Arrays Antenna

An antenna array (or array antenna) is, much like it sounds, several elements
interconnected and arranged in a regular structure to form an individual antenna. The
purpose of an array is to produce radiation patterns that have certain desirable
characteristics that a single element would not. A stacked dipole array, as shown in
figure, is comprised of vertical dipole elements.



This dipole array has an omnidirectional pattern like the element dipole does; but has higher gain and a narrower main lobe beamwidth in the vertical plane. Figure 22
shows how the vertical-plane gain of the dipole element can be “enhanced” by making an array of them. Figure 22(a) represents the radiation pattern of one element. The Figure is kindly the pattern antenna array as well as array schema.




The omnidirectional coaxial collinear antenna (often referred to as an “omni”) is a
very popular array design for base stations. It is comprised of quarter-wave coaxial
sections with inner and outer conductors transposed at each junction. A conceptual illustration is shown in figure. Although more complex than the illustration, this antenna array behaves like a series of vertical dipoles stacked one above the other. The more stacked sections, the greater the gain and the narrower the vertical beamwidth. A vertical-plane pattern for this type of antenna is shown in figure 24. Variations in electrical design can produce a downward tilt of the vertical-plane pattern as shown in figured. This antenna often is enclosed in a fiberglass sheath, called a radome, and appears as a simple pole that can be mounted off the side or on top of a mast or tower.

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LDPA (Log Dipole Periodic Antenna)

A somewhat novel, but very useful, design is the log-periodic antenna. This antenna is based on the dipole element. As shown in the illustration of figure 18, it is in fact comprised of a set of dipoles, all active, that vary in size from smallest at the front to largest at the rear.



Usually, this antenna is constructed so the antenna terminals are located at the front (on the shortest dipole).



The Figure shows a typical installation. The key features of this antenna are, first of all, its broadband nature, and second, its relatively high front-to-back gain ratio. The latter feature is evident in the typical radiation pattern shown below:

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Yagi Antenna

Another antenna design that uses passive elements is the Yagi antenna. This antenna, is inexpensive and effective. It can be constructed with one or more (usually one or two) reflector elements and one or more (usually two or more) director elements. Figure below shows a Yagi antenna with one reflector, a folded-dipole active element, and seven directors, mounted for horizontal polarization.



Figure is a typical pattern for a threeelement (one reflector, one active element,
and one director) Yagi antenna. Generally, the more elements a Yagi has, the higher the gain, and the narrower the beamwidth. This antenna can be mounted to support either horizontal or vertical polarization and is often used for point-to-point applications, as between a base station and repeater-station sites.

Yagi antena usually have radiation signal as below

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Dipoles and Monopoles Antenna

The vertical dipole—or its electromagnetic equivalent, the monopole—could beconsidered one of the best antennas for LMR applications. It is omnidirectional (in azimuth) and, if it is a half-wavelength long, has a gain of 1.64 (or G = 2.15 dBi) in the horizontal plane. Although this is a simple antenna, it can be difficult to mount on a mast or vehicle. It is half a dipole placed in halfspace, with a perfectly conducting, infinite surface at the boundary.



A monopole over an infinite ground plane is theoretically the same (identical gain,
pattern, etc., in the half-space above the ground plane) as the dipole in free space. In practice, a ground plane cannot be infinite, but a ground plane with a radius approximately the same as the length of the active element, is an effective, practical solution. The flat surface of a vehicle’s trunk or roof can act as an adequate ground plane. Below shows to you typical monopole antennas for base-station and mobile applications.

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TV Antenna

Its usually used in home to received tv signals. There’s no different model between digital tv and analog tv for antenna kind.

Below Some terms of antenna:
• Gain – a measure of how much signal the antenna will collect.
• Beam width – how directional an antenna is.
• Bandwidth – how the gain varies with frequency. A narrow band antenna will receive some channels well, but other channels poorly.
This is an easy page. But if you want, you can skip or skim the rest of this page without compromising your understanding of the pages that follow.



The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie (which has wider bandwidth), the folded-dipole (which can solve an efficiency problem) and the loop (a variation on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dBi. “dBi” means “dB of improvement over an isotropic radiator, which is an antenna that radiates equally in all directions. This sounds like a discussion of transmitting antennas, and it could be. An antenna will have the same gain when receiving as when transmitting, and also the same radiation pattern.




The dipole has positive gain because it does not radiate equally in all directions. This is a universal truth. To get more gain, an antenna must radiate in fewer directions. Imagine a spherical balloon. Now press on it from opposite sides with a finger of each hand. Push in until your fingers meet. The result looks like the torroid above. But more importantly, the balloon expanded in the other directions. A-hah! Gain! That’s the way antennas work.

