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Antenna Theory Crash Course Part 3: A Look at Common Antenna Types - From Wires to Loops

Building upon our discussion of fundamental antenna characteristics, we now turn our attention to some of the most common and basic antenna types: wire antennas and loop antennas. These forms are significant due to their simplicity, versatility, and widespread use in various applications.

Wire Antennas

Wire antennas are familiar to many and are seen in numerous places such as on vehicles, buildings, and aircraft. They come in various shapes, including straight wires (dipoles), loops, and helices. Linear or curved wire antennas are some of the oldest, simplest, cheapest, and often the most versatile.

  • The simplest form is a straight wire, commonly referred to as a dipole. Dipoles are discussed in more detail in Chapter 4 of Balanis' "Antenna Theory". 
  • Variations of the straight wire include monopoles, often used for mobile cell and cordless telephones, sometimes in a helical form with a plastic cover to increase input resistance for better matching.
  • Long wire antennas were among the first travelling wave antennas.
  • V antennas and bent wire antennas are other practical forms, useful for applications like aircraft communication.
  • Arrays of wire elements are also common, such as the Yagi-Uda antenna, which typically uses linear dipoles with one driven element and parasitic directors and reflectors. There are also Yagi-Uda designs using loops.
  • Helical antennas, although technically wire antennas, are often considered separately due to their unique properties like circular polarisation and higher frequency operation. They can operate in normal (broadside) or axial (end-fire) modes.
  • Log-periodic wire antennas are a type of frequency-independent antenna, often designed with a wire structure based on the edges of logarithmically periodic surfaces.
  • Computer codes like NEC and MININEC have been developed for analysing the characteristics of wire antennas.

Loop Antennas

Loop antennas represent another fundamental antenna type, known for being simple, inexpensive, and very versatile. They can take various forms, including circular, rectangular, square, elliptical, and other configurations. The circular loop is particularly popular due to its simplicity in analysis and construction.

  • A key characteristic is that a small loop (circular or square) is electrically equivalent to an infinitesimal magnetic dipole with its axis perpendicular to the loop's plane.
  • Chapter 5 of Balanis' book provides a more detailed discussion of loop antennas.
  • Loops can be used as single elements or in arrays. They are also a component in other antenna designs such as helical antennas operating in the normal mode and some variations of the Yagi-Uda array.
  • A solenoid with a ferrite rod inside, essentially a loop configuration, is used in transistor radios as a receiving antenna and tuning element.

Both wire and loop antennas serve as foundational elements in antenna theory and are employed in a wide array of communication and sensing systems. Understanding their basic properties and variations is crucial for further exploration into more complex antenna designs.

Resource: Balanis, C. A. (2016). Antenna Theory: Analysis and Design (4th ed.). John Wiley & Sons, Inc.

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