Antenna Theory - Wire

Wire antennas are the basic types of antennas. These are well known and widely used antennas. To have a better idea of these wire antennas, first let us have a look at the transmission lines.

Transmission Lines

The wire or the transmission line has some power, which travels from one end to the other end. If both the ends of transmission line are connected to circuits, then the information will be transmitted or received using this wire between these two circuits.

If one end of this wire is not connected, then the power in it tries to escape. This leads to wireless communication. If one end of the wire is bent, then the energy tries to escape from the transmission line, more effectively than before. This purposeful escape is known as Radiation.

Transmission Lines

For the radiation to take place effectively, the impedance of the open end of the transmission line should match with the impedance of the free-space. Consider a transmission line of a quarter-wave length size. The far end of it is kept open and bent to provide high impedance. This acts as a half-wave dipole antenna. Already, it has low impedance at one end of the transmission line. The open end, which has high impedance, matches with the impedance of free space to provide better radiation.

Dipole

The radiation of energy when done through such a bent wire, the end of such transmission line is termed as dipole or dipole antenna.

The reactance of the input impedance is a function of the radius and length of the dipole. The smaller the radius, the larger the amplitude of the reactance. It is proportional to the wavelength. Hence, the length and radius of the dipole should also be taken into consideration. Normally, its impedance is around 72Ω.

This is better understood with the help of the following figure.

Dipole Half-Wave Antenna

The figure shows the circuit diagram of a normal dipole connected to a transmission line. The current for a dipole is maximum at the center and minimum at its ends. The voltage is minimum at its center and maximum at its ends.

The types of wire antennas include Half-wave dipole, Half-wave folded dipole, Full-wave dipole, Short dipole, and Infinitesimal dipole. All of these antennas will be discussed in further chapters.

Comments

Post a Comment

Popular posts from this blog

Gate Questions paper 2022 with answer

Image
1:-  In the circuit shown below, the initial charge on the capacitor is 2.5 mC, with the voltage polarity as indicated . The switch is closed at time t = 0 the current it at a time t after the switch is closed is  (A) i(t)=15 exp(-2×10³t)A. (B) i(t)= 5 exp(-2×10³)A. (C) i(t) =10 exp(-2×10³)A. (D) i(t) = -5 exp(-2×10³)A. ANSWER :-   i(t)=15 exp(-2×10³t)A 2:- The Z- parameter matrix [z¹¹z¹² z²¹z²²] for the two port network shown is (A):- 0 (B):-  99 (C):- 63 (D):-  12 ANSWER:-               0 3:- If Ē = -(2y³-3yz²)x^ -(6xy²-3xz²)6y^+(6xyz)z^ is the electric field in a source free region a valid expression for the electrostatic potential is (A):- xy³-yz² (B):- 2xy³-xyz² (C):- y³+xyz² (D):-  2xy³-3xyz² ANSWER:-         2xy³-3xyz² 4:-  Two Silicon diodes,with a forward voltage drop of 0.7 v are used in the circuit shown in the figure. The range of input voltage...
Read more

Antenna Theory - Short Dipole

Image
A  short dipole  is a simple wire antenna. One end of it is open-circuited and the other end is fed with AC source. This dipole got its name because of its length. Frequency range The range of frequency in which short dipole operates is around 3KHz to 30MHz. This is mostly used in low frequency receivers. Construction & Working of Short Dipole The  Short dipole  is the dipole antenna having the length of its wire shorter than the wavelength. A voltage source is connected at one end while a dipole shape is made, i.e., the lines are terminated at the other end. The circuit diagram of a short dipole with length L is shown. The actual size of the antenna does not matter. The wire that leads to the antenna must be less than one-tenth of the wavelength. That is L &lt; &#x03BB; 10 " role="presentation"> L < λ 10 L < λ 10 Where L  is the length of the wire of the short dipole. λ  is the wavelength. Another type of short dipole is infinitesimal dip...
Read more