Wire antennas are one of the basic types of antennas. They are well‑known and widely used. To better understand wire antennas, let us first take a look at the transmission line.
Transmission Line
A transmission line, or wire, carries electrical energy from one end to the other. If both ends of the transmission line are connected to circuits, information can be transmitted or received between the two circuits through this wire.
If one end of the wire is left open, the electrical energy will try to escape. This leads to wireless communication. If the end of the wire is bent, the energy will escape from the transmission line more efficiently than before. This purposeful escape is called radiation.
To achieve efficient radiation, the impedance at the open end of a transmission line needs to be matched to the impedance of free space. Consider a transmission line with a length of one-quarter wavelength. Its far end is left open and bent, thereby presenting a high impedance. This structure forms a half-wave dipole antenna. While the feed end of the transmission line has a low impedance, the high impedance at the open end matches the impedance of free space, resulting in improved radiation performance.
Dipole
When energy is radiated through such a bent wire, the end of the transmission line is called a dipole or dipole antenna.
The reactance of the input impedance is a function of the dipole’s radius and length. The smaller the radius, the larger the magnitude of the reactance, and the reactance is proportional to the wavelength. Therefore, both the length and the radius of the dipole need to be taken into account. Typically, the impedance of a dipole antenna is about 72 Ω.
The figure below helps illustrate this concept more clearly.
The figure shows a circuit diagram of a simple dipole connected to a transmission line. In a dipole antenna, the current is maximum at the center and decreases to zero at the ends, while the voltage is minimum at the center and increases to maximum at the ends.
Common types of wire antennas include: half-wave dipole antenna, half-wave folded dipole antenna, full-wave dipole antenna, short dipole antenna, and infinitesimal dipole antenna. All these antenna types will be discussed in the following chapters.
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Post time: May-29-2026
