Microstrip feed Spanner Shape Monopole Antennas for Ultra Wide Band Applications

An optimized design for the spanner shape regular hexagonal monopole (RHM) patch antennas has been reported to yield ultra wide band (UWB) impedance bandwidth. Impedance and radiation characteristics are presented and discussed. From the results, it has been observed that, the impedance bandwidth, defined by 10 dB return loss, can reach a value of 8.63GHz. A rectangular slit in the radiating patch is used to reduce the loweredge frequency resulting in improved bandwidth. This technique yields the ability to construct a smaller antenna in comparison to a simple printed planar monopole antenna for a given frequency range. The printed metallic patch is defected for increasing the operating bandwidth. The antennas considered in this paper operate between 2.95 GHz to 11.58 GHz, making them suitable for numerous DECT, WLAN, remote sensing, radar, imaging, localization and medical applications. The etched area of the antenna can be used for communication circuit components.


I. INTRODUCTION
US -Federal Communication Commission (US-FCC) released a 10dB bandwidth of 7.5 GHz (3.1 -10.6 GHz) for ultra-wideband (UWB) wireless communication [1].With the development of the high data rate wireless communication systems, antenna engineers face an increasing demand for smaller antennas which still retain broadband characteristics.Printed planar monopole antennas have become very popular due to their attractive features of low profile, light weight, easy fabrication, and conformability to mounting hosts, large bandwidth and radiation properties [2], [3].Highperformance miniaturized front-end printed circuit boards are essential in portable systems [4].Planar

Microstrip feed Spanner Shape Monopole Antennas for Ultra Wide Band Applications
Tapan Mandal 1 and Santanu Das 2   1 Department of Information Technology, Government College of Engineering and Textile Technology, Serampore, Hooghly, India, PIN-712201 2

Department of Electronics & Tele-Communication Engineering,
Bengal Engineering and Science University, Shibpur, Howrah, India, PIN-711103 1 tapanmandal20@rediffmail.com, 2 santanumdas@yahoo.comUltra Wide Band (UWB) monopole antennas with rectangular, square, disk, elliptical and triangular shapes have been reported [5][6][7][8][9].The Hexagonal monopole antennas are found to have 10 dB return loss bandwidth for UWB application [10], [11].Rectangular slot with resistive load hexagonal for GPR application has been presented [12].Surface current distribution analysis is very effective to investigate the radiation area of the antenna and to give guidance to design and optimization [13].The antennas integrated on a printed circuit board can be viewed as a suspended monopole broad band antenna.The existence of many possible current modes explains the reason of such a broad antenna bandwidth.The current modes that could exist on a circular metal patch are in the form of higher order Bessel functions of the first kind, where current is less at the center of the patch and mostly concentrated on its periphery [13].
In this paper, miniaturized spanner shape monopole antenna characteristics are proposed.It is an enhancement of the work presented in [11].The design initially begins with a printed regular hexagonal monopole antenna (PRHMA).The inner aperture area of the spanner shape can be used for communication circuit components.The proposed antenna has advantages of low cost, compact and easy fabrication due to simple configuration.The simulation is carried out by a Method of Moment (MoM) based IE3D TM simulator.The simulation results confirm ultra wideband (UWB) characteristics of the antenna.The detail simulation responses are presented and discussed here.
In this research work, the return loss measurements are taken out by Agilent make Vector Network Analyzer (N5230A).
The organization of this paper is as follows.Section II presents the configuration design criteria of the proposed antenna.The details of the antenna parameters and observations are given in section III.
Section IV concludes the findings of this paper.

