Fractal antenna
A compact antenna using self-similar designs for multiband operation.
A fractal antenna is designed using self-similar patterns that repeat at different scales, known as iterations. This structure increases the effective electrical length or perimeter of the conductive material without requiring a larger physical surface area or volume. These antennas are often called multilevel or space-filling curves. Their compact size and ability to operate across multiple frequency bands make them useful in cellular phones and microwave communications.
Unlike traditional antennas, which must be cut to a specific size to work well at a single frequency, fractal antennas can perform well at many different frequencies at once. The fractal shape also reduces the antenna's physical size without needing extra components like inductors or capacitors.
Log-periodic antennas, invented in 1952 and commonly used for TV reception, were created before the term "fractal" was coined in 1975. Some researchers consider them an early form of fractal antenna because of their infinite self-similarity across scales. However, even in theory with an infinite number of elements, they have a finite length and do not have a fractal dimension exceeding their topological dimension—one common definition of fractals. Most authors treat log-periodic antennas as a separate but related class.
The first self-similar antenna elements were built by Nathan Cohen starting in 1988, and the first scientific publication on fractal antennas appeared in 1995. Many fractal element antennas use their structure as a virtual combination of capacitors and inductors, creating multiple resonances that can be tuned by selecting the right fractal design. The current on these structures is complex due to inductance and self-capacitance. Although their effective electrical length is longer, the antennas themselves are physically smaller because of this reactive loading.
Fractal element antennas are smaller than conventional designs and do not need extra components if the structure naturally provides the desired resonant input impedance. However, the fractal dimension of an antenna is a poor predictor of its performance. Not all fractal antennas work well for every application, so computer simulations and search methods are often used to find the best design. Studies in the 2000s showed advantages of fractal element technology in real-world uses like RFID and cell phones.
- First publication
- 1995
- First created by
- Nathan Cohen
- First creation year
- 1988
- Commercial use since
- 2010s
- Related class
- log-periodic antennas
- Invented year of log periodic
- 1952
Lore & Background
Antenna elements made from self-similar shapes were first created by Nathan Cohen, then a professor at Boston University, starting in 1988. Cohen's efforts with a variety of fractal antenna designs were first published in 1995, which marked the inaugural scientific publication on fractal antennas. Many fractal element antennas use the fractal structure as a virtual combination of capacitors and inductors, giving the antenna many different resonances that can be chosen and adjusted by choosing the proper fractal design. This complexity arises because the current on the structure has a complex arrangement caused by the inductance and self capacitance. Although their effective electrical length is longer, fractal element antennas are themselves physically smaller, due to this reactive loading. Fractal element antennas are shrunken compared to conventional designs and do not need additional components, assuming the structure happens to have the desired resonant input impedance. In general, the fractal dimension of a fractal antenna is a poor predictor of its performance and application. Not all fractal antennas work well for a given application or set of applications. Computer search methods and antenna simulations are commonly used to identify which fractal antenna designs best meet the needs of the application. Studies during the 2000s showed advantages of the fractal element technology in real-life applications, such as RFID and cell phones. Fractals have been used commercially in antennas since the 2010s. Their advantages are good multiband performance, wide bandwidth, and small area. The gain with small size results from constructive interference with multiple current maxima, afforded by the electrically long structure in a small area.
Reader's Guide
Fractal antennas represent a distinct approach to antenna design, leveraging self-similar geometry to achieve multiband and wideband performance in a compact form. Their significance lies in their ability to operate at many different frequencies simultaneously without needing to be 'cut' for a single frequency, unlike standard antennas. This capability, combined with their small physical size achieved without extra components such as inductors or capacitors, has made them useful in cellular telephone and microwave communications, as well as in RFID and cell phones since the 2000s. However, the field includes dispute: some researchers have observed that antenna geometry alone does not uniquely determine the electromagnetic properties of a small antenna, and others have concluded that fractal antennas offer no advantage over fat dipoles, loaded dipoles, or simple loops, with non-fractals always being better. Some reports have found fractal antennas equivalent in performance to the electrically small antennas they were compared to. Log-periodic antennas, invented in 1952, are considered by some authors to be an early form of fractal antenna due to their infinite self similarity at all scales, though they do not have a fractal dimension exceeding their topological dimension. The self-scaling aspect of some fractal element antennas relates to frequency invariance, with self-similarity identified in 1999 as the requirement for antennas to be frequency and bandwidth invariant, leading to the Hohlfeld-Cohen-Rumsey (HCR) principle based on Maxwell's equations. Beyond antennas, fractals have found application in other antenna system components including loads, counterpoises, ground planes, inductors, tuned circuits, filters, and metamaterials, such as in the first wideband metamaterial 'invisibility cloak' at microwave frequencies.
Did You Know?
- Fractal antennas were first created by Nathan Cohen starting in 1988, with the first scientific publication in 1995.
- Fractal antennas can operate at many different frequencies simultaneously without being 'cut' for a single frequency.
- Fractal antennas have been used commercially since the 2010s.
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