COMPACT UWB PLANAR ANTENNA FOR BROADBAND
APPLICATIONS
Rezaul Azim1, Mohammad Tariqul Islam2, Norbahiah Misran3 and Ahmed Toaha Mobashsher4
1,3,4Department of Electrical, Electronic and Systems Engineering, Universiti
Kebangsaan Malaysia, 43600 UKM Bangi, Selangor D., E., Malaysia. 2,3Institute of Space Science (ANGKASA), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor D., E., Malaysia.
Key words: Broadband, Planar antenna, ultrawideband (UWB) antenna, partial ground plane.
Abstract: In this paper, a planar antenna for broadband applications has been proposed. The antenna consists of a rectangular 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 14.75 x14.5mm2. The result shows that the impedance bandwidth (VSWR < 2) of the proposed antenna is 12.87 GHz (3.02 to 15.89 GHz), which is equivalent to 136.12%. Details of the proposed compact planar UWB antenna design is presented and discussed.
Kompaktna UWB planarna antena za širokopasovne aplikacije
Kjučne besede: Širokopasoven, planarna antena, ultra široko pasovna (UWB)antena, delna ozemljitvena ploskev
Izvleček: V članku predstavimo planarno anteno za širokopasovne aplikacije. Antena je sestavljena iz pravokotne zaplate, ozemljitvene ploskve in reže v ozemljitveni ploskvi. Antena z lahkoto integriramo v mikrovalovna vezja, s čimer dosežemo nizke proizvodne stroške. Ploskovna antena ima kompaktno strukturo in je velika 14.75 x14.5mm2. Rezultati nam pokažejo, da je impedančna pasovna širina (VSWR < 2 ) antene 12.87 GHz (3.02 do 15.89 GHz), kar je enako 136.12 %. V članku predstavimo podrobnosti izvedbe kompaktne UWB antene in rezultate.
1. Introduction
Ultra-wideband (UWB) communication systems draw great attention in the wireless world because of their advantages, like high speed data rate, extremely low spectral power density, precision, high precision ranging, low complexity and low cost since the Federal Communications Commission(FCC) allowed 3.1 to 10.6 GHz unlicensed band for UWB communication/1/. UWB also have wide applications in short range and high speed wireless systems, such as ground penetrating radars, medical imaging system, high data rate wireless local area networks (WLAN), communication systems for military and short pulse radars for automotive even or robotics. The antenna is one of the crucial components which determine the performance of UWB system. The UWB antennas proposed in the open literature mainly focus on the slot and monopole antenna /2-5/. Printed wide slot antennas have an attractive property of providing a wide operating bandwidth, especially for those having a modified tuning stub, such as the fork-like stub /6/-/8/, the rectangular stub /9/, and the circular stub /10/ inside the wide slot. Broadband planar monopole antennas have received considerable attention owing to their attractive merits, such as ultrawide frequency band,
good radiation properties, simple structure and ease of fabrication. The typical shapes of these antennas are half-disc /11/, circle, ellipse /12/, and rectangle /13/.
One of the popular UWB antenna types requires a perpendicular ground plane, which resulted in increased antenna size and difficult to integration with microwave-integrated 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 broadband 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 broadband applications that achieves a physically compact planar profile, sufficient impedance bandwidth and highly stable bi-directional radiation pattern is proposed. The planar antenna consists of a rectangular shaped radiating patch and partial ground plane with a rectangular slot on the upper edge to cause a broad bandwidth from 3 to 16.0 GHz frequency. The antenna structure is flat, and its design is simple and straightforward.
2.	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 14.75x14.5mm2, 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 30x7.5mm2 in this study. The dimension of the slot is 6.0x0.9mm2 and 3.5mm away from the left edge of the ground plane. The bottom of the patch is connected by a microstrip line, which is fed by a 50 W 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: Lsub = 22mm, Wsub = 30mm, Lp =14.75mm, Wp = 14.5mm, LG = 7.5mm, lf = 7.25mm, Wf = 4mm, We = 3.5mm, ws = 6.0mm and ls = 0.9mm.
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.
3.	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 12.87 GHz (from 3.02 GHz to 15.89 GHz) which is equivalent
Fig. 1.
Geometry of the proposed UWB antenna (a) front view (b) back view(c) side view.
