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In radio communication system the receiving antenna is linked to the transmitting antenna through the electromagnetic wave. This arrangement is somewhat similar to that we find in transformer circuits. In the case of transformer, the coupling is strong and the field involved is entirely magnetic. In the case of antenna, however, the coupling is weak and the field involved is electromagnetic.
The antenna coupling system can be represented by a four terminal network. This representation is very useful because we can then apply the well-known network theorems to solve antenna problems. The important general results obtained there by are applicable to all kinds of antennas. A network theorem which is particularly useful in antenna theory is the reciprocity theorem.
 
Fig shows a high frequency generator, connected to a two parallel wire transmission line. If the line is kept sufficiently far away from any metallic or conducting objects, equal and opposite current will flow in the two wires at any given position on the line. Therefore, at any appreciable distance, from the line, the field effects of two wires will almost cancel each other. The end of the line, remote from the generator, is sorted with a straight segment of wire. The field effect of this wire may be observed at any distance remote from the line, because there is no source of equal and opposite fields to cancel them.
Elemental Antenna

This shorted segment is known as elemental antenna.
 
Radiation resistance:
When a high frequency current flows through an antenna, there is also an energy-loss due to the radiation of electromagnetic energy. This in turn, concludes that the transmission line, feeding the elemental antenna, must see a resistance component of load. If the load were all-resistive, the average power delivered to the antenna would have to be zero. It is un-important that our energy is not dissipated as heat. There is still an energy-loss to be accounted for and the amount of energy lost is given by

Pavg=789(dl/λ)2I02 or Pavg=789(dl/λ)2Irms2 or Pavg=˝RradI02
The radiation resistance of elemental antenna is R rad =789(dl/λ)2


Antenna Directivity And Gain:
Directivity and gain is two very useful terms in Antenna System. For a particular direction the ratio of power per unit solid angle to the power per unit solid angle for a Reference isotropic antenna is called the Directivity. Gain is the ratio of maximum radiation intensity of an antenna to the maximum radiation intensity of a reference antenna provided both antennas have the same power input.


Polarization:
Polarization is the direction of electric field of the incoming electromagnetic waves.


The Half-Wave Dipole
There is only one part of a receiving aerial that is active, i.e. does the receiving and is connected to the TV/radio set. This active element is called the dipole. The simplest design of antenna would consist of a dipole only:

Dipole Antenna
          A half-wave dipole
In the diagram above, there are two wires marked 'to receiver.' For UHF and VHF, one wire will be the copper-core and the other the copper braiding of a co-axial cable.
Before we precede, a quick word about gain. Although having a technical definition, for us 'gain' can mean "the effectiveness with which a receiving aerial receives a signal."
The diagram below shows the reception pattern of a half-wave dipole. The blue area is where the gain is higher than a certain value; the dipole is in the center:

Dipole Antenna Polar Diagram
We can change the directivity of the aerial by adding other elements. Any other elements that we add to the basic half-wave dipole are called passive elements and are not connected electrically to the dipole.
There are two types of passive elements:


Directors


Directors alter the directivity of the aerial so that the aerial's gain is improved in front of the dipole. Most aerials have more than one director, and the more directors the aerial has the better the aerial is at picking out the signal from the required source and rejecting signals from other angles.

Dipole and Directors
These diagrams do not show the cross-bar that holds all the elements in place as it does not much affect the characteristics of the aerial.
The spacing between the directors, diameter of the tubing used and the spacing between the first director and the dipole are important in practice but will be disregarded here. The length of the directors governs the bandwidth of the aerial (over which channels it is effective), but suffice it to say that it is about 75% the length of the dipole.
The gain of the dipole with directors in place looks like this:

Dipole/Director Polar Diagram
Notice how the gain is now more focused in the direction of the directors.
As stated earlier, the more directors an aerial has the more focused the gain is in the direction of the directors. Every new director added becomes less effective though, and in practice it is only worth adding 18-20 directors to the aerial, as any more than this wouldn't increase the gain very much.
On the diagram above, the aerial still has some gain at the rear - in other words, it can still receive signals from behind. This is known as a low front-to-back ratio.


The Reflector
To improve the front-to-back ratio we can add the second type of passive element, a reflector. The reflector reflects signal coming in from the back of the aerial whilst improving the forward gain.

Yagi Antenna
This design is called a Yagi-Uda array, after its creators.
Again, the length, size and position of the reflector affect the aerial's properties, but we won't go into that here.
The reflector can take the shape of a metal plate (with holes in it, making the aerial more impervious to wind) or several rods spaced equidistant from the center of the dipole.
The result is that there is less gain behind the aerial and more, where we want it to be, in front:
Yagi Polar Diagram


Folded Dipoles
In order to minimize signal loss it is important that the impedance (a sort of resistance for AC) of the dipole matches that of the feeder cable and the receiving set.
The impedance for the type of dipole discussed above is about 75 ohms. More often than not though the impedance needs to be altered to match the cable and receiving set characteristics.
This change of impedance is achieved by folding a rod over so that its folded length is still half-a-wavelength:
Folded Dipole
Now we know what each constituent part of an aerial is called and what its function is, let's look at some examples in the field.
 
Monopole Antenna:
Consider a transmitter perpendicular to the ground. The electrons in the antenna, when a signal is applied, are changing their velocities continuously (i.e. moving up and down very quickly) in response to the applied signal.
For a station that broadcasts at a wavelength of 1500m, the antenna needs to be 750m long. This is because there is a 'virtual antenna' caused by the aerial being earthed in the ground:
  Virtual Antenna
The transmitting aerial (and the receiving aerial) need only be half-the-wavelength tall.
Now if this transmitter has no directional properties (i.e. it radiates in all directions equally), it has a coverage area, assuming completely flat ground that is a perfect circle:
Circular Coverage Area
(View from above - antenna in center; blue is coverage area)
 
 

Electromagnetic wave propagation
 


Different mechanisms are involved in the propagation of radio waves from transmitting to receiving antennas, the important ones being:
 

1. Ground wave or Surface wave propagation
2. Space wave or Tropospheric wave propagation
3. Sky wave or Ionospheric wave propagation

 

Ground wave or Surface wave propagation:


Due to the presence of the ground, near the transmitting and receiving antennas, the propagation of the ground waves takes place along the surface of the earth. In the case of long and medium wave signals, the ground wave propagation is common. Daytime reception of all radio signals is possible due to the ground wave propagation.
 
Space wave or Tropospheric wave propagation:


The portion of the earth’s atmosphere situated in the first 15 km adjacent to the earth’s surface is known as the earth’s Troposphere. The propagation of the space wave takes place through the earth’s troposphere. In case of radio waves from television, radar and frequency modulated transmitter, where the frequencies are above 50Mhz, the tropospheric space waves are the important means of radio communications.
 
Sky wave or Ionospheric wave propagation:


An ionized region situated at height of 90km or more is known as the “Ionosphere”, which contains electrons, positive ions and neutral atoms. The sky wave propagation takes place due to reflection of the radio wave from the lower surface of the ionosphere and earth’s surface. All long distance radio communications are possible due to the sky wave reflection from the ionospheric and as well as reflection from the satellites.

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