Analysis and Design of a Novel Compact Multiband Printed Monopole Antenna

A compact multiband printed monopole antenna is presented. The proposed antenna, composed of a modified broadband Tshaped monopole antenna integrating some band-notch structures in the metallic patch, is excited by means of a microstrip line. To calculate the bandwidth starting frequency (BSF) of the T-shaped broadband antenna, an improved formula is proposed and discussed. The multiband operation is achieved by etching three inverted U-shaped slots on the radiant patch. By changing the length of the notch slots, operation bands of themultiband antenna can be adjusted conveniently.The antenna is simulated inAnsoft HFSS and then fabricated andmeasured.Themeasurement results show that the proposed antenna operates at 2.25–2.7 GHz, 3.25– 3.6Hz, 4.95–6.2GHz, and 7-8GHz, covering the operation bands of Bluetooth, WiMAX,WLAN, and downlink of X-band satellite communication system and thus making it a proper candidate for the multiband devices.


Introduction
With the rapid development of the communication technology, there is a great demand for antennas suitable to operate with dual-or multibands characteristics in wireless communication devices, such as mobile phones and laptops.Printed antennas have been paid great attention in recent years because of their compact size, low profile, light weight, and low cost.A great quantities of printed antennas for dual-or multibands operations have been reported in the literature [1][2][3][4][5][6][7][8][9][10][11][12].In [1][2][3][4][5][6][7], the authors have presented several kinds of printed monopole or dipole antennas for dualband operation.In [8][9][10][11][12], slot antennas have been utilized.Antennas mentioned above achieve dual-or multibands operation; however, they usually have complicated structures or narrow bandwidth, and their operation bands cannot be adjusted easily either.
Recently, a lot of wideband antennas have been proposed because of the wide operation band and high date rate [13][14][15].To avoid the interference between UWB (ultrawideband) antennas and narrow bandwidth communication systems, antenna designers have proposed several UWB antennas with band-notch characteristics [16][17][18][19].The above-mentioned design solutions provide us a different way to achieve the multiband operation.
In this paper, we present a novel compact multiband monopole antenna based on the broadband antenna theory and employing band-notch technique.A T-shaped monopole antenna is designed to achieve a broad impedance bandwidth.Using the techniques suitable to widening of the operative frequency band, three inverted U-shaped slots are etched on the metallic patch to reject the undesired bands; in this way the multiband operation is achieved.The operation bands of the proposed antenna can be adjusted conveniently by changing the length of each band-notch slot.
The organization of the paper is as follows.In Section 2, the configuration of the proposed antenna is introduced.An improved formula for computing the bandwidth starting frequency (BSF) with higher accuracy is proposed and discussed.Then the broadband characteristic of the Tshaped monopole antenna is analyzed.Finally, the frequency behaviour of the band-notch structures consisting of three inverted U-shaped slots etched on the metallic patch is investigated.Results of the proposed antenna (return loss, normalized radiation pattern, and peak gain) are presented and discussed in Section 3, while some conclusions are drawn in Section 4.

Antenna Analysis and Design
Figure 1 shows the geometry of the proposed multiband antenna.The proposed antenna is printed on a low-cost FR4 substrate with relative permittivity of 4.4, dielectric loss tangent of 0.02, and thickness of 1 mm.A T-shaped patch is printed on one side of the substrate and a truncated ground plane on the other.The T-shaped patch is realized by removing two symmetric metal notches at the bottom of a rectangular patch in order to improve significantly the impedance matching of the monopole antenna at high frequency.Three inverted U-shaped slots with different sizes are etched on the T-shaped patch to reject the undesired frequency bands of the proposed multiband antenna.The commercial software Ansoft HFSS has been adopted for the analysis and design of the proposed antenna.The optimized geometrical parameters describing the antenna are reported in Table 1.
An improved formula useful to calculate the BSF in terms of the antenna parameters is proposed and discussed firstly.Then the effects of the antenna parameters are investigated.Finally, the inverted U-shaped band-notch structures are studied.

