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Author Topic: Fractal Antennas?  (Read 8089 times)
N5RWJ
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« on: January 19, 2011, 02:18:46 PM »

Any info out there on HF Fractal antennas' for us hams ? Is it going to be the future wonder for us on earth, or is it only for talking to outer space Aliens on Mars.
« Last Edit: January 20, 2011, 09:51:45 AM by N5RWJ » Logged
KE4DRN
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« Reply #1 on: January 19, 2011, 04:36:50 PM »

hi Charles,

http://www.antennex.com/preview/Jan504/intro-fract.htm

73 james
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N3OX
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« Reply #2 on: January 19, 2011, 06:07:20 PM »

This is pretty much the only source I know of for construction plans for a fractal HF antenna:

http://www.scribd.com/doc/18788401/FYIFQY

Otherwise, you're kind of on your own for experimentation.    I might build one of those fractal quads just for the heck of it.  Might do it on 2m because it's easier to measure things about it.

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W8JI
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« Reply #3 on: January 22, 2011, 05:22:45 AM »

Any info out there on HF Fractal antennas' for us hams ? Is it going to be the future wonder for us on earth, or is it only for talking to outer space Aliens on Mars.

There is no magic to a fractal shape. Packing a longer and longer wire into a fixed volume of space for a compact antenna can only increase losses from the most simple systems that are properly designed.
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K9FV
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« Reply #4 on: January 22, 2011, 05:56:10 AM »

http://www.scribd.com/doc/18788401/FYIFQY

Boy is that a terribly slow site - got so much of special format, pdf or something - S L O W!!! loading - even with my cable modem.  I wanted to read the site, but I kept thinking my computer was hung.  I finally killed the page and left. 

rant mode off (I've always preferred fast loading webpages)

73 de Ken H>
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N3OX
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« Reply #5 on: January 22, 2011, 08:27:53 AM »

http://www.scribd.com/doc/18788401/FYIFQY

Boy is that a terribly slow site - got so much of special format, pdf or something - S L O W!!! loading - even with my cable modem.  I wanted to read the site, but I kept thinking my computer was hung.  I finally killed the page and left. 

Yes, it's terrible.  There's not much else.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5DXP
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« Reply #6 on: January 22, 2011, 09:18:46 AM »

As Tom says, there is no magic to a fractal shaped antenna and I don't recall anyone ever saying that fractal antennas possess magical powers. Dr. Balanis, author of Antenna Theory, and one of the foremost active antenna authorities says: "Fractal antennas exhibit space-filling properties that can be used to miniaturize classic antenna elements, such as dipoles and loops, and overcome some of the limitations of small antennas. ... Because of the space-filling properties of fractal antennas, their Q can be lower than that of a classic cylindrical dipole, ..." (Lower Q = greater bandwidth.)

From: Fractal Antennas, by Poprzen and Gacanovic: "Abstract: This report will describe the theories and techniques for shrinking the size of an antenna through the use of fractals. Fractal antennas can obtain radiation pattern and input impedance similar to a longer antenna, yet take less area due to the many contours of the shape. Fractal antennas are a fairly new research area and are likely to have a promising future in many applications. ... Advantages of fractal antenna technologie are: minituratization, better input impedance matching, wideband/multiband, frequency independent, reduced mutual coupling in fractal array antennas. ... The space saving abilities of fractals to efficiently fill a limited amount of space create distinct advantage of using integrated fractal antennas over Euclidean geometry."
--
73, Cecil, w5dxp.com
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K4SAV
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« Reply #7 on: January 22, 2011, 04:47:26 PM »

When reading the claims for fractal antennas you will often see things like higher gain, wider bandwidth, and smaller size.  That is meaningless.  Wider bandwidth than what? Smaller than what?  Usually they don't say. 

Some fractal antennas can have a very wide bandwidth but that is usually when the fractal is many wavelengths long.  In that case it is not very small in terms of wavelengths.  (It might still look small if the antenna is for 10 GHz.)  When you look at the fundamental resonant point of a fractal dipole, usually it has less bandwidth than a conventional half wave dipole. 

Example: A K2 20 meter fractal dipole has a bandwidth of 700 kHz.  A 20 meter K3 fractal dipole with 128 segments has a bandwidth of 650 kHz.  A 20 meter K3 fractal dipole with 64 segments has a bandwidth of 550 kHz.  A Koch quadratic 20 meter dipole has a bandwidth of 180 kHz.  A conventional 20 meter half wave dipole has a bandwidth of 1100 kHz.  (All at 30 ft height, using #12 wire, and a lossless matching network.)

How do you compare the size of two antennas that are of such different dimensions.  If you compare in terms of area, the conventional antenna is usually smaller.  If you compare a single dimension only, like width, and ignore the second dimension, the fractal antenna is often smaller.  In either case it is an apples and oranges comparison and declaring one as "smaller" is meaningless in the absence of any other data.

Jerry, K4SAV
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KB1NXE
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« Reply #8 on: January 23, 2011, 09:27:20 AM »

Did you know Log Periods and Quad antennas are 'fractal' antennas? 

