IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013 1057

Circularly Polarized Meshed Patch Antenna forSmall Satellite Application

Tursunjan Yasin, Student Member, IEEE, and Reyhan Baktur, Senior Member, IEEE

Abstract—A circularly polarized meshed patch antenna designis presented. The proposed antenna consists of two square meshedpatches, which generate two orthogonal linear polarizations at twoslightly different frequencies. A common proximity feedline is uti-lized to excite the two patches, and a 90 phase difference can beachieved between the two resonant frequencies, which is neededfor circular polarization. The overall structure of this antenna ishighly compatible with solar panels; thus, it is of great significancefor small satellite applications. The measured results of the an-tenna prototype show that it has a 3-dB axial-ratio (AR) bandwidthof 15 MHz, a 10-dB impedance bandwidth of 2%, and a gain of5.15 dB at its center frequency 2.47 GHz.

Index Terms—Circular polarization, meshed patch antenna,solar cell integration.

I. INTRODUCTION

O NE of the challenges of Cube Satellites (CubeSats), anemerging crucial tool for space exploration, is their lim-

ited surface area that imposes restrictions on amount of solarcells and positions of antennas and space instruments. There-fore, optically transparent antennas that can be integrated on topof solar cells are a desirable potential solution for the limitedreal estate issue [1]–[3]. Although transparent conductors suchas indium tin oxides (ITO) can be applied for transparent an-tenna design [2], [3], ITO antennas have been shown to havevery low efficiency at lower frequencies (e.g., S-band) and thetransparencies are too low for solar cell integration. On the otherhand, meshed antennas can be designed with very high trans-parency ( ) and good efficiency for S-band [1], [3], wheremost CubeSat applications lie. When a meshed patch antenna isachieved with a circular polarization (CP), it will be immunefrom the Faraday effect [4] in the earth’s ionosphere, whichmakes it even more favorable for satellite applications.CP for regular patch antennas can be generated by exciting

two orthogonal patch modes with 90 phase difference [5]. Onecommon method to realize this is to apply two feeds in phasequadrature to two adjacent sides of a square patch [6]. Anothertypical technique is to design a square patch with a pair of trun-cated diagonal corners that is excited using a single feed [6].For a meshed antenna, however, neither of these two methods

Manuscript received May 31, 2013; accepted August 06, 2013. Date of publi-cation August 29, 2013; date of current version September 12, 2013. This workwas supported by the National Science Foundation under Award No. 0801426.The authors are with the Department of Electrical and Computer Engineering,

Utah State University, Logan, UT 84322 USA (e-mail: tursunjan.y@aggiemail.usu.edu).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2013.2280131

Fig. 1. Solar panel on a CubeSat: (a) isometric view of a CubeSat; (b) typicalstructure of the solar panel.

is practical. While the dual-feed method is bulky and the feed-lines would inevitably hinder the integration with solar cells, themethod of corner truncation requires a very careful and low-tolerance design of mesh geometry to efficiently conduct thecurrents of complicated patterns to support the two resonantmodes for a CP. Similarly, due to either complex mesh de-sign or inappropriate feed structure, many other available tech-niques [7]–[11] to achieve circular polarization are not suitablefor solar cell and meshed patch integration.This letter presents a circularly polarized meshed antenna

composed of two meshed patch elements that are coupled witha single feedline. Because the nontransparent feedline can beplaced along the gaps [G in Fig. 1(a)] between solar cells, thedesign is highly compatible with solar panels of small satellitessuch as a CubeSat [Fig. 1(a)]. The meshed patches, which arehighly transparent to the solar light, can be placed on top of solarcells. The performances of the proposed antenna were verifiedby the measurement results, such as return loss, radiation pat-tern, and axial ratio (AR).

II. ANTENNA DESIGN

The typical assembly of a surface-mounted solar panel on aCubeSat is schematically displayed in Fig. 1(b), where the solarcells are mounted on the conductive shielding of the satelliteand covered with a layer of cover glass in order to protect thesolar cells. In our design, the cover glass acts as the dielectricsubstrate for the transparent meshed antennas, and the metallicshielding is used as the ground. The antenna’s loss due to theactive junctions of solar cells is not the objective of this letter,as it has been experimented in [12]. The effect of the earth onthe antenna as well as the noise temperature are not discussedeither because the first issue is not prominent for a CubeSatcommunication link budget, and the second issue is consideredsimply by using the sky’s temperature [1].

1536-1225 © 2013 IEEE

1058 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 2. Coupling mechanisms of coplanar proximity feed for solid patch:(a) capacitive coupling; (b) inductive coupling.

Fig. 3. Coupling mechanisms of coplanar proximity feed for meshed patch:(a) capacitive coupling; (b) inductive coupling.

