The length of an antenna is often chosen to be a fraction of the wavelength to achieve efficient transmission or reception of electromagnetic waves. The specific value of 0.25 (or 1/4) of the wavelength is associated with a type of antenna called a quarter-wave antenna or quarter-wave monopole.
Quarter-wave antennas are widely used in radio communication systems because they exhibit desirable characteristics, such as impedance matching and radiation pattern. Here's a brief explanation of why the length of a quarter-wave antenna is approximately 0.25 of the wavelength:
Reflection and Current Distribution: When an electromagnetic wave encounters an antenna, a portion of the wave is reflected, and the remaining energy is absorbed or radiated. For an antenna to efficiently radiate or absorb energy, it needs a proper distribution of electric current along its length.
Voltage and Current Nodes: A quarter-wave antenna is designed to have voltage and current distribution that results in a standing wave pattern along its length. At the open end of the antenna (the top), there is a voltage node, meaning the voltage is at its minimum, while at the base of the antenna (where it is connected to the transmitter or receiver), there is a current node, meaning the current is at its minimum. This configuration provides a good match between the antenna and the transmission line.
Impedance Matching: Quarter-wave antennas have a characteristic impedance that matches the impedance of the transmission line or the feeding circuitry, typically 50 ohms. This impedance matching ensures efficient transfer of power between the antenna and the connected system, minimizing signal loss.
By selecting a quarter-wavelength length for the antenna, the voltage and current distribution along its length align properly, leading to efficient radiation or reception of electromagnetic waves. The 0.25 wavelength length is an approximate value, as other factors like the dielectric properties of the medium surrounding the antenna can slightly affect the precise length.
It's important to note that while quarter-wave antennas are commonly used, there are other types of antennas with different lengths and configurations that are also utilized in various applications, depending on the desired performance characteristics and frequency range.