Volume 2, Spl. Issue 2 (2015)
e-ISSN: 1694-2310 | p-ISSN: 1694-2426
A Review Paper on Stacked Microstrip Antenna for MIMO Application Ayushi Agarwal1, Amanpreet Kaur2 1,2
ECE department, Thapar University, Patiala
1
aaayushi562@gmail.com, 2amanpreet.kaur@thapar.edu
Abstract---- MIMO system consists of multiple number of antennas at transmitter and receiver which in turn offers high capacity to wireless system over a SISO system. The stacked microstrip antenna has particular characteristics such as high gain and wide bandwidth which make it suitable for MIMO applications. This paper presents study on the stacked microstrip antenna and the parameters governing its operation. The application of stacked antenna to WLAN is also presented in the paper. Methods to obtain circular polarisation with rectangular patches are also presented in the current article. Keywords----Microstrip antenna, stacked, widebandwidth, high gain, high capacity, MIMO system, WLAN.
I. INTRODUCTION The idea of microstrip antenna dates back to 1950’s, but it was 1970’s in which microstrip antenna received considerable attention [1]. The basic configuration of microstrip antenna consists of a metallic patch printed on a thin ground dielectric substrate as shown in fig(1). The radiating element and the feed lines are usually photo etched on the dielectric substrate. The radiating patch may be square, rectangle, circle, thin strip(dipole) and many other configurations. There are many configurations that can be used to feed the antenna. The four most popular are the microstrip line, coaxial probe, aperture coupling, proximity coupling[1]. The microstrip patch is designed so that by excitation beneath the patch its pattern maximum is normal to the patch. The microstrip antenna is a broadside antenna. The microstrip antenna has a very low profile and can be made conformable and potentially at low cost. Other advantages include versatility in terms of electromagnetic induction [2] and easy fabrication into linear and planar array [3]. Although conventional microstrip antenna has various advantages but there are three main disadvantages associated with it namely their capability to resonate at single frequency, narrow bandwidth and low gain. This can be overcome by increasing thesubstrate height or alternatively stacking different layers to form a stacked antenna. But increasing the substrate height introduces the surface waves [1]. Therefore stacking is preferable [4] [5].
Fig 1. Microstrip antenna Development of wireless technology has raised the capacity and reliability requirements of wireless communication. It has become increasingly difficult to fulfil the requirements with traditional SISO systems due to the limitation in channel capacity. MIMO is a new wireless technology conveyed in mid 90s. One of the main benefits of MIMO is that the additional paths can be used to increase the capacity of the link without increasing transmitpower [6]. MIMO is able to transmit multiple de-correlated signals with same power level, simultaneously through spatially parallel channel. For any MIMO application wide band antenna is desirable, hence we can use stacked antenna to achieve wide band in conjunction with MIMO systems. The space consumption of MIMO antennas is vital in applications such as access points, modems and end terminal equipment in WLAN system. When regularly spaced antenna elements are used in MIMO systems, the channel capacity and transmission quality is dependent on the distance between the array elements. For compactness in MIMO system pattern diversity, multimode diversityand polarization diversityin conjunction with space diversity can be used. The paper presents a study on the use of stacked antennas for MIMO and WLAN applications and it is organised as follows. Section II gives the brief introduction of stacked microstrip antenna. Section III describes the various parameters associated with the stacked antenna. Next sections are dedicated towards the application of stacked antenna. Section IV presents a design strategy of stacked microstrip antenna for MIMO WLAN system. Section V deal with the analyses of the designed antenna. II.STACKED MICROSTRIP ANTENNA In spite of various advantages of microstrip antenna the bandwidth provided by the single patch antenna is narrow. It is well known that a multilayer structure is a useful method to improve the problem. In stacking a parasitic
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patch is constructed over the feed patch and both the patches are electromagnetically coupled to each other.By stacking a parasitic patch on a feed patch an antenna with high gain or wide band can be realised. These characteristics of the stacked antenna can be controlled by the distance between the parasitic patch and feed patch [5]. When the distance between the parasitic patch and feed patch is less than the stacked antenna has two resonant frequencies hence in turn it increases the bandwidth. When the distance between the patches is approximately half a wavelength, these patches form a leaky resonator. As a result, resonant field increases the gain.
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Fig 3(b). Square patch driven at adjacent sides through 90o hybrid[1]
Fig 3(c). square patch with slot for circular polarization[1]
Fig 2: Stacked patch aperture coupled to a microstrip line
Stacked antenna can be fed with various techniques but it has been reported that an aperture coupled stacked structure has a relative bandwidth of about 70% [7] [8]. Fig 3 shows aperture coupled stacked microstrip antenna. When the stacking is done the patch size of feed patch or parasitic patch can be varied. Usually the parasitic patch is constructed larger than the feed patch but vice versa can be done according to the desired application. Both linear and circular polarizations can be produced [4]. There are various methods of creating circular polarization in square patch [9] [10]. Fig(4) shows some of the techniques used to produce circular polarization in square patch and fig(5) shows the method to produce circular polarization in circular patch.
