaced to provide signal reception over a wide range of frequencies. While the dipole, monopole, microwave dish, horn, Yagi-Uda, and log-periodic are among the most common types of antennas, many other designs also exist for communicating at different frequencies. Submarines traveling underwater can receive coded radio commands from shore by using extremely low frequency (ELF) radio waves. In order to receive these signals, a submarine unravels a very long wire antenna behind as it travels underwater. Television camera crews broadcasting from locations outside the studio use powerful microwave transmitter antennas, which can send signals to satellites or directly to the television station. Amateur, or “ham,” radio enthusiasts, who generally use frequencies between those of AM and FM radio, often construct their own antennas, customizing them for sending and receiving signals at desired frequencies.

CURRENT AND VOLTAGE DISTRIBUTION

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When an RF signal voltage is applied at some point on an antenna, voltage and current will result at that point. Travelling waves are then initiated, and standing waves may be established, which means that voltage and current along the antenna are out of phase. The radiation pattern depends chiefly on the antenna length measured in wavelengths, its power losses and the terminations at its end (if any). In addition, the thickness of the antenna wire is of importance. The figure below shows the voltage and current distribution along a halfwave dipole. We can recognize the similarity to the distribution of voltage and current on a section of quarter wavelength transmission line open at far end.These voltage and current characteristics are duplicated every antenna.

λ length along the 2

ANTENNA PARAMETERS There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.

A. Resonant frequency The resonant frequency is related to the electrical length of the antenna. The electrical length is usually the physical length of the wire multiplied by the ratio of the speed of wave 11

propagation in the wire. Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties. Antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log periodic, but its gain is usually much lower than that of a specific or narrower band aerial.

B. Gain In antenna design, gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an isotropic antenna, the gain is often expressed in units of dBi (decibels over isotropic). For example, a dipole antenna has a gain of 2.14 dBi [2]. Often, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to build), in which case the gain of the antenna in question is measured in dBd (decibels over dipole). The gain of an antenna is a passive phenomena - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a positive gain in some directions, it must have a negative gain in other 12

directions as energy is conserved by the antenna. The gain that can be achieved by an Antenna is therefore trade-off between the range of directions that must be covered by an Antenna and the gain of the antenna. For example, a dish antenna on a spacecraft has a very large gain, but only over a very small range of directions - it must be accurately pointed at earth - but a radio transmitter has a very small gain as it is required to radiate in all directions. For dish-type antennas, gain is proportional to the Aperture (reflective area) and surface accuracy of the dish, as well as the frequency being transmitted/received. In general, a larger aperture provides a higher gain. Also, the higher the frequency, the higher the gain, but surface inaccuracies lead to a larger degradation of gain at higher frequencies. Aperture, and radiation pattern are closely related to gain.

Aperture

is the shape of the "beam" cross section in the direction of highest gain, and is two-dimensional. (Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.)

Radiation pattern is the three-dimensional plot of the gain, but usually only the two-dimensional horizontal and vertical cross sections of the radiation pattern are considered. Antennas with high gain typically show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes detract from the antenna quality whenever the system is being used to determine the direction of a signal, as in radar systems and reduce gain in the main lobe by distributing the power.

C. Bandwidth The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by 13

several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency.

D. Impedance Impedance is similar to refractive index in optics. As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system. Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

E. Polarization The polarization of an antenna or orientation of the radio wave is determined by the electric field or E-plane. The 14

ionosphere changes the polarization of signals unpredictably, so for signals which will be reflected by the ionosphere, polarization is not crucial. However, for line-of-sight communications, it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization. Polarizations commonly considered are linear, such as vertical and horizontal, and circular, which is divided into right-hand and left-hand circular.

F.Efficiency Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and therefore, reduces efficency.

