An Antenna also called an aerial, an antenna is a conductor that can transmit, send and receive electromagnetic signals such as VLH, LF, HF, VHF, UHF, Microwave includes all types of radio or satellite signals. A high-gain antenna increases signal strength, where a low-gain antenna receives or transmits over a wide angle. An antenna is required to radiate and receive the signals and therefore their performance is key to the operation of the overall radio system. If the antenna performance is poor, then it will limit the performance of the overall system. As such, maximizing the performance of the antenna is very important. An understanding of basic antenna theory will help the maximum be gained from any aerial system.
Key antenna theory topics
There are several basic topics that are common to all antenna types and which form part of the basic antenna theory.
Polarization: Radio antennas are sensitive to polarization. In just the same way that electromagnetic waves can be polarized, so to are antennas. It will be seen that some antennas have their elements in a vertical fashion and others are horizontal. This is to accommodate vertical and horizontally polarized electromagnetic waves.
Resonance & bandwidth: Resonance and bandwidth are key issues for antenna theory. Most antennas operate in a resonant fashion and in this way, they are able to perform to their best. Associated with this is the bandwidth over which they are able to operate.
Gain & directivity: Antennas do not radiate equally in all directions – only an isotropic source radiates equally in all directions and this is a theoretical entity only. In some directions practical antennas exhibit gain where the available power is focused in a particular direction, and they have a directional pattern. Antenna theory for directivity and gain is important in many areas.
Feed impedance & matching: The input connection to an antenna presents an impedance to the feeder to which it is connected. For optimum power transfer source and load should be matched. Accordingly, antenna theory associated with the feed impedance is important for the optimum operation of the antenna.
Dipole Antenna
A dipole antenna is the simplest type of radio antenna, consisting of a conductive wire rod that is half the length of the maximum wavelength the antenna is to generate. This wire rod is split in the middle, and the two sections are separated by an insulator. Each rod is connected to a coaxial cable at the end closest to the middle of the antenna.
Radio frequency voltages are applied to dipole antennas at the center, between the two conductors. They are used alone as antennas, especially in rabbit-ear television antennas and as the driven elements in other types of antennas.
ipole provides the best performance if it is more than a half-wavelength above the ground, surface of a body of water or horizontal conducting medium such as sheet-metal roofing. The element should also be a certain wavelength away from electrically conducting obstructions such as supporting towers, utility wires and other antennas.
Dipole antennas are oriented vertically, horizontally or in slants. Polarization of electromagnetic fields radiated by dipole-transmitting antennas correspond to element orientation. Radio frequency (RF) current in dipoles is at its maximum at the centers of the dipole and at its minimum at ends of the element, and vice versa for RF voltages.
Dipole antennas were invented in 1886 by a German physicist named Heinrich Hertz. These antennas are also referred to as a doublet and make up the main RF radiating and receiving element in different sophisticated type of antennas. Dipole antennas are balanced in that they are bilaterally symmetrical, and they are fed with balanced, parallel wire RF transmission lines.
There are three types of dipoles:
Ideal half-wavelength dipole
Folded dipole
Hertzian dipole
HF Loop Antenna
A loop antenna is a type of a radio antenna, which consists of a loop (circular electrical conductor) with ends connected to the transmission line. There are different types of shapes. They are triangular, circular, elliptical, and square shape antennas.
Depends on loop’s circumference the loop antenna is classified as two types electrically small and electrically large.
VHF Yagi Antenna
A Yagi antenna is a directional antenna consisting of a driven element such as dipole or folded dipole and additional parasitic elements, typically a reflector and one or more directors. It radiates in only one direction and is most commonly used in point-to-point communications.
A Yagi antenna is used for communications in a medium range of three to five miles between two points. It can also be used as a bridge antenna to connect clients to an access point.
This term is also known as a Yagi-Uda array or patch antenna.
The Yagi antenna was invented by Shintaro Uda and his colleague Hidetsugu Yagi in 1926. A similar design to the Yagi antenna is found all over the United States and is referred to as a log-periodic antenna.
