I have been hearing about fractal antennas for a little while and wanted to try making my own fractal antenna to try out the concept. Some of the benefits promoted in research papers are the ability of fractal antennas to receive multiband RF signals, and the ability to shrink the size of the antenna while maintaining signal strength. I decided to create a prototype fractal antenna based upon the Sierpinski Gasket fractal pattern.
This antenna is a prototype antenna. I am posting this article on my blog for the benefit of other antenna building enthusiasts. There is still a lot of work required to finish tuning and improving the design. If you are looking for a general purpose WiFi antenna, either a 2.4 GHz patch or grid antenna are simple and effective solutions for most wireless links. I can recommend L-com as a quality supplier of traditional wireless antennas.
I designed my fractal antenna to have a connector that is compatible with my Linksys WRT54GS 802.11g router. This was my first test of the fractal antenna concept and I learned a lot. The antenna has a low gain design and through preliminary testing on a 1/2 km WiFi link with a few trees in the path I achieved a reliable link.
There is certainly lots of room for improvement through more testing, computer simulation, and better design but fractal antennas do work.
You can download a PDF version of the antenna pattern I used.
Fractal Reference Materials
Here are three PDFs that were helpful when I created my fractal antenna:
ECE416 Project Report
Design and Implementation of Compact Microstrip Fractal Antennas (copied from the WaybackMachine Archive)
by Paul Simedrea
www.imaging.robarts.ca/~simedrea/paul-416-report.pdf (Original Link Broken)
Sierpinski Gasket Patch And Monopole Fractal Antenna (copied from the WaybackMachine Archive)
By Abd Shukur Bin Ja’afar
http://eprints.utm.my/4429/1/AbdShukurJaafarKPFKE2005TTT.pdf (Original Link Broken)
by Philip Felber
Building the Prototype
This is a photo of my completed fractal antenna prototype.
I attached a Linksys WRT54GS style RP-TNC Connector to the fractal antenna for testing.
When I designed my first prototype fractal antenna I was concerned that my home printed circuit board etching process would isolate the Sierpinski triangles so I added coupling patches. Note: Since the final toner transfer process ended up being more accurate than I expected the next version of the prototype fractal antenna will feature finer triangle contact points between each of the Sierpinski fractal triangle iterations.
The antenna design was laser printed onto Pulsar Pro FX toner transfer paper. This process allows me to copy the antenna design onto the copper clad PCB material.
The laser printed antenna design was then transferred onto the sheet of copper clad printed circuit board by a thermal bonding process using modified laminator.
This is the copper PCB material after the first stage of the toner transfer process. The laser printed design has been temporarily bonded to the copper PCB.
The next step was to use a laminator to apply the Pulsar Pro FX "Green TRF Foil" to the circuit board. The green foil is used to fill in any toner gaps or thicken uneven coatings in the toner transfer.
This is the cleaned circuit board with the antenna design freshly transferred. The board is now ready for etching.
For this antenna prototype I masked the backside of the PCB with electrical tape.
I used the direct etch method with ferric chloride (FeCL) to etch the circuit board in 10 minutes. The direct etch method is done using a sponge to slowly wipe the ferric chloride chemical over the entire circuit board. As you wipe the PCB board surface the sponge takes away the etched material and coats the board with fresh Ferric Chloride. This is a quick way to make a custom circuit board in your home workshop. Due to the health hazards of using ferric chloride I wore safety glasses and gloves.
This is the board after the etching process is complete. The toner transfer coating still remains on the etched PCB.
I wiped the etched circuit board down with a swab coated in acetone to remove the toner transfer coating. I used nitrile gloves when cleaning the board down because acetone will soak through the typical latex disposable gloves.
I drilled the hole for the antenna connector using a drill press and a small PCB drill bit.
For my first prototype I used an RP-TNC connector harvested from a standard Linksys router 'rubber ducky' antenna.
This is a closeup of the Linksys compatible RP-TNC antenna connector.
I applied a droplet of water washable flux to the PCB prior to soldering.
The next step was to solder the wire from the RP-TNC connector to the base of the Sierpinski antenna PCB.
It only takes a few moments to solder the RP-TNC connector's wire to the fractal antenna.
I soldered the antenna connector's 2nd wire to the PCB ground plane.
Lastly, I finished the antenna by securing the RP-TNC connector to the antenna with a large glob of hotglue.
RF Simulating the Sierpinski Fractal Antenna
A blog reader Alexander Pelevin was very generous and ran a FEKO RF simulation on the Sierpinski fractal antenna design. He has provided the following images.
Alexander commented that it is important to make sure the Sierpinski gasket triangle elements are in contact with each other and the connection points should be kept as small as possible. Based upon his tip I will have to reduce the size of the coupling patches on my next prototype of the Sierpinski Fractal Antenna down to the fine tips of the triangles.
As a personal observation I find his visualization of the fractal antenna very intriguing because they show the antennas multiband properties. When you look at the following RF simulation images the lower frequency 1.731 GHZ signal has the strongest currents split between the outermost 2nd iteration triangles of the Sierpinski pattern, while the higher frequency 14.268 GHz signal has the highest currents on the innermost 4th iteration of the Sierpinski pattern.
This is the fractal antennas' simulated RF currents when operating at 1.731 GHz.
This is the fractal antennas' simulated RF currents when operating at 3.774 GHz.
This is the fractal antennas' simulated RF currents when operating at 7.393 GHz.
This is the fractal antennas' simulated RF currents when operating at 14.268 GHz.
I would like to thank Alexander Pelevin for contributing the RF antenna simulation images to this article.