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Types of Antenna

Types of Antenna •

Parabolic

The Parabolic antenna is a high-gain, reflector antenna used 16

for radio, television and data communications, and also for radiolocation (RADAR), on the UHF and SHF frequencies. The relatively short wavelength of electromagnetic (radio) energy at these frequencies allows reasonably sized reflectors to exhibit the very desirable highly directional response for both receiving and transmitting. A typical parabolic antenna consists of a parabolic reflector illuminated by a small feed antenna. The reflector is a metallic surface formed into a paraboloid of revolution and (usually) truncated in a circular rim that forms the diameter of the antenna. This paraboloid possesses a distinct focal point by virtue of having the reflective property of parabolas in that a point light source at this focus produces a parallel light beam aligned with the axis of revolution. The feed antenna is placed at the reflector focus. This antenna is typically a low-gain type such as a half-wave dipole or a small waveguide horn. In more complex designs, such as the Cassegrain antenna, a sub-reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. The feed antenna is connected to the associated radio-frequency (RF) transmitting or receiving equipment by means of a coaxial cable transmission line or hollow waveguide. Considering the parabolic antenna as a circular aperture gives the following approximation for the maximum gain: or where: is power gain over isotropic is reflector diameter in wavelengths

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Practical considerations of antenna effective area and sidelobe suppression reduce the actual gain obtained to between 35 and 55 percent of this theoretical value. Applying the formula to just one of the 25-meter-diameter VLA antennas shown in the illustration for a wavelength of 21 cm (1.42 GHz, a common radio astronomy frequency) yields an approximate maximum gain of 140,000 times or about 50 dBi (decibels above the isotropic level). With the advent of TVRO and DBS satellite television, the parabolic antenna became an ubiquitous feature of urban, suburban, and even rural, landscapes. Extensive terrestrial microwave links, such as those between cellphone base stations, and wireless WAN/LAN applications have also proliferated this antenna type. Earlier applications included ground-based and airborne radar and radio astronomy. The largest "dish" antenna in the world is the radio telescope at Arecibo, PR, but, for beamsteering reasons, it is actually a spherical, rather than parabolic, reflector. •

Omnidirectional

An omnidirectional antenna is an antenna system which radiates power uniformly in all directions. The only 3 dimensional omnidirectional antenna is the isotropic antenna, a theoretical construct derived from actual antenna radiation patterns and used as a reference for specifying antenna gain and radio system effective radiated power. Practical antennas approach omnidirectionality by providing uniform radiation or response only in one reference plane, usually the horizontal one parallel to the earth's surface. Common omnidirectional antennas are the whip antenna, a vertically oriented dipole antenna, the discone antenna, and the horizontal loop antenna.

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Whip

A whip antenna is the most common example of a monopole antenna, an antenna with a single driven element and a ground plane. The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a ground plane. The whip antenna can also be called a half-dipole antenna, and as such, has a toroidal radiation pattern where the axis of the toroid centers about the whip. The length of the whip determines its wavelength, although it may be shortened with a loading coil anywhere along the antenna. Whips are generally a fraction of their actual operating wavelength, with half-wave and quarter-wave whips being very common. These antennas are widely used, especially for mobile applications and hand-held radios. They are usually attached to a vehicle and designed to be flexible, so that they don't break when struck; their name is derived from their whip-like motion when disturbed. Being vertically mounted causes the whip antenna to have vertical polarization. Whips are thought of as omnidirectional, because they radiate equally in all directions in a horizontal plane, although they have a conical blind zone directly above them.

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Discone

A discone antenna is a version of a biconical antenna where one of the cones is replaced by a disc. It is usually mounted in vertical orientation, with the disc at the top 19

