Antenna Theory: Introduction
Introduction to Antennas:
In radio engineering, an antenna is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. A few definitions of an Antenna are as follow:
“An antenna is a device which either transmits electromagnetic (EM) waves into the space around it, or receives radio waves from that space”.
“An antenna is a transducer of electrical and electromagnetic energy”.
“An antenna is a transducer that converts radio frequency fields into alternating current or vice versa. There are both receiving and transmission antennas for sending or receiving radio transmissions”.
“That portion, usually wires or rods, of a radio transmitter or receiver station used for radiating waves into or receiving them from space. It changes electrical currents into electromagnetic waves, and vice versa”.
It is possible to make a simple, working antenna from a simple piece of wire. On the other hand some antenna designs are very complex devices with multiple, precised dimension conductors which are spaced at precise distances from one another. It is usually used with a radio transmitter or radio receiver. In transmission, a frequency alternating current to the antenna’s terminals, and the antenna radiates the energy from the current as electromagnetic waves or radio waves. In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, which is applied to a receiver to be amplified.
What is a smart antenna?
A smart antenna system combines multiple antenna elements with a signal processing capability to optimize its radiation and/or reception pattern automatically in response to the signal environment. Beam the radio signal directly at a user to follow the user as they move.
Important Parameters while designing an antenna:
• Operating frequency and bandwidth
• Input impedance that varies with frequency
• Radiation pattern or Gain of antenna
• Power handling capacity
• Size and weight
• Vulnerability to weather and physical abuse
Applications of Antennas:
|RADAR||Phased array RADAR, air traffic control, synthetic aperture RADAR|
|SONAR||Source location and classification|
|Communications||Smart antenna systems, Directional transmission and reception, sector broadcast in satellite communications|
|Imaging||Ultrasonic, optical, tomographic|
|Geophysical Exploration||Earth crust mapping, oil exploration |
|Astrophysical Exploration||High resolution imaging of universe|
|Biomedical||Neuronal spike discrimination, fetal heart monitoring, tissue hyperthermia, hearing aids|
Working Principle of an Antenna:
The working principle of an antenna is that it converts electrical currents (carried along by metallic conductors) into EM radiation in free space so they can travel sometimes thousands of kilometers around the Earth or even into space and back — or vice versa. Therefore, an antenna is used to both transmit and receive EM waves.
How does an antenna work?
According to NASA (National Aeronautics and Space Administration) report, there are two different types of antennas:
- High Efficiency (HEF) and
- Beam Waveguide (BWG).
Both antennas receive and transmit information to and from the satellites in space but are different in how they receive, transmit and process the data.
High Efficiency (HEF)
Information from satellites comes in the form of an analog signal – a continuous wave of a radio signal on specific frequencies. That signal is captured in the bowl part of the antenna and then feeds into the subreflector of the antenna (the small part at the top) where that energy from that signal becomes focused and finally proceeds into the receiver. The information then travels to a computer where it is processed into binary code – a computer language of ones and zeros. That data then travels to another computer where it translates the information into pictures and/or information that we can read and see.
The HEF antenna is different from the BWG antenna in that it only transmits on specific frequencies in the antenna feeds. In order for the HEF to transmit on another frequency, a new antenna feed would need to be added. The feed is very difficult to access because it is very high up on the dish and requires other special equipment.
Beam Waveguide (BWG)
Like the HEF, analog signals from satellites bounce off the dish and focus on the subreflector. Unlike the HEF, the BWG has a series of five mirrors which reflect the radio signals along the beam waveguide tube to the pedestal room below ground that houses the computer equipment where the signal is translated on the spot to information we can read and see.
The BWG allows for multiple frequencies to be transmitted and received by the turning of the mirror in the pedestal room. The antenna is much easier to maintain and access than the HEF and provides a variety of support to many different missions in space.