Broadband Gap-Coupled Slot Cut Rectangular Microstrip Antennas

Amit A. Deshmukh1, A. Joshi1, T. Tirodkar1 1. DJSCOE, Vile-Parle (W), Mumbai – 400 056

Email: amitdeshmukh76@rediffmail.com

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 using multi-resonator gap-coupled technique or by cutting the slot inside the patch. The gap-coupled configuration increases the antenna size whereas the slot cut designs maintains the low profile nature of the antenna and also adds to the bandwidth. The slot is said to introduce a mode near the fundamental mode resonance frequency of the patch and increases the bandwidth. In this paper, broadband proximity fed E-shaped microstrip antenna is discussed. The gap-coupled configuration of E-shaped antenna with rectangular microstrip antennas is proposed. This gap-coupled configuration gives a bandwidth of 450 MHz at center frequency of around 1000 MHz with broadside radiation pattern and gain of more than 7 dBi over the bandwidth. Further gap-coupled configuration of pair of rectangular slot cut rectangular patches with E-shaped antenna is proposed. This configuration yields a bandwidth of more than 550 MHz with broadside radiation pattern and peak gain very close to 9.5 dBi. Key terms: Rectangular microstrip antenna, Broadband microstrip antenna, E-shaped microstrip antenna, Rectangular slot, Proximity feeding

I. INTRODUCTION The broadband microstrip antenna (MSA) is realized by fabricating the patch on lower dielectric constant thicker substrate [1 – 3]. However for substrate thickness (h) more than 0.04λ0, the bandwidth (BW) is limited by the feed probe inductance. For h > 0.06λ0, the BW is increased by using proximity feeding technique [4]. The BW is also increased by using multi-resonator gap-coupled configurations [1 – 3]. However this method increases the antenna size. More commonly the antenna BW is increased by cutting the slots of shapes like, U-slot, V-slot, and rectangular slot at an appropriate position inside the patch [5 – 10]. The slot is said to introduce a mode near the fundamental mode of the patch and yields larger BW. In most of the reported designs the slot length is either taken equal to half wave or quarter wave in length at the desired slot frequency. The gain of the antenna is increased by using the gap-coupled configurations or by using the arrays of the individual patch elements [1, 11, 12]. The broadband E-shaped MSA is realized by cutting the pair of rectangular slots on one of the radiating edges of the patch [10]. 978-1-4673-5952-8/13/$31.00 ©2013 IEEE

In the reported design the slot length is assumed to be nearly quarter wave in length. In this paper, broadband proximity fed E-shaped MSA is discussed. The E-shaped MSA gives a bandwidth of nearly 350 MHz with broadside radiation pattern with gain of more than 7 dBi over the VSWR BW. Further a gap-coupled configuration of parasitic rectangular MSAs (RMSA) with an E-shaped MSA is proposed. This configuration gives a BW of more than 450 MHz at center frequency of around 1000 MHz. The proposed antenna gives broadside radiation pattern with peak gain of nearly 9 dBi. Further increase in the BW of above gap-coupled configuration is realized by cutting the pair of rectangular slots along the non-radiating edges of the gap-coupled RMSAs. This configuration gives a BW of more than 550 MHz at center frequency of 1000 MHz. The radiation pattern is in the broadside direction with peak gain of more than 9 dBi. All these configurations have been first analyzed using IE3D software followed by experimental verifications [13]. The dimensions of the individual patches were optimized such that they cover 800 – 1200 MHz frequency band. The air substrate is used to maximize the radiation efficiency and gain. In the measurements the antennas were fabricated using copper plate and were suspended in air using the foam spacer support placed towards the antenna corners. The antenna is fed using N-type connector of 0.32 cm inner wire diameter. The measurement was carried out using R & S vector network analyzer. Since an infinite ground plane is used in the measurements, a larger square ground plane of side length 80 cm (2.67λ0) is used in the measurements. The radiation pattern was measured in minimum reflection surrounding with the required minimum far field distance between the reference antenna and the antenna under test [14]. The antenna gain was measured using three antenna method [14].

II. BROADBAND E-SHAPED MSA Broadband proximity fed E-shaped MSA is shown in Fig. 1(a, b). The MSA is optimized on air substrate of thickness (h) 3.0 cm. The pair of rectangular slots introduces a resonant mode near the TM10 mode frequency of the rectangular patch and yields broader BW. In this configuration two resonant modes are present and therefore two loops are present in the input impedance locus as shown in Fig. 1(c). The simulated and measured BW is more than

350 MHz (> 36%). The antenna has broadside radiation pattern over the BW with gain of more than 7 dBi. Further increase in the BW and gain of this configuration is realized by gap-coupling parasitic RMSAs of equal dimensions along the two radiating edges of the rectangular patch as discussed in the following section.

