# High Efficiency and Broadband Microstrip Leaky-Wave Antenna

Authors: Onofrio Losito
Journal: Active and Passive Electronic Components
 (2008)

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## Abstract

A novel layout of leaky-wave antennas based on tapered design has been proposed
and investigated. The new tapered leaky-wave antenna (LWA) was designed running a simple procedure
which uses an FDTD code, and using a suitable metal walls down the centerline along the length of the
antenna connecting the conductor strip and the ground plane, which allows to use only half of the structure,
the adoption of a simple feeding, and the reduction of sidelobes. The good performance of this new
tapered microstrip LWA, with reference to conventional uniform microstrip LWAs, is mainly the
wider band of 33% for
VSWR<2, higher gain (12 dBi), and higher efficiency (up to 85%). Furthermore, from the theoretical analysis we can see that, decreasing the relative dielectric constant of the substrate, the bandwidth of the leaky-wave antenna becomes much wider, improving its performance.


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## Body

## 1. Introduction

Progress in the
recent years has been obtained on the development of leaky-wave antennas based
on the higher order mode of microstip [1, 2]. The LWAs possess the advantages
of low-profile, easy matching, fabrication simplicity, and frequency/electrically
scanning capability. But in some application especially for point-to-point
communication, the main beam variation of LWA should be as low as possible. A
tapered steps microstip LWA, in which each step can irradiate in subsequent
ranges of frequency, is a possible solution to obtain a fixed mainbeam LWA [3].
Unfortunately, in this antenna the impedance mismatch between subsequent steps
reduces the bandwidth, furthermore the excitation of higher order mode without
dominant mode perturbation requires more elaborate feeding scheme. A curved
design of tapered antennas with a physical grounding structure along the length
of the antenna allows to reduce the impedance mismatch, suppressing the
dominant mode (bound mode). This solution improves the band the gain, the
efficiency, and simplifies its feeding. Moreover, due to the image theory it
is possible to design only half antenna with the same property of entire,
reducing the dimensions of the uniform miscrostrip LWA. The performance of the
efficiency and band of this LWA can be improved further, if we use a substrate
with relative dielectric constant that approached 1.In this paper,
we have proposed such new curve tapered LWA as discussed in the following
section.


## 2. Design of Microstrip Curve LWA

We can explain
the character of microstrip LWAs through the solution of the
dispersion characteristic equation
obtained with full-wave analysis methods. This solution allows to evaluate the
radiation region of leaky-wave through the complex propagation constantk=β−jα,
where β is the phase constant of the first higher
mode, and α is the leakage constant. The complexity of
full wave analysis [4, 5] to solve the propagation characteristics suggests to
use an easy FDTD algorithm which use a PML boundary condition, as proposed in [6],
to obtain the normalized phase constant and attenuation constant. The leakage
radiate phenomena, from the propagation characteristics, can only be noted
above the cutoff frequency of higher order mode, and below the frequency such
that, the phase constant is equal at the free space wave number.From these
characteristics of curves we know the frequency range of the leaky-mode radiation,
that can be indicated in the more useful way for the design of our antenna [3]:(1)c2weffεr=fc<f<fcεrεr−1.From
(1) we can see
that as the width of the antenna decreases, the cutoff frequency increases
shift toward high frequency. This behavior allows to design a multisection
microstrip LWA according [3] superimposing different sections in which each
section can radiate in a different and subsequence to frequency range, obtaining a
broadband antennas. In this way, each section should be into bound region,
radiation region, or reactive region, permitting the power, to uniformly
radiated at different frequencies. Moreover, we note from (1) that using a
substrate material with relative dielectric constant which approached 1, the
leaky-wave antenna bandwidth become much wider and increased drastically. Unfortunately, this multisection LWA (layout Type I in
Figure 1(a))  shows ripples in return loss curve, spurious
sidelobes, and impedance mismatch. Our idea to reduce ripples and sidelobes is
to design a smooth contour antenna using the same contour of the cutoff phase
constant or attenuation constant curve (αc = βc), This curve was obtained varying the frequency for different width
and length of each microstrip section, as mentioned in [7], employing the equation(2)βc=c1f2+c2f+c3. This equation is
given by linear polynomials interpolation of the cutoff point of the dispersion
characteristic, where c1=0.13,c2=5.4,c3=−10.41. Further reduction of ripples and
spurious sidelobes can be obtained using a suitable metal walls down the
centreline connecting the conductor strip and the ground plane that allows the
adoption of a simple feeding planar line (layout Type II in
Figure 2). This
physical grounding structure suppresses the fundamental mode, forcing the
energy to propagate in the next higher mode. As mentioned previously, it is
possible to design only half of the entire curve tapered LWA. The half curve
tapered LWA is characterized by the same property of the entire, but with the
dimensions reduced of the 20% from the entire tapered LWA and reduced of the
60% from the uniform microstrip LWA. The antenna layout Type III (half curve
tapered LWA) shown in Figure 3 was optimized trough a 3D electromagnetic
simulator (CST Microwave Studio), and the return loss, the
radiation pattern, and antenna efficiency were compared with the multisection
LWA (Type I) and with curve tapered LWA (Type II). A prototype of tapered LWA
was designed and fabricated on RT/Duroid 5880 substrate, with εr=2.32 and a thickness of 0.787 mm. The length of
the LWA was chosen to be 120 mm. to allow 90% radiation at an upper frequency of
9.5 GHz, with a 15 mm start width, and 8.9 mm of final width. A sequence of covered wires was inserted in the holes made in
the centerline of the antenna to obtain a simple physical grounding structure
along all the length of antenna (see Figure 4).Figure 1
Layout of multisection LWA.(a) Layout of curve tapered LWA Type II. (b) Cross-section of LWA Type II with a physical grounding structure along the length of the
antenna which connects the conductor strip and the ground plane.
(a)(b)(a)  Layout of half tapered LWA. (b) Cross-section of half LWA.
(a)(b)(a) A prototype of tapered LWA with wires in the holes.
(b) The soldering made in the centerline of the antenna to cover the
wires in the holes.
(a)(b)Moreover, a
prototype of half tapered LWA was designed and fabricated with the same
substrate mentioned previously, applying an adhesive conductor at the edge
centerline, to connect the conductor strip and the ground plane, along all the
length of antenna (see Figure5).Figure 5
A prototype of half curve tapered leaky-wave antenna..This layout,
therefore, improves the band, (33% for VSWR < 2), the gain (12 dBi), and the
efficiency (up to 85%) with reference to the conventional uniform microstrip
LWAs [8] (which have band of 22% for VSWR < 2, peak gain up to
10 dBi, and
efficiency up to 75%).