To quote:

"The first fractal "antennas" were, in fact, fractal "arrays", with fractal arrangements of antenna elements, and not recognized initially as having self-similarity as their attribute. Log-periodic antennas are arrays, around since the 1950s (invented by Isbell and DuHamel), that are such fractal arrays. They are a common form used in TV antennas, and are arrow-head in shape."

So to answer your question,  Yes, there are many sites with information of fractal antennas of these two basic designs.  Folded dipoles also meet the definition of a fractal antenna.  As do Moxons, Spider Beams, etc.  Are they the new savior of antenna designs?  You decide for yourself.  Just because a re-aired Nova segment on them has re invigorated the concept, doesn't mean it's new or great.  Just new to you.

They have their place in the vhf/uhf/shf world for low power, low gain situations where compact design and space is a vital consideration.  Cell phones/RFID/Wireless Data (I hate the term WiFi - what does that mean?  Wireless Fidelity?) are situations the latest designs work well.

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K4SAV
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« Reply #9 on: January 23, 2011, 01:55:08 PM »

So to answer your question,  Yes, there are many sites with information of fractal antennas of these two basic designs.  Folded dipoles also meet the definition of a fractal antenna.  As do Moxons, Spider Beams, etc.  

Wikipedia's definition of a fractal antenna is:

A fractal antenna is an antenna that uses a fractal, 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 radiation within a given total surface area or volume.  Such fractal antennas are also referred to as multilevel and space filling curves, but the key aspect lies in their repetition of a motif over two or more scale sizes,[1] or "iterations".

Wikipedia also says a log periodic antenna is a fractal antenna.  They said:

The first fractal "antennas" were, in fact, fractal "arrays", with fractal arrangements of antenna elements, and not recognized initially as having self-similarity as their attribute. Log-periodic antennas are arrays, around since the 1950s (invented by Isbell and DuHamel), that are such fractal arrays.

I can see where there are parts of a log periodic antenna that could be considered as "repetition of a motif over two or more scale sizes", or maybe close to that.  A multiband quad might also qualify, even though most fractals exist mostly in two dimensions and a quad is in three dimensions, but I can see no reason a fractal can't exist in three dimensions.

I fail to see the repetition of the motif over "two or more scale sizes" for folded dipoles, Moxons, and Spider Beams.   Do I have to cross my eyes and stare?

Jerry, K4SAV
« Last Edit: January 23, 2011, 01:59:02 PM by K4SAV » Logged
WB2WIK
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« Reply #10 on: January 23, 2011, 02:37:58 PM »

(invented by Isbell and DuHamel), that are such fractal arrays.[/i]

I fail to see the repetition of the motif over "two or more scale sizes" for folded dipoles, Moxons, and Spider Beams.   Do I have to cross my eyes and stare?

That's so silly.

You have to stand on your left leg leaning to the right looking through a sextant during a full moon and you'll see this very clearly.

Must I explain everything?
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K7KBN
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« Reply #11 on: January 23, 2011, 03:01:25 PM »

If you're younger than eighteen years old, you cannot LEGALLY look through a sextant! Grin
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73
Pat K7KBN
CWO4 USNR Ret.
K1BQT
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« Reply #12 on: January 26, 2011, 04:31:39 AM »

Any info out there on HF Fractal antennas' for us hams ? Is it going to be the future wonder for us on earth, or is it only for talking to outer space Aliens on Mars.

Fractal shaping is a form of distributive element loading that uses mathematically defined self-similar patterns. Other distributive examples include helical, linear, meander line, serpentine, and random. I built a few lower-order fractal elements and compared them with other forms of loading on 2 Meters back in the mid 90’s, but was never able to document any over-riding performance benefit. Far more rigorous research conducted by Steve Best, VE9SRB, and others tend to confirm that observation. I also found that most lower-frequency fractals constructed from wire, rod, and tubing are somewhat difficult to shape and support.

From an anecdotal perspective, there’s little doubt antennas using fractal loading do “work”. Build and match them in correctly, and you’ll likely receive gratifying signal reports. However, 15 years after the fractal’s vigorously promoted introduction to the ham fraternity, it remains more a curiosity than a practical element-compressing solution for builders and manufacturers. Were that not the case, I suspect we’d all be using fractal loading methods – much as we’ve embraced SMT, DSP, SDR, NEC, SteppIR, Hex Beams, and dozens more examples of technical innovation that emerged to enhance the hobby during the same time frame. That’s not to diminish the hard work of fractal proponents or dispute the utility of fractal structures as applied to specialized non-ham applications. If the concept interests you, I’d encourage you to build one up and draw your own conclusions!
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N5RWJ
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« Reply #13 on: January 26, 2011, 12:26:59 PM »

Who will be the first to design a 40 or 20 meter fractal for this topic ?
« Last Edit: January 26, 2011, 12:31:58 PM by N5RWJ » Logged
WB6BYU
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« Reply #14 on: January 26, 2011, 12:37:48 PM »

Designing it isn't difficult.  Building takes more effort.  Getting it to actually outperform
other, simpler alternatives is the most difficult part.
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