Design Simulation and Fabrication of a Dual Band Frequency Reconfigurable Monopole Antenna for Wi-Fi and WiMAX Applications

Nowadays, wireless systems require compact multi band antennas that can dynamically change some of its fundamental parameters such as frequency band, polarization and radiation pattern. The novelty of the proposed work lies in using two conducting strips that are perpendicular to each other. The longer strip resonates at 2.4 GHz and the shorter strip resonates at 3.5 GHz. The length and width of the Ground plane is made equal to length of the main arm and length of the side arm respectively. The results are obtained by simulating the antenna structure using simulation tool Ansoft HFSS v15.0. The Simulated antenna Gains were 6.2364 dBi and 7.1758 dBi for frequencies of 2.4 GHz and 3.5 GHz respectively. The antenna dimensions are 50 × 25 × 1.6 mm on a 1.6 mm thick FR-4 substrate.


Introduction
Monopole antennas have become the most preferred choice in the domain of the Wi-Fi and WiMAX applications and other fields like Bluetooth cards and wireless communications.In the present work, Monopole classes of Antennas are selected which are compact in size and normally the gains are low.To excite dual bands, two conductors of different lengths are chosen that are perpendicular to each other.The length of the longer strip is designed for resonating at 2.4 GHz while the shorter strip is designed for 3.5 GHz.The novelty of the proposed work lies in making the length of the Ground plane equal to the length of the Open Journal of Antennas and Propagation Ground plane has been proposed for WLAN and WiMAX applications.By inserting inverted L shaped strips and perturbations in the Ground plane resulted in a resonant frequency of 2.4 GHz, 3.5 GHz and 5.2 GHz.The structure size reported were 20 mm × 27 mm × 1 mm.The Bandwidth of the proposed antenna was 180 MHz, 380 MHz and 280 MHz covering both the 2.4, 5.2 GHz WLAN and 3.5 GHz WiMAX bands.The Antenna Gains reported were between 2.06 to 2.78 dBi for the lower band, 2.14 dBi for the 3.5 GHz and the 5.2 GHz bands respectively.In [8], a pentagonal shaped monopole Antenna with defected Ground structure is presented.The Antenna is etched on a FR4 substrate and compact in shape.The Antenna resonates at 1.8 GHz and 3.5 GHz.The antenna gives a gain of 0.611 dB and 3.56 dB for frequencies of 1.847 GHz and 3.474 GHz respectively.The Bandwidth of the proposed antenna was 90 MHz and 150 MHz respectively for DCS and WiMAX bands respectively.In [9], dual-band monopole antenna is presented for resonating at 900 MHz and 1.8 GHz covering the GSM 900 and GSM 1800 bands respectively.The proposed design can be used as an Energy Harvesting system.The Antenna was printed on a FR4 substrate.The return loss values were −30 dB and −25.5 dB for GSM 900 and 1800 bands respectively.The Gains reported were −1.64 dB and 0.85 dB for the lower and the upper bands respectively.Bandwidths of the proposed Antenna were 124 MHz and 196 MHz covering lower and upper GSM bands respectively.
In [10], a compact slotted multi-band monopole antenna has been proposed for WLAN and WiMAX applications.The proposed antenna has a size of 30 mm × 25 mm × 1 mm.The designed antenna obtained three frequency bands namely 2.4 GHz, 3.5 GHz and 5.8 GHz covering the WLAN and the Wi-Max bands.The current path is changed by using inverted E and C shaped slot.The structure resulted in a Bandwidth of 51 MHz, 450 MHz for the WLAN 2.4 GHz and 5.5 GHz and 251 MHz for the Wi-Max band.In [11], a simple multiband metamaterial-loaded monopole antenna suitable for WLAN and Wi-Max has been proposed.The rectangular Monopole antenna was originally designed to resonate at 5.2 GHz.When the inverted-L slot is etched, the antenna produces a second resonance at around 4.1 GHz.Then, with the addition of the metamaterial reactive loading, a third resonance covering the 2.4-GHz band occurs.Consequently, the antenna can cover the 2.4/5.2/5.8GHz WLAN and 2.5/3.5/5.5 GHz WiMAX bands with a very compact size.In [12], design of a dual-band monopole antenna with microstrip fed for use in wireless devices in the WiMAX system is presented.The antenna radiator has a compact size of only 14.5 × 8.7 mm 2 and consists of a short stem and two branches resonating at around 2.4 GHz and 3.5 GHz for the WLAN and WiMAX bands.In [13], a circularly polarized planar monopole antenna is presented.By inserting the L and the C shaped strips on the patch, two frequency bands were obtained.The structure has a compact size of 40 × 47 × 1.5 mm 3 .The Bandwidths of the proposed Antenna were 380 MHz and 1240 MHz for the 2.4 GHz and the 5.2 GHz bands respectively.The patterns have the characteristic of bidirectional radiation, and the gains are stable in both bands.In [14], multi band Monopole Antennas, covering three frequencies using Defective Ground plane structures were realized.The novelty of the proposed work lies in dual inverted L-shaped strips and is fed by a cross-shaped stripline.This technique resulted in creating additional bands and improved Bandwidth.The size of the Antenna reported were 20 mm × 30 mm 2 , covering the WLAN and Wi-Max Applications.In [15], a multiband bow-tie monopole antenna with Co-planar waveguide feed is proposed.To cover different frequency bands, slots of different lengths were etched in the bow-tie patch.The Antenna resulted in a peak Gain of 3.75 dBi, 3.56 dBi and 3.93 dBi covering the 2.5 GHz, 3.5 GHz and 5.5 GHz bands respectively.

