Introduction
A usually metallic device (as and or wire) for radiating or receiving radio waves, is a definition of an antenna according to Webster`s Dictionary.
Let`s see what IEEE standard says about it?
The IEEE standard (145- 1983) says “a means for radiating or receiving radio waves.” Generally, it is the transitional structure between free space and a guiding device, figure shown as below:
A high gain antenna increases signal strength, where a low gain antenna receives or transmits over a wide angle.
TYPES
We can divide types as directional and non – directional.
Let’s see about non-directional are simple and they are basic diploes and monopoles.
The vertical dipole or vertical monopole could be considered one of the best antennas for LMR applications. It is unidirectional. Below figure shown it is half a dipole placed in half-space, with a perfectly conducting, infinite surface at the boundary.
Now let`s describe types of antenna on a directional base:
1). Wire antennas
2). Aperture antennas
3). Microstrip antennas
4). Array antennas
5). Reflector antennas
6). Lens antennas
Let me take one by one:
1). Wire antennas: are commonly used antenna, in such as automobiles, buildings, ships, aircrafts, spacecrafts, and so on. Shapes may be changed like straight wire (dipole), loop and helix. See figures below:
2). Aperture antennas:
These types of antennas are very useful for aircraft and spacecraft applications, because they can be very conveniently flush-mounted.
Watch out figures below:
3). Microstrip antennas: are used for government and commercial applications and they consist of metallic patch on a grounded substrate. They are low profile, comfortable to non-planer & planer surfaces, simple and inexpensive to fabricate using modern printed-circuit technology. They can be mounted on surface of aircraft, spacecraft, satellites, missiles, cars and even mobile telephones.
4). Array antennas:
Instead of single element, we use aggregation of radiating elements in an electrical & geometrical arrangement means an array, so it will result in the desired radiation characteristics. Typical examples of arrays are shown as below:
According to Josefsson and Persson, there is also one type named as conformal array, which defined as antenna that conforms to a surface whose shape is determined by considerations other than electromagnetic. Other means Aerodynamic and Hydrodynamic.
Figures are conformal array antenna, Microstrip conformal array antenna and conical conformal array antenna, respectively, in which last one is used for data communication from satellite.
5). Reflector antennas: these types of antennas are designed to be used in order to transmit & receive signals that had to travel millions of miles, it can be built with 305m large as diameter & such large dimensions are needed to achieve high gain.
Two types of reflectors are there:
1). Parabolic antenna & 2). Corner reflector antenna and they are shown as:
The disadvantage in this type of antenna is fading due to snow accretion in some northern. And temperature below 0 C, fading going to be longer.
6). Lens antennas:
By properly shaping the geometrical configuration and choosing the appropriate material of the lenses, they can transform various forms of divergent energy into plane waves. Some forms are shown on last page.
But what is the definition of ideal one?
So, Ideal antenna will radiate all the power delivered to it from the transmitter in desire direction or directions.
These are general concepts about antennas, now let`s get deeper.
The Gain Function:
The field set up by any radiating system can be partitioning into two components: the induction field and the radiation field.
The induction field is only important in the immediate locality of the radiating system, but at large distances the radiation field is dominant.
The antenna is treated as an effective point source, radiating power that, per unit solid angle, is a function of direction only.
The directive properties of an antenna are most conveniently expressed in terms of “Gain function” G (θ, ø).
Where θ is elevation angle and
Ø is azimuth angles in a set of polar co-ordinates centered at the antenna.
Let P (θ, ø) be the power radiated per unit solid angle in direction θ, ø and P, the total power radiated.
G (θ, ø) = P (θ, ø)/Pt/4π
Means the gain function is the ratio of the power radiated in a given direction per unit solid angle to the average power radiated per unit solid angle.
The Receiving Cross section for the performance of an antenna as a receiving device.
The amount of energy absorbed in the load (the detector) will depend on the orientation of the antenna, the polarization of the wave and the input impedance match in the receiving system.
The absorbed power can be described as the power incident on an effective absorbing area, which called as “the receiving cross section” or “absorption cross section” Ar, of the antenna. And let S as the power flux density in the incident wave, the absorbed power Pr is,
Pr = S* Ar
But, now you are thinking, that`s OK, but what is input antenna impedance?
So, input antenna impedance is the effective resistance to an alternating electric current presented by the antenna at its terminals.
And what about antenna Polarization?
Ok, it is the polarization of the wave radiated by the antenna in a given direction.
If the electric and magnetic field vectors of an electromagnetic wave lie in a fixed plane at all times, it is called plane polarized wave.
Now let`s discuss the radiation mechanism of one. Let me take dipole antenna as an example, where the time of travel is neglible.
I am trying to explain the mechanism by which electric lines of force are detached from the antenna to form the free-space waves.
The figures below display the lines of force created between arms of a small center-fed dipole in the first quarter of the period during which time the charge has reached its maximum value and the lines have traveled outwardly a radial distance λ/4 and the number of lines formed three. During the next quarter of period, the original 3 lines travel an additional λ/4(a total of λ/2 from the initial point) and the charge density on the conductors begins to diminish. The lines of force created by the opposite charges are three and travel a distance λ/4
during the second quarter of the first half and they are shown dashed in figure (b). The end result is that there are three lines of force pointed upward in the first λ/4 distance and the same number of lines directed downward in the second λ/4. As shown in figure (c), there is no net charge on it, then lines of force detaching from the conductors and to unite together to form closed loops. In the remaining second half of the period, the same procedure is followed in opposite direction.
Historical Aspects:
Prior to World War II, most antenna elements were of the wire type (long wires, dipoles, helices, rhombuses, fans, etc.) and they were used either as single elements or in arrays.
During and after World War II, many other radiators, some of which may have been known for some and others of which relatively new, were put into service.
This created a need for the better understanding and optimization of their radiation characteristics.
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