Planar Finger-Shaped Antenna Used in Ultra-Wideband Wireless Systems

Recently, extensive requirements have developed for ultra-wideband (UWB) technology that provides high activity and small size for use in small communication systems, remote sensing, and radar applications. Thus, we concentrated with high-resolution radar UWB antenna to cover Federal Communication Commission’s (FCC) standard UWB range (3.1-10.6 GHz). The proposed ideal-size and low-cost finger-shaped patch antenna of 24 mm x 19 mm printed on 40 mm x 35 mm rectangular Taconic TLY-5 material was designed and established through experiments and simulations. Results show that the two cut notches of 1.5 mm x 2 mm at the bottom corners of the patch can increase the bandwidth. To increase the radiation area and achieve more resonance frequencies, two cut slots at the top edge of the patch were designed in depth of 10 mm. Four parameters were considered in the analysis of the proposed antenna design, namely the feeder width, number of slots, number of notches, and feed gap space. The simulated and measured results of the main antenna parameters make the design suitable in applications of UWB wireless systems.


Introduction
There are many applications for UWB technology at the present time, such as mobile radio and wireless communication with pulses that are less than a nanosecond.UWB techn0logoy is used extensively because it provides several advantages, including 1) low complexity and low cost, 2) high quality service, 3) high transmission rates, and 4) operational flexibility [1].To meet the requirements of applications in wireless communication systems, small microstrip antennas can be designed that have ultra-wide bandwidths of frequency that allows them to achieve high-capacity bit rates of several hundred Mbps or even Gbps for distances up to 10 meters.The UWB frequencies are allocated by the Federal Communications Commission (FCC) in the United States, and the frequency spectrum ranges from 3.1 to 10.6 GHz with a low-power spectral density of -41.3 dBm/MHz [2].Thus, we proposed a sensitive microstrip-patch antenna with a reflection coefficient of less than -10 dB that has high gain, good directivity, small size, and transmission-line feeding but which is also lightweight.Many shapes have been designed to achieve the desired UWB characteristics, such as the diamond antenna [3] for the operational band 3.38 to 14 GHz; the T-slotted, ground-plane antenna [4] with a frequency range of 3.1 to 11.5GHz; the 'smart antenna' for spatial rake receivers [5]; UWB slotted microstrip patch antenna using FR4 material of 4.4 dielectric constant and dimensions are 30mm x 55mm [6]; the dual-band, notched antenna with curved was proposed by [7] to operate from 2.5 to 12 GHz; the two-port, circular-patch antenna for divers applications [8]; the bow-tie, planar, wideband dipole with a frequency range between 4 and 9 GHz [9]; a four-element, microstrip, antenna array that was presented in [10] to attract the radiated data in the FCC frequency range; a planar monopole antenna with an octagonal-shaped patch that was produced by [11] and multi-band and wideband antenna was designed by [12] to operate in the allocated bandwidth for UWB wireless communications; fork-shaped radiating patch was proposed by [13] to cover Bluetooth and UWB frequency bands and the fabricated substrate  ISSN: 1693-6930 TELKOMNIKA Vol. 12, No. 2, June 2014: 419 -428 420 was made of FR4 material; a four-filter antenna with three split resonators that was suggested to offer dual band WiMAX and WLAN applications [14,15,16], while a CPU-fed, planar, inverted-cone antenna was fabricated to cover the bandwidth from 1.3 to 11 GHz with maximum gain [17].In order to reduce the cost of antennas and also for reducing the cost, antenna was designed in [18] with substrate made of textile material that used to produce jeans, in addition the patch and the ground plane were made of copper to be suitable for UWB bandwidth.A tuning fork type was proposed and simulated by [19] for operating frequency range of 3.7 GHz to 13.8 GHz with printed patch antenna on a FR4 substrate.
In this paper, we concentrated on the design of ultra wideband rectangular patch with two notches on each bottom side of the patch to achieve the bandwidth around 3 GHz to 12 GHz.The antenna had three slots to extend the radiating element reasonably over the operating band 3 -12 GHz, which is sufficient for UWB operation.The proposed antenna is smaller in size compared with the previous designs to be suitable for small UWB communication systems.In Section 2, detailed design and the dimensions are presented and followed by Section 3 of fabricated antenna design.After that simulated and measured results with their discussions and conclusions are presented in Sections 4 and 5, respectively.

