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Characteristics Study of Four Coplanar Waveguide Feeding
Devices
Wenwen Chai1,2 , Xiaojuan Zhang1 , and Jibang Liu1,2
1
Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, China
2
Graduate School of Chinese Academy of Sciences, Beijing 100039, China
Abstract— Four types of coupling slots on the coplanar waveguide (CPW) are studied and
their effects on resonance frequency and bandwidth of microstrip antennas are simulated by the
finite-difference time domain (FDTD) method. The simulation results are explained by the near
field distributions. Feeding characteristics of four CPW coupling devices obtained by analysis
and comparison greatly improve the flexibility of antennas design and will be very useful for
wireless communication.
DOI: 10.2529/PIERS060904222227
1. INTRODUCTION
The microstrip patch antennas have a lot of merits such as low-cost, low-profile, conformability,
and ease of manufacture, which make them very attractive. The primary barrier to implementing
these antennas in many applications, however, is their limited bandwidth — only on the order of a
few percent for a typical patch radiator [1, 2]. Because of this fact, much work has been devoted to
increasing the bandwidth of microstrip patches. A straightforward method is to use a thick substrate
with a low dielectric constant for the antenna. Another technique which can be implemented in the
aperture coupled configuration is to use a near-resonance aperture in combination with the thick
antennas substrate [3, 4] or multilayer substrates to achieve wide frequency band. But the common
aperture coupled structure where the slot in the ground plane is fed via a microstrip line on an
additional substrate layer which complicates the antenna design. Coplanar waveguides have been
suggested as an alternate to microstrip-line for feeding the microstrip antenna [5], and they have
been used increasingly in the design of millimeter-wave microstrip antennas.
However, there haven’t been yet any special papers which systematically analyze feed characteristics of the coplanar waveguide with coupling slots. Although the coplanar waveguide with a
rectangular slot has been studied in some papers [6], the effects of feeding structure dimensions
on the antenna performance aren’t presented concretely. In this paper, the feeding performance
of coplanar waveguides with rectangular, dual-T shape, and H-shape slots are studied respectively,
and the relations between antennas structure and behavior are shown and explained by the near
field distribution.
2. MODEL ANALYSIS
In a coplanar waveguide-fed microstrip antenna, the antenna and coplanar line are placed on the
opposite sides of the same dielectric substrate and the coupling from the coplanar line to microstrip
antenna is accomplished via a slot in the ground plane connected directly to the end of the coplanar
line. In general, slot coupling may involve an electric polarisability, a magnetic polarisability,
or both. In slot coupling to a microstrip antenna, the magnetic polarisability is the dominant
mechanism for a slot near the center of the patch [7]. Because the polarisabilities strongly depend
on the shape of the slot as well as the size, it is desirable to improve the antenna performance by
optimizing the shape and size of coupling slot for given antenna dimensions.
Microstrip patch antennas model fed by the CPW are shown in Fig. 1. Antennas A, B, C, D
are fed respectively through inductive rectangular, capacitive rectangular, dual-T shape, and Hshape slots. The width ws of parallel and vertical slots which equals to 1 mm affects the coupling
level very weakly, so antennas performances are analyzed only by changing the slot length ls and ld
(ld ≥ ws). The tendency for the variation of parameters is derived by FDTD method. In the FDTD
simulation, the selected space step lengths are ∆x = 0.1 mm, ∆y = 0.2 mm, and ∆z = 0.2 mm,
respectively, and the selected time step length is to ∆t = 0.27 ps, satisfy the courant stability
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condition. A modulated Gaussian pulse is used as the excitation source. The perfectly matched
layer (PML), introduced by Gedney, is used to truncate the FDTD lattices.
Figure 2 shows the evolution of resonant frequency and impedance bandwidth (VSWR< 2) of
antenna A as a function of the slot length (ls). The resonant frequency decreased nearly linearly
with the increase of ls. The bandwidth was greater than 0 only when ls was limited to a span
(lsmin , lsmax ), and the maximum bandwidth was achieved as ls was around (lsmin + lsmax )/2.
(a) Antenna A
(b) Antenna B
(c) Antenna C
(d) Antenna D
(e) Side view
Figure 1: Microstrip patch antenna fed respectively by CPW with different shape slots.
Fixed dimensions: a = 23, b = 17.6, ε = 2.2, h = 1.6, ws = 1, w = 3 unit: mm.
