Broadband Sectoral Slot Cut Microstrip Antenna

Amit A. Deshmukh1, Ankita R. Jain1

1. DJSCOE, Vile-Parle (W), Mumbai – 400 056 Email : amitdeshmukh76@rediffmail.com

nks_j@yahoo.co.in

K. P. Ray2 2. RFMS, SAMEER, I.I.T. Campus, Powai,

Mumbai – 400 076, Email: kpray@rediffmail.com

Abstract: The broadband microstrip antenna is more commonly realized by cutting the slot at an appropriate position inside the patch. The slot is said to introduce a mode near the resonance frequency of the fundamental mode of the patch and yields broadband response. The compact microstrip antenna is realized by placing the shorting post or the plate along the zero field line at the fundamental mode of the patch. In this paper, the compact variation of equilateral triangular microstrip antenna, a shorted sectoral microstrip antenna is discussed. A broadband proximity fed rectangular slot cut shorted sectoral microstrip antenna is proposed. The analysis to study the effects of slot on the broadband response is presented. The slot reduces the higher order mode resonance frequencies of the shorted sectoral patch and along with the fundamental patch mode yields broadband response. The simulated and measured bandwidth of more than 550 MHz is realized. Since the shorted patch is used, the antenna shows radiation pattern in end-fire direction with a gain of more than 6 dBi over the bandwidth. Key terms: Broadband microstrip antenna, Equilateral Triangular microstrip antenna, Shorted sectoral microstrip antenna, Rectangular slot, Proximity feeding

I. INTRODUCTION The broadband microstrip antenna (MSA) is more commonly realized by fabricating the slot cut patch on thicker substrate having lower dielectric constant and further by using the proximity feeding technique [1 – 7]. The slot can take different shapes like U-slot, V-slot and rectangular slot. It is a general understanding in these slots cut MSAs that it introduces a resonant mode near the fundamental mode resonance frequency of the patch and yields broadband response. The compact MSA is realized by placing the shorting post or the plate along the zero field line at the fundamental mode of the patch [8, 9]. This converts conventional half wavelength resonator into quarter wavelength resonator. The bandwidth (BW) in these shorted antennas is increased by cutting the slot inside the patch [10]. While designing these slot cut MSAs in desired frequency band the slot length is either taken equal to half wave or quarter wave in length at the desired slot frequency. However this simpler approximation does not give accurate results for different slot length and its position inside the patch. The formulations for slot length at the desired frequency, is not reported. 978-1-4673-5952-8/13/$31.00 ©2013 IEEE

The analysis to study the broadband and dual band response of slot cut rectangular MSA is reported [11]. In that configuration four resonance frequencies were observed. The approximate equations for the four resonant frequencies in terms of slot and patch dimensions are given. However a clear description of the modes at the individual frequencies and the comparison between the simulated and measured results against the calculated results obtained using the equations is not given. The analysis to study the effects of slot on the broadband response of slot cut MSA is reported [12]. The resonance curve plots, surface current distribution and the radiation pattern plots for different slot lengths were studied. It was observed that the slot does not introduce any additional mode but modifies the resonance frequency of higher order mode of the patch and along with the fundamental mode yields broadband response. The slot also modifies the directions of surface currents on the patch at higher order mode and aligns them in the same directions as that of the currents at fundamental patch mode and gives broadside radiation pattern over the complete BW. In this paper, the compact variation of equilateral triangular MSA (ETMSA), a shorted 600 Sector MSA is discussed [9]. The fundamental and higher order modes of ETMSA and shorted sectoral MSA are studied. The proximity fed rectangular slot cut shorted plate 600 Sectoral MSA is proposed. The analysis to study the effects of the rectangular slots on the frequency response of shorted patch is presented. The slot reduces the resonance frequencies of higher order modes like, TM1/4,1 mode and along with the fundamental TM10 mode yields broadband response. The simulated and measured BW’s of more than 550 MHz is obtained. Since the shorted patch is used the antenna shows radiation pattern in the end-fire direction with higher cross-polarization levels. The antenna has gain of more than 6 dBi over the complete BW. The proposed antenna is first analyzed using IE3D software followed by experimental verification [13]. The air substrate is used to maximize the radiation efficiency and the gain. The antenna was fabricated using the copper plate and was suspended in air using the foam spacer support placed towards the antenna corners. It is fed using the N-type connector of 0.32 cm inner wire diameter. The measurement was carried out using R & S vector network analyzer using square ground plane of side length 50 cm. The proposed antenna is optimized in 1000

MHz frequency band as it can find application in mobile communication environment.

II. SHORTED PLATE 600 SECTORAL MICROSTRIP ANTENNA

The suspended ETMSA and its compact variation, shorted plate 600 sectoral MSA are shown in Fig. 1(a – c). The sectoral MSA is obtained by placing the shorting plate along the zero field line at the fundamental TM10 mode of the ETMSA. For air substrate of thickness h = 3.0 cm, the side length ‘S’ of the ETMSA is calculated such that it operates in its fundamental TM10 mode at around 1000 MHz [9]. The ‘S’ was found to be 13.8 cm. To realize the impedance matching and larger BW, the patch is fed using the proximity feeding method. The coupling strip is placed in the same plane of the patch to control the coupling between the patch mode and the strip. The ETMSA was simulated using IE3D software and its resonance curve plot is shown in Fig. 1(d). It shows two peaks due to TM10 (1057 MHz) and TM20 (1949 MHz) modes. The peaks due to other modes are absent as the impedance matching for given feed point location is not realized. The surface current distributions at these two frequencies are shown in Fig. 2(a, b).