Fig 3(d). Trimmed square patch for circular polarization[1]
Fig 4. Circular patch fed with 90o hybrid[1]
Fig 3(a). Square patch driven at adjacent sides through a power divider.[1]
III.STACKED ANTENNA PARAMETERS The designing of stacked patch antenna is complicated as here are many design parameters that are associated with it and for desirable operation features they must be determined. These parameters and their effects are summarized below [9] [11]: Bottom Antenna Substrate Thickness and Dielectric Constant: A thick substrate with a low dielectric constant yields large bandwidth, but coupling to the bottom patch decreases with the substrate thickness, requiring a compensating increase in aperture size and thus increased back radiation.
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e-ISSN: 1694-2310 | p-ISSN: 1694-2426
Bottom Patch Length and Width: As the dimension of the bottom square patch is increased the top patch becomes isolated from the excitation field of the slot. It is tempting to think that patches in stacked antenna resonates independently but their strong coupling between the patches, the resonant sizes of the patches must be determined simultaneously. Top Antenna Substrate Thickness and Dielectric Constant: The substrate thickness and dielectric constant control the bandwidth of the resonance associated with the top patch as well as the coupling between the patches. Top Patch Length and Width: The coupling level is strongly dependent on the separation of the two resonant frequencies. The coupling is also affected by whether the top patch is larger or smaller than the bottom. Aperature Length and Width: Coupling from the feedline to the bottom patch is controlled primary by the length of the aperture. A longer aperture increases coupling but also increases undesirable back radiation. The aperture width also affects the coupling strength but to a lesser degree than the length.
Where k=2ᴨ/λ is the wave number, Jm(x) is the Bessel function of second kind of order m, a is the patch radius, Vmis the peak input voltage of mth order, ∅o is the reference azimuth angle. Stacked Square Patch:There can be many variations for patch designs for WLAN application. Square shaped patch is one of the them which is very simple to design. Author K. Shambhavi proposed Identical Dual Square Microstrip with air gap (IDSMA) [13]. Proposed antenna is shown in fig(6). The stacked configuration consists of two identical square patches stacked on top of the other. The lower patch is fed with a coaxial probe and the top parasitic patch is electromagnetically coupled to the lower one. In this paper an air gap was introduced which further increased the bandwidth. Inserting an air-gapbetween the two identical patch reduces the average relativepermittivity of the substrate and increases the total substratethickness between the patches. As a consequence thebandwidth is increasedA maximum bandwidth of 12.21% is achieved for an air-gap of 9mm which is more desirable for WLAN.
Distance Between the Patches: When the patches are very close to each other they appear to the slot as a single patch, through the coupling between them generates resonance at high frequency. Conversely, for larger values the behaviour of the overall structure is close to the behaviour of bottom patch alone since the coupling to the patch is very weak. IV. VARIOUS DESIGNS OF STACKED MICROSTRIP ANTENNA Stacked Circular Patch: Authors Alper Ocalan, OzgurErtung designed a multimode stacked circular microstrip antenna designed for MIMO-OFDM WLANapplication [12], in which the upper patch was excited at the TM11 mode and the bottom patch is excited at the TM21 mode to meet the compactness requirements since the radius of a circular microstrip patch scales up with the mode number m excited given by [1]:
=
χ
2ᴨ√
Where mindicates the first order zero of the derivative of second kind Bessel function of order mJm(x).however, including the fringing effect the actual effective radius of the circular patch is larger than the estimated above. The far-field radiation pattern for a circular microstrip patch antenna excited with mth mode is given by [1]:
,
∅ ))
=−
,
⃗=
=
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[
(
,
⃗+
( )−
( )+
⃗
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( )]cos ( (∅ − ∅ ))
( )] cos( ) sin ( (∅ −
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Fig 5: Proposed antenna
V. ANALYSIS OF STACKED ANTENNA There are various parameters that are analysed for the performance of stacked antenna. The driven patch should be of high dielectric and the parasitic patch must be of low dielectric to avoid fringing. A decrease in the dielectic constant will increase the resonance frequency and widens the operating bandwidth which is desirable for WLAN. In addition antenna efficiency will also be improved due to fringing fields. Another very important parameter is the distance between the two patches. When the two patches are very close to each other they appear to slot as a single patch. Conversely, for large values of the distance between the patches the behaviour of the overall structure was close to the behaviour of the bottom patch alone [11] [13]. As the distance increases, degree of coupling reduces, bandwidth increases which is desirable for WLAN operation. But we cannot increase the distance beyond certain limit due to the physical constraints. In the stacked antenna with air gap[13] it was clear that as the air gap increases between the patches there was a considerable increase in the gain and the
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bandwidth. Bandwidth comparision of the proposed antenna with single patch is shown in fig6. Further increase in the gap results in decrease in bandwidth. As the dimension of the lower patch increases, upper patch becomes isolated from the excitation field of slot and The coupling to the fringing field reduces. As the lower dimension is decreased lower frequency decreased and upper frequency
Fig 6: Bandwidth comparision of IDSMA and single patch [13]
increased. Similarly when the upper patch dimension is reduced the upper frequency decreases. Fringing field explains this behavior, as the size of the top patch is reduced, its coupling to the fringing field of the bottom patch becomes negligible. For WLAN we need compact antenna but the variations explained must be taken care of will designing.
