 Myself Piyusha Shedgar, Assistant Professor, Walchand Institute of Technology, Solapur. In today's session we will discuss the modes of helical antenna. These are the learning outcomes for today's session. At the end of this session, students will be able to differentiate between different modes of helical antenna. They will be able to list the applications of helical antenna. These contents will be discussed in today's session. So before going to start actually the modes of a helical antenna that is actual working of the helical antenna, you must know what is the helical antenna. So you can pause video here for a second and recall that what is helical antenna. Yes, the helical antenna or it is also called as a helix antenna is the antenna in which the conducting wire is wound in helical shape and connected to the ground plate with a feeder line. Now this is the structure of a helix antenna or helical antenna. So the helical antenna which having the shape of a helix that is the number of turns can be used in shape of a helix with the spacer are provided between each turn. The helical aerial element is used to make the number of turns. The central support is provided at the centrally which is also called as a axis of the helix and the base is used at the left side. So it is also called as a reflector or the ground plate with the feeding line. So here the coaxial cable is used. The inner conductor is connected to the helix whereas the outer conductor is connected to the ground plane. There are the different types of the ground planes. So according to the structure of the helix for designing of the helical antenna to get a proper radiation pattern these elements are considered these elements are nothing but the spacing between the turn, length of the turn or the length of the helix and the circumference. So circumference depend upon the diameter of the helix. Diameter of the helix is given by the capital D and the circumference is given by this capital C. C is given by the equation pi times of the D whereas alpha is nothing but the peach angle. Peach angle made between this normal line of the circumference and its perpendicular to the consideration with the length of the helix. Here the spacing D and the C that is circumference and the diameter of the helix antenna decides the alpha value. So by using the Pythagoras theorem you can write the equation for the length of the helix. It is given by under root s square plus c square whereas the alpha can be calculated by taking the tangent value of the alpha tan alpha is equal to the opposite side by adjacent side. So s upon c where c is given by the pi times of D. So alpha is equal to tan inverse of s by pi D or it is also can be written as tan inverse of s upon c. So this is the structure of the helix antenna. Now consider the different modes of operation of the helical antenna. So there are the different modes of helical antennas but generally the two main modes of operations are there. So one is the normal mode of operation whereas the axial mode of the radiation of helical antenna. So normal mode it is also called as the perpendicular mode of radiation whereas the axial mode is also called as a end fire or beam mode of radiation. Now what is actually the normal mode? In normal mode the radiation field is normal to the helix axis. So as you know that the helix axis is provided at the centre to the number of turns the major radiation is perpendicular to this helix axis with the minor lobes are also present. Radiated waves are circularly polarized. It is the property of the helical antenna. It is having the circular polarization that is the EM wave which having the horizontal as well as the vertical components. Mode of the radiation is obtained if the dimension of the helix are small as compared to the wavelength. So for obtaining the proper radiation this should be the condition for the normal mode of operation for the helical antenna. For the normal mode of radiation dimension of the helix are must be small as compared to the wavelength whereas the radiation pattern of this helical antenna is the combination of short dipole antenna and the loop antenna. So actually how the radiation for the normal mode of operation is shown in this figure. So suppose the helical antenna which having the number of turns in this direction with the axis of the helix is provided at the centre then for the normal mode of operation the major radiation pattern which is perpendicular to the axis of the helix. Therefore at the 90 degree and 270 degree you are getting the major loop. Obviously the minor lobes are also present here. So from this you can says that the radiation pattern depends upon the values of the diameter of the helix denoted with the capital letter D and the spacing between the turn that is capital S. So what is actually the drawback of this mode of operation? So it having the low radiation efficiency along with the narrow bandwidth therefore very hardly it is used. Next mode of operation is the axial mode. In axial mode of operation the radiation is in the direction of the helical axis whereas in case of the normal mode it is perpendicular. So the major radiation is in the direction of the helical axis itself. Waves are again same the circularly or nearly circularly polarized in case of the axial mode. In axial mode the dimension is obtained by raising the circumference in order to get one wavelength and the spacing of approximately should be lambda by 4. So this is how the pattern for the axial mode is defined. So here the major radiation is in the direction of the axis of the helix. So antenna can be designed for the right-handed or the left-handed circularly polarized wave. So whenever the antenna is designed for right-handed circularly polarized well it will not receive the left-handed circularly polarized wave and vice versa. So mode of operation is generated with greater is and therefore more practically this type of a mode of operation is used. So for the proper radiation the circumference of the helix should be greater than 3 lambda by 4 and it should be less than 4 lambda by 3. So the condition for the proper radiation for this type of a mode of operation the circumference should lie between 3 lambda by 4 to 4 lambda by 3. Some of the advantages of the helical antennas, the helical antennas are simple to design. It having the highest directivity along with the wider bandwidth. It can achieve the circular polarization. It can be used for the ultra high frequency very high frequency and high frequency bands also. Along with the advantages some of the disadvantages of the helical antennas. As you know that the helical antennas are having the larger shape so it requires the most space and the efficiency decreases with the number of turns of the helical antenna. These are the applications the single helical antenna or its array is used to transmit and receive the very high frequency signal. It can be frequently used for satellite and the space communication purpose. It is used for the telemetry links with ballistic missiles and satellites at earth stations whereas it also used to establish communication between the moon and the earth. Again one of the application in radio astronomy thus the helical antennas can be used for the different applications. These are the references used for today's session. Thank you.