and the cone under it. It may be made of solid metal sheets, which is practical for small indoor high-frequency antennas, such as for Wi-Fi, or of discrete metal elements assembled to a "star" at the top and a cone of beams going down from the star's center, which makes it less vulnerable to wind. The cone and the disc are separated by an insulator. A discone antenna is omnidirectional, vertically polarized and wideband—allowing frequency ranges of up to 10:1, and its radiation pattern in the horizontal plane is quite narrow, making its sensitivity highest in the plane parallel to the Earth. It is suitable for a wide range of applications, from amateur radio to various commercial and military uses. While it can be used for transmitting, its wideband characteristics make it more likely to transmit undesired spurious frequencies, and it is less efficient than some other designs. The discone antenna has three components: the disc, the cone, and the insulator. Each of them determines the antenna's parameters. The disc elements should have an overall length of 0.7 times a quarter wavelength of the antenna's minimum frequency. The insulator keeps the disk and the cone a fixed distance apart. This distance determines part of the antenna's properties. It should he about a quarter of the diameter of the top of the cone, which is usually about 3 mm.The antenna's feed point is in the center of the disc. The number of the elements is usually not critical. Often as few as six are used, though the simulation of the electrically complete disk and cone gets more accurate with increasing number of elements. The result is usually a compromise between cost, performance, and resistance to wind. •

Dipole

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A dipole antenna, invented by Heinrich Rudolph Hertz around 1886, is an antenna with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view.

DIPOLE CHARACTERISTICS Frequency versus length Dipoles that are much smaller than the wavelength of the signal are called Hertzian dipoles. These have a low radiation resistance and a high reactance, making them inefficient, but they are often the only available antennas at very long wavelengths. Dipoles whose length is half the wavelength of the signal are called half-wave dipoles, and are more efficient. In general radio engineering, the term dipole usually means a halfwave dipole. A half-wave dipole is cut to length according to the formula l = 468 / f, where l is the length in feet and f is the center frequency in MHz [1]. The metric formula is l = 143 / f, where l is the length in meters. The length of the dipole antenna is about 80% of half a wavelength at the speed of light in free space. This is because the velocity of propagation of electromagnetic waves in wire is slower than that in free space.

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Radiation pattern and gain A dipole's radiation pattern Dipoles have a toroidal (doughnut-shaped) reception and radiation pattern where the axis of the toroid centers about the dipole. The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dBi. The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15 dBi.

Feeder line Ideally, a dipole should be fed with a balanced line matching the theoretical 73 ohm impedance of the antenna. A folded dipole uses a 300 ohm balanced feeder line. Many people have had success in feeding a dipole directly with a coaxial cable feed rather than a ladder-line. However, coax is not symmetrical and thus not a balanced feeder. It is unbalanced, because the outer shield is connected to earth potential at the other end. When a balanced antenna such as a

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dipole is fed with an unbalanced feeder, common mode currents can cause the coax line to radiate in addition to the antenna itself, and the radiation pattern may be asymmetrically distorted. This can be remedied with the use of a balun.

APPLICATIONS Common applications of dipole antennas

Set-top TV antenna The most common dipole antenna is the "rabbit ears" type used with televisions. While theoretically the dipole elements should be along the same line, "rabbit ears" are adjustable in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio equipment on the ground or the ends on the ground with the center supported. Shorter dipoles can be hung vertically.

Folded dipole Another common place one can see dipoles is as antennas for the FM band - these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. The main advantage of this arrangement is an improved bandwidth over a standard halfwave dipole.

Shortwave antenna Dipoles for longer wavelengths are made from solid or stranded wire. Portable dipole antennas are made from wire that can be rolled up when not in use. Ropes with weights on the ends can be thrown over supports such as tree branches and then used to hoist up the antenna. The center and the connecting cable can be hoisted up with the ends on the ground or the ends hoisted up between two supports in a V shape. While permanent antennas

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can be trimmed to the proper length, it is helpful if portable antennas are adjustable to allow for local conditions when moved. It is important to fit a good insulator at the ends of the dipole, as failure to do so can lead to a flashover if the dipole is used with a transmitter. One cheap insulator is the plastic carrier that holds a pack of beer cans together. This beer can insulator is an example of how a household object can be used in place of an expensive object sold for use as an item of radio equipment. Other objects that can be used as insulators include buttons from old clothing.

