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Design principles for monopole antennas: How to use wavelength to optimize performance?
A monopole antenna is a radio frequency antenna consisting of a straight rod-shaped conductor, usually mounted vertically on some conductive surface, called a ground plane. The transmit and receive signals of this type of antenna are transmitted through the connection between the lower end of the monopole antenna and the ground plane. Compared with dipole antenna, monopole antenna has its own unique working mode and design principle. To maximize the performance of the monopole antenna, the choice of wavelength becomes a critical factor.
The length of a monopole directly determines its performance when transmitting a specific frequency, which is closely related to the expected radio wavelength.
Monopole antennas are usually designed as quarter-wavelength antennas, which means that their length is approximately one-quarter of the wavelength of the desired radio waves. However, in broadcast applications, the eight-fifth wavelength design is also popular because this length maximizes the power radiated outward. At both lengths, the antenna's performance is optimized for efficiency from different angles, particularly in terrestrial broadcasting.
Historical BackgroundThe invention of the monopole antenna dates back to 1895, when Guglielmo Marconi conducted historic experiments in radio communications. In these early experiments, Marconi used two identical horizontal wires as dipole antennas. He found that he could transmit farther by connecting the transmitter with one wire suspended in the air and grounding the other end. distance. Therefore, the monopole antenna is also called a Marconi antenna.
Radiation Pattern
Like the vertically suspended dipole, the monopole has an omnidirectional radiation pattern, radiating energy evenly in all azimuths, and the amount of energy it radiates varies with elevation angle. At the zenith on the antenna axis, the radiation decreases to zero.
The unique feature of the monopole antenna is that it has a more pronounced effect on the radiation pattern from the ground, which requires consideration of ground characteristics during design.
Technically, the principle of a monopole antenna can be imagined as replacing the lower half of a vertical dipole antenna with a vertical conductive plane (ground plane). On an infinite and conductive ground plane, such a structure demonstrates a radiation pattern identical to that of the upper half of a dipole antenna. For the designer, understanding these basic radiation characteristics is an important step in optimizing the monopole antenna.
Gain and Input Impedance
Since a monopole only radiates upward, its gain can be considered similar to twice that of a dipole, and its radiation impedance is only half that of a dipole. For example, a quarter-wavelength monopole antenna has a gain of approximately 5.2 dBi and a radiation impedance of approximately 36.5 Ohms. In this design, the input impedance is clearly purely resistive, but its reactive nature exhibits different responses at different wavelengths as the antenna length varies.
In practical applications, the use of different types of artificial ground planes or natural ground planes will significantly affect the performance and gain of the antenna.
As the length of the antenna increases, the effect of the metal conductor becomes more and more significant, which becomes more important for enhancing the transmitted signal. For most wireless communications or broadcast applications, designers carefully consider these factors to ensure that the monopole antenna maximizes its operating efficiency at the specified frequency.
Types of Monopole Antennas
Monopole antennas are widely used in a variety of applications, from transmissions below 20 MHz to higher frequency communication systems. Traditional designs include vertical antennas, T-antennas, and umbrella antennas to improve gain and efficiency. At VHF and UHF frequencies, since the required ground plane becomes smaller, artificial ground planes become common, and such designs allow the antenna to operate in mid-air.
As wireless technology has advanced, variations of monopole antennas have emerged, such as the inverted-F antenna in mobile devices, further expanding their potential applications.
Today, monopole antennas remain a key component in wireless communications, from basic portable radios to complex wireless networking systems, allowing this technology to demonstrate its unique advantages. But for future wireless technologies, can we expect continued innovation in monopole antenna design and performance to meet evolving needs?
