Wednesday, August 10, 2011

Fractal Antenna


Fractal fail. This one wins the most-annoying-home-project-ever award. I found some online instructions for a "fractal" HDTV antenna on Instructables. [here] On first glance it looked more simple than the Bowtie model I made a few weeks ago. Flush with spare coat hangers and scrap wood from that prior project; I gave it a go.  It started off very simple. Cut some plywood...

...Sand and prime it.

...Sand again, and paint it.(This part ain't rocket science)

Then measure and mark out all the holes... cut the 8" lengths of wire for the aerials. I won't rehash all the instructions, it's not wildly different than the home-brew antenna I made late last month. [here]  I didn't stop until I got partway through the instructions. Then something went wrong. With a fractal antenna the idea is that you are shaping the aerials with self-similar patterns, the angles and lengths are very specific.You'll probably see it right off, but I just wrote down the numbers and made my cuts and bends.
The problem is that some of those 60° angles are acute, and others are obtuse. All I know is that it's wrong. If all the angles and lengths were the same it'd be shaped like a wave. What I think they meant was 120°.  But I don't know that for sure. Anyway, realizing the angles were wrong I made some educated guesses. But re-bending all the aerials twisted them, which certainly didn't help. I plugged it in just for a dry run and got 3 stations. It is substantially inferior to the previous home-brew antenna. Below is a somewhat mangled fractal antenna. I will have to cut all new wires and begin again.

For the record, fractal antennas are a very recent discovery. They were first discovered in 1988 by boy genius Nathan Cohen and patented in 1995. The fractals resonate to tuned frequencies and make the antenna much more effective. Most manufacturers just print fractal designs directly on circuit boards. That allows them to make smaller and more intricate fractal designs without the changes in resistance you get bending wire. The antenna gets more sensitive the more self-similar repetitions occur within the design. The one I messed up was actually quite simple. Next time I get some scrap copper wire I'll take another shot at this.

3 comments:

Paul said...

The picture looks off, the bend in the middle of the wire is not 60 degrees acute or obtuse. Cell phones make great use of Fractal Antennas. The theory is simple enough, I just think this guys instructions are not that great. Try this one, it has a good way to make a template, which is easier than using a protractor.

http://www.bobmc.net/doku2/doku.php?id=hdtv_fractal_indoor_antenna

Jose Fritz said...

I know that real fractal antennas really work. What screwed me up was just writing down the measurements and angles on scrap paper then building it. If I'd stopped to think about it for a sec,I'd have realized they can't all be 60 degrees. The result was mangled coat hangers and nothing that resembled a real fractal antenna.

Next time I get a bit of scrap copper I'll try again with that diagram. Looks simple enough. I wish a had a better handle on the math. Now understanding the concepts better I think I could build something much more complex.

Anonymous said...

I know this is an old post but the easiest way to make the designs is to create your own template. You just have to print out the design, place the paper on a scrap piece of wood, nail in headless nails at the points of the design. Follow the design on the paper and shape the wire around the nails.