Fig.2.
4 6 8 10 12 14 16 18 20 Frequency (GHz) Return loss of the proposed antenna
to 136.12 %. Its covers the entire UWB frequencies mentioned earlier.
Fig. 3 shows the antenna gain in a frequency range of 3-10 GHz. The maximum gain is 1.60 dBi with an average of 0.52dBi, which can meet the usual requirement of -4 dBi for broadband applications. The gain is affected by the size of the ground plane.
The radiation efficiency of the antenna in the frequency range of 2-10 GHz is shown in fig. 4. The antenna has a maximum of 65.34% radiation efficiency at 3 GHz. The use of a substrate with high dielectric constant and a direct microstrip feeder may be the cause for deterioration in the radiation efficiency.
Fig. 5 shows the radiation patterns of the proposed antenna at 4.4, 6.2 and 8.1 GHz. It is seen that proposed antenna has a main beam in the broadside direction (0°, 180°) over all operating frequency. At 8.1 GHz although,
6 7 Frequency (GHz)
Fig.3. Gain of the proposed antenna in 3-10 GHz
23456789 10 Frequency (GHz)
Fig.4. Radiation efficiency of the proposed antenna
the third harmonic radiation pattern (in cross-polarization) is observed, the antenna has a good stable radiation in the broadside direction without gain degradation unlike the existing UWB monopole antennas. At 8.1 GHz the difference of radiation level between copolarization (9 = 0°) and cross-polarization (9 = 90°) is approximately 6.69 dBi. The radiation pattern at 4.8 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 78" at 4.8 GHz, 68.5° at 6.2 GHz and 87.9° at 8.1 GHz and the radiation patterns are almost symmetric. In the H- plane, the 3 dB beamwidth is 81.3° at 4.8 GHz, 78.7° at 6.2 GHz and 60.8° at 8.1 GHz.
Fig. 6 illustrates the current distributions of the proposed antenna at different frequencies. Through a numerical study of the vector current distributions on the antenna, three characteristics current modes are found to exist over the bandwidth from 3.0 to 16.0 GHz. Using fig. 2 as a reference, each current mode is dominant at each resonance, 8.1, 9.7 and 14.4 GHz respectively. It is seen that, the strongest currents are concentrated on the edges of the patch, which can be easily from fig.6. At 8.1 GHz the polarity of the current on the left portion of the patch is upward (along vertical direction), while at the right portion it is downward as shown in fig.6 (a). At 9.7 GHz the polarity
Crosspolarization
Copolarization
0 0
270
90 270
180 180 f= 4.8 GHz
0
270
180 180 f= 6.2 GHz
0
270
f= 8.1 GHz
Fig.5. Radiation pattern of the proposed antenna (a) E-plane and (b) H-plane
on the upper portion of the patch (along vertical direction) is opposite to that of lower portion and very little amount of current is found on the centre of the patch as shown in fig.6 (b), while at 14.4 GHz the polarity of current on patch is downward in the upper portion and almost upward in the lower portion as shown in fig.6 (c). However, the current is uniformly distributed elsewhere.
4. Conclusions
A low cost, compact microstrip-fed planar UWB antenna has been proposed and implemented. The antenna size is 14.75x14.5 mm2. 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 136.12% (3.02 -15.89 GHz) at -10 dB. The relatively constant bidirectional radiation patterns and rather flat gain throughout the whole bandwidth makes the proposed antenna suitable for broadband applications.
(a)
(b)
(c)
Fig. 6.
Current distributions on the antenna at (a) 8.1 GHz, (b) 9.7 GHz and (c) 14.4 GHz.
Acknowledgment
The authors would like to thank the Institute of Space
Science (ANGKASA), Universiti Kebangsaan Malaysia for
sponsoring this work.
References
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Rezaul Azim1, Mohammad Tariqui /slam2, Norbahiah Misran3 and Ahmed Toaha Mobashsher4
1,3,4Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor D., E., Malaysia. rezaulazim@yahoo.com, i_toaha@yahoo.com, bahiah@vlsi.eng.ukm.my
2,3/nstitute of Space Science (ANGKASA), University Kebangsaan Malaysia, 43600 UKM Bangi,
Selangor D., E., Malaysia. titareq@yahoo.com