Design Methodology
In our proposed work, the main antenna is designed to resonate at 2.4 GHz Open Journal of Antennas and Propagation which is shown by the longer monopole of length L 2.4 and side arm is inserted towards the right of the longer monopole antenna so as to create an additional resonant frequency of 3.5 GHz.The substrate chosen here is a FR4 type which has a dielectric loss tangent of 0.02 and permittivity of 4.4.The substrate thickness is kept equal to 1.6 mm.The length and width of the ground planes are selected such that the antenna size becomes compact, with the length of the ground plane kept equal to L 2.4 and width of the ground plane is selected such that it is approximately equal to the length of the side arm of the monopole.

Antenna Design
In the proposed design, the Ground plane has been partially removed to give it a Monopole like structure.

Design Equations
where L 1 = Length of the Main Monopole resonating at 2.4 GHz.(vii) Optimize the antenna dimensions and repeat the steps (v) and (vi).

Proposed Algorithm
(viii) Insert a side arm perpendicular to the main arm using the theoretical length calculated for 3.5 GHz.
(ix) Repeat steps (v) to (vii) until all the parameters are as desired. P.

Simulation & Measured Results
The Antenna has been simulated under HFSS version 15.0 and also fabricated and tested with VNA.The results are indicated as shown.

Results and Discussions
The Dual band Antenna resonating at Wi-Fi and Wi-Maxband which is reported here is fabricated and experimentally tested with dimensions as reported in Ta- The bandwidth reported in the upper band was the highest.From the Figure 6, the measured value of VSWR reported in the Wi-Fi band was 1.53 and that in the Wi-Max band was 1.09 indicating a good impedance match at the upper band compared to the lower band.Comparing Figure 7 and Figure 8, the simulated Bandwidth reported is 330 MHz and 412 MHz for Wi-Fi and the Wi-Max Frequency bands respectively.From Figure 10, peak value Gain is 6.23 dB for 2.4 GHz, and 7.17 dBi in the 3.5 GHz band.The measured radiation pattern as reported in Figure 13 and Figure 14, show that the pattern is omnidirectional for both the 2.4 GHz and the 3.5 GHz frequency bands respectively.Referring to            Ohms in the lower band compared to the higher band.Both the Gain and the Bandwidths are increased by a factor of 0.9 dB and 88.5 MHz in the upper compared to lower band respectively.Comparing Figure 11, we see that the peak boresignt gain is 6.23 dBi under both E and the H plane at 2.4 GHz.But the pattern in H plane is Figure 8. Comparing Figure 12, we see that the peak boresight Gain is 6.5 dB and 7.17 dB under H and E planes.Referring to Table 3, we see that the measured Bandwidth reported is very high close to 2.16 GHz in the upper band compared to lower band and also there is a shift in the resonant Frequency from 2.4 GHz to 2.7 GHz in the lower band.Referring to Table 4 and Table 5, we see that both the Gain and Bandwidths reported in the present work are high compared to other designs reported in [6] [7] and [10] at 2.4 GHz and 3.5 GHz Frequency.Figure 15 indicates that there is a good improvement in the  From the pattern it is clear that there are some side lobes are appearing at −180 degrees.
Keynote 13: The measured radiation pattern is nearly omni directional in E plane for both the frequencies.

Figure 1
Figure1shows the snap shot of the antenna designed in HFSS clearly indicating the substrate, the Antenna and the Ground plane.Figure2shows the fabricated Figure 2 shows the fabricated Keynote 1: L 2.4 represents the length of the monopole resonating at 2.4 GHz and b represents the section contributing for resonance at 3.5 GHz.

Figure 1 .
Figure 1.Snapshot of the proposed Dual frequency Monopole Antenna in HFSS.

gλ=
Guide wavelength.reff ε = Effective electrical permittivity of the dielectric substrate.0 λ = Free space wavelength corresponding to frequency of operation.

