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- Slot Antenna Impedance Equation. Following is the slot antenna impedance equation: Zs = ηo 2 / (4. Zd) ηo = 120.π Ohm Zd = Impedance of complementary dipole = Rd + j.Xd Where, Rd = Real Part and Xd = Reactive Part Antenna related links. Antenna Tutorial. Table of Antenna types and gain equations. Manufacturers of RF antenna Set Top Box manufacturers Set Top Box basics TV Antenna Booster.
- Gain, radiation pattern, return loss, axial ratio and size of an antenna. The slots on the patch or on the ground plane will help to design a antenna with improved bandwidth and effeciency. Key words: microstrip patch antenna, slots, bandwidth, gain, efficiency. 1.INTRODUCTION Microstrip slot antennas invented in 1938 by Alan Blumlein.
And width of the patch and slot. Broadening the antenna bandwidth is achieved by U-slot technique. The antenna analysis is carried out with full wave simulation Ansoft HFSS by using the parameters Return loss (dB), Bandwidth, VSWR, Radiation Pattern and gain. The antenna operating frequency range is 1.85GHz - 2.49GHz.
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On-line version ISSN 2179-1074
http://dx.doi.org/10.1590/2179-10742017v16i3867
Articles
Dual U-Slot Loaded Patch Antenna with a Modified L-Probe Feeding
1Department of Electronics and Communication Engineering, Raffles University, Neemrana, Rajasthan, INDIA-301705, Email: srakeshnath@gmail.com
2Department of Physics, Galgotias University, Greater Noida, Uttar Pradesh, INDIA-201308, Email: prabhakarsingh3@gmail.com
3School of Computational and Integrative Sciences, Jawaharlal Nehru University, New Delhi, INDIA-110067, Email: bkkanaujia@yahoo.co.in
In this paper, a modified L-strip fed patch antenna is theoretically analyzed for wideband applications. Dual U-shaped slots are incorporated in the radiating patch and a maximum bandwidth of 16.93% (2.65 GHz to 3.14 GHz) is achieved. Further, when two parasitic elements are used, antenna bandwidth improves up to 25.89% (2.90 GHz to 3.77 GHz). The maximum gain of dual U-slotted patch and with parasitic elements is 8.23 dBi and 8.46 dBi respectively. Antenna parameters are calculated by CST Microwave studio and equivalent circuit model theory is presented. The proposed antenna is fabricated and the measured results compare well with the theoretical as well as simulated results.
Index Terms Dual U-slot patch antenna; modified L-probe; multilayer patch antenna; gap coupled patch antenna; parasitic elements
I. INTRODUCTION
With the rapid development and attractive solution for various wireless communication systems and demands of their applications, compact and wideband antenna designs have been given immense priority [1], [2]. In this process, various methods were used to improve the bandwidth such as by loading the slots of different size and shape, etching notches and introducing discontinuities in the radiating patch as well as in the ground plane [3], [4]. There are several structures reported to improve the antenna characteristics such as E-shaped [5], [6], C-shaped, U-slot loaded and modified L-strip [7-9]. Different feeding methods also increase the antenna bandwidth such as proximity feed patch, asymmetric CPW fed patch antennas [10], [11]. Substrate with low dielectric constants, multilayer structures and use of air gaps between the dielectric layers increases the impedance bandwidth and gain of the microstrip antennas [12-14]. Besides the fed patch, some parasitic inverted-L wire improves the radiation performance of patch antennas [15], [16]. Apart from that some other types of parasitic element design such as slot type, shorted strip type etc [17], [18] can improve the antenna bandwidth and gain.
The present paper reports a radiating structure to study the antenna bandwidth, gain, efficiency and the radiation pattern. Dual U-slot is incorporated in the radiating patch such that one U-slot is lying within another U-slot forming dual U-slot loaded patch antenna (DUSPA). Further, to increase the bandwidth, two parasitic elements are used above this DUSPA. Both designs are fed by modified L-probe. All the calculations are made by using CST Microwave studio. Also, a theoretical analysis for the proposed antenna is developed based on cavity model. The proposed design is fabricated and various antenna parameters are measured. The details of antenna design and results are discussed in the following sections.
