fractal antennas - pt 1
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N
B
FR CT L NTENN
P RT
Introduction and the ractal Quad
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of the antenna. a
tures. It has been
has dominated a
the electromagn
In 1985.Land
monogra*'h on a
point about Max
tioned the assum
and antenna reso
reversed the pro
give dipoles and
results look far f
domly bent wire
far better results
simple geometr
duce the best an
be, a defined ad
sic geometric de
antennas. This p
use a branch of
ly unexploited i
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segment seems simple enough; but after doing
this, we can continue with another iteration and
put a triangle on each one of those line seg-
ments. For the third iteration we put triangles
on top of each of those line segments, and so
on, ad infinitum. The result is a structure that
on every scale has triangles, and looks the same
at all magnifications.
One can construct a star-like structure by
attaching several iterated Koch fractal ends into
a closed unit, which for obvious reasons mathe-
maticians call "islands."
Figure
6 is a Koch
island star, the result of this amusing labor.
Now let's consider a strange trick of fractals.
Looking at the star reveals that the perimeter is
unrelated to the re of the island. In fact, as
the number of iterations becomes large, the
perimeter of the star has triangles on triangles
down to an infinitesimal scale, and the perime-
ter goes on to infinity Contrast this to a circle,
square, or other closed Euclidean shape: the
re andperimeter are intimately related, and
Figure
5
A von Koch fractal for iterat
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as a Euclidean
this simulation
product: a seri
recording stud
In the 1960s
clear use of sel
I've been unab
his work, but m
recalls a huge 2
of the spreader
supported anot
quad elements
motivation in c
another, simply
of distributing
Any way it's an
was fractally fi
D.L. Jaggard
apply the conc
objective, like
tioned above, w
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antennas use double ridges to decrease the
resonant fr eq ue n~ y. ~variety of shrunken
quads (the Maltese quad, for example), delta
loops, and other useful antennas all incorporate
some type of loading using rectangles, boxes,
and triangles to shorten the element dimen-
sions. Pfeiffer brought this to a logical limit
recently with a fan-shaped quad design of unre-
ported gain and
impedance.8 Yet all of these
designs use a fractal motif of thef irs t iteration;
they basically load Euclidean structures with
another Euclidean structure in a repetitive fash-
ion, using the same size in the repetition. They
do not exploit the multiple scale self-similarity
of real fractals. They could just be an exotic
loading scheme and, therefore, of no particular
importance when compared to other (lossy)
methods of shrinking antennas, such as loading
with
L
circuits, capacitive hats, dielectrics,
Table
1
Some resonant fr
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are better resonators than others through
Commandment 4:
Optimized fractal antennas approach their
asymptotic PCs in fewer iterations than nonop-
timized ones.
In other words, the best fractals for anten-
nas have large values of A and C. They shrink
the most and the fastest.
Finally, the odd property of non-small radia-
tion resistance invites yet another command-
ment, Commandment 5:
The radiation resistance of a fractal antenna
drops as some small power of the perimeter
compression, PC. A fractal island always has a
substantially higher radiation resistance than a
small Euclidean loop of equal size.
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Freq
42 3 HHz
J . .
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general corroborati
of the Minkowski f
before ELNEC mo
I ran ELNEC in
One source was att
was divided into at
pulse densities cou
ond iteration Mink
memory and softw
to keep the pulse d
various iterations.
Minkowski Island
ment, each segmen
cent of a waveleng
es was thus very hi
I show the antenna
derive the lowest r
terns, up to and inc
designs were const
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Table 4.2-meter antenna measurements.
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Table 5. MI2
MI3
efficiencies.
MI2
d B
QUAD
attenuation, wh
ance) for the re
at resonance for
attempt to meas
ticular antennas
of a noise bridg
With this pro
duced quite not
several dB diffe
ones. Removal
excess of a 20 d
this way, I was
tortions in my r
in a meaningful
For each ante
ward gain and o
made no attemp
nor to measure
demonstrate the
The results of th
rized in Table 4
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on a side will be discussed in Part 2.
At this point, it's important to know the effi-
ciencies of MI2 and MI3, because these com-
pact designs offer real advantages over a full-
sized quad element. As I stated, it wasn't possi-
ble to attempt this for the 2-meter versions, but
I built and tested 6-meter versions of MI2 and
MI3 (see Figure 13). I attached an RX-noise
bridge between these antennas and a JST 245
transceiver. By nulling the receiver at about 54
MHz, and calibrating the 50-ohm resistance
bridge with
5
and 10-ohm resistors, I obtained
the results of Table 5.
Efficiency was defined as:
where Z is the measured impedance. R was
obtained by subtracting the ohmic and reactive
3
PC
I
/
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Part
2
will continue this intriguing saga of tiny,
multiband fractal antennas and arrays.
cknowledgments
Paradigm shifts12 are challenging and some-
times pleasurable. This one had its share of
both. My thinking and resolve were sharpened
by comments from individuals in many differ-
ent fields. I would like to thank them all, in a
random fractal order: Alexander Filippov,
Frank Drake, Benoit Mandlebrot, Bruce Tis,
Carl Helmers, Andrew Pfeiffer, Paul Pagel,
Paul Horowitz, Bernard Steinberg, Seth
Shostak, Darren Leigh, William Vetterling,
Maury Peiperl, Robert Hohlfeld, D.L. Jaggard,
Eugene Hastings, Enders Robinson. For obvi-
ous reasons, I acknowledge the management of
Charles River Park
commentary on frac
REFERENCES
1.
F.
Landstorfer and R. Sacher,
York, 1985.
2. N. Cohen,
G r u ~ i t y s L e n s ,
. W
3. B. Mandlebrot, Fractal Geom
1984.
4. M. Schroeder, F~ ac tnl s , hao
1992.
5. M. Berry, Diffractals, Journ
6. Y. Kim and D. Jaggard, The
IEEE,
74. 1278-1280, 1986.
7. J. Kraus, Antennas, McGraw-H
8. A. Pfeiffer,
The
Pfeiffer Qua
28-32.
9 . E. D a v ~ d ,
F
Anrenna Collec
10. J. Krau$,Antennas, 1st editio
11 H. Lauwener, Frar.talr, Prin
12. T. Kuhn, Str.uctur.e ofSc ienti
York, 1986.