II. ANTENNA CONFIGURATION
For UWB antennas, lower band edge frequency and entire band width become the design parameters, instead of resonance or operating frequency [2].The design initially begins with a microstrip fed printed regular hexagonal monopole antenna (PRHMA).The prototype PRHMA element is designed on a substrate having dielectric constant (ε r ) = 4.4, thickness (h) = 1.59 mm and loss tangent = 0. 002 (Fig. 1).Provided that the lower frequency (f l ) operation is 2.98 GHz, the entire length (L) and width (W) of the proposed antenna [10,[14][15][16] are calculated by equations ( 1) and (2).
Finally, L×W is selected as 31×52 mm 2 , approximately half an effective wavelength of the lowest operating frequency [15].
where c is the velocity of light in free space.The lower band edge frequency (f l ) has been determined for UWB using formulas [2], [10] given in equation ( 3) where H is the height from ground plane edge to the top of the hexagon and r is the radius of an equivalent cylindrical monopole antenna in cm and p is the length of the feed line in cm.With reference to configurations in Fig. 1  In another form (Fig. 2) the monopole metallic patch element is made on the same substrate with the dimensions same as before.However the hexagonal element is defected by a rectangular shape.
The optimum dimensions of Ls and Ws mm are found to be 10 mm and 12 mm respectively by way of parametric study.It is observed from the simulation that the current on the hexagonal patch radiator are mostly concentrated on its periphery with very low current density towards center.The idea of cutting out a rectangular slot derives from the examination of current distribution of hexagonal patch antenna.A prototype spanner antenna is proposed that occupies only 65% area of the PRHMA and fabricated and tested.The fabricated prototype and spanner antennas are shown in Fig. 3.

A. Return Loss
Here the BW of the antenna depends on how the impedance of various modes of the patch is matched with feed line.For the PRHMA the return loss characteristics are shown in Fig. 4. It exhibits simulated impedance BW of 7.66 GHz (2.98 -10.64 GHz) (defined by 10 dB) having a ratio 3.57:1.
The measured response yields the BW of 6.63 GHz (3.07 -9.7 GHz).The discrepancy between simulated and measured results should be mostly attributed to the loss tangent (tan δ) of the substrate, tolerance in manufacturing.Thus it may be considered as a good Ultra Wide Band (UWB) antenna.The return loss characteristics of spanner shape (Fig. 2) are shown in Fig. 5 and Fig. 6.It is found that the lower-edge frequency will decrease when the slit length Ws increases.At the same time its return loss decreases (absolute value increases).The length of the defected slot can be also used to extend the lower edge frequency of the impedance bandwidth.While comparing the results in Fig. 5 and Fig. 6, it is apparent that for the spanner and PRHMA antenna, longest current path on the patch is approximately half the wave length at the lowest band frequency.Further simulation shows that the slot perimeter of the spanner antenna is half wave length of the lowest band frequency.If the slot perimeter becomes more than of half wave length then mismatches at 5.45 GHz and the second mode resonant frequency will not appear in UWB band.

B. Radiation Pattern
The influences of the slot on the radiation patterns has also been investigated.At lower frequencies H-plane radiation patterns are omnidirectional, whereas in E-plane, it is a figure of eight because of very small ground plane.Radiation patterns, at higher frequencies, their occurs little change in the H-plane pattern however in the E-plane reduces to an omnidirectional one with slight increase in its cross polarization level.It is noted that adding a slot in the radiating patch does not affect the radiation pattern in the H-plane.Thus it may be considered as a good UWB antenna.

C. Gain and Efficiency
The efficiency and gain of the proposed antennas are shown in Fig. 9 and Fig. 10 respectively.It can be seen that the proposed antenna have almost the same efficiency.Therefore the slot does not have appreciable effects on the performance throughout entire operation band.But the antennas provide more than 90% antenna efficiency.The spanner antenna has lower gain at the high frequency band.At higher frequencies, there are rapid variations of surface currents along the periphery of the patch.This decrease of gain at higher frequency can be attributed partially to the increase in losses and to the surface current variation between various modes of the antenna configuration.However, the gain variation of the spanner antenna is less than 1.5 dB from PRHMA throughout the whole UWB frequency.For practical applications in UWB systems, the slot region may be used for RF circuits without affecting antenna operation subject to further invertigation.The overall metallic area of this antenna is reduced by 35% as compared to a PRHMA.

Fig. 5 .
Fig.5.Simulated return loss characteristics for Fig.6.Simulated return loss characteristics for various lengths of Ws various widths of Ls

Fig. 7 .
Fig.7.Comparison of the return loss characteristics (simulation and measured) of PRHMA and spanner shape monopole antenna

Fig. 9 .
Fig. 9. Simulated antenna efficiency Vs. frequency Fig. 10.Simulated gain Vs. frequency of proposed of proposed antenna antenna

TABLE I :
THE ANTENNA DESIGN PARAMETERS