TECHNICAL FEATURE
ENHANCEMENT OF
MICROSTRIP STEPPED-
IMPEDANCE LOW-PASS
FILTERS USING FRACTAL-
SHAPES
Microstrip stepped-impedance low- pass filters using fractal shapes, such as Koch islands and Sierpinski carpets, are proposed for the first time. Conventional stepped-impedance low-pass filters are
very popular, due to their ease of implementation in either microstrip or coplanar technology.
However, to achieve a low-pass filter with high
stopband performance, techniques such as increasing the high- to low-impedance ratio will
degrade the passband performance because of
sharp current discontinuities in the high-low
steps. By proper design, the sharp current discontinuities can be smoothed by constructing
the low-impedance sections with fractal-shaped
geometries, thus the passband performance of
the microstrip stepped-impedance low-pass filters is enhanced greatly. To verify the proposed
method, low-pass filter prototypes, constructed
with different fractal-shapes of different iteration orders, are designed, fabricated and measured, and the results show that the maximum
return loss level is greatly reduced, either using Koch or Sierpinski fractals, while the other
performance parameters are not changed.
A compact and high performance microwave low-pass filter (LPF) is highly desirable to
suppress harmonics in wireless communication
systems, such as satellite and mobile communication systems. One of the conventional filters
used is the stepped-impedance low-pass filter,
mainly because of its ease of fabrication in either microstrip or coplanar technology. The
filter is normally composed of alternating low
and high impedance regions, where the change
in impedance is controlled by the line width of
the strip. In order to achieve a high degree of
attenuation in the stopband, it is necessary to
increase the order of the filter or obtain a large
high to low impedance ratio (Zh / Zl).1,2 However, if the filter order is increased, the size
of the entire circuit will be larger, and if the
impedance ratio is large enough, the passband
performance will be degraded because of the
sharp current discontinuities in the high-low
steps.
To enhance the passband performance for
a low-pass filter with a large impedance ratio,
electromagnetic bandgap (EBG) structures
are used.
3-6 However, since the frequency se-
WEN-LING CHEN AND GUANG-MING
WANG
Missile Institute of Air Force Engineering
University, Shaanxi Province, P.R. China
GUO-DONG CHEN, GUANG-FENG
YAO, YING-CHEN HAN AND CHUN-
WANG XIANG
Equipment Academy of Air Force
Beijing, P.R. China
