Introduction to antennas
Traditionally, people had problem with communication especially where it involved long distances. Wireless communication began in 1901, when Marconi received the first radio signal across the Atlantic Ocean. From that period, technology kept on advancing until the development of the present day antennas. Antennas form the basic components of any communication system that changes energy from one form into another. A receiving antenna changes electromagnetic energy into electric or magnetic energy. They work through transmitting RF signal that travels on a conductor into an electromagnetic wave in free space. An antenna maintains the same characteristic regardless of whether in use or not. In order for an antenna to work, the frequency of the receiving band of the radio must be similar to the radio where it is connected to avoid transmission impairment. Once a signal is fed into an antenna, it emits space-distributed radiations in a certain way in a geographical representation referred to as radiation pattern (v 2001).
Basics of antennas and terms used
The quarter wave vertical forms the most basic antenna that has a quarter wavelengths long with a vertical radiator. Some good examples of basic antenna are found in vehicles and are used in two-way communication. In addition, the isotropic radiator forms another basic antenna that radiates wavelengths in all directions. This type of antenna is used as a standard for comparison with other antennas. Several terms are used while describing an antenna. These are;
Input impedance: in order for an antenna for work efficiently, the input impedance of the radio and the transmitting cable that connects the two must be the same. For example, a trans-receiver with 50 ohms impedance must operate on a transmission line with 50 ohms impedance.
Bandwidth: this is the range of frequencies that an antenna is allowed to operate. It is calculated using the below formula,
BW = 100 x FH-FL FC (1)
where;
FH is the highest frequency in the band,
FL stands for the lowest frequency, and
FC is the centre frequency of the band.
Directivity and Gain: The antenna has the ability to focus energy in a certain direction during transmission and receive energy from a particular direction. The direction of an antenna can be used to get directivity and concentrate a radiation beam in any direction.
Types of antennas
The classification of antenna is based on the following ;
Frequency and size
Antennas used for VHF frequencies differ from those used for HF frequencies, and not differ from antennas used for microwaves. The wavelength of an antenna is different at various frequencies that make them come in different sizes in order to radiate signals at the expected wavelength.
Directivity
Antennas classification falls under three categories depending on the direction of transmission. The Omni-directional antennas transmit receive transmissions from all corners in a 360 degree pattern. These include the Dipole-Type and Ground Plane. The sectional antennas radiate wavelengths from a specific direction. The wavelength beam ranges between 60 degrees and 180 degrees. The third type is called directive antenna and a beam width narrower than of sectional antenna. Directive antennas have the highest gain and are used for long distance transmissions (Harish & Sachidananda, 2007).
How an antenna operates
An antenna operates with electromagnetic spectrum. The wavelength of the transmitted wave relates to the frequency under the following equation;
Wavelength = 3 x 10^8 / frequency .. (2)
Transmission process
A transmitting antenna is connected with a signal generator that gets the audio or visual input and converts them into an AC signal. The AC signal is taken into a transmitting antenna that radiates an electromagnetic wave having the same properties as the input signal. The receiving antenna operates opposite to the transmitting antenna. An induced current in the receiving antenna is translated into audio or visual information making the transmission process complete (123HelpMe.com, n.d).
Antenna applications
Antennas find applications in millions of ways but some of the most important applications will be discussed below. The first application of antennas is in the United States Navy’s Extreme Low Frequency (ELF) system. US Navy operates two antennas, the one in Clam Lake, Wisconsin and the other in Republic, Michigan. The two antennas are dipoles working at a frequency of 76 Hz and using over 80 miles of wire. The main reason for using low frequency in the Navy is to allow the signal to penetrate seawaters thousands of feet in order to reach submarines. The second application of antennas is wireless communications. The most common used types of antennas for this application are the quarter wave helical and quarter wave whip. The Whip antennas are commonly used in cellular phones, and uses frequencies of range between 400 and 500 MHz. the quarter wave helical antenna is small but also works the same as whip. It uses frequencies of 800 to 1000 MHz range (Carr, 2001).
References
Carr, J. J. (2001). Practical antenna handbook (4th ed.). New York: McGraw-Hill.
Harish, A. R., & Sachidananda, M. (2007).Antennas and wave propagation. New Delhi: Oxford
Huang, Y., & Boyle, K. (2008). Antennas: from theory to practice. Chichester, UK: John Wiley
& Sons Ltd.
123HelpMe.com. (n.d). The Physics of Basic Antenna Theory and Design. Retrieved July 22,
2013 from: http://www.123HelpMe.com/view.asp?id=153286.