Configuration of the Proposed Antenna
The small size and easy to fabricate antennas are two features strongly desired for modern UWB communication systems.Another consideration has to be taken into account is to provide good matching between feed section and radiator part.The geometry of the proposed antenna consists of the following parts, which are suitable for integration with a printed circuit board (PCB).

The substrate
The length of rectangular microstrip antenna is one-half wavelength if the substrate is air gap and the length decreases when using substrate material of valuable dielectric constant [23].Taconic TLY-5 material was used in the fabrication of the antenna which was manufactured with very lightweight woven fiberglass and had the following characteristics: dimensional stability; a low dielectric constant range ( r = 2.2), a low dissipation factor (approximately 0.0009 at frequency of 10 GHz); made of high-speed digital material; and a wide range of bandwidths.The width, length, and height are denoted by W, L, and h respectively, and the proposed rectangular dimensions were: W = 40 mm, and L = 35 mm, and h = 1.575 mm.The small size of this antenna provides the flexibility required for use in small, wireless communication systems, such as mobile systems, laptop devices, indoor receivers and transmitters, and narrow band radio systems.The effective dielectric constant ( reff ) of the patch was determined to be 2.1 from the following equation [20]:

Ground plane
The rectangular ground plane was made of copper that had a thickness of 0.035 mm, and it was printed on the back of the substrate.The ground plane has a length (L gp ) of 11.6 mm and a width (W gp ) of 40 mm, which were optimized to obtain the best return loss (S 11 ) (less than -10 dB) and impedance bandwidth.The various dimensions were chosen to increase the effective length bellow the 4 GHz frequency band.

Feed gap
The bandwidth of the antenna was independent of the feed gap of the ground plane since the ground plane, served as an impedance-matching circuit .The size of the feed gap allowed us to obtain a wider impedance bandwidth and omni-directional radiation patterns .The optimal value of the thickness of the feed gap (L gap ) was determined to be 0.

Microstrip feed
Four popular configurations are used to feed microstrip antenna, i.e., microstrip line, coaxial probe, aperture coupling, and proximity coupling.The configuration we used in the proposed antenna was the microstrip line feed made of copper with t = 0.035 mm (thickness), L f = 12 mm (length), and W f = 4.7 mm (width).

Microstrip patch element
Many shapes of the microstrip patch have been designed as radiators for UWB radiation.There are two degrees of freedom (length and width) for controlling a rectangular microstrip antenna, so that the order of changing the mode can be done by varying the relative dimensions of the width and length of the patch based on resonant frequency.The patch dimensions of W p x L p can be determined according to the following procedure when the resonant frequency (f r ) is 5 GHz [21].
where, c is the speed of the light in free space (3 10 m/s), and the width (W p ) of the patch was determined to be 24 mm.The extended length of the patch ( L) can be calculated as: The actual length of the patch (L p ) can be calculated by applying equations ( 4) and ( 5) to get 19 mm: where r  is the resonant wavelength ( r  = c/f r ) .
Fig. 1 illustrates that each radiating edge of the patch is represented by the admittance (Y) of real part denoted by conductance (G) and the imaginary part denoted by susceptance (B), so that Y = G + jB.To enhance the conventional rectangular UWB antenna, we divided the upper patch edge into three sub-edges (fingers) by making two slots of equal dimensions through the upper patch edge.These three fingers were proposed to increase the radiation area and to get three resonant frequencies that used to expand the bandwidth.As the total resonant input admittance is real, the total input resistance also is real, and it is given by [22]:  ISSN: 1693-6930 TELKOMNIKA Vol. 12, No. 2, June 2014: 419 -428 When the mutual effects are taken into account to calculate the exact value of the input impedance, mutual conductance exists between the edges of the patch, so the input resistance can be expressed as: where G 1 is the conductance of the bottom edge of the patch and G 12 is the mutual conductance between upper and lower edges of the patch.The signs (+) and (-) are used for modes by odd and even resonant voltage distribution, respectively.
where R in (L s = value) is the input impedance at L s inset feed point and can be taken as 50 Ω.
R in (L s = 0) is the input impedance at the top edge of the patch, and, when W p <  : The mutual conductance (G 12 ) can be determined by equation (11), where J 0 is the Bessel function of the current density at zero order.By equations (9)(10)(11), the depth of each slot (L s ) was calculated to be 10 mm through the antenna patch [20].The schematic simulated diagram was derived from the reference rectangular antenna with rectangular printed patch on the front side of the Taconic TLY-5 substrate of thickness 1.575 mm and dielectric constant of 2.2.This geometrical diagram of front and back view is shown in Fig. 2 with all dimensions are defined in Table 1.In this design, there four parameters: feeder width, feed gap space, number of cut slots, and number of cut notches were modified to achieve a wider impedance bandwidth and a better impedance matching.