Improved Formula for BSF of the Broadband Antenna.
For a broadband antenna, the BSF and bandwidth are two important factors to evaluate its frequency performance.An accurate formula to calculate BSF of a broadband antenna is quite necessary for antenna designers to save simulation time and accelerate the design process.In this paper, an improved formula is presented to provide a much more accurate prediction of BSF of the T-shaped monopole.
Kumar and Ray have proposed a formula to calculate the BSF of the planar monopole [19].Thomas and Sreenivasan improved the formula by considering the effect of the substrate [20].Equation ( 1) is the formula proposed by Thomas and Sreenivasan: where  denotes the length of the monopole (both the planar monopole and the equivalent cylinder monopole),  denotes the gap between the ground plane and the monopole, and  denotes the radius of the equivalent cylinder monopole.The equivalent radius  is expressed as where  denotes the area of the radiant patch and the area of the side face of the equivalent cylinder monopole,   is the effective dielectric constant of the air-substrate composite dielectric and can be calculated by   = (1 +   )/2, and   denotes the relative constant of substrate.The parameters , , and  appearing in (1) and ( 2) are expressed in millimeters.However, (1) is not accurate enough to calculate the BSF of the T-shaped monopole antenna because the parameter  does not take into account the effect of the two bevel cuts on the feeding gap.Therefore, we propose to replace it by an effective parameter   defined as follows: Here  2 denotes the width of the higher edge of the radiant patch, and , , and  have the same meanings as in (1), while in this paper,  =  1 +  2 and  =  1  1 +  2  2 .Then (3) can be rewritten as The modified formula to calculate BSF of the T-shaped monopole is After performing some numerical simulations it is found that the values of the BSF calculated by (5)   the simulated ones.So a calibration factor   with its value of 1.145 is introduced.Then (5) can be modified as Figure 2 shows the calculated and simulated BSF of the Tshaped monopole with different  1 ,  2 ,  1 ,  2 , respectively, while the other parameters of the broadband antenna stay unchanged.From Figures 2(a) and 2(b) it can be observed that, for  1 ,  2 , the values of BSF calculated by ( 1) and ( 6) almost have the same accuracy.For  1 ,  2 , however, the values of BSF calculated by ( 6) are obviously much more accurate than those calculated by (1) compared to the simulated ones as shown in Figures 2(c) and 2(d), validating the accuracy of ( 6).The relative error of the proposed BSF formula comparing with the simulation is calculated; the maximum and mean values are 11.07%and 4.06%, respectively.

Broadband Antenna Design.
In this section, the parameters of the broadband antenna are analyzed and discussed in detail.Figure 3 shows the frequency behavior of the scattering parameter  11 for the different dimensions of the T-shaped structure.From Figures 3(a) and 3(b), it can be observed that the two cuts at the lower edge of the radiant patch have a significant effect on the impedance matching at higher frequency.Correspondently, the impedance match at higher frequency is improved.It is also seen in Figures 3(a that the BSF of the broadband antenna decreases when  1 decreases or  1 increases.The reason is that the effective gap between the ground plane and the radiant patch increases while the width or length of the cuts increases.Because of the same reason, the BSF decreases when  2 increases as shown in Figure 3(c) and when  2 increases as shown in Figure 3(d), and we can see that with the increase of  2 , the BSF of the monopole antenna decreases and the impedance matching at higher frequency degrades.Because longer  2 provides a longer current path at lower frequency, thus a lower BSF.

