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Author Topic: Has anyone successfully designed antennas for HF based on fractals?  (Read 3665 times)
W1TXT
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« on: March 11, 2013, 09:59:36 AM »

Something I've been wondering about...wonder if anyone has tried and if so, can share their designs.
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G0VKT
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« Reply #1 on: March 11, 2013, 10:11:44 AM »

There is an old thread on here if that helps.

http://www.eham.net/ehamforum/smf/index.php?topic=72394.0
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N4CR
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« Reply #2 on: March 11, 2013, 04:34:48 PM »

It's interesting that the overall length of a loaded antenna seems to set the overall characteristic of it's gain. Whether you bend the wire into a zig-zag, spiral or fractal, it's reduced length sets the loss over a full length 1/2 wave dipole.

The U.S. government dumped a lot of money into this wishing well and pulled up an empty bucket.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
K3VAT
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« Reply #3 on: March 12, 2013, 08:47:41 AM »

It's interesting that the overall length of a loaded antenna seems to set the overall characteristic of it's gain. Whether you bend the wire into a zig-zag, spiral or fractal, it's reduced length sets the loss over a full length 1/2 wave dipole.
<snip>

Phil, Good Points !!  Also ...
A shorted dipole, loaded or not, actually has very little loss in gain as it reduces size.  For example, a shortened dipole of 60% has <0.3 db loss when compared to a full-size dipole; a shortened dipole of 40% has 0.4 db loss - even a 10% shortened dipole has < 1 db loss.  The problem with these reduced length dipoles (and verticals) is feedpoint impedance.  That 40% shortened dipole has a feedpoint impedance of 8-j1244 which is a difficult job for many tuners.  At 10% the feedpoint Z = 0.5-j4600 !!  Impedance matching devices for antennas this short are always lossy and they are expensive to construct (vacuum variable capacitors; high Q coils, etc.), that I believe that is the reason why reduced size antennas are not more prominent at least in the amateur community.

The numbers for the above come from this article by LB Cebik, Table 1:  http://w4rnl.net46.net/download/40hb.pdf

73, Rich, K3VAT
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N3OX
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« Reply #4 on: March 12, 2013, 09:49:33 AM »

It's interesting that the overall length of a loaded antenna seems to set the overall characteristic of it's gain. Whether you bend the wire into a zig-zag, spiral or fractal, it's reduced length sets the loss over a full length 1/2 wave dipole.

That's more or less true for GOOD designs, but a poor short antenna design can add many dB extra loss over a good short antenna design!

Specific designs should be compared by measurement or reliable models or both.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #5 on: March 12, 2013, 10:39:54 AM »

Fractals may be useful for wide band antennas on the higher (UHF+)
bands where available lengths are several wavelengths.  But they
don't offer any significant improvement for loading short antennas.
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G8HQP
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« Reply #6 on: March 12, 2013, 11:59:10 AM »

As K3VAT said, the length of a short antenna sets its radiation resistance and hence efficiency when the tuner is taken into account. Not gain, which varies very little from half-wave to infinitesimal dipoles (2.15dBi vs. 1.5dBi IIRC). People often get this confused, including magazine article authors!

Fractal antennas can be good when you have a lot of space and need a broadband antenna. Typically the opposite of amateur radio: we have limited space and (broadly) harmonically-related narrow bands.
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W5DXP
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« Reply #7 on: March 12, 2013, 02:26:58 PM »

http://www.fractenna.com/
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KB4QAA
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« Reply #8 on: March 12, 2013, 06:34:25 PM »

Fractal antennas have special and limited uses, typically broad bandwidth, and space savings in the UHF and above bands.

Neither of these feature imply gain, nor better performance than common designs.

Common sense and practical experiments show that fractal antennas suffer from weight and strength problems in the HF bands.

*No true fractal antennas are commonly known for amateur uses

*Despite all the hubbub about fractals, the well known amateur who holds patents on certain fractal principles has never contributed ANY functional design or formulas to the community, in almost 20 years of shameless self promotion. Wink (and I say that with a smile).
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W5DXP
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« Reply #9 on: March 12, 2013, 07:39:51 PM »

... the well known amateur who holds patents on certain fractal principles has never contributed ANY functional design or formulas to the community, in almost 20 years of shameless self promotion.

Actually, Chip did provide a fractal vertical for 20m about 15 years ago when I was living in AZ. If I remember correctly, it had about 3 dB gain over a 1/4WL vertical. The basic problem with fractal antennas on HF is they are not self-supporting.
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KB4QAA
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« Reply #10 on: March 12, 2013, 07:51:59 PM »

Last I recall, he provided only a photo/outline of a PORTION of what he said was an antenna, but without formula or dimensions, making it impossible to realize.  In other words, it was a nice piece of Fibonacci art.

I believe his first patent runs out in 2015, so perhaps we may see entrepreneurs and experimenters introducing workable designs.
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N3OX
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« Reply #11 on: March 13, 2013, 04:17:10 AM »

I believe his first patent runs out in 2015, so perhaps we may see entrepreneurs and experimenters introducing workable designs.

Not sure why experimenters can't introduce workable designs.

Here's one side of an element for 10m with actual dimensions:

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

Here are some EZNEC results for something very close... slightly enlarged reflector I think; would have to look it up because I did it a while ago.

http://n3ox.net/files/fqy.jpg
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K2DC
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« Reply #12 on: March 13, 2013, 11:14:49 AM »

Before retiring as a Microwave Antenna Engineer I saw several Fractal designs.  Nearly all of the practical designs had dimensions of multiple wavelengths with smaller repeating features, often log periodic in nature.  The result was very wide bandwidth, but it would make a practical HF Fractal a real monster to try to physically realize.

73,

Don, K2DC
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KB4QAA
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« Reply #13 on: March 13, 2013, 05:49:15 PM »

Thanks for the files and article Dan!!!!
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KA4NMA
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« Reply #14 on: March 13, 2013, 09:11:55 PM »

Oh no!  I remember the fractal wars several years ago.

Randy ka4nma
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