Fractal antennas

[Joe Hillmann]

I am playing around with making fractal antennas for tv reception. At the moment I am making them by engraving the fractal pattern halfway through a piece of plexiglass, filling the engraving with solder paste then bake it in a toaster oven that only has the top element hooked up. Once the paste melts I let it cool then attach my leads. Right now I am just testing smaller antennas and haven't built a full size one yet.

Can anyone suggest any other ways to make the fractal circuitry? My toaster oven can only fit about a 9 by 13 inch piece of plexiglass and I would like to make an antenna about 16 by 32 inches. I have though of trying to color fill the engraving with conductive paint but it is expensive and has a very high resistance (something along the lines of 2 ohms per inch) so I don't think that will work at all.

[David Fairfield]

I cant help you but wanted to say the melted solder paste idea is really cool! Would you post a photo ofy our results? People here might be able to use that for some artistic things.

Dave

[Joe Hillmann]

Dave,
I'll post pictures next week, although I don't think it could be used for anything artistic, unless they make a lower temp solder paste, the plexi starts to brown and it isn't too pretty at the moment, although I have only done two tests so maybe if I paid closer attention I could get the solder to melt and the plexi to stay clear.

[David Fairfield]

OK... still wanna see the pictures :D Really cool idea. Thanks!

[Rich Harman]

You can use copper. Spay paint the copper then laser away the parts that you don't want.

Put the copper sheet in a ferric chloride bath for a few minutes and it will eat away the exposed copper.

I have made printed circuit boards this way.

[George Carlson]

I'm amazed that you can reflow the solder without the Plexiglass burning up. Solder is a very lossy material for RF. Etching the pattern on copper is much better. If you spray the copper with clear acrylic lacquer, or better yet clear nitrocelulose lacquer, the lacquer burns away without ash. It works great. I've cut planar antennas from thin copper foil tape using my vinyl cutter. Then transfered the copper pattern to a suiable sunstrate. FR4 is very lossy at microwave frequencies.

[Chris DeGerolamo]

You could [vector] cut channels into the plexi to run wire through, assuming that your fractal pattern is contiguous. This would allow you to get the perfect/proper bends. If you layered 2 pieces of plexi, you could likely spray the rear piece with metallic spray paint to act as a back scatter thingy.

[Joe Hillmann]

As it turns out my method did not work when I tested the antenna with a multimeter I found many many breaks in the circuit where the solder didn't melt all the way. I did a bunch of testing and about the longest section I could find in the solder without breaks was about 1 inch. After several tries of rubbing in more solder into the breaks and heating it again I ended up with several pieces of badly burned and bubbled plexi with dots of melted solder on them. At this point I think I am just going to make an antenna using wire, a pattern and push pins.

[Joe Hillmann]

I had thought of doing something like that but the sides of the smallest parts are only .010" long, I just thought the laser would be a good use for this idea since it can do small cuts like that with more accuracy than any other way I have at my disposal.


You could [vector] cut channels into the plexi to run wire through, assuming that your fractal pattern is contiguous. This would allow you to get the perfect/proper bends. If you layered 2 pieces of plexi, you could likely spray the rear piece with metallic spray paint to act as a back scatter thingy.

[Joe Hillmann]

Did the copper foil work on the antennas? And as it turns out I can't get the solder to flow into the plexiglass, it looked like it did but when I tested it with a multimeter there were many places it didn't melt.


I'm amazed that you can reflow the solder without the Plexiglass burning up. Solder is a very lossy material for RF. Etching the pattern on copper is much better. If you spray the copper with clear acrylic lacquer, or better yet clear nitrocelulose lacquer, the lacquer burns away without ash. It works great. I've cut planar antennas from thin copper foil tape using my vinyl cutter. Then transfered the copper pattern to a suiable sunstrate. FR4 is very lossy at microwave frequencies.

[George Carlson]

Yes, the copper foil works quite well. I was designing Yagis for 915 and 2450MHz. I have a roll of 4" wide, .002" thick, foil with an adhesive back. I got it back when I had to observe the FCC and CE testing of products I designed. The foil was great for improving shielding when the designed-in sheilding was not quite good enough. The foil cuts easy in the vinyl cutter. I would trasnfer it to a substrate just like it was vinyl.