Keep this balloon analogy in mind. More complicated antennas work by reducing radiation in most directions. They distort the balloon considerably, but the volume of the balloon remains constant.

Another rating system for antennas uses dBd, which means dB of improvement over a dipole antenna. To convert dBd to dBi, just add 2.15. Antenna makers specify their gains in dB. They actually mean dBd, but given the way they exaggerate their claims, dBi is usually closer to the truth.

In the US, TV antennas are always horizontal. If you rotate an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization.

Stacked Dipoles

Two heads are better than one, and so it is with dipoles. N dipoles will take in N times as much RF power as one dipole, provided they are not too close to each other. Thus a 4-dipole antenna would have a gain of 8.15 dBi. (That is 2.15 dBi doubled once (plus 3 dB) and doubled again (plus another 3 dB).) This assumes their positions and cable lengths are adjusted so that their signals add in-phase. This explanation of gain may seem at odds with the balloon explanation, but ultimately they are equivalent. (Adding dipoles does not increase the volume of the balloon because phase cancellation occurs in some directions.)

`Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and in echelon (end-fire).



When dipoles are stacked horizontally, the horizontal beam width becomes very narrow. This is because they do not add in-phase for directions not straight ahead. Similarly, when stacked vertically, the vertical beam width becomes narrower.

Lets say you are 20 miles from a city and TV transmitters are scattered all over the city. A medium gain antenna might be too weak, but a high gain antenna would be so directional you would need a rotor. Solution: A bunch of dipoles stacked vertically would give you the gain you need. The vertical narrowness of the resulting beam is of little importance, but the horizontal broadness of the beam means no rotor needed.





Reflector Antennas

Radio waves will reflect off of a large conducting plane as if it was a mirror. A coarse screen will serve as well. Reflector antennas are very common.



The double bow-tie above has an average gain of 6 dBi. With a bigger screen it would have more. The parabolic reflector focuses the signal onto a single dipole, but its bandwidth is a little disappointing. The corner reflector has a little less gain but much greater bandwidth. The corner reflector has roughly the gain of three dipoles. It is a good medium gain antenna, widely used for UHF. If you need more than 25 dBi then the paraboloid dish is the only practical choice, but they are huge.

Log-Periodic Dipole Arrays (LPDA)

The LPDA has several dipoles arranged in echelon and criss-cross fed from the front. The name comes from the geometric growth, which is logarithmic.



This is a very wideband antenna with a gain of up to about 7 dBi. For any frequency, only about three of the elements are carrying much current. The other elements are inactive. As frequency increases, the active elements “move” toward the front of the array. Most VHF TV antennas are LPDAs.

TV LPDAs come in two types: straight and Vee. The Vee type (LPVA) has a very slightly higher gain for channels 7-13. But this author often favors the straight type since it has nulls 90 to each side that can be used to cancel out interference.


Yagi Antennas

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.



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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Pengarah antena (Director Antenna)

Director antenna digunakan sebagai pemberi arah/direction dari sebuah antena. Director ini merupakan elemen pasif saja, dimana tidak mendapatkan catuan dari sumber.



Antena yang menggunakan jenis director ini biasanya jenis antena array, misalkan saja:
1. Antenna Yagi
2. Antenna Log Periodic.

Fungsi director antena selain digunakan sebagai pengarah,dia juga dapat gunakan memperbaiki faktor gain (penguatan) antena.

Namun semakin banyak director yang ditambahkan belum pasti gain antena semakin besar. Ada titik saturasi dari director antenna ini.

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Reflector antenna

Reflector antenna atau juga bagian antenna yang berfungsi memantulkan sinyal radiasi yang melewatinya. Sehingga diharapkan rugi-rugi antena akan semakin kecil atau perbandingan front to back ratio besar.

Jenis Reflector antenna banyak macamnya, misalkan saja:
1. Jenis plannar




2. Jenis Parabolic





3. Jenis Corner

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Macam Jenis Antena

Antena secara garis besar dibedakan dalam tiga jenis:
1. Bagian yang meradiasikan (driven antenna)
2. Bagian yang memantulkan (Reflector antenna)
3. Bagian yang mengarahkan sinyal radiasi (Director antenna).

Dari ketiga jenis bagian antena tersebut dibahas dibawah ini:
1. Bagian yang meradiasikan sinyal (Driven antenna)
Banyak macam dan jenis yang akan digunakan untuk meradiasikan antena, diantaranya:
a. Jenis Monopole




b. Jenis Dipole



c. Jenis Patch



d. Jenis Horn

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