DIPOLE TYPES Ideal half-wavelength dipole This type of antenna is a special case where each wire is exactly one-quarter of the wavelength, for a total of a half wavelength. The terminal impedance is about 73 ohms if wire diameter is ignored. If the dipole is not driven at the centre then the feed point resistance will be higher. If the feed point is distance x from one end of a half wave (λ/2) dipole, the resistance will be described by the following equation. Rx = 75 / sin2(2πx / λ) If taken to the extreme then the feed point resistance of a λ/2 long rod is infinite, but it is possible to use a λ/2 pole as an aerial; the right way to drive it is to connect it to one terminal of a parallel LC resonant circuit. The other side of the circuit must be connected to the braid of a coaxial cable lead and the core of the coaxial cable can be connected part way up the coil from the RF ground side. An alternative means of feeding this system is to use a second coil which is magnetically coupled to the coil attached to the aerial.

Folded dipole 24

A folded dipole is a dipole where an additional wire (λ/2) links the two ends of the (λ/2) half wave dipole. The folded dipole works in the same way as a normal dipole, but the radiation resistance is about 300 ohms rather than the 75 ohms which is expected for a normal dipole. The increase in radiation resistance allows the antenna to be driven from a 300 ohm balanced line. Infinitesimal dipole The length of this antenna is significantly smaller than the wavelength:

The radiation resistance is given by:

The radiation resistance is typically a fraction of an ohm, making the infinitesimal dipole an inefficient radiator. In the far field, the maximum directive gain is 1.5. The maximum effective aperture is:

A surprising result is that even though the infinitesimal dipole is minute, its effective aperture is comparable to antennas many times its size.

Dipole as a reference standard Antenna gain is sometimes measured as "x dB above a dipole", which means that the antenna in question is being compared to a dipole, and has x dB more gain (has more directivity) than the dipole tuned to the same operating

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frequency. More often, gains are expressed relative to an isotropic radiator, which is an imaginary aerial that radiates equally in all directions. As it is impossible to build an isotropic radiator, gain measurements expressed relative to a dipole are more practical when a reference dipole aerial is used for experimental measurements. A dipole antenna cut from an infinitely large sheet of metal, with sufficient thickness, is complementary to the slot antenna, both giving the same radiation pattern.

Dipole with baluns

Coax acting as a radiator instead of the antenna. When a dipole is used both to transmit and to receive, the characteristics of the feedline become much more important. Specifically, the antenna must be balanced with the feedline. Failure to do this causes the feedline, in addition to the antenna itself, to radiate. RF can be induced into other electronic equipment near the radiating feedline, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation (and reception) pattern may be distorted asymmetrically. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder coax, this becomes a

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more significant problem. One solution to this problem is to use a balun. Several type of baluns are commonly used to transmit on a dipole: current baluns and coax baluns.

Current balun

Dipole with a current balun.

A current balun is a bit more expensive but has the characteristic of being more broadband.

balun

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Coax balun

Magnetic Loop

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Sleeve

Magnetic loop antennas (also known as Small Transmitting/Receiving Loops) have a small antenna size compared to other antennas for the same wavelength. The antenna is typically smaller than 1/4 wavelength of the intended frequency of operation. Antennas for shortwave communication are normally very large sometimes covering hundreds of feet or meters in length. The advantage of the magnetic loop is that with its small size it maintains very high efficiency levels. The technical mechanism is to use a capacitor to "enlarge" the antenna and bring it to resonance. The disadvantage of this method is the low bandwidth of the antenna, also known as "high Q". However, a "high Q" antenna also has advantages as well. In reception: Since Magnetic Loop antennas only function within a narrow range of frequency when tuned, they reject harmonic noise from other radio sources. This rejection of interfering noise from other harmonically related frequencies keeps the noise level down compared to other antennas like the common 1/4 wave vertical antenna. As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna needs to be retuned by changing the capacitive value of the antenna. Bandwidth is the usable frequency range of an antenna in relation to the area of desired operation. When the antenna is operated outside of its bandwidth, the energy from the transmitter is reflected back from the antenna, down through the feedline back to the transmitter. The term bandwidth relates to the concept of Standing Wave Ratio or SWR. When the reflected power exceeds a 2.5:1 power reflection ratio (too much energy being reflected from the antenna back into the feedline) the antenna will not maintain its performance characteristics. This type of condition relates specifically to the antenna's ability to transmit radio energy from the transmitter to the antenna.

