Analysis and Design of L-strip Proximity Coupled Circular Microstrip Antenna

An L-strip proximity coupled circular microstrip antenna is proposed. The structure is investigated using circuit theoretic approach and simulated using IE3D simulation software. The patch is designed on a thick substrate of thickness of 11 mm for a design frequency of 3.74 GHz and provides ultra wide band operation. The numerical results for input impedance, VSWR, radiation pattern, efficiency and gain are presented. Bandwidth is found to be dependent on length of horizontal part of L-strip. A bandwidth of 69.52% is achieved (for VSWR≤2) for y0=0.112λ0 and h2=0.097λ0. The beam of antenna rotates with operating frequency.

overcome band limitation problem.Many researchers have used L-shaped microstrip line as a feed line.M.K. Meshram used L-strip to to achieve a bandwidth of 56.67% [9] in rectangular microstrip antenna.Zhongbao Wang, Shaojun Fang, and Shiqiang Fu used modifies L-strip to 22% bandwidth with improved gain of 9 dBi [10].
In this paper, ultra-wideband proximity coupled L-strip fed Circular Microstrip Antenna (CSMA) has been presented.Using a foam layer of thickness 11 mm as a substrate, an impedance bandwidth of 69.52% and gain of upto 8 dBi has been achieved which is better than earlier reported results by T.
Huynh and K.F. Lee [8].No optimization was adopted in the design.The antenna is simulated using IE3D software.The computed results using circuit theoretic approach agree well with simulated data.
The simulation for antenna efficiency, radiation efficiency, radiation pattern, gain and directivity has also been carried out.

II. THEORETICAL INVESTIGATION
The L-strip proximity coupled circular microstrip antenna is analyzed using circuit theoretic approach and cavity model.Broad banding is achieved by using thick substrate.But this reduces coupling between patch and microstrip feed.Various techniques have been used to counter this problem.The antenna given Y. X. Guo, K. M. Luk and K. F. Lee [11] is taken as reference for comparison.In the present analysis L-shaped micro-strip feed is used.The proposed structure is shown in fig. 1.The antenna structure contains a thick substrate of thickness H.An L-shaped strip line is designed to couple the power to patch electromagnetically.This L-shaped feed is connected to a standard microstrip feed which in turn is connected to source.The fig. 2 shows equivalent circuit of proposed antenna.The length of horizontal part of L-strip under patch is kept less than quarter wavelength because up to λ/4 length of an open circuited stub, the nature of impedance is capacitive.
The capacitance thus introduced is suppressed by the inductance arising from vertical part of L-strip.
Apart from these, a series resistance arises due to finite conductivity of copper used.The expressions of series resistance (R s ) and series inductance (L s ) as given by R. K. Huffman (1987) [12] Where w s is width and t s is thickness of strip in mm, h 2 is height of L-strip, f is operating frequency in GHz, ρ is specific resistance of the strip (Ω cm) and ρ 0 is specific resistance of copper.
All antenna metallization is taken as perfect except vertical portion.There is a capacitance (C s1 ) arising due to vertical electric fields between horizontal part of L-strip and ground plane in series with above L s and R s and is calculated as Where y 0 is penetration of L-strip into patch ε r is relative dielectric constant and ε 0 is dielectric constant of vacuum.Where ε e is effective dielectric constant of material buried under the microstrip line and ground plane.From T. C. Edward [13] the associated fringing capacitance is calculated as Where l e is extension in length of L-strip feed, c is velocity of light in vacuum, Z 0 is characteristic impedance of feed and ε reff is effective dielectric constant.The fringing capacitance between horizontal part of L-strip and ground plane (C f1 ) is calculated by putting h=h 1 +h 2 and the two capacitances between patch and horizontal part of L-strip (both C f2 ) is calculated by putting h=h 3 .
Fringing capacitance between patch and L-strip is calculated using equations ( 4) and ( 5), ignoring curvature of patch.The capacitance due to vertical electric field between horizontal part of L-strip and patch is calculated as The equivalent circuit of L-strip fed circular microstrip antenna is shown in fig. 2. The structure contains a series RLC resonant circuit in series with a parallel RLC resonant circuit.The parallel RLC circuit is equivalent of circular microstrip antenna.The resonance resistance R p of patch, antenna capacitance C p and inductance L p are calculated by Stuart A. Long, Liang C. Shen, Mark D. Walton and Martin R. Allerding [14] and is given as Where Q T is total quality factor, G T is total conductance of patch of radius a incorporating radiation loss, conduction loss and dielectric loss [15] and f res is resonant frequency of patch [16].