Geometry of the Fabricated and Printed Antenna
The overall structure of the antenna was successfully fabricated using Taconic TLY-5 material for substrate and copper of 0.035 mm thickness to print radiator and ground plane to be integrated with other system elements.The geometry of the fabricated design is shown in Fig. 3 of prototype mounted on the mentioned material substrate with connector of 50 Ω input impedance.

Simulated and Practical Results and Discussion for the Proposed Antenna
The performance of an antenna can be measured based on its reduction of the reflection power requirement on the transmission line to the source, which depends on the voltage reflection coefficient at the input terminals of the antenna.The practical and simulation frequency band was 3-12 GHz (ratio bandwidth was 4:1) for the designed antenna to determine its stable radiation characteristics.Fig. 4 illustrates the experimental and simulated return loss To do antenna design optimization, parametric studies for considered four parameters are shown in Figure 6 when CST Microwave Studio Software was carried out in simulation.In parametric study of each parameter, the CST software was run at various parameters when other parameters were kept invariant.From Fig. 6 (a), the effect of notch cut is illustrated to improve the parameter of using two notches cut at each lower corner of the patch.The return loss curve of two notches is covering the UWB frequency while that of one notch covers up to 8 GHz.Second parameter was proposed is the number of slots at the top edge of the patch.The return loss curves are shown in Fig. 6 (b) with no slot, one slot, and two slots.One can conclude that, a wider impedance bandwidth can be obtained with a patch of two slots at the upper edge in order to cover the UWB bandwidth which is from 3.1 to 10.6 GHz.The other considered parameter is feed gap size (L gap ) parameter that was used to optimize the widest bandwidth; the CST software was run at various values of L gap .Fig. 6 (c) shows the calculated return loss curves with three values of L gap .It can be seen that at increasing feed gap size results reductions in bandwidth and return loss.Hence, we conclude that wider impedance bandwidth can be obtained by tuning the feed gap size.After that the software also was run at various feeder width (W f ) values.The results of this variation are presented in Fig. 6 (d) to show the optimal W f was found to be at 4.7 mm that kept the input impedance equals to 50 Ω, so that no need to decrease or increase W f and this size is suitable for tolerance with the soldered 50 Ω connector.Return Loss / dB -