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The magnetic loop antenna is an old antenna, however, many military, commercial, and ham radio operators still use them today. The Magnetic Loop was widely used in the Vietnam War due to its high portability. •

Helical Antenna A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. Helical antennas can operate in one of two principal modes: normal (broadside) mode or

axial (or endfire) mode. In the normal mode, the dimensions of the helix are small compared with the wavelength. The far field radiation pattern is similar to an electrically short dipole or monopole. These antennas tend to be inefficient radiators and are typically used for mobile communications where reduced size is a critical factor. In the axial mode, the antenna produces true circular polarization. These antennas are best suited for space communication, where the orientation of the sender and receiver cannot be easily controlled, or where the polarization of the signal may change. Helical antennas can have either a clockwise (right-handed) or counter-clockwise (left-handed) polarization. Helical antennas can receive signals with any type of polarization, such as horizontal or vertical polarization, but clockwise polarized antennas suffer a severe gain loss when receiving counterclockwise signals, and vice versa. Helical antennas are made of a single driven element which is coiled in a spiral, or helix. The direction of the coil determines 29

its polarization, while the space between the coils and the diameter of the coils determine its wavelength. The length of the coil determines how directional the antenna will be and its gain; longer antennas will be more sensitive in the direction in which they point. A reflector is almost always used to increase the sensitivity, or gain, in one direction (away from the reflector). Terminal impedance in axial mode ranges between 100 and 200 Ω. The resistive part is approximated by:

where R is resistance in ohms, C is the circumference of the helix, and λ is the wavelength. The maximum directive gain is approximately:

where N is the number of turns and S is the spacing between turns. •

Horn Antenna

A horn antenna is used for the transmission and reception of microwave signals. It derives its name from the characteristic flared appearance. The flared portion can be square, rectangular, or conical. The maximum radiation and response corresponds with the axis of the horn. In this respect, the antenna resembles an acoustic horn. It is usually fed with a waveguide. In order to function properly, a horn antenna must be a certain minimum size relative to the wavelength of the incoming 30

or outgoing electromagnetic field. If the horn is too small or the wavelength is too large (the frequency is too low), the antenna will not work efficiently. Horn antennas are commonly used as the active element in a dish antenna. The horn is pointed toward the center of the dish reflector. The use of a horn, rather than a dipole antenna or any other type of antenna, at the focal point of the dish minimizes loss of energy (leakage) around the edges of the dish reflector. It also minimizes the response of the antenna to unwanted signals not in the favored direction of the dish. Horn antennas are used all by themselves in short-range radar systems, particularly those used by law-enforcement personnel to measure the speeds of approaching or retreating vehicles. •

Microstrip

In telecommunication, There are several types of microstrip antennas (also known as a printed antennas) the most common of which is the microstrip patch antenna or patch antenna. A patch antenna is a narrowband, widebeam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating substrate. Because such antennas have a very low profile, are mechanically rugged and can be conformable, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile radio communications devices. Microstrip antennas are also relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. They are usually employed at UHF and higher 31

frequencies because the size of the antenna is directly tied to the wavelength at the resonant frequency. •

Rhombic Antenna

A rhombic antenna is a broadband directional antenna, mostly used in HF (high frequency, also called shortwave) ranges. It is named after its "rhombic" diamond shape, with each side typically being of wavelength size. Each vertex is supported by a pole, several meters tall. It is fed at one of the sharp angles through a balun transformer, and is terminated at the opposite sharp angle with a non-inductive resistor. It is directional toward the resistor end, so it "points" toward the region of the world it is designed to serve. Rhombic antenna signal-gathering action compared to other end-fire, backfire and traveling-wave types. The rhombic antenna can radiate close to the horizon or at a higher angle depending on its length, width, and height relative to the operating frequency. Likewise, its beam can be narrow or broad. A proper combination of size, height, and operating frequency make it fit for short- medium- or long-range communication. Due to its considerable size, it is not very practical as the sole antenna of a radio station if operating conditions are expected to change rapidly. Moreover, it plainly requires a lot of land - especially if several must be combined to serve a variety of geographic targets at different distance ranges and on widely different frequencies. On the other hand, it is one of the best options for predictable point-to-point circuits. Its very size gives it a degree of 32

gain, and allows it to capture energy from a wide area, thus making it a little less susceptible to sharply localized fading than smaller antennas. •

Slot Antenna

A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation distribution pattern. Slot antennas are often used instead of line antennas when greater control of the radiation pattern is required. Slot antennas are often found in standard desktop microwave sources used for research purposes.