A Yagi antenna has two to three straight antenna elements, which are set to a length of roughly half the electrical wavelength they are designed to support. It is considered a balanced type but can also be unbalanced depending on whether it is used with a balun at the feed line joint, which joins the drive element of the antenna.
The benefits of the Yagi antenna include good range and ease of aiming the antenna compared to other directional dishes and designs. Since the Yagi antenna is directional, it focuses its entire signal in a cardinal direction. This results in increased gain over an antenna dispersing energy in a 360-degree circle, such as the omni-directional model of other antenna designs.
One disadvantage of the Yagi design is its large size, especially for the range achieved.
VHF UHF Ground Plane Antenna
A ground-plane antenna is a form of dipole antenna designed to work with an unbalanced feed line. A ground-plane antenna is more of less one half of the dipole and mounted above the ground plane. Ease of fabrication and cost makes the ground-plane antenna one of the popular antennas in communication systems. A ground-plane antenna is also known as a monopole antenna. A ground-plane antenna looks similar to coaxial antenna. The lower section of the antenna consists of two or more radials, which are straight elements. The radials measure ¼ of the wavelength and are connected to the shield of the feed line cable or to the outer connector. The main element that is used can be of any length, but is arranged to function in and around a specific frequency. This adjustment in the antenna is done with the help of a tuning coil. The main element is connected to the center conductor.
One feature of the ground-plane antenna is that it is omnidirectional. The horizontal radiation pattern is in the shape of a circle, and the antenna radiates in all directions and in equal measures. However, in the case of its vertical radiation pattern, it has a lower angle, unlike a dipole antenna. At frequencies below or around 50 MHz, this gives ground-plane antenna a longer-range propagation advantage. In order to get a directional antenna with gain, an arrangement of two or more ground-plane vertical antennas can help. However, the ground-plane antenna has a narrow bandwidth.
VHF UHF OMNI directional Antenna
An omnidirectional antenna is a wireless transmitting or receiving antenna that radiates or intercepts radio-frequency (RF) electromagnetic fields equally well in all horizontal directions in a flat, two-dimensional (2D) geometric plane. Omnidirectional antennas are used in most consumer RF wireless devices, including cellular telephone sets and wireless routers.
In theory, a vertically oriented, straight conductor such as a dipole antenna measuring no more than 1/2 wavelength from end-to-end always exhibits omnidirectional properties in a horizontal (azimuth) plane. Multiple collinear (in-line) vertical dipoles also exhibit omnidirectional behavior in the azimuth plane; they can offer improved performance over a single dipole in some applications.
If the conductor axis is not oriented vertically, then the antenna radiates and receives equally well in all directions in the plane through which the conductor passes at a right angle. However, this ideal state of affairs exists only in the absence of obstructions or other nearby conducting objects. In practice, surrounding objects (such as the user of a cell phone set or a computer next to a wireless router) distort the radiation and reception pattern. In the usual context, a so-called omnidirectional antenna does not perform equally well in all possible directions in three-dimensional (3D) space. Such a device, which can exist only in theory but can be approached in practice, is called an isotropic antenna or isotropic radiator.
Antennas that offer enhanced performance in some directions, at the expense of other directions, are called directional antennas or directional radiators. The most common example is the dish antenna used with satellite internet, satellite television, and space-communications installations. Other examples include the Yagi antenna, quad antenna, billboard antenna, and helical antenna.
Rubber Duck Antenna
The rubber ducky antenna (or rubber duck aerial) is an electrically short monopole antenna that functions somewhat like a base-loaded whip antenna. It consists of a springy wire in the shape of a narrow helix, sealed in a rubber or plastic jacket to protect the antenna.
Rubber duck antennas on hand-held radios are a severe compromise on efficiency. On the plus side is their short size and flexible forgiveness to brutish handling. On the negative side is their terrible radiation inefficiency, probably worse than many of you expected.