Rectangular and Circular Antenna Design on Thick Substrate
JJOURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 58 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/
RECTANGULAR AND CIRCULAR ANTENNA DESIGN ON THICK SUBSTRATE
Harsh Kumar & Shweta Srivastava
Abstract - Millimeter wave technology being an emerging area is still very undeveloped. A substantial research needs to be done in this area as its applications are numerous. In the present endeavor, a rectangular patch antenna is designed on thick substrate and simulated using SONNET software, also a novel analysis technique is developed for circular patch antenna for millimeter wave frequency. The antenna is designed at 39 GHz on thick substrate and has been analyzed and simulated.The results of the theoretical analysis are in good agreement with the simulated results.
Index Terms — Microstrip, thick substrate, coaxial feeding . —————————— (cid:1) ——————————
1 I
NTRODUCTION
ILLIMETER
Wave can be classified as electromag-netic spectrum that spans between 30GHz to 300 GHz, which corresponds to wavelengths from 10mm to 1mm. Despite millimeter wave technology has been known for many decades, it is still undeveloped. The major applications of this technology being high speed point to point wireless local area network and broad band access[1]. The mm wave systems have mainly been deployed for military applications and low-cost in-tegration solutions. It has started to gain a great deal of momentum from academia and industry due to its advan-tages limited access till date. In this paper, however, the focus is specifically on 38 to 40 GHz frequency range, which is used for high speed microwave data link. Work has been done on rec-tangular antenna on thick substrate and empirical formu-la for input impedance has been also derived. Using this, a rectangular patch antenna is designed on thick substrate and simulate on SONNET software. The empirical for-mula for input impedance of the circular patch antenna on thick substrate is developed and a circular patch mil-limeter wave antenna is designed for 39 GHz on thick substrate. Comparison has been done between the simu-lated and the calculated results. When thick substrate is used surface wave excitation takes place so surface wave loss also has to be considered. James & Henderson [2] proposed that for thick substrate h/λ > 0.09 For ε r =2.32 and h/λ > 0.03 For ε r =10 where ‘h’ is the thickness of sub-strate and λ is the free space wavelength. D ESIGN OF R ECTANGULAR PATCH MILLIMETER WAVE ANTENNA ON THICK SUBSTRATE
Fig. 1 Rectangular microstrip patch antenna The formulas available for the design of thin substrate patch antenna are not applicable for thick substrate. The antenna which is designed from these formula is not re-sonate when antenna thickness is greater then 0.09λ . R ECTANGULAR PATCH MILLIMETER WAVE ANTENNA
Patch resonant length and patch width were determined by analyzing the shape and slope of curve substrate thickness with experimental Length/calculated Length and substrate thickness with experimental Width / calcu-lated Width respectively. The resulting formulas are [13] (1) M ———————————————— • Harsh kumar is with the Department of ECE, Birla Institute of Technology, Mesra, Ranchi, India. • Dr. Shweta Srivastava is with the Department of ECE, Birla Institute of Technology, Mesra, Ranchi, India.
OURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 59 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/ (2) (3) D ETERMINATION OF INPUT RESISTANCE OF RECTANGULAR PATCH WITH THICK SUBSTRATE
A formula based on the cavity model and equiva-lent loss resonant circuit has been developed to calculate the resonant input resistance of the rectangular antenna elements with substrate thickness satisfied the criteria h>=0.0815λ . The antenna elements were consider in its fundamental mode and assumed infinite ground plane. The resonant input resistance of such antenna can be cal-culated from R in = R c + R d + R r + R s (4) The resistance due to conductor loss is calculated from (5) Where (6) Where Є ew is the effective permittivity of the substrate and is calculated from (7) The resistance due to dielectric loss is calculated from The radiation resistance can be calculated from (8) Where L ef is given by (9) Where Weq is the equivalent patch width (10) The existence of dielectric substrate over the conducting ground plane in microstip antenna can cause the surface wave excitation along air dielectric interface. The resis-tances due to surface wave excitation Rs can be derive from ratio of power loss to surface wave Ps and radiation power given by James (11) (12) Where (13) Z ew is the characteristic impedance of the substrate filled strip line of width W and strip conductor of zero thick-ness. It is given for W/h=< 3.3 [13] (14) For W/h>= 3.3, then (15) Where R =120π is vacuum medium resistance. Z is the characteristic impedance of an air filled substrate charac-teristic impedance of an air filled substrate ε r =1. Input impedance (16) Where (17) D ESIGN S PECIFICATIONS
Fig 2: Rectangular patch with coaxial feeding
OURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 60 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/ D ESIGN DETAILS (cid:2)
Frequency of operation ( fo ): The resonant fre-quency selected for this design is 39 GHz. (cid:2) Dielectric constant of the substrate (ε r ): ε r= 4.7 (cid:2) Height of dielectric substrate ( h ): h=0.8 (cid:2) Length=1.06mm (cid:2)
Width=0.98mm (cid:2)
Feed point (a)=0.05mm R ESULTS
The theoretical results for return loss and VSWR were obtained using the above derived expressions (Fig.) and the antenna was simulated using sonnet software and the resulting graph for return loss is shown in Fig..