•

Yagi – Uda Antenna

A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna works the same.

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The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable. Yet the other elements carry almost as much current. The Yagi is the most magical of all antennas. No attempt will be made here to explain why it works. The more directors you add, the higher the gain becomes. Gains above 20 dBi are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band. An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna.

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The formula for the aperture area of any TV antenna is A=Gλ 2/4π where λ is the wavelength and G is the gain factor over an isotropic antenna (not dB). The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency of the band while sizing the directors for the highest. But this decreases the best gain of the antenna. (It is said that the gainbandwidth product remains the same.) A better way to increase the bandwidth is to replace the reflector element with a cornerreflector assembly. This boosts the performance on the lower numbered channels without hurting the high channels. Although the Yagi/CornerReflector might not be the best antenna, it is the most common UHF TV antenna, mainly because it can be mounted on the front of a VHF antenna without degrading the VHF antenna. A UHF Yagi today is designed for channel 69. If you see an old Yagi, it might be intended for channel 82. In the future they will be cut for channel 51. It is not possible to tell by looking at a Yagi which era it belongs to, so be careful. The Yagi antenna or more correctly, the Yagi - Uda antenna was developed by Japanese scientists in the 1930's. It consists of a half wave dipole (sometimes a folded one, sometimes not), a rear "reflector" and may or may not have one or more forward "directors". These are collectively referred to as the "elements".

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The Yagi antenna This particular antenna has been optimized for dual band operation. It is designed to pick up both VHF and UHF transmissions. Because I live in a regional of NSW in Australia, TV antennas tend to be single channel types designed either for higher gain or better directivity. Different examples will be presented later. A practical Yagi TV antenna Looking from left to right on this dual band Yagi we have six UHF "director" elements which improve gain and directivity. Next is the UHF half wave dipole which could have easily been a folded dipole but is in fact a plain half wave dipole. The next three much longer elements form a "phased array" for the VHF band. I am unsure of the function of the three remaining smaller elements, information is quite scant here but one would certainly be a UHF "reflector". Likely the other two also fulfill this function also. Note: This is a horizontally polarised antenna and is orientated roughly NNW, 315 degrees.

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You will notice the effect of very strong storms from the sea have had in bending the second larger elements. In my locality storms are a problem but not as much as roosting parrots such as large sulphur crested cockatoos.

Comparing a Yagi/CornerReflector to an 8-DipoleReflector

The graph above shows the gain functions for four TV antennas: • Plot A is the Channel Master 4228 8-Bay, a stacked dipole reflector antenna. • Plot B is the Channel Master 4248, a Yagi/Corner-Reflector. • Plot C is the 4248 with all of its directors removed, making it a pure corner reflector antenna. • Plot D is the 4248 with its corner reflector removed and replaced by a single reflector element, making it a standard

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Yagi. The D2 plot shows the backward gain where this exceeds the forward gain. The point of this graph is that a Yagi/Corner-Reflector performs like a Yagi for the high numbered channels and a corner reflector for the low numbered channels. For the middle channels it outperforms the sum of the two types.

Radiation patterns

As you can see, the 8-Bay is a very directional antenna. If miss-aimed by 5° you can lose 1 dB of signal. If the horizon is more than 5° above horizontal, you should tilt the antenna up to point at the horizon.

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The overhead view shows nulls at 30° and 90° to both sides. These can be used to eliminate multi-path (ghosts) or interference. You simply rotate the antenna until the offending signal is in one of the nulls.

