5.3 Antennas

Antennas play a crucial role in radio communications by converting electrical signals into radio waves for transmission and converting received radio waves back into electrical signals. They are designed to operate efficiently at specific frequencies, which is directly related to their physical dimensions.

Purpose of Antennas:

• Transmission: When transmitting, an antenna takes the electrical signal from the transmitter and radiates it as electromagnetic waves into the air.
• Reception: When receiving, an antenna captures electromagnetic waves from the air and converts them back into electrical signals for the receiver to process.

Relationship Between Antenna Length and Frequency: The length of an antenna is inversely related to the frequency of the radio waves it is designed to transmit or receive. This relationship is based on the principle that the wavelength of a radio wave is the distance it travels in one complete cycle. The formula for the wavelength (λ) in metres is given by:

​

where c is the speed of light (approximately 300,000,000 metres per second) and  is the frequency in hertz (Hz).

Common Antenna Types and Their Lengths:

1. Half-Wave Dipole Antenna:
   • Length:  ​
   • This is one of the simplest and most commonly used antennas. Its total length is half the wavelength of the operating frequency.
2. Quarter-Wave Vertical Antenna:
   • Length:  ​
   • This type of antenna is a quarter of the wavelength long and is often used for its simplicity and ease of installation.
3. Yagi-Uda Antenna:
   • Length: Varies with elements (directors, reflectors) designed for directional gain.
   • This type of antenna is commonly used for its high gain and directional properties, with multiple elements cut to specific fractions of the wavelength.
4. Folded Dipole Antenna:
   • Length: λ
   • This is a variant of the dipole antenna, where the elements are folded back, providing a broader bandwidth.

Practical Application:

• Calculation Example: For a frequency of 100 MHz (megahertz), the wavelength is calculated as:

Thus, a half-wave dipole antenna for 100 MHz would be approximately 1.5 metres long.

Understanding the relationship between antenna length and frequency is essential for designing and installing antennas that perform optimally at the desired frequencies.