II. ANTENNA DESIGN AND GEOMETRICAL CONFIGURATION
The top view and the side view of the proposed antenna configuration are shown in Fig. 1. The radiating patch is printed on lower side of a substrate of thickness h2. On the other side a conducting strip of dimension ls × ws is printed. The relative position of the strip is at a distance ‘p’ from the edge of the patch. This strip is excited by center conductor of coaxial probe. The patch is suspended at a height h1 from the ground plane. An inverted U-slot of dimensions L2 × W2 with width d1 is etched within another U-slot of dimensions L1 × W1 with same width d1. This DUSPA which is printed on the lower side of the substrate of thickness h2, is energized by a conducting strip printed on the upper side of this substrate. Further two parasitic elements of dimensions Lp × Wp separated by gap D are placed at thickness h3 from the conducting strip. These parasitic patches are excited by electromagnetic coupling with DUSPA. A detail design specification is given in Table-1.
Fig. 1 Geometry of the proposed antenna (a) Top view (b) Side view.
Table 1 Design specifications of the proposed antenna:
| Parameter | Value |
|---|---|
| Wg × Lg | 80 × 52 mm2 |
| W × L | 70 × 48 mm2 |
| W1 × L1 | 42 × 29 mm2 |
| W2 × L2 | 30 × 23 mm2 |
| c × d | 10 × 8 mm2 |
| Air gap (h1) | 3.5 mm |
| Height of dielectric substrate (h2) | 1.55 mm |
| Height of dielectric substrate (h3) | 1.50 mm |
| Substrate used | RT/duriod 5880 (εr = 2.2) |
| Conducting strip (ws × ls) | 3.0 × 10.5 mm2 |
| Wp × Lp | 31.2 × 25 mm2 |
| d1 | 1.0 mm |
| W3 | 5.0 mm |
| W4 | 28.0 mm |
| W5 | 14.0 mm |
| D | 5.6 mm |
| H | 6.85 mm |
| l3 | 7.0 mm |
| p | 5.0 mm |
The proposed antenna is fabricated on Rogers RT duriod substrate (dielectric constant 2.2) with ground plane dimensions (Wg × Lg) 80 × 52 mm2 (Fig. 2).
Fig. 2 Photograph of proposed fabricated antenna.
III. THEORETICAL INVESTIGATION
(a) Analysis of modified feeding
Modified L-probe feeding can be analyzed into two parts i.e. vertical probe and horizontal strip.
Vertical Probe
Vertical probe can be analyzed as series combination of resistance Rv and inductance Lv, and can be given as [19].
where μ = permeability of the probe conductor, f = frequency in GHz, dp = diameter of probe
and
Horizontal Strip
A series combination of distributive resistance Rs and inductive Ls is developed due to horizontal conducting strip and can be given as [20].
here, ws = width of strip, ts = thickness of strip, ρ0 = ratio of specific resistance of strip and copper.
The distributive capacitance Csp between horizontal strip and the radiating patch can be given as
Since the open ends of the horizontal strip above the radiating patch will have fringing field, so the effective length of the strip is increased. The increment of length will cause some extra capacitance which is fringing capacitance and it can be calculated as
here,
in which εes is effective dielectric constant for the material under conducting strip [19]. The fringing capacitance Cfp between open end of the strip and the radiating patch is calculated by putting the substrate height h = h2 and the fringing capacitance Cfg can be given by putting h = (h1 + h2). The fringing capacitance Cpf1 between the parasitic elements and the strip is calculated by putting h = h3. The entire feeding acts as a series L-C resonant element and connected in series with the radiating patch. The impedance of this modified L-probe can be calculated using Fig. 3 as
where,
(b) Analysis for DUSPA
The value of capacitance C1, inductance L1 and resistance R1 for a rectangular patch can be given as [20].
where, L = length of the patch, W = width of the patch, f1 = resonance frequency, ls = feed point location, ω = 2πf1 and Qr is the quality factor of the resonator.
where, c = velocity of light, f1 = design frequency, εe = effective permittivity of the medium
in which, εrs can be calculated as
where, n is the number of stacked layers and εr = relative permittivity of the substrate material.
