Compact planar antenna for UWB applications

In this paper, a planar antenna for UWB applications has been proposed. The antenna consists of a square patch, a partial ground plane and a slot on the ground plane. The proposed antenna is easy to be integrated with microwave circuitry for low manufacturing cost. The flat type antenna has a compact structure and the total size is 14.5×14.5mm2. The result shows that the impedance bandwidth (VSWR≤ 2) of the proposed antenna is 12.49 GHz (2.95 to 15.44 GHz), which is equivalent to 135.8%. Details of the proposed compact planar UWB antenna design is presented and discussed.

One of the popular UWB antenna types requires a perpendicular ground plane, which resulted in increased antenna size and difficult to integration with microwaveintegrated circuits. Compared with the three dimensional type of antenna, flat type UWB antenna printed onto a piece of printed circuit board (PCB) is a good option for many applications because it can be easily embedded into wireless devices or integrated with other microwave-integrated circuits. However, the antenna design for UWB applications faces many challenges.
A low profile and embeddable unidirectional antenna is required for certain UWB communication, imaging, localization, and radar applications. The lower and upper UWB spectrums are 3.1-4.8 GHz (43%) and 6.0-10.6 GHz (55%), respectively. The existing broadband directional antennas, such as the Vivaldi, log-periodic, cavity-backed, waveguide, horn, and dish antennas, cover the entire 3.1-10.6 GHz band (109%). However, they are electrically large, and have a high profile in the direction of wave propagation. Omni-and bi-directional antennas, such as the planar monopoles [14,15], disc cone [16], and slot antennas [17], have a low gain and back radiation pattern, therefore they are not suitable for sectorial or unidirectional communication. Also, it is a challenge to maintain a stable radiation pattern across the whole frequency band, since the radiation aperture is frequency dependent.
In this paper, a microstrip-fed antenna for the UWB applications that achieves a physically compact planar profile, sufficient impedance bandwidth and highly stable bidirectional radiation pattern is proposed. The planar antenna consists of a square shaped radiating patch and partial ground plane with a rectangular slot on the upper edge to cause a wide bandwidth from 3 to 10.6 GHz for UWB application. The antenna structure is flat, and its design is simple and straightforward.

II. ANTENNA GEOMETRY AND DESIGN
Antenna is the key element in UWB systems. The motivation of UWB antenna design is to design a small and simple antenna that introduces low distortions with large bandwidth. Fig. 1 illustrated the configuration of the proposed antenna, which consist of a squarer patch, a partial ground plane and a single rectangular slot on the ground plane. The antenna, which has a compact dimension of 14.5×14.5mm 2 , is printed in the front of a FR4 PCB substrate of thickness 1.6 mm and relative permittivity 4.6. The dimension of partial ground plane which is printed in the back side of the substrate is chosen to be 29×7.5mm 2 in this study. The dimension of the slot is 5.5×0.9mm 2   ground plane. The bottom of the square patch is connected by a microstrip line, which is fed by a 50Ω coaxial probe from the side of the antenna. The microstrip line was etched on the same side of the substrate as the radiator. The antenna has the following parameters: L sub = 22mm, W sub = 29mm, L P =14.5mm, W P = 14.5mm, L G = 7.5mm, l f = 7.5mm, W f = 4mm, w s = 5.5mm and l s = 0.9mm. There is no gap between radiating patch and ground plane. Three techniques: the use of (i) square radiating patch, (ii) a partial ground plane and (iii) a single slot on the ground plane applied to the proposed design lead to a good impedance matching. The geometric parameters of this structure can be adjusted to tune the return loss and bandwidth over wide range of frequency.

III. RESULTS
The performance of the proposed antenna has been analyzed and optimized by using commercially available method of moments based full-wave electromagnetic simulator IE3D [18]. The simulated return loss of the proposed antenna is depicted in fig.2.
The plot of the return loss shows that the impedance bandwidth of the proposed antenna is 12.49 GHz (from 2.95 GHz to 15.44 GHz which is equivalent to 135.8 %. Its covers the entire UWB frequencies mentioned earlier. Fig. 3 shows the antenna gain in a frequency range (3-10 GHz) and the maximum gain is 1.15 dBi. The gain is affected by the size of the ground plane. The radiation efficiency of the antenna in the frequency range of 3-10 GHz is shown in fig. 4. The antenna has a maximum of 65% radiation efficiency. The use of a substrate with high dielectric constant and a direct microstrip feeder may be the cause for deterioration in the radiation efficiency. GHz shows that the difference in radiation level is relatively low compared to other frequency data. The differences of polarized radiation levels against the frequencies provide the advantage of minimizing fading effects by multicurrent paths in wireless communications [19]. In the E-plane, the 3 dB beam width is 85. 1 GHz respectively. It is seen that for the antenna mode, the square patch is more like a resonance curved halfwave length dipole, rather than a low Q factor disc resonator. It is also seen that, the strongest currents are concentrated on  the edges of the patch. At 3.3 GHz the current's polarity on the patch is upward (along + y-axis), at 8.0 GHz the polarity is downward while at 10.1 GHz the polarity on the left side of the patch (along x-direction) is opposite to that of right side and very little amount of current is found on the centre of the patch. However, the current is uniformly distributed elsewhere.

IV. CONCLUSION
A low cost, compact microstrip-fed planar UWB antenna has been proposed and implemented. The antenna size is 14.5×14.5 mm 2 . The use of rectangular slot on the upper side of the partial ground plane improves not only the impedance matching in high frequency band but also the radiation characteristics at high frequencies. The antenna structure is flat, and its design is simple and straightforward, so it is easy to fabricate. The proposed antenna achieved a bandwidth of 135.8% (2.95 -15.44 GHz) at -10 dB. The relatively constant bidirectional radiation patterns and rather flat gain throughout the whole bandwidth makes the proposed antenna suitable for UWB communication applications.