A Yagi also has some forward nulls that can be used as ghost killers. But a Yagi/Corner-Reflector acts more like a corner reflector for most channels, and has no nulls. At channel 60 you can finally see the Yagi pattern start to emerge. 39

•

Log – Periodic Antenna

In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array) is a broadband, multielement, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA). It is normal to drive alternating elements with a circa 180o (π radian) phase shift from the last element. This is normally done by wiring the elements alternatingly to the two wires in a balanced transmission line. The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other.

Sample Problems 1. A half-wave dipole antenna is capable of radiating 1 kW and has a 2.15 dB gain over an isotropic antenna. How much power

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must be delivered to the isotropic (omnidirectional) antenna, to match the filed strength directional antenna?

SOLUTION: A( dB ) = 10 log10 2.15 = 10 log10

P2 P1 P2 1000

 P  10 0.215 =  2   1000   P  1.64 =  2   1000  P2 = 1640 W

2. Calculate the beamwidth between nulls of a 2 m paraboloid reflectror used at 6GHz. Note: Such reflectors are often used at that frequency as antennas in outside broadcast television microwave links.

SOLUTION: 70λ (but, the problem involves 2 paraboloid antenna, thus this equation becomes D 70λ 0.05 φO = 2 x = 140 x D 2 O φ O = 3.5

φO =

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Design Proper Theory of Operation 42

Basics A transmitting antenna takes waves that are generated by electrical signals inside a device such as a radio and converts them to waves that travel in an open space. The waves that are generated by the electrical signals inside radios and other devices are known as guided waves, since they travel through transmission lines such as wires or cables. The waves that travel in an open space are usually referred to as free-space waves, since they travel through the air or outer space without the need for a transmission line. A receiving antenna takes free-space waves and converts them to guided waves. Radio waves are a type of electromagnetic radiation, a form of rapidly changing, or oscillating, energy. Radio waves have two related properties known as frequency and wavelength. Frequency refers to the number of times per second that a wave oscillates, or varies in strength. The wavelength is equal to the speed of a wave (the speed of light, or 300 million m/sec) divided by the frequency. Low-frequency radio waves have long wavelengths (measured in hundreds of meters), whereas highfrequency radio waves have short wavelengths (measured in centimeters). An antenna can radiate radio waves into free space from a transmitter, or it can receive radio waves and guide them to a receiver, where they are reconstructed into the original message. For example, 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 length of the antenna and back. Each oscillation pushes electromagnetic energy from the antenna, emitting the energy through free space as radio waves.

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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 loudspeaker, reproducing the original message.

The Log-Periodic Antenna One of the major drawbacks with many antennas is that they have a relatively small bandwidth. This is particularly true of the Yagi-Uda antenna. However, in 1957, a new antenna design, the log-periodic type antenna, a better way of reception and transmission was obtained. The log periodic antenna is used in a number of applications where a wide bandwidth is required along with directivity and a modest level of gain. It is sometimes used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained. The main feature of this antenna is its frequency independence for both radiations resistance and pattern.

Why choose Log-Periodic? A special case of a driven array antenna, log periodic, offers reasonably good gain over an extremely wide range of frequencies. It is also highly directional. This is an advantage against a Yagi type or other array because log-periodic provides a very wide bandwidth. Most Yagis and other driven arrays are designed for a very specific frequency or a narrow band of 44

frequency. For the log periodic type, one can design in a greater frequency band, the reason why it is useful for multiband transceiver operation and as a TV receiving unit to cover the entire VHF and UHF bands. There are many types of log-periodic antenna. For this design, a log-periodic dipole type was implemented because it provides between 4 and 6 dB gain over a bandwidth of 2:1 while retaining an SWR level of better than 1.3:1. With this level of performance it is ideal for many applications, although a log periodic antenna will be much larger than a Yagi that will produce equivalent gain. However the Yagi is unable to operate over such a wide bandwidth.