A slot in the radiating structure can be analyzed using the duality relationship between the dipole and slot [21]. The radiation resistance of an inclined slot in the patch is given by
The input reactance of the inclined slot is given as [22]
here, α is the inclination angle of slot with respect to x-axis, Si and Ci are the sine and cosine integrals, d1 = thickness of the slot, L1 = length of the slot. Impedance for this inclined slot is given by [22]
here
Now, U-slot in a patch is analyzed by assuming two slots along the y-axis as the vertical slot of length ‘L1’ at angle α = 00 and a slot along x-axis as the horizontal slot of dimension ‘W1’ at angle α = 900.
The input impedance of the vertical slot can be calculated by using equations (9), (10) and (11) as:
here, ZV1 is calculated by putting α = 00. Similarly the impedance of the horizontal slot can be calculated as:
here, ZH1 is calculated by putting α = 900. Thus equivalent circuit for U-slot in the patch is given by Fig. 4.
Fig. 4 Equivalent circuit of U-slot in the patch.
Thus, the equivalent circuit for modified L-probe fed patch can be given as shown in Fig. 5.
The input impedance of DUSPA is calculated by using Fig. 5 as:
in which, ZU1 and ZU2 are the impedances of two U-slots in the patch and can be calculated using Fig. 4 and ZP is the impedance of the rectangular patch and can be calculated as:
(c) Analysis of the parasitic elements
The parasitic elements are excited through the electromagnetic coupling with DUSPA. Each parasitic element is considered equivalent to parallel combination of resistance Rp, Lp and Cp. These two parasitic elements are coupled with each other by the gap coupling and the equivalent circuit is given in Fig. 6. The equivalent circuit of the gap can be given as a π-circuit, consisting of the gap coupling capacitance Cg and the plate capacitances CP1. Now two radiating structures (DUSPA and parasitic elements) are coupled through the electromagnetic coupling.
Fig. 6 Equivalent circuit for the gap coupled parasitic elements.
The accurate equations for the coupling capacitance Cg and the plate capacitances CP1 of the microstrip gap can be calculated from the hybrid mode analysis [23], [24]. Now using the equivalent circuit as shown in Fig. 7, the total input impedance of DUSPA with parasitic elements can be calculated as:
and
in which,
here, LM, CM are the mutual coupling inductance and capacitance between two radiators and ZP1 is the impedance of parasitic elements.
Fig. 7 Equivalent circuit of the modified L-probe fed DUSPA with parasitic elements.
Using equation (16), we can calculate the reflection coefficient, VSWR and return loss of the proposed antennas.
where, Z0= characteristic impedance of the coaxial feed (50 Ω)
and
IV. Result and discussion
Fig. 8 shows the simulated return loss obtained from CST Microwave studio for different values of dual U-slot width (d1). From the graph it is observed that entire operating band shifts towards higher side for increasing value of d1, however, antenna bandwidth decreases with increasing value of d1. The bandwidth of the antenna is calculated for return loss < −10 dB and found maximum (16.67%, 2.66 − 3.14 GHz) at d1 = 1.0 mm and below d1 = 1.0 mm the antenna exhibits dual band nature.
Fig. 8 Variation of return loss with frequency for different slot thickness (d1).
A comprehensive analysis has been done to study the bandwidth variation of DUSPA for different combination of dual U-slot length L1, L2 and slot width d1. From Fig. 9, it is observed that the bandwidth of the antenna increases with increasing value of slot length L1. The maximum bandwidth (16.67%) is obtained at d1 = 1.0 mm and L1 = 30.0 mm. Fig. 10 reveals the similar variation of bandwidth as observed in Fig. 9. The highest bandwidth (16.93%, 2.65 to 3.14 GHz) is seen when d1 = 1.0 mm, L1 = 30.0 mm and L2 = 24.0 mm.