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Simulated S11 with 0.15 mm gap Simulated S11 with 0.25 mm gap Simulated S11 with 0.4 mm gap (c) (d) Figure 6.Proposed parametric study; (a) number of notch, (b) number of slot, (c) feeder width, (d) feed gap size Fig. 7 shows the three-and two-dimensional, omni-directional patterns at frequency of 7.5 GHz, which describe antenna's performance over the entire UWB bandwidth.Simulated current distributions are illustrated in Figure 8 at resonant frequencies of 5 and 3.8 GHz to facilitate the understanding of the antenna's performance.For two set notches and two slots, the figure shows that the surface current on the radiator surface of the antenna was greater at resonant frequency of 5 GHz than that at 3.8 GHz.The peak values of the surface current distribution of the UWB radiator at the notches and the slotted edge was 63.7 A/m at 5 GHz and 23.1 A/m at 3.8 GHz; these values were greater than those presented in [19] at the steppers and convex corners of the radiator.From the simulation results, the values of the proposed antenna design parameters are presented in Table 2 antenna for the chose frequencies over the coverage UWB bandwidth.Fig. 9 shows the simulated radiation patterns of the far-field antenna at the x-y H-plane (Theta = 0 o , 90 o ) and at the y-z E-plane (Phi = 0 o , 90 o ).These patterns were compared with different values of frequency, i.e., 4.5, 7, and 10 GHz for the H-plane and the E-plane.The radiation patterns essentially were omni-directional in the E and H planes for the simulation of the proposed antenna.
The fabricated antenna was tested using anechoic chamber by fixing the antenna on a stand using 50 Ω connector which is soldered at the lower edge of the feed line.So that by Anechoic chamber and network analyzer, the elevation (E-plane) and azimuth (H-plane) patterns were measured for the proposed antenna in a planes containing feed.These measured radiation patterns are shown in Fig. 10 at three operating frequencies: 4.5 GHz, 7 GHz, and 10 GHz in H-plane (xz-plane) and E-plane (xy-plane) and a little change in patterns due to frequency increase.The measured E-planes and H-planes show mostly omni-directional radiation patterns over the specified frequencies.In measured patterns, there are many ripples at the radiation amplitude due to into the field reflections from antenna holder, chamber scattering, and from inside of the anechoic chamber.To the proposed design with those of [4] and [7], the proposed finger-shaped antenna is smaller in dimension size, higher gain, using lighter and cheaper substrate material, and more stable omni-directional radiation patterns.

Conclusions
For UWB wireless applications and to reduce costs, a small size of planar finger-shaped patch antenna was designed to cover the wideband frequency range (3 -12 GHz) to be suitable for indoor and outdoor propagation of wireless UWB communication systems.The cutting slots and notches at the top and bottom edges of the patch radiator contributed to the extension of the radiation area and the evaluation of the current distribution behavior.These cutting slots and notches enhanced the performance of small antenna design over UWB frequency range.The antenna was fabricated and tested, and there was a good agreement between simulated and measured results for the most important antenna parameters such as return loss (S 11 <-10 dB) and omni-directional radiation patterns.
4 mm in order to TELKOMNIKA ISSN: 1693-6930  Planar Finger-Shaped Antenna Used in Ultra-Wideband .... (Rashid A. Fayadh) 421 produce nearly-pure resistive input impedance to cover the UWB range in wireless communication systems.
Fig.1illustrates that each radiating edge of the patch is represented by the admittance (Y) of real part denoted by conductance (G) and the imaginary part denoted by susceptance (B), so that Y = G + jB.To enhance the conventional rectangular UWB antenna, we divided the upper patch edge into three sub-edges (fingers) by making two slots of equal dimensions through the upper patch edge.These three fingers were proposed to increase the radiation area and to get three resonant frequencies that used to expand the bandwidth.For resonant input resistance analysis, the admittances of the fingers are Y 21 , Y 22 , and Y 23 , where Y 21 = G 21 + jB 21 , Y 22 = G 22 + jB 22 , and Y 23 = G 23 + jB 23 .Y 2 is the parallel equivalent admittance of Y 21 , Y 22 , and Y 23, and it is equal to G 2 + jB 2 .Since the fingers are identical, G 2 = G 21 = G 22 = G 23, and B 2 = B 21 = B 22 = B 23 , therefore G 1 = G 2 and B 2 = -B 1 .As the total resonant input admittance is real, the total input resistance also is real, and it is given by[22]:

Figure 1 .
Figure 1.Equivalent circuit of the transmission model

Figure 2 .
Figure 2. Dimensions of the proposed antenna (a) front view, (b) back view

Figure 3 .
Figure 3.The fabricated antenna being tested on anechoic chamber and network analyzer: (a) front view; (b) back view

FrequencyFigure 4 .
Figure 4. Simulated and measured S 11 of Figure 5. Characteristics of antenna gain with the proposed antenna increasing bandwidth for the proposed design

Table 2 .
The values of antenna parameters at specified UWB frequencies