( i )
Select the Antenna dimensions require using Equations (1) to (3) (ii) Select the substrate type required, thickness and relative permittivity (iii) Determine the value of effective permittivity using (3) (iv) Simulate the Antenna for 2.4 GHz with the theoretical length.(v) Determine the values of Return loss, VSWR, Impedance bandwidth and Gain.(vi) Is the Return loss well below −10 dB and VSWR < 1.5.

ble 1 .
The substrate lengths and widths were set equal to 50 mm by 25 mm.The Length and width of the ground plane was theoretically set equal to quarter of the Guide wavelength corresponding to 2.4 GHz and 3.5 GHz respectively.The Length of the longer monopole arm was set equal to a quarter of the Guide wavelength corresponding to 2.4 GHz frequency and the side arm length which is perpendicular to main arm is set equal to quarter of the Guide wavelength corresponding to 3.5 GHz frequency.After theoretical calculations it was found to be 17.1 mm.The length of the side arm (a + b) was calculated at Quarter wavelength resonance condition to arrive at 11.7 mm.Further optimization was done on the longer arm and the side arm resulting in a length of 22.6 mm and 14.3 mm respectively.The width of the feed was calculated using Equation (4) taking the port impedance as 50 Ohms substrate height as 1.6 mm and effective permittivity as 4, to arrive at the feed width of 3.0 mm.Figure3and Figure4show the simulated and measured return loss of the proposed Antennas.The Return loss of the fabricated Dual band Antenna was measured using VNA MASTER MS2308C.Two resonant modes were excited 2.4 GHz and 3.5 GHz.The measured values were 2.7 GHz and 3.426 GHz with an impedance Bandwidth of 0.26 GHz and 2.16 GHz.The bandwidth reported in the upper band was the highest.The measured value of VSWR reported in the Wi-Fi band was 1.53 and that in the Wi-Max band was 1.09 indicating a good impedance match at the upper.The measured values as seen from Figure4were 2.7GHz and 3.426 GHz with an impedance Bandwidth of 0.26 GHz and 2.16 GHz as reported in Figure9.The VSWR reported in Figure5is close to 1 indicating a good impedance match.
markers m1 and m2 indicate a return loss of −31.79 dB & −24.72 dB for 2.4 GHz and 3.5 GHz.

Figure 3 .
Figure 3.Return Loss plot of the Monopole Antenna resonating at 2.4 and 3.5 GHz.

Figure 4 .
Figure 4. Measured Return Loss plot of the Fabricated Monopole Antenna.The Return loss value is −13.38 dB at 2.72 GHz as indicated by marker MK2 and −27.53 dB at 3.5 GHz corresponding to marker MK3.

Figure 6 .
Figure 6.Measured VSWR plot of the Monopole Antenna resonating at 2.4 GHz and 3.5 GHz as shown by markers MK2 and MK3.The corresponding values are 1.53 and 1.09 for lower and upper band respectively.

Figure 7 .
Figure 7. Simulated Bandwidth of the Monopole Antenna resonating at 2.4 GHz indicating a Bandwidth of 330.2 MHz.

Figure 8 .
Figure 8. Simulated Bandwidth of the Monopole Antenna resonating at 3.5 GHz indicating a Bandwidth of 412.7 MHz.From the figure, the lower −10 dB frequency is 3.33 GHz and the upper −10dB frequency is 3.747 GHz and the difference gives the Bandwidth.

Keynote 9 :
Horizontal Axis represents the Frequency in GHz and the Vertical Axis represents the Return loss in dB.

Figure 9 .
Figure 9. Measured Bandwidth of the Dual band Antenna resonating at 2.4 GHz 3.5 GHz as shown by markers MK3, MK4, MK5 and MK6 indicating a Bandwidth of 262 MHz and 2.158 GHz for Wi-Fi and Wi-Max.The markers MK7, MK8 and MK9 indicate the resonant frequencies of 1.5 GHz, 2.7 GHz and 3.429 GHz.
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Figure 10 .
Figure 10.(a) Simulated Gain plot for 2.4 GHz.The peak; (b) Simulated Gain plot for 3.5 GHz indicating a Gain reported is 6.236 dB.peak Gain of 7.17 dBi.

Figure 11 .
Figure 11.Simulated radiation pattern of Antenna at 2.4 GHz (a) under H plane and (b) under E plane.
the radiation pattern indicates a peak bore sight gain of 6.5 dB under H plane and 7.17 dB under E plane for 3.5 GHz.

Figure 12 .
Figure 12.Simulated radiation pattern of Antenna at 3.5 GHz (a) H plane (b) E plane.From the pattern it is clear that there are some side lobes are appearing at −180 degrees.

Figure 13 .
Figure 13.Measured Normalized E-plane radiation pattern of Antenna at (a) 2.4 GHz and (b) 3.5 GHz.

Keynote 14 :
The radiation pattern is nearly omni-directional for both the frequencies.The pattern is normalized to 0 dB.

Figure 14 .
Figure 14.Measured normalized H-plane radiation pattern of Antenna at (a) 2.4 GHz and (b) 3.5 GHz.

Table 2 ,
we see that the return loss value is more negative in the 2.4 GHz band

Table 1 .
Geometrical specifications of the Antenna.

Table 2 .
Showing the comparison of the Simulated Antenna parameters at 2.4 GHz and 3.5 GHz.

Table 3 .
Showing the comparison of the Simulated and Fabricated Antenna Results.

Table 4 .
[10]ing the comparison of the Simulated results reported in the paper with[6][7] and[10]at 2.4 GHz.