Fig. 9 Bandwidth variation of DUSPA for different (a) outer slot length L1 (b) inner slot length L2.
Fig. 10 Bandwidth variation of DUSPA for combination of L1 and L2.
In the process of improving the antenna bandwidth, two parasitic elements are placed at a distance 3.05 mm from DUSPA. Fig. 11 shows the variation of return loss with frequency for different values of parasitic patch length (Lp). It is observed that the bandwidth is almost invariant with Lp. The optimum bandwidth achieved is 25.26 % varying from 2.88 to 3.71 GHz at Lp = 26.0 mm. However, when Lp = 27.0 mm, the antenna exhibits dual nature.
Fig. 11 Variation of return loss with frequency for different values of Lp.
From Fig. 12, it is observed that bandwidth decreases as gap D increases. The maximum bandwidth obtained is 24.59% (from 2.92 to 3.74 GHz) at D = 5.0 mm. At the higher value of D, the gap coupling effect between two parasitic elements is ceases to exist and the antenna starts behaving like DUSPA. Below D < 5.0 mm the increment in the bandwidth is almost constant.
Fig. 12 Variation of return loss with frequency for different value of gap ‘D’.
From Fig. 13, it is observed that the bandwidth of the antenna increases with decreasing the value of Wp. From this graph it is observed that the highest operating bandwidth of 25.89% (from 2.90 to 3.77 GHz) is achieved for the optimized value of Wp = 31.2 mm. Further, below 31.2 mm the antenna shows dual nature.
Fig. 13 Variation of return loss with frequency for different values of Wp.
The simulated return loss is compared with the theoretical and the measured results of DUSPA and DUSPA with parasitic elements respectively (Fig. 14). From the graph it is clear that simulated and theoretical results of both the antennas are agreeing quiet well with the measured one. Fig. 15 depicts the simulated and measured gain for both the antennas. The simulated peak gain for DUSPA is 8.23 dBi at 2.8 GHz while for DUSPA with parasitic elements peak gain is 8.46 dBi at 3.0 GHz. For DUSPA, the maximum gain variation is 0.63 dBi for the entire band of operation (2.75-3.10 GHz) while for DUSPA with parasitic elements it is 0.96 dBi for the entire band of operation (2.82-3.75 GHz).
Fig. 14 Measured return loss for DUSPA and DUSPA with parasitic elements.
Fig. 15 Measured gain for DUSPA and DUSPA with parasitic elements.
Radiation efficiency is calculated for both the antennas and found quite acceptable (above 97.0%) for entire operating frequency band (Fig. 16). The radiation patterns of the proposed antennas are measured using anechoic chamber. Fig. 17 shows the measured and simulated radiation patterns for DUSPA at 2.80 GHz and 3.09 GHz. The cross polarization level is quite low at φ = 900 than that at φ = 00. A good agreement between the measured and simulated results is observed. Radiation patterns for DUSPA with parasitic elements are plotted at 3.09 GHz, 3.45 GHz and 3.63 GHz (Fig. 18). The cross polarization level at φ = 900 is again quite low than that at φ = 00 for all three resonant frequencies. This is primarily because of the feed location which is along y-axis. Also, the inherent asymmetry property of probe feed which generates higher order modes and hence increases the cross-polarization level. In the measured results, some ripples are observed below ground plane due to reflection of radiation by the conducting strip. The simulated radiation pattern is compared with measured results however, some mismatch is observed in radiation pattern due to fabrication inaccuracy and numerical methods used in simulator.
Fig. 16 Simulated radiation efficiency for DUSPA and DUSPA with parasitic elements.
Fig. 17 Radiation pattern of DUSPA at (a) 2.80 GHz, and (b) 3.09 GHz.
