Fractal Element Antennas and Performance
Antenna elements (as opposed to antenna arrays) 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 (thus the first scientific publication on fractal antennas), and a number of patents have been issued from the 1995 filing priority of invention. Most allusions to fractal antennas make reference to these "fractal element antennas".
Many fractal element antennas use the fractal structure as a virtual combination of capacitors and inductors. This makes the antenna so that it has many different resonances which can be chosen and adjusted by choosing the proper fractal design. Just like any other antenna, the physical size of the antenna is related to its potential bandwidth, and the resonant frequency changes depending on the amount of fractal reactive loading. In this manner, the fractal antenna, just like any other antenna with reactive loading (i.e. dielectric, ferrites, capacitors, inductors, etc.), can have its resonant frequency lower than that of the typical free-space half-wavelength fundamental resonant frequency predicted by setting the largest physical dimension of the antenna equal to half a wavelength. False claims that these antennas do not follow the basic laws of electromagnetics that govern antenna behavior abound.
This complexity arises because the current on the structure has a complex arrangement caused by the inductance and self capacitance. In general, although their effective electrical length is longer, the fractal element antennas are themselves physically smaller, again due to 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 need of the application.
Although the first validation of the technology was published as early as 1995 (see ref.1), recent independent studies show advantages of the fractal element technology in real-life applications, such as RFID and cell phones.
One researcher has stated to the contrary that fractals do not perform any better than "meandering line" (essentially, fractals with only one size scale, repeating in translation) antennas. Specifically quoting researcher Steven Best: "Differing antenna geometries, fractal or otherwise, do not, in a manner different than other geometries, uniquely determine the EM behavior of the antenna." However, in the last few years, dozens of studies have shown superior performance with fractals, and the below reference of frequency invariance conclusively demonstrates that geometry is a key aspect in uniquely determining the EM behavior of frequency independent antennas.
Read more about this topic: Fractal Antenna
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