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Category : Antennas
Antenna basic theory
2011-08-22 20:41:59 - by : admin

An antenna is “a usually metallic device (as a rod or wire) for radiating or receiving radio wave”. [Webster’s dictionary]
Antenna radio may be defined as the structure associated with the region of transition between a guided wave and a free space wave or vice versa. [JOHN D KRAUS ]
The antenna or aerial is “a means for radiating or receiving radio waves”. [IEEE Std 145-1983]
Antenna is the transitional structure between free-space and a guiding device [Prof. Constantine A. Balanis], as shown in figure 1.1 below
 antenna as transition devices
Examples of this guiding devices are coaxial line cable and wave guide cable (hollow pipe) , and it’s transport the electromagnetic energy from transmitting source generator to the antenna or from antenna to receivers.

The easy way to understand antenna concept, we can look the antenna as Thevenin’s transmission line. The source is represented by an ideal generator Vg, transmission line is represented by a line with characteristic impedance Zo, and the antenna is represented by load ZA while the formula of  [ZA = (RL+RR) + jXA]
It’s shown In Figure 1.2.
antenna and transmission line

In the practical system there are conduction dielectric losses due to the lossy nature of transmission line and the antenna as well as due to reflections (mismatch) in the interface losses between transmission line and antenna. The reflected power or transmitted power from the antenna will produce VSWR (Voltage Standing Wave Ratio), here tell about how much power will be reflected to the source and how much transmitted power out from antenna.

Antenna radiation pattern is a picture of the distribution of the signal emitted by an antenna is measured from the reception signal received by an antenna at each receiver to the antenna direction.
Radiation pattern will form a three-dimensional transmitting power so it can be described into two two-dimensional field that is vertically and horizontally
Both of the above pattern will form three-dimensional pattern. There are some radiation pattern that is formed from several types of antennas.
Isotropis antenna radiation pattern will form into a ball, will form a dipole antenna radiation pattern such as donuts, directional antenna will have a certain radiation pattern containing the main loops, side loops and back loops as shown in figure 1.3
 antenna radiation patern
Another popular antenna specification is the "front-to-back" (F/B) ratio. It is defined as the difference in dB between the maximum gain or front of the antenna (usually 0 degrees) and a point exactly 180 degrees behind the front. The problem with specifying only the F/B ratio is that it does not account for any lobes in the rear two quadrants

Beam width antenna is usually a benchmark for measuring the width of degrees that would be obtained if the received power at 3 dB below the maximum power received. Maximum power must be in main loop so that at the point of intersection between the radiation pattern with a power worth 3 dB below the maximum power that we can determine the antenna beam width
Beam width divided two part also correspond to the radiation pattern of vertical and horizontal, depiction can be seen in Figure 1.3

The gain of an antenna (reffered to a lossless isotropic source) depend on both directivity and its efficiency. If efficiency is not 100%, the Gain is less than the directivity, thus the GAIN

G = kD (dimensionless)

Where k = efficiency factor of antenna (0 ≤ k ≤1). dimensionless
          G = antenna gain
          D = directivity

Here’s a various antenna types in order to get a glance as to what will be encountered in the remainder of the next articles.

  • Wire Antenna

  • Aperture Antenna

  • Micro strip Antenna

  • Array Antenna

  • Reflector Antenna

  • Lens Antenna

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