Fig. 18 Radiation pattern of DUSPA with parasitic elements at (a) 3.09 GHz, (b) 3.45 GHz, and (c) 3.63 GHz.
It is observed that all simulated co-polar graphs are showing almost semi circular back lobe bellow the ground plane. It may be due to the size and thickness limitation of the ground plane taken in our design for the simulation. Both the antennas exhibit linearly polarized, broadside and symmetrical radiation patterns for entire operating frequency band.
V. CONCLUSION
A dual U-slot loaded patch with a modified L-strip feeding technique is presented. The equivalent circuit model is successfully implemented for the calculation of various antenna parameters. The modification in the feeding technique makes the antenna fabrication convenient as compared to proximity coupled L-strip feeding. From this investigation it is inferred that L-probe fed dual U-slot loaded patch increases the bandwidth up to 16.93%. Moreover, when DUSPA is stacked with two parasitic elements, the gap (D) between the two parasitic elements and width Wp are the key parameters to increase the bandwidth. In this stacked configuration the antenna bandwidth increases up to 25.89% with maximum gain of 8.46 dBi. The dimension of conducting strip can be optimized to further improve the antenna characteristics. Antenna bandwidth can also be controlled with inner and outer U-slot dimensions. This antenna is operating in S-band which can be used in weather radar, surface ship radar and other communication satellites.
REFERENCES
[1] K.L. Wong, Compact and Broadband Microstrip Antennas, New York, Wiley, 2002. [ Links ]
[2] G. Kumar and K. P. Ray, Broadband Microstrip Antennas, Artech House, Norwood, MA, 2003. [ Links ]
[3] B. L. Ooi, “A double-π stub proximity feed U-slot patch antenna,” IEEE Trans. Antennas Propag., 52, (2004), 2491-2496. [ Links ]
[4] Y. Sung, “Printed Wide-Slot Antenna With a Parasitic Center Patch,” IEEE Trans. Antennas Propag., 60 (2012), 1712-1716. [ Links ]
[5] Y. Chen, S. Yang, and Z. Nie, “Bandwidth enhancement method for low profile E-shaped microstrip patch antennas,” IEEE Trans. Antennas Propag., 58, (2010), 2442-2447. [ Links ]
[6] Y. Chen, and C. F. Wang, “Characteristic-mode-based improvement of circularly polarized U-slot and E-shaped patch antennas,” IEEE Antennas Wirel. Propag. Lett., 11 (2012), 1474-1477. [ Links ]
[7] S. Bhardwaj, and R. Y. Samii, “A comparative study of C-shaped, E-shaped, and U-slotted patch antennas,” Microw. Opt. Technol. Lett., 54 (2012), 1746-1757. [ Links ]
[8] K. F. Lee, S. L. S. Yang, A. A. Kishk, and K. M. Luk, “The versatile U-slot patch,” IEEE Antennas and Propag. Magaz., 52 (2010), 71-88. [ Links ]
[9] Z. Wang, S. Fang, and S. Fu, “Wideband dual-layer patch antenna fed by a modified L-strip,” Journal of Microw. Opt. and Electro. Applic., 9, (2010), 89-100. [ Links ]
[10] A. A. Deshmukh, and K. P. Ray, “Broadband proximity-fed modified rectangular microstrip antenna,” IEEE Antenna Propag. Mag., 53 (2011), 1-5. [ Links ]
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[15] J.-Y. Jan, and L.-C. Tseng, “Small planar monopole antenna with a shorted parasitic inverted-L wire for wireless communications in the 2.4, 5.2, and 5.8-GHz bands,” IEEE Trans. Antennas Propag., 52 (2004), 1903–1905. [ Links ]
[16] J. H. Lee, and J. G. Yook, “Improvement of radiation performance of mobile phone antenna using parasitic element,” IEEE Trans. Cons. Electron., 56 (2010), 2411–2415. [ Links ]
[17] Tsien-Ming Au, Kin-Fai Tong, and Kwai-Man Luk, “Theoretical and experimental studies of a microstrip antenna with two parasitic patches,” Int J RF and Microwave CAE, 8 (1998), 49–55. [ Links ]
[18] K. C. Lin, C. H. Lin, and Y. C. Lin, “Simple printed multiband antenna with novel parasitic-elements design for multistandard mobile phone application,” IEEE Trans. Antenna Propag., 61 (2013), 488–491. [ Links ]
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Received: December 26, 2016; Revised: December 29, 2016; Accepted: May 26, 2017
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The standard rectangular microstrip patch is a narrowband antenna and provides 6-8 dBi Gain with linear polarization. This example based on the work done in [1],[2], models a broadband patch antenna using a slot in the radiator and develops a dual-band and a tri-band variation from it. In the process, the single wide response has been split into multiple narrow band regions catering to specific bands in the WiMAX standard. These patch antennas have been probe-fed.