Multiband Antenna Design.
Based on the broadband antenna design, three inverted U-shaped slots are etched on the T-shaped radiant patch to reject the undesired frequency bands, thus achieving the multiband operation.The resonant frequency of each inverted U-shaped slot can be approximately calculated by (7) reported in [21]: denotes the resonant frequency of the th bandnotch structure and l  denotes the length, expressed in millimeters, of the ith band-notch structure with  = 1, 2, 3. Equation ( 7) predicts a decrement of the resonant frequency   as the parameter   is increased.
Figure 4 shows the frequency behavior of the scattering parameter  11 for different values of the geometrical parameters  1 ,  2 , and  3 .It can be seen that with the increase of the length of the band-notch structures, the resonant frequency decreases and the bandwidth also changes, verifying the behavior predicted by (7).It is also found that the   width   and the position   of the inverted U-slots also have effects on the frequency performance of the band-notch structures, with  = 1, 2, 3.
Figure 5 shows the frequency behavior of the scattering parameter  11 of the broadband antenna with only one inverted U-shaped slot and with all the three slots (the proposed multiband antenna).It can be seen that each band-notch structure can work independently and has little effect on the frequency performance of the other band-notch structures.
The maps of the surface current distributions, excited on the monopole antenna at each one of the operative frequency bands, are shown in Figure 6.

Results and Discussion
The proposed antenna has been fabricated and then measured using Agilent Vector Network Analyzer E5071C.Figure 7 shows the simulated and measured  11 of the proposed multiband antenna.It can be seen that reasonable agreement has been achieved between the simulated and measured results.Because of the fabrication tolerances and of the perturbation effect caused by the SMA connector, there are some discrepancies between the two results.The fluctuation of relative permittivity and loss tangent of the FR4 substrate at high frequency also contributes to the disagreement.
The radiated electric field has a linear polarization for the proposed antenna.Figure 8 shows the computed radiation patterns of the proposed antenna at the working frequency of 2.4, 3.5, 5.5, and 7.5 GHz.It can be seen that the proposed antenna, similar to the typical monopole antennas, has nearly omnidirectional radiation pattern on H-plane except at 7.5 GHz.The degradation at 7.5 GHz can be explained as the following: with the increasing frequency the electrical length of the antenna is more than the half wavelength; then the surface current distributed on the radiant patch will be destructive, thus degradation of the radiation pattern at this frequency is observed.
International Journal of Antennas and Propagation  Figure 9 shows the simulated peak gain of the multiband antenna at the proposed frequency band.From the figure it appears that the peak gain increases as the frequency increases.The deviation between the maximum and minimum peak gain in each operation band is less than 1.5 dBi.International Journal of Antennas and Propagation

Figure 2 :
Figure 2: Calculated and simulated BSF for different values of the geometrical parameters.

𝑙 2 Figure 3 :
Figure 3: Frequency behavior of the scattering parameter  11 of the T-shaped antenna for different values of the geometrical parameters.

Figure 4 :
Figure 4: Frequency behavior of the scattering parameter  11 for different values of the geometrical parameters.

Figure 5 :
Figure 5: Frequency behavior of the scattering parameter  11 for the multiband antenna with one, two, and three U-slots.

A
multiband antenna based on a broadband antenna and the band-notch structures has been presented.A compact T-shaped monopole antenna with three inverted U-shaped slots has been adopted to achieve a multiband frequency behavior.After simulation and optimization in Ansoft HFSS, the proposed antenna is fabricated and measured.The measured results have shown that the frequency range of the proposed antenna can cover the operation bands of Bluetooth (2.4-2.484GHz), WiMax (3.3-3.69GHz), WLAN (5.15-5.875GHz), and downlink of X-band satellite communication system (7.25-7.75GHz).An improved formula useful to calculate the BSF of a general the T-shaped monopole

Figure 8 :
Figure 8: Simulated gain patterns in the E-plane (phi = 90 degree) and H-plane for the proposed antenna at the frequency of (a) 2.4 GHz, (b) 3.5 GHz, (c) 5.5 GHz, and (d) 7.5 GHz.

Figure 9 :
Figure 9: Peak gain for the proposed antenna.

Table 1 :
Optimized geometrical parameters of the proposed multiband antenna.
ẑ Figure 1: Geometry of the proposed multiband antenna.