Fractal antenna
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A fractal antenna is an antenna that uses a self-similar design to maximize the length, or increase the perimeter (on inside sections or the outer structure), of material that can receive or transmit electromagnetic signals within a given total surface area. For this reason, fractal antennas are very compact, are multiband or wideband, and have useful applications in cellular telephone and microwave communications.
Fractal antenna response differs markedly from traditional antenna designs, in that it is capable of operating optimally at many different frequencies simultaneously. Normally standard antennae have to be "cut" for the frequency for which they are to be used—and thus the standard antennae only optimally work at that frequency. This makes the fractal antenna an excellent design for wideband applications.
The first fractal antennas were arrays, and not recognized initially as having self similarity as their attribute. Log-periodic antennas are arrays, around since the 1950's (invented by Isbell and DuHamel), that are such fractal antennas. They are a common form used in TV antennas, and are arrow-head in shape.
Antenna elements made from self similar shapes were first done by Nathan Cohen, a professor at Boston University, in 1988. Most allusions to fractal antennas make reference to these 'fractal element antennas'.
Many fractal element antennas use the fractal structure as a virtual comnbination of capacitors and inductors. This makes the antenna so that is has many different resonances, that can be chosen and adjusted, by choosing the proper fractal design. It also shrinks the antenna compared to conventional designs, and does not need components.
Not all fractal antennas work well for a given application, much as not all conventional antennas are suitable for a given need.
In 1999, it was discovered that self-similarity was one of the underlying requirements to make antennas 'invariant' (same radiation properties) at a number or range of frequencies. Previously, under 'Rumsey's Principle', it was believed that antennas had to be defined by angles for this to be true; the 1999 analysis, based on Maxwell's equations, showed this to be a subset of the more general set of self-similar conditions. Hence fractal antennas offer a closed-form and unique insight into a key aspect of electromagnetic phenomena.
Contrast: Waveguide antenna.
