Design and Analysis of a Novel Compact Wideband Antenna with Two Excited Modes

A novel planar compact wideband antenna with coplanar waveguide (CPW) feeding configuration is presented in this paper. The proposed antenna is fed and coupled by the CPW feeding line at the same time. Therefore, two distinct excited modes are realized. Besides, detail investigation is presented as well to demonstrate the properties of the proposed antenna. Eventually, the prototype of the proposed antenna with optimal design is fabricated and tested. The proposed antenna, with a fairly compact size of 24 mm× 18 mm× 1 mm, has a measured 10 dB impedance bandwidth spanning from 3.05 GHz to 6 GHz, about 65.2%. Moreover, stable radiation patterns over the operating band are obtained. Furthermore, time-domain characteristics of the proposed antenna are also investigated.


Introduction
The need of high data rates wireless communication becomes more and more urgent, and various solutions have been brought forward.Wideband antenna techniques have been paid the most attention for many advantages, such as higher data rates, immunity to multipath cancellation, increased communications operational security, and low interference to legacy systems [1].The wideband antenna techniques will play an important role in the short-range wireless communication systems.One of the major challenges in the design of wideband antennas is how to achieve small size with desired electrical properties in the band of interest.On the other hand, planar wideband antennas have attracted much interest for applications due to their simple geometry and easier-to-integrate as well as good impedance properties.
In the design of one planar wideband antenna, the shape of the antenna patch, the ground plane, and the feeding structure are of great importance.Different methods such as the truncated slot on the antenna patch or on the ground plane have been proposed for increasing the frequency range and improving the radiation pattern characteristics [2][3][4][5].Many planar wideband antennas presented in [6][7][8] possess compact antenna size.However, as another important transmission property, the time-domain characteristic of wideband antennas is investigated rarely.
This paper presents a compact wideband antenna with CPW feeding configuration.Because the antenna can be coupled and fed by CPW feeding line at the same time, two distinct excited modes were obtained.The proposed antenna demonstrated both a compact size and a wide impedance bandwidth.The size of the proposed antenna is only 24 mm × 18 mm, and the operating band is ranging from 3.05 GHz to 6 GHz.Details of the proposed antenna designs are presented.Measured results of the impedance bandwidth, the radiation patterns, and the simulated timedomain characteristics are given and discussed.

Antenna Design
Figure 1(a) shows the physical structure and dimensions of the proposed antenna.A pair of stepped structures and a coplanar waveguide feeding line are etched on the dielectric substrate (in this study, the FR4 substrate of thickness 1 mm and relative permittivity 4.4 was used).The length and the width of the dielectric substrate are 24 mm and 18 mm, respectively.The right stepped structure is directly connected to the CPW feeding line by a metal line, and the left stepped  structure links to the ground via the other metal line.As a result, the right stepped structure is directly fed by CPW, and the left stepped structure is coupled by the CPW feeding line.

Results and Discussion
The antenna prototype, as shown in Figure 1  of the excited methods between the left portion and the right portion of the proposed antenna, the current distributions at two frequencies are completely different.At 4.5 GHz, the current mainly distributes around the right portion and the stepped slot.However, the surface current of the proposed antenna appears mostly in the left portion at 5.5 GHz.The two excited modes can generate two resonant frequencies, and when they are close to enough, a wide bandwidth can be achieved.
Figure 4 shows the measured radiation patterns of the proposed antenna in the x-y and x-z plane for 4.5 GHz and 5.5 GHz.Clearly, the x-y plane radiation pattern is close to omnidirectional, and the x-z plane radiation pattern is monopole-like at 5.5 GHz.Meanwhile, similar radiation patterns with only few limited distortions are observed at 4.5 GHz.Compared with at 5.5 GHz, when the antenna is mainly coupled by the CPW feeding line, the antenna is directly fed by the CPW feeding line basically at 4.5 GHz.Therefore, more energy is excited, and some extra currents distribute along the stair slots at 4.5 GHz, which leads to broadside radiation pattern.
Considering the requirements of UWB communications, both good frequency-domain and time-domain characteristics are essential to the quality of the communications.Thus, the time-domain characteristics of the proposed antennas are investigated by placing the virtual probes at a distance of 1 m from the antennas.We can assume that the probes are in the far-field region of the antenna, since the distance is more than 10 wavelengths at the lowest frequency for the proposed antenna.The polarization of the probes is linear.Moreover, the copolarizations of the probes point to the copolarizations of the antennas.The whole time-domain test system operates over the frequency band ranging from 3 to 6 GHz. Figure 5 shows the transmitted signal and received signals of the proposed antenna with different angles.And considering the quasiomnidirectional characteristic of the antenna and the space limitations, only three different angles are presented here.From the results, it can be seen that the received signals of the proposed antenna are similar to the transmitted signal with only few limited distortions introduced.
According to the definition in [9], the fidelity factor values of the proposed antenna with different angles of phi are calculated and listed in Table 1, from which it can be seen that the fidelity factor of the proposed antenna is very close to unity.However, slight tailing phenomenon of the received signal is still can be observed because of the dispersion of the antenna and the loaded slots.On the other hand, the levels of the tailing are different, since the direction of the proposed antenna is not strictly omnidirectional.

Conclusion
In this paper, a novel compact wideband antenna with two distinct excited modes is proposed and investigated.The measured impedance bandwidth of the proposed antenna is from 3.05 GHz to 6 GHz.Furthermore, the antenna also demonstrates a fairly compact size of 24 mm × 18 mm × 1 mm.Good and stable radiation patterns are observed.Featuring of good frequency-domain characteristics and time-domain characteristics as well as easy design, the proposed wideband antenna is a very good candidate for short-range wireless communication.

2 InternationalFigure 1 :Figure 2 :
Figure 1: The configuration of the proposed antenna and photograph of prototype.(a) Configuration of the proposed antenna, (b) photograph of prototype.

Figure 3 :
Figure 3: The current distributions of the proposed antenna (a) at 4.5 GHz (b) and 5.5 GHz.
(b), was fabricated and tested.Figure2shows the measured reflection coefficient of the proposed antenna.It can be seen that the impedance bandwidth of the proposed antenna is from 3.05 GHz to 6 GHz, about 65.2%.The current distributions of the proposed antenna at 4.5 GHz and 5.5 GHz (simulated by CST) are illustrated in Figure3.Because of the difference x

Figure 4 :
Figure 4: Measured radiation patterns of the proposed antenna (a) at 4.5 GHz (b) and 5.5 GHz.

4 InternationalFigure 5 :
Figure 5: Transmitted signal and received signals of the antenna with different angles of phi, at theta = 90 degree.(a) Transmitted signal, (b) received signal of the proposed antenna with angle phi = 240 degree, (c) received signal of the proposed antenna with angle phi = 270 degree, and (d) received signal of the proposed antenna with angle phi = 300 degree.

Table 1 :
Fidelity factors of the antenna with different angles of phi at theta = 90 degree.