Thus total input impedance of the circuit is given as
where C total is total capacitance arising due to L-strip (i.e. C 1 , C s1 , C f1 , and C f2 ) and is calculated as The reflection coefficient of the antenna is given as and the VSWR is calculated as

III. DESIGN PARAMETERS
The basic design parameters of the proposed antenna are same as taken by Y. X. Guo, K. M. Luk and K. F. Lee [11] for comparison purpose.The radius of patch (a) is 17 mm, total height (H) of substrate is 11mm, and dielectric constant is 1.07 (foam layer).The parameters which are new for the design are -height of microstrip feed (h 1 = 1.6 mm or 0.02λ 0 ), height of L-strip (h 2 = 7.8 mm or 0.097λ 0 ) and gap between circular patch and horizontal part of L-strip (h 3 = 1.6 mm or 0.02λ 0 ).The width and length of L-strip are 5mm and 9.5 mm (0.097λ 0 ) respectively.The design frequency of the antenna is 3.74 GHz (λ 0 = 80.2 mm).A 50 ohms microstrip line on 1.6 mm thick substrate was taken to feed the power to L-strip (w s = 5 mm).

IV. RESULTS AND DISCUSSIONS
The L-strip proximity coupled microstrip CMSA is analyzed and the results are compared with the ones obtained by Y. X. Guo, K. M. Luk and K. F. Lee [11].The variation of input impedance with frequency for different horizontal length of L-strip of proposed structure is shown in fig. 3 At the same time, bandwidth decreases due to increased quality factor of the structure.The bandwidth for different y 0 is given in Table I. it is clear that bandwidth decreases with increase in y 0 at constant value of h 2 .The simulated result is also given in the table which shows a close resemblance with calculated bandwidth.II.The bandwidth decreases with height of L-strip.It is very similar to bandwidth variation with length of horizontal part of L-strip (y 0 ).Again the simulated and calculated results are in good agreement.The antenna was simulated on Zealand IE3D v 14.0 software [17].The variation of VSWR and input impedance at y 0 =0.112λ 0 and h 2 =0.097λ 0 are shown in figs.3, 4, 5 and 6.The calculated results using circuit theoretic approach and simulated results were in good agreement.The return loss of an antenna shows how well antenna port is matched with source.A good return loss or VSWR alone is not measure of a good antenna as it does not tell how well the radiation is taking place.Hence investigation of Directivity, radiation efficiency, antenna efficiency and antenna gain is required.Radiation efficiency of an antenna is defined as ratio of power radiated to power given to antenna excluding return loss at antenna port.However, the antenna efficiency is defined as ratio of radiated power to actual power fed to antenna (includes return loss at port).at 0.46 0 , 5.07 0 , and at 10.24 0 for 3.1GHz, 3.8 GHz and 4.5GHz frequencies respectively.The variation of beam width and its direction shift is shown in

V. CONCLUSION
A novel L-strip fed circular microstrip antenna has been presented for ultra wideband application.An equivalent circuit was given for the structure and calculations were carried out for circular patch of 11 mm thickness.Various antenna properties were investigated using circuit theoretic approach and results were verified with simulation.The proposed antenna has an operating frequency range from 2.85GHz to 5.45GHz (2.9GHz to 5.5GHz simulated) and bandwidth of 69.52% which is better than earlier reported bandwidth (35%) and gain (8 dBi).It may also be concluded that the input impedance is very sensitive to variation in horizontal length and height of L-strip feed.The beam of antenna is rotating with the frequency of operation.
. The capacitive nature of antenna increases with horizontal length of L-strip.The resonance resistance decreases as open end of L-strip moves towards center of patch.This indicates that open end is working as feed point.The variation of VSWR with frequency for different horizontal length of L-Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol.11, No. 1, June 2012 Brazilian Microwave and Optoelectronics Society-SBMO received 10 Oct. 2011; for review 14 Oct. 2011; accepted 15 June 2012 Brazilian Society of Electromagnetism-SBMag © 2012 SBMO/SBMag ISSN 2179-1074 197 strip is shown in fig. 4. The fig. shows that matching improves with the horizontal length of L-strip.

Fig 4 .Fig 5 .
Fig 4. Variation of VSWR with frequency for different horizontal length of L-strip at h 2 =0.097λ 0 .

TABLE I .
BANDWIDTH FOR DIFFERENT y 0 AT h 2 =0.097Λ 0 .

table III .
The wave takes definite time to reach at the Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol.11, No. 1, June 2012 get coupled to the patch.This time delay causes phase difference which in turn affects the total field in the far field zone.Hence antenna beam rotates for different frequency of operation.Table III.Beam Rotation at Different Operating Frequency.