(a) Resonant frequency
(b) Impedance bandwidth
Figure 2: Behavior evolution of antenna A as a function of the slot length (ls).
Figure 3 shows the influence of the length of the slot (ls) on resonant frequency and impedance
bandwidth of antenna B. As ls increased, the resonant frequency decreased linearly which was similar to antenna A. But bandwidth curves of two antennas were different obviously. From Fig. 3(b)
it was seen that the bandwidth varied relatively weakly, and ls could guarantee good antenna
bandwidth in a large span.
The influence of the length of the vertical feeding slot (ld) of antenna C at several ls value
points is represented in Fig. 4. Resonant frequency was reduced as ls or ld increased, and it was
seen that the resonant frequency changed with ld more strongly if ls became larger. When ls was
fixed, ld had a max value ldmax that could assure bandwidth greater than 0, and ldmax became
smaller with the increase of ls. The maximum bandwidth value achieved about at ldmax /2 varied
very little when ls < 0.22λ0 (λ0 ≈ 62.5 mm), but when ls ≥ 0.22λ0 the maximum bandwidth which
would be achieved at ld = ws was obviously reduced with the increase of ls.
Figure 5 presents the resonance and bandwidth characteristics of antenna D as a function of
the length of vertical feeding slot (ld) at several ls value points. As ls or ld was increased, the
resonant frequency decreased linearly. The bandwidth curve differed sharply with that of antenna
C because ls and ld could assure good antenna bandwidth in a large span as shown in Fig. 5(b).
PIERS ONLINE, VOL. 3, NO. 1, 2007
(a) Resonant frequency
50
(b) Impedance bandwidth
Figure 3: Behavior evolution of antenna B as a function of the slot length (ls).
(a) Resonant frequency
(b) Impedance bandwidth
Figure 4: Behavior evolution of antenna C as a function of the vertical slot length (ld).
3. RESULT COMPARISON AND DISCUSSION
Dual-T shape slot and H shape slot are both formed by loading the rectangular slot, while the
difference lies in their connection way with coplanar waveguide: the former is inductive, while the
latter capacitive. Based on above result curves, it can be also concluded:
(1) with the same feed slot, the bandwidth and gain of inductively fed antennas are better than
that of capacitively fed antennas, but the resonant frequency of the former is greater.
(2) when the length of feed slot (ls) is fixed, dual-T shape slot and H shape slot on coplanar
waveguide can attain far greater coupling level than rectangular slot by appropriately tuning the
length ld of vertical feed slot, therefore resulting in a wider impedance band.
(3) For a given antenna, different feeding types can lead to different resonant frequencies. For
example, when ls = 13 mm and ld = 1 mm, 400 MHZ frequency shift can be achieved from antennas
A to antennas B, while 500 MHZ from antennas C to antennas D. Therefore, it is reasonable to
(a) Resonant frequency
(b) Impedance bandwidth
Figure 5: Behavior evolution of antenna D as a function of the vertical slot length (ld).
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tune or switch the frequency of operating using a varactor diode that can modify the coupling level
in order for the antenna optimization.
It is visualized to analyze the effect of coupling slot on resonant frequency and bandwidth of
antennas by the near field distribution. The current distributions on the patch without slot and
with slot on the coplanar waveguide are presented respectively in Fig. 6(a) and (b). It is obvious
that the existence of feed slot makes patch current mainly concentrate between the slot and patch
edge, and the equivalent current path deviate more seriously as ls becomes larger. The deviation
could lengthen the equivalent path and hence reduce the resonant frequency. On the other hand, for
inductively fed antennas, oversize and undersize slots both lead to impedance mismatch, decreasing
coupling degree and increasing energy loss, so antennas impedance bandwidth gradually reduced to
0 with the variation of slot size. Moreover, oversize slot also increases back radiation, consequently
reducing antennas efficiency.
(a) Without slot
(b) With slot on CPW
Figure 6: Current distribution on the patch.
4. CONCLUSIONS
Resonant frequency and impedance bandwidth of antennas fed by four types of coupling slots on
coplanar waveguide are studied and compared. Feed rules of these coupling devices are attained
and explained by near field distribution. The antennas can be designed flexibly based on the
experimental conclusion. The freedom in modifying the coupling simply by tuning the varactor
diode helps to realize the optimum impedance characteristics in practice.
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