Fig.3 Theoretical results of variation of return loss with fre-quency
Fig.4: Theoretical results of variation of VSWR with fre-quency
Fig 5: Radiation patern
Fig 6: Simulated Return loss Vs frequency
Fig 7: Simulated VSWR Vs frequency
In theoretical analysis resonance takes place at 38.9 GHz and the return loss is -41.36dB and vswr is 1.014 but in simulation the resonance takes place at 38.3 GHz and re-turn loss is-23.8dB, vswr is1.171and gain of the antenna is 1.2dB
OURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 61 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/ CIRCULAR
PATCH
ANTENNA
The expressions to find out the dielectric conductor radia-tion and losses in a microstrip antenna are given as fol-lows [3, 4]: (1) (2) (3) P d is dielectric loss, P c is conductor loss, P r is radiation loss, tanδ loss tangent, W T is the total power absorbed, ω is the resonant frequency. D ETERMINATION OF INPUT IMPEDANCE FOR CIRCULAR PATCH ANTENNA ON THICK SUBSTRATE
Fig. 8: Circular patch antenna
The relations for radiated power and radiation resistance for a circular patch antenna are given as [5]: (4) (5) where is Radiation resistance The existence of dielectric substrate over the conducting ground plane in microstrip antenna can cause the surface wave excitation along air dielectric interface. The resis-tances due to surface wave excitation R s can be derive from ratio of power loss to surface wave P s and radiation power given by James[6] (6) where (7) is the total energy stored in the patch at the resonance frequency for circular patch and is given by the expres-sion (8) Substituting the value of in eqn. 1 and 2 the value of power loss due to conductor and power loss due to di-electric for thick substrates is determined as (9) (10) Using these relations the value of resistance due to dielec-tric and conductor can be determined (11) (12) Total resistance can be determine by using above equ-ations (13) As given by Kara [7] the total resistance can be equated to input resistance of the microstrip antenna. R T = R in (14) Using the above expression the reflection coefficient and return loss of the antenna can be determined. Radiation efficiency can be defined as the ratio of radiated power to input power, that is (15) Directivity is defined as the ratio of the maximum power density in the main beam direction to the average ra-diated power density and can be expressed as [9] (16) (17) y a x z
OURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 62 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/
For thick substrate the height of the substrate is decided by the given condition h=0.8 mm, ε r =2.32. The disk metal-lization radius ‘a’ can be determined by the resonance condition, that is, =0. For the lowest order mode n=1 and the 1 st root of occurs at 1.841[9]. The feed point can be determined by the following expression [8]. Using the above relations the dimensions of the patch were determined as radius, a=1.21 mm, feed point loca-tion ε=0.46 for optimum matching Fig.9:Block diagram of designed antenna (sonnet) R ESULTS
The theoretical results for return loss were obtained using the above derived expressions and the antenna was simu-lated using sonnet software and the resulting graph for return loss is shown in Fig.10.
Fig.10 Theoretical and Simulated results of variation of return loss with frequency
The graphs show a good agreement of the theoretical and simulated results, thus proving the correctness of the technique developed. Both the graphs are resonating at 39GHz. The bandwidth shown by the theoretical results (320 MHz) is lower than the simulated results (250 MHz). The VSWR value for theoretical analysis at reson ance comes out to be 1.38 and for simulated results it has the value of 1.3.
Gain is calculated using eqns 15, 16 and 17 and the re-sults plotted in Fig. 5 showing the value for maximum gain obtained at resonance as 4.76dB
Fig.12:gain vs frequency(theoretical)
Fig.12: Simulate radiation patern
For simulated result we can see the gain of the antenna is 5.7dB at 39 GHz frequency
Theoretical return loss Simmulated return loss
OURNAL OF TELECOMMUNICATIONS, VOLUME 2, ISSUE 2, MAY 2010 63 © 2010 JOT http://sites.google.com/site/journaloftelecommunications/ C ONCLUSION
It can be concluded from the above analysis that an ef-ficient technique has been developed for analysis of re-tangular and circular patch antenna at millimeter wave frequencies for thick substrates. The feasibility of the technique is proved by simulations. The expressions to find out input resistance and gain for circular patch an-tenna on thick sustrate have been developed and verified R EFERENCES [1]
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Mr.
Harsh kumar received his B.Tech in 2008 from B N Mandal University, Madhepura, Bihar, India. Presently he is pursuing his M.E from Birla Institute of Technology Mesra, Ranchi, India, in the field of Microwave engineering. His areas of interest are Microstrip Antennas and RF circuit.
Dr. Shweta Srivastava received her Ph.D. from BHU, Varanasi (U.P.)-India. Presently she is working as senior lecturer in the Department Elec-tronics & Communication Engg., B.I.T, Mesra, Ranchi, India. She is currently supervising a project named “ Design of Active Microstrip An-tenna for Wireless Communication ””