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REFERENCES 1. C.A. Balanis,Antenna Theory: Analysis and Design, 2nd ed. UnitedStates of America, John Wiley&Sons Inc., 1997. 2. David M. Pozar and Fellow: ‘Microstrip Antenna’, Proceedings of theIEEE,Vol. 80, No. 1, pp. 79-91, January 1992. 3. Celal Alp Tunc, UgurOlgun, VakurB. Erturk and AyhanAltintas: ‘On the Capacity of Printed Planar Rectangular Patch Antenna Arrays in the MIMO Channel: Analysis and Measurement’, IEEE Antennas and Propagation Magazine, Vol. 52, No. 6, pp. 181-192, December 2010. 4. Shigeru Egashire and EisukeNishiyama: ‘Stacked Microstrip Antenna with Wide Bandwidth and High Gain’, IEEE Transactions on Antenna Propagation, Vol. 44, No. 11, pp. 1553-1534, November 1996 5. E. Nishiyama, M. Aikawa and S. Egashira: ‘Stacked Microstrip Antenna for Wideband and High Gain’, IEEProc-Microwave Antenna Propagation, Vol. 151, No. 2, pp. 143-148, April 2004. 6. Harshal Nigam and Mithilesh Kumar: ‘Capacity Enhancement and Design Analysis of UWB MIMO OFDM over SISO System Using Microstrip Antennas’, IEEE International Conference on Recent Advances and Innovations in Engineering (ICRAIE-2014), May 0911, 2014 7. Targonski, S.D. Waterhouse, R.B. and Pozar, D.M: ‘Design of WideBand Aperture-Stacked Patch Microstrip Antennas’, IEEE Transactions Antennas Propagation, 46(9), pp. 1245-1251, 1998. 8. Garg, R. Bhartia, P. Bahl, I. and Ittipiboon, A: Microstrip Antenna Design Handbook, Artech House, Boston, 2001. 9. David M. Pozar, Fellow and Sean M. Duffy: ‘A Dual-Band Circularly Polarized Aperture-Coupled Stacked Microstrip Antenna for Global Positioning Satellite’, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 11, pp. 1618-1625, November 1997. 10. SumitaShekhawat, PratibhaSekra, Deepak Bhatnagar, Virender Kumar Saxena and Jaswant Singh Saini: ‘Stacked Arrangment of Rectangular Microstrip Patches for Circularly Polarized Broadband Performance’, IEEE Antennas and Propagation Letters, Vol 9, pp. 910-913, 2010. 11. Fredric Croq and David M. Pozar, Fellow: ‘Millimeter-Wave Design of Wide-Band Aperture-Coupled Stacked Microstrip Antennas’, IEEE Transactions on Antennas and Propagation, Vol. 39, No. 12, pp. 1770-1776, December 1991. 12. Alper Ocalan, AsumanSavascihabes, Ibrahim Gorgec, OzgurErtug and ErdemYazgan: ‘Compact Space-Multimode Diversity Stacked Circular Microstrip Antenna Array for 802.11n MIMO-OFDM WLANs’, Loughborough Antennas and Propagation Conference, November 16-17 2009. 13. K. Shambavi: ‘Gain and Bandwidth Enhancement Technique in Square Microstrip Antenna for WLAN Applications’, Proceedings of Asia-Pacific Microwave Conference, 2007.
The parameters explained above will be similar for both the antenna designed in section IV but the square patch antenna will have greater bandwidth due to the air gap present between the upper and lower patches. VI. CONCLUSION This paper dealt with the stacked microstrip antenna for MIMO and WLAN applications. With the advancement in wireless technology it has become difficult to fulfil the requirements with SISO, hence MIMO came into the picture. Stacked antenna has been developed to overcome the limitations of the microstrip antenna for applications in MIMO and WLAN systems. But there are certain parameters like dielectric, distance between patches which are to be taken care of for desired results of stacked structure. All those parameters were also discussed. Finally for the application part, the design and analysis of 2 stacked antenna one comprised with stacked circular patch and another square patch with an air gap was studied. Due to the low correlation and compactness of stacked antenna, it is a promising solution for next generation WLAN systems.
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