Building the Single U-Slot Patch
Define Parameters The basic U-slot patch antenna consists of a rectangular patch radiator within which a U-shaped slot has been cut out. As discussed in [1], the patch itself is on an air substrate and thick so as to enable higher bandwidths to be achieved. The presence of the slot structure achieves additional capacitance within the structure which combines with the inductance of the long probe feed to create a double resonance within the band. The geometry parameters based on [2] are defined and shown in a drawing below.
Define radiator shape - Single U-slot
Use the rectangle shape primitives in Antenna Toolbox™ to create the U-slot patch radiator shape. Boolean subtraction operation is used among the shape primitives for this purpose.
Define ground shape
Create the ground plane shape for the antenna. The groundplane in this case is rectangular and 71 mm x 52 mm in size.
Define stack
U Slot Patch Antenna Calculator Online
Use the pcbStack to define the metal and dielectric layers and the feed for the single U-slot patch antenna. The layers are defined top-down. In this case, the top-most layer is a metal layer defined by the U-slot patch shape. The second layer is a dielectric material, air in this case, and the third layer is the metal ground plane.
Calculate and Plot Reflection Coefficient
Mesh the structure by using a maximum edge length which is one-tenth the wavelength at the highest frequency of operation which is 6 GHz for this example. Compute and plot the reflection coefficient for this antenna over the band. The reflection coefficient is plotted with a reference impedance of 50 ohms.
Calculate and plot pattern
Plot the radiation pattern for this antenna at the frequencies of best match in the band.
Dual-band U-Slot Patch Antenna
Define Parameters
To achieve dual-band behavior as shown in [1], [2], the double resonance is modified such that the two contributing resonances, i.e. from the patch and from the slot do not merge. To do so the existing slot parameters are adjusted and a second slot is introduced into the structure. The parameters for the double U-slot are listed below as per [2] and a figure annotated with the variables used is shown.
Create Double U-slot radiator
As before use the shape primitives, to create the geometry by using Boolean operations.
Modify Layers in Stack
Modify the existing stack by introducing the new radiator in the Layers property.
Mesh and Plot Reflection Coefficient
Mesh the structure at the highest frequency of operation and calculate the reflection coefficient.
Triple-Band U-slot Patch Antenna Parameters
For triple-band operation a third U-slot is introduced and the existing slot parameters are adjusted. The parameters are shown below based on [2].
Create Triple U-slot radiator
Modify Layers in Stack
Patch Antenna Design Calculator
Mesh and Plot Reflection Coefficient
Conclusion
The models of the multi-band single layer U-slot patch antenna as discussed in [1], and [2] have been built and analyzed and agree well with results reported.
Reference
[1] K. F. Lee, S. L. S. Yang and A. Kishk, 'The versatile U-slot patch antenna,' 2009 3rd European Conference on Antennas and Propagation, Berlin, 2009, pp. 3312-3314.
[2] W. C. Mok, S. H. Wong, K. M. Luk and K. F. Lee, 'Single-Layer Single-Patch Dual-Band and Triple-Band Patch Antennas,' in IEEE Transactions on Antennas and Propagation, vol. 61, no. 8, pp. 4341-4344, Aug. 2013.