Fig. 1 (a) Top and (b) side views of ETMSA, (c) shorted plate 600 sectoral MSA and (d) resonance curve plots for, (_____) ETMSA, (__ __ __) shorted 600 Sectoral MSA

At TM10 mode, the surface currents show one half wavelength variations along the patch side length. At TM20

mode, surface currents shows two half wave length variation along the patch side length. To realize the shorted 600 sectoral MSA, the shorting plate is placed along the zero field lines at TM10 mode. The resonance curve plot for the shorted patch is shown in Fig. 1(d). The resonance curve plot shows peaks due to TM10 and TM21 modes. The peak due to TM20 mode is absent as the impedance matching at that mode in the shorted patch is not realized. In shorted patch the field shows quarter wavelength variations from the shorted point on the ground plane and towards the open circuit edge of the patch. This reduces the resonance frequency of TM10 mode as compared to that in ETMSA. The surface currents at TM10 and TM21 modes in shorted patch are shown in Fig. 2(c, d). It shows one quarter wavelength and three quarter wavelength variations along the shorted patch length. The resonance curve plots for shorted 600 sectoral MSA for two different feed point locations as shown in Fig. 3(a), is shown in Fig. 3(b). In this case the feed point is placed inside the patch. For the feed point at point ‘A’, the resonance curve shows the excitation of TM10 and TM20 modes. For the feed point location at point ‘B’, the curve shows the excitation of another mode near the TM20 mode frequency. At this mode, the surface currents shows quarter wavelength variation along shorted length and half wave length variation along shorted width as shown in Fig. 3(c). Due to this surface current variation this mode is referred as TM1/4,1 mode. To increase the BW, the rectangular slot is cut inside the proximity fed sectoral MSA as discussed in the following section.

Fig. 2 Surface currents distribution for ETMSA at (a) TM10 and (b) TM20 modes and for shorted 600 sectoral MSA at (c) TM10 and (d) TM21 modes

Fig. 3 (a) Probe fed shorted 600 sectoral MSA, its (b) resonance curve plot for two feed point locations and (c) surface current distribution at TM1/4,1 mode

III. BROADBAND PROXIMITY FED SHORTED RECTANGULAR SLOT CUT 600 SECTORAL MSA

The broadband proximity fed slot cut sectoral MSA is shown in Fig. 4(a). The parametric study for the variations in slot length is carried out. For slot width of 0.4 cm, slot length is increased in steps of 0.5 cm from 0 to 6.5 cm. For each of the lengths, the resonance curve plots, surface currents distributions were studied. The resonance curve plots for different slot lengths are shown in Fig. 4(b, c). The slot length is parallel to the surface currents at shorted TM10 mode, hence reduction in its frequency is negligible. For smaller slot lengths (less than 4.5 cm), using the proximity feed, the impedance matching at TM1/4,1 mode is not realized. Therefore for those slot length a prominent peak

due to the same is absent. Since the surface currents at TM1/4,1 mode varied along the horizontal as well as vertical direction inside the patch with an increase in the slot length its frequency reduces and prominent peak due to that mode starts appearing in the resonance curve. The surface current distribution at this modified TM1/4,1 mode is shown in Fig. 5(a). The contribution of currents in horizontal direction inside the patch is increasing with the slot length.

Fig. 4 (a) Proximity fed rectangular slot cut 600 sectoral MSA, its resonance curve plots for l equal to (b) (____) 0 cm, (____ ____) 3.5 cm, (___ _ ___) 4.5 cm, and (c) (____) 0 cm, (____ ____) 5.5 cm, (___ _ ___) 6.5 cm Thus the slot length realizes the tuning of TM1/4,1 mode resonance frequency with respect to TM10 mode frequency and the broader BW will be realized when the loop formed due to the coupling between them lies inside the VSWR = 2 circle. This is realized for slot length of l = 7.0 cm. However for the given proximity feed position the coupling between the patch and the strip is smaller. To optimize coupling, the coupling strip is placed inside the inset slot cut on the edges as shown in Fig. 5(b) and the optimized input impedance plots is shown in Fig. 5(c).

Fig. 5 (a) Surface current distribution at TM1/4,1 mode for l = 5.5 cm, (b) optimized proximity fed rectangular slot cut 600 sectoral MSA and its (c)

input impedance plot, (_____) simulated, (___ ___) measured The simulated BW is 565 MHz (45%). The antenna was fabricated using the copper plate and it was supported in air using the foam spacer support. The measurement was carried out using R & S vector network analyzer. The measured BW is 576 MHz (47%) as shown in Fig. 5(c). The fabricated prototype of the configuration is shown in Fig. 6(a, b). The

radiation pattern at the center frequency is shown in Fig. 7(a). Due to the shorted patch the radiation pattern is in the end-fire direction with higher cross-polarization levels. The antenna gain as shown in Fig. 7(b) is more than 6 dBi over the complete BW. The gain plot shows two peaks due to the two dominant modes.

Fig. 6 (a, b) Fabricated prototype of proximity fed rectangular slot cut 600 sectoral MSA

IV. CONCLUSIONS

The proximity fed ETMSA and compact shorted plate 600 sectoral MSA are discussed. The compact and broadband proximity fed rectangular slot cut 600 sectoral MSA is proposed. The analysis to study the effects of slot on wide frequency range is presented. The slot reduces the resonance frequency of TM1/4,1 mode of the sectoral patch and along with the shorted TM10 mode yields broadband response. The measured and simulated BW of more than 550 MHz (>45%) has been realized. Due to the shorted patch the radiation pattern is in the end-fire direction with higher cross-polarization levels. The antenna gain is more than 6 dBi over the complete BW. The proposed antenna can find application in the mobile communications in 1000 MHz frequency band.

Fig. 7 (a) radiation pattern at center frequency and (b) gain variation over the BW for proximity fed rectangular slot cut 600 sectoral MSA, (_____)

simulated, (__ __ __) measured

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