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Fractal antenna constructions

2017-12-22 02:10  
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Among usual antennas used in todays data transfer there are also different type of antenas used. First publications about electrodynamic characteristics of fractal structures were published in 1980′s , but first practical approach appeared after 10 years. Dr. Nathan Cohen professor of boston university designed, engineered and patented many practicalfractal antennasolutions and founded a€?Fractal Antenna Systemsa€? in 1995.

As Nathan states in centre of Boston there were forbidden to use external antennas in city so he managed to hide antenna within design of amateur radiostation. He took aluminum foil and made antenna as decoration according toVan Kochfigure:

Koch_figure.PNG

 

This figure builds as follows: first line (length is z)is cut in 3 even pieces z/3. In the middle the triangle is formed with same side lengths z/3 and same angles. This way wee get single element template. Then repeat this process with other segments where sizes diminishes 3 times (z/9) then follows again 3 times (z/27) and so on. This way fractal size doesn’t change.

Today fractal antena technology is in early stages, because engineers are doing empirical experiments to find out what geometrical structures give better results. During these experiments they found out that using ordinar type antenna templates building fractals give better gain coefficient. Like this antenna type:

ordinar_antenna_fractals.PNG

 

This type of structure is so called recursive tree. Each new iteration multiply branches by two and lowers resonance frequency. So using iterations it is possible to use antenna at lower frequencies without increasing antenna size.

Dipole antennas usually have narrow band-about 2.4% around frequency carrier. If 5thiteration is used then this parameter grows up to 3.1%. If 3D tree is used (when there are 4 branches used instead of 2) then this parameter grows up to 12.7%.

Besides Dipole antennas there are resonant loop antenas used. Loop antenas are build using Koch figures:

loop_fractal antena.PNG

 

Ordinary frame antenas have low input impedance what makes difficult to connect to feeder. Fractal loops allow to increase impedance even for frequencies lower resonance and this way effectiveness increases.

Another thing what makes fractal antennas so attractive is that they can be fabricated using PCB making methods. Because of compact size they can be put directly on PCB inside like cell phone.

Additionally to narrow band antennas there one type of antennas a€“ wide band. These are Sierpinski antennas are frequency independent and have several bands of resonance and can be compared to log-periodics and spirals. Frequency independence is a result of retaining similar shape at many scales. Sierpinski fractals doesn’t require additional space while frequency band grows as it happens with spiral and log-periodic antennas:sierpinski.PNG

 

Sierpinski fractal antennas can be successfully used in automotive where transparent antennas are sticked to the front window (or other) and can receive multiple bands independently to to standard and country.

car_fractal_antenna.PNG

 

Another widely used example is cell phones. They are long time using internal fractal antennas:

cell_antenna.gif

 

This allow to use multiple bands like 900MHz, 1800MHz, 1900MHz and so on.

Source: RA-2002-09


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