## 3. Theoretical and Experimental Results

To  excite the higher-order mode TE10 in our LWA is
necessary simply by an asymmetrical planar feedline of 50Ω.
In fact, the physical grounding structure along the centreline
of the LWA allows to suppress the dominant mode excited in the antenna
. The multisection tapered antenna
Type I was made as an open circuit, with a 15 mm start width and 8.9 mm of final width
designed using for-section calculation according to [3], while the smooth contour
of the LWA Type II was designed through (2). As said previously, the antenna Type III was
obtained using only half of transversal dimension of antenna Type II, and both
(Type II and Type III) were made as an open-circuit LWA like Type I. We have
implemented the simulations using a substrate of thickness of 0.787 mm and relative
dielectric constant of 2.32. From the return loss (S11) of Type I, we note that
it is below −10 dB only in three short-range frequencies (see
Figure 6), while
S11 of Type II and Type III is practically the same, below −10 dB from 8 to
11.2 GHz as shown in Figure 7. Moreover, the VSWR of LWAs Type II and Type III
(VSWR<2  between 8.01 and 11.17 GHz) are in agreement like the antenna
efficiency (about 85% in the same range frequency) as shown in
Figures 8(a)-8(b) and
Figures 9(a)-9(b). The main
lobe pattern of LWAs Type I and Type III at 9.5 GHz is shown in
Figures 10 and
11. From these figures, we can see a reduction of sidelobe, the peak of
gain of LWA Type III up to 12 dBi, and only few degree of mainlobe variation
between Type I to Type III. The results of the measured return loss of half LWA
(Type III) and the measured E-field pattern (at 8 GHz) of the half LWA (Type
III), compared to the simulated return loss and the simulated E-field pattern
(at 8 GHz) plotted in Figures 12 and
13, show a good agreement.Figure 6
Simulated return loss of multisection LWA Type I.Figure 7
Simulated return loss (S11) of tapered LWA (Type II) and half
tapered LWA (Type III) obtained by 3D electromagnetic simulator.(a) Simulated VSWR of LWA Type II.
(b) Simulated VSWR of LWA Type III.
(a)(b)(a) Simulated efficiency LWA Type II.
(b) Simulated efficiency LWA Type III.
(a)(b)Figure 10
Radiation patterns of  electric field (H plane) LWA  Type III, at 9.5 GHz.Figure 11
Radiation patterns of  electric field (H plane) LWA  Type I, at 9.5 GHz.Figure 12
The measured and simulated return loss of  half tapered LWA.Figure 13
The measured and simulated radiation patterns of E field of  half tapered LWA (at
8 GHz).These results
indicate a high performance of antenna Type III (33% for VSWR  < 2, high
antenna efficiency, and high power gain) compared with uniforms LWAs (20% for
VSWR, peck power gain up to 10 dBi) as mentioned in [8]. Nevertheless, this
performance were obtained reducing up to 60% the uniform antenna’s dimensions.Finally, from
(1) we note also that if the relative dielectric constant approaches 1, the
cutoff frequency shifts toward high frequency and the upper limit of radiation
region frequency becomes infinite, increasing drastically the bandwidth of LWA,
as we can see from the simulation return loss of antenna Type III obtained with
a εr 1.1 shown in Figure 14. Moreover, from the
dispersion equation solved
with FDTD for εr 1.1 and for an average width of antenna Type
III (12mm)
we note (see Figure 15) flat values of β near 0.8 and flat values of α near 0. This produces a low variation of
mainlobe angle (ϑ=cos−1(β/K0)) for a wide range of high frequency and a
narrow shape of  the same mainlobe.Figure 14
Simulated return loss LWA type III with relative dielectric
constant 1.1.Figure 15
FDTD dispersion characteristics of LWA Type III first
high mode with relative dielectric constant 1.1 and width 12 mm.


## 4. Conclusions

In this study, a
new design of broadband microstrip leaky-wave antenna from 8 to 11 GHz was
proposed with high added value. An FDTD code was used to determinate the
propagation constant of LWA, which was necessary for designing a smooth contour
of LWA, obtained from the interpolation of the cutoff phase constant or
attenuation constant curve calculated by varying the frequency of a multisection
broadband microstrip LWA. The simulations and the measured results, of this
curve tapered microstrip leaky-wave antenna with a physical grounding structure
along its length, demonstrate the good performance compared to conventional
uniform microstrip LWAs (wider band and higher gain). This performance improves
whenεr approaches 1 and indicates that this structure
is attractive for the design of high-performance microstrip leaky-wave antennas
for microwave and millimeter wave applications.



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*Source: 742050-2008-08-03.xml*