Design Computation The group decided to use a frequency range of 100MHz – 500MHz. The antenna, which is of a dipole array type of log periodic antenna, was design with such frequency to ensure signal reception from the very high frequency’s (VHF) low-band, high band and at the ultra high frequency range (UHF). The summary of the computations used in the design of the logperiodic dipole array antenna is as follows: Frequency range:

100MHz – 500MHz

Design factor:

0.7

Longest element:

λ 3 x 10 8 m/s 3m = = = 1.5m 6 2 100 x 10 Hz 2

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Limiting factor of element:

λ 3 x 10 8 m/s 0.6m = = = 0.3m 6 2 2 500 x 10 Hz

L1 = 1.5m

L2 = τ x L1 = 0.7(1.5) = 1.05m

L3 = τ x L2 = 0.7(1.05) = 0.735m

L4 = τ x L3 = 0.7( 0.735) = 0.5145m

L5 = τ x L4 = 0.7( 0.5145) = 0.36015m

Distance from the longest element to the next longer element: D1 = 0.09λ =

3 x 10 8 m / s = 0.27 m 100 x 10 6 Hz

D1 = 0.27m

D2 = 0.7( 0.27 ) = 0.189m

D3 = 0.7( 0.189) = 0.1323m

D4 = 0.7( 0.1323) = 0.09261m

Construction Procedure Materials antenna element square boom wire element holder screws clam element cover impedance matching device(matching transformer) boom cover

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Procedure 1. Prepare all the materials for the antenna construction. 2. Cut the antenna element according to the computed length as governed by the designed frequency range. 3. Measure the appropriate distances of each element on the antenna boom with reference to your calculations. 4. Mark the measured distances, cut the boom into the desired length, and then bore holes according to the markings. 5. Place the element holder on the antenna boom length, and screw it in place. 6. Screw the antenna elements to its holder. 7. Connect each dipole element through wires. 8. Place the element and boom cover. 9. Test the antenna for reception.

Applications Like those of the rhombic, the applications of log-periodic antenna lie mainly in the filed of high-frequency communications, where such multiband steerable and fixed antenna are very often. It has an advantage over the rhombic in that there is no terminating resistor to absorb power. Antennas of this type have also been designed for use in television signal reception, with one antenna for all channels including those of the UHF range. As a matter of fact, most TV antennas in use today are the log-periodic variety so that they can provide high gain and directivity for both VHF and UHF TV channels. Log-periodic antennas are also used in 47

other two-way communications frequencies must be covered.

systems

where

multiple

Cost Analysis MATERIAL

QUANTITY

Boom Antenna element Boom cover Element cover

1

UNIT PRICE (Php) 60

3

25

75

2

1.50

3

10

0.25

2.50

10(small), 5(big)

0.50(small), 1(big)

10

5

8

40

1 4 meters 1

20 9 35

20 36 35

Screws Element Holder Clam Coaxial cable Balun

TOTAL (Php) 60

Total

281.50

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Conclusion and Recommen dation 49

Conclusion After careful analysis and study, the group obtained with the designed and constructed antenna, the group was able to conclude that the design of a Log-Periodic dipole array antenna is a good antenna design to consider for television (TV) signal reception. With a log-periodic design, one can obtain TV signal of all channels included in the very high frequency (VHF) and the ultra high frequency (UHF) band, without suffering video and audio quality. Less noise and ghosting is attained by properly tuning and directing the antenna to the direction of highest reception by installing in an appropriate location and height. It should be noted here that the main beam of the antenna is coming from the smaller front element as in the figure below.

With the above mentioned findings, the group was able to prove the theoretical principle of a log-periodic antenna – its superiority in reception of VHF and UHF signals against other driven array antenna types like the famous Yagi-Uda antenna.

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Recommendation The conclusion of the group had made its based only on the direct observation from the TV signals received as pictured in a television monitor. It is therefore recommended that for a more specific, detailed, and accurate facts to support the log-periodic antenna’s efficiency, testing through computer software simulations should be implemented. With this method, numerical data and precise evaluation of the antennas behavior will be obtained – its beam width, directivity, its exact gain, and even the effects of external conditions such as the prevailing climate/weather sate.

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