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In essence, the difference between a monopole and dipole antenna, is that a dipole antenna uses an additionally radiator to generate a synthetic ground plane between the symmetric radiator elements, where a monopole antenna requires a physical ground plane. For a dipole antenna, the radiator elements are connected 180 degrees out-of-phase to each other, such as with the inner and outer conductor of a coaxial cable. For a monopole antenna, the reference plane of the transmission line connection, outer conductor of a coaxial cable, is the ground plane of the monopole.
Dipole electromagnetic field pattern
Monopole antenna electromagnetic field pattern
Both monopole and dipole antennas exhibit similar radiation patterns and performance, except that monopole antennas are not symmetric vertically. Dipoles tend to be more common. The size and design constraints of requiring a ground plane for monopole antennas is often restrictive and the radiation pattern of the monopole depends on the orientation of the ground plane. Dipole antennas, however, have a vertically symmetric radiation pattern, and can relatively easily be oriented in the direction of optimum reception/transmission.
As dipoles tend to be more common than monopole antennas, there are more varieties and typologies of dipole antennas, including the common half-wavelength dipoles. Common monopole antennas include automotive AM antennas, naval low frequency antennas. There are types of monopole antennas used in the very high frequency (VHF) broadcast applications that use several grounded radial wires as the ground plane of the monopole antenna, and these conductive wires can be designed and oriented to optimize the radiation pattern of the “ground-plane” antenna for ideal broadcasting performance.
All antennas require two terminals, just as a circuit element such as resistor requires two terminals for current flow. It is easy to see that a resistor can have AC (RF) current applied to its two terminals, and dissipate energy.
With a dipole, the two terminals are intuitive as well. Each “arm” of the classic dipole is ¼ of a wavelength long, symmetrical, and each is connected to a terminal. However, the arms of a dipole do not have to be symmetrical, or even be the same shape. In an extreme version of this, one arm is a large shape.
Such large arm or shape of the antenna may be thought of as a “ground plane”. It could be a large metal disk, an array of radial wires, or even the rectangular ground plane of a PCB. In an extreme, it could even be the surface of the planet earth. In a true monopole antenna, we can see only one arm, and the other connection could be the earth. In all cases, there are still two connections!
The following graphic has been created to illustrate the similarities. For more antenna and antenna tutorials, you may visit the education area of Antenna Test Lab Co.
Two terminals are ok if one it talking about TEM (coax, two wire etc|) line (or similar) feeding; but of course one can use waveguide feeds towards higher frequencies and thats why a" pole pair" is usually treated as "port" in the S-parameter language.
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