Fig. 1 (a) Side and (b) top views of proximity fed E-shaped MSA and its (c) input impedance plot, (______) simulated, (___ ___) measured

III. BROADBAND GAP-COUPLED E-SHAPED MSA AND RMSA

The gap-coupled configuration of RMSAs with proximity fed E-shaped MSA is shown in Fig. 2(a). The two RMSAs of equal dimensions are gap-coupled along the two radiating edges of the proximity fed E-shaped MSA. To realize different resonant frequencies the length of gap-coupled RMSAs is taken to be smaller than the length of E-shaped MSA. Thus in this configuration, three resonance frequencies are present (i.e. TM10 mode on the proximity fed E-shaped MSA, a mode introduce by the slot and TM10 mode

of parasitic RMSAs). The dimensions of the patches are selected such that the broadband response is realized in 1000 MHz frequency band. The gap-coupled RMSA length and the air gap between RMSA and E-shaped MSA is optimized such that all the loops will lie inside the VSWR = 2 circle. The optimized input impedance and VSWR plot are shown in Fig. 2(b). The simulated BW is 446 MHz (45.6%). The antenna was fabricated using the copper plate and the measurement was carried out using square ground plane of side length 80 cm. The measured BW is 460 MHz (47%) as shown in Fig. 2(b). The fabricated prototype of the configuration is shown in Fig. 3(a). The radiation pattern over the BW is measured and at the center frequency it is shown in Fig. 3(b). The gain over the BW is shown in Fig. 3(c). The radiation pattern is in the broadside direction with E and H-planes aligned along Φ = 00 and 900, respectively. The antenna gain is more than 8 dBi over most of the BW.

Fig. 2 (a) Proximity fed E-shaped MSA gap-coupled to RMSA and its (b) input impedance and VSWR plots, (______) simulated, (___ ___) measured

Fig. 3 (a) Fabricated prototype (b) radiation pattern at center frequency and (c) gain variation over BW for RMSAs gap-coupled to proximity fed E-shaped MSA

Further increase in the BW of above configuration is realized by cutting the pair of rectangular slots inside the gap-coupled RMSAs as discussed in the following section.

IV. PROXIMITY FED E-SHAPED MSA GAP-COUPLED TO PAIR OF RECTANGULAR SLOT CUT RMSAs

The broadband proximity fed E-shaped MSA gap-coupled to pair of rectangular slot cut RMSA is shown in Fig. 4(a). Total four resonance frequencies are present in this configuration (i.e. TM10 mode on E-shaped MSA and parasitic RMSA and modes introduce by the slot on E-shaped MSA and gap-coupled RMSAs). The resonance curve plot for proximity fed E-shaped MSA gap-coupled to RMSA and for gap-coupled pair of slots cut RMSA for ls = 5.0 cm, is shown in Fig. 4(b). As shown by the arrow, the pair of slot introduces a mode near the resonance frequencies of E-shaped MSA and RMSAs. The tuning of this mode with respect to other modes present in the resonance curve is realized by increasing the slot length (ls).

Fig. 4 (a) Proximity fed E-shaped MSA gap-coupled to pair of slot cut RMSA, (b) resonance curve plots for, (______) ls = 0, (___ ___) ls = 5.0 cm

The simulated input impedance plot for ls = 8.2 cm, is shown in Fig. 5(a). Four loops are present in the input impedance plot. However there positions is not optimized inside the VSWR = 2 circle. Also it was observed that for given patch dimensions, only three loops are optimized inside VSWR = 2 circle thereby realizing smaller BW. The surface current distribution at the frequency of first loop is shown in Fig. 5(b). The surface currents are directed along the horizontal direction in the proximity fed E-shaped MSA indicating the presence of TM10 mode on the patch. To optimize the placement of the loop due to TM10 mode of E-shaped MSA, its frequency is needed to be increased. This can be realized by changing the slot dimension which is cut to place the proximity strip or by changing the patch dimensions. The changes in the slot and hence the strip dimensions will also

affect the positions of other loops in the input impedance plot. Therefore the width of E-shaped MSA is reduced to increase the TM10 frequency. This helps in optimizing the four loops inside VSWR = 2 circle and also achieve the broadband response in 800 to 1200 MHz frequency band. The optimized configuration and its input impedance plots are shown in Fig. 5(c) and 6(a) respectively. The simulated BW is 547 MHz (53.4%). The antenna was fabricated and the measurement was carried out. The measured BW is 575 MHz (55.2%). The fabricated prototype of the configuration is shown in Fig. 6(b). The radiation pattern at the center frequency is shown in Fig. 7. The pattern is in the broadside direction with cross polar levels less than 15 dB as compared to co-polar levels. The antenna gain is more than 8 dBi over the BW as shown in Fig. 8.

Fig. 5 (a) Input impedance plot for ls = 8.2 cm, (b) surface current distribution and (c) optimized configuration of proximity fed E-shaped

MSA gap-coupled to pair of rectangular slot cut RMSAs

Fig. 6 (a) input impedance plots, (______) simulated, (___ ___) measured and fabricated prototype of proximity fed E-shaped MSA gap-coupled to pair of

rectangular slot cut RMSA

Fig. 7 Radiation pattern at center frequency for proximity fed E-shaped MSA gap-coupled to pair of rectangular slot cut RMSA

Fig. 8 Gain variation over BW for proximity fed E-shaped MSA gap-coupled to pair of rectangular slot cut RMSA

V. CONCLUSIONS

The broadband proximity fed E-shaped MSA is discussed. The gap-coupled configuration of RMSA with proximity fed E-shaped MSA is proposed. This configuration gives BW of more than 450 MHz (>45%) with broadside radiation pattern and gain of more than 8 dBi. Further increase in the BW of gap-coupled configuration is realized by cutting pair of rectangular slots inside the gap-coupled RMSA. To optimize all the loop position and for realizing the broadband configuration in the 1000 MHz frequency band, the width of the proximity fed E-shaped MSA is optimized. This configuration gives a BW of nearly 550 MHz (> 52%) with broadside radiation pattern and gain of more than 8 dBi over most of the BW. The proposed configuration can find applications in mobile communication applications in 800 – 1200 MHz frequency band.

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