A. Capacitive and Inductive Proximity-Fed Square Patch

The basis of the proposed design is a coplanar proximity feedas reported in [13] and [14], where the feed is an open-endedmicrostrip line. There are two different coupling mechanismsfor this type of feeding technique: capacitive and inductive cou-pling. As an example, a square patch with the capacitive cou-pling is depicted in Fig. 2(a), where the central line of the patchis aligned at a half-wavelength from the tip (T) of the open-ended feedline. In this configuration, the voltage standing waveon the feedline has a magnitude maximum that coincides atthe patch center and, through coupling across the gap, excitesa fundamental resonant current pattern [illustrated with arrowsin Fig. 2(a)] in the direction perpendicular to the feedline, indi-cating a linearly polarized radiation in the far field. In the caseof inductive coupling [Fig. 2(b)], the edge of the square patch isaligned with the tip of the feedline. The patch is driven by themagnitude maximum of the current standing wave, which is 90out of phase with the voltage standing wave. The surface currentand the radiation polarization of this patch are both orthogonalto their counterparts from the capacitive coupling case.The two proximity feed methods in Fig. 2 can be applied

to meshed square patches as shown in Fig. 3. The number ofcurrent carrier lines (the ones providing paths for the resonantcurrents) of the meshed patch can be made higher than thatof equipotential lines (the ones only responsible for cross-con-necting current carrier lines) in an attempt to promote the de-sired polarization while suppressing the cross-polarization levelof the antenna.

B. Circular Polarization Design

ACP can be decomposed into two linear polarizations that areorthogonal in space and quadrature in phase. The two meshedpatch antennas displayed in Fig. 3 radiate two linear polariza-tions orthogonal to each other. By combining these two antennas

Fig. 4. Meshed patch antenna for circular polarization.

Fig. 5. Mechanism details of a CP meshed antenna.

into a more compact configuration without changing their polar-ization directions, the proposed meshed antenna for circular po-larization, as shown in Fig. 4, can be achieved provided that thetwo element patches resonate at slightly different frequencies.The reason for having two very close but different frequenciesis explained in the following paragraph.The common feedline in this design is able to provide the two

patches with the two types of coupling methods described ear-lier in Section II-A. In the instant the increasing voltage mag-nitude and decreasing current magnitude, as conceptually illus-trated in Fig. 5, are formed along the open-ended feedline, thecurrent on the feedline induces decreasing currents, through in-ductive coupling, on the patch below the feedline (illustratedwith black arrows in Fig. 5). At the same time, the voltage onthe feedline creates the decreasing surface currents, via capac-itive coupling, flowing from the gap side toward the oppositeend of the other patch (illustrated with black arrows in Fig. 5).Thus, the currents on the two patches appear in phase if thetwo patches resonate at the same frequency. Therefore, the twopatches should be designed to resonate at two slightly differentfrequencies.Therefore, one can achieve a CP from the antenna config-

uration illustrated in Fig. 4 by carefully adjusting the sizes ofthe two patches such that the difference between their resonantfrequencies results in a required 90 phase difference [5]. Forthe configuration in Fig. 4, a left-handed circular polarization(LHCP) can be achieved by having the capacitively coupledpatch resonate at a higher frequency, whereas having the in-ductively coupled patch resonate at a higher frequency yieldsa right-handed circular polarization (RHCP).

YASIN AND BAKTUR: CIRCULARLY POLARIZED MESHED PATCH ANTENNA FOR SMALL SATELLITE APPLICATION 1059

Fig. 6. Geometry of an RHCP meshed antenna.

III. RESULTS AND DISCUSSION

Although in the actual implementation of integration withsolar cells, a transparent dielectric (e.g., cover glass) will be thesubstrate for meshed patch antennas, the design philosophy ofthe CP meshed antennas is the same for transparent and non-transparent substrates. Therefore, in the current laboratory vali-dation, Roger’s RO4003C laminate ( , mm,

) was chosen to examine the design. The twomeshed patches are transparent to light, and the transparency isdefined as the ratio of the see-through area over the entire patcharea. The meshed patches can be placed on solar cells, whereasthe nontransparent feedline can be placed along the gap betweensolar cells [G in Fig. 1(a)]. The transparency of the meshed an-tenna can be controlled by the width and the number of the meshlines. The relation between the transparency and antenna prop-erties can be found in [15]. The operational frequency is chosento be in S-band, where the size of the meshed patch is realisticto be placed on top of a commercial triple-junction solar cellsused in space applications.The prototyped antenna was designed using Ansys’s HFSS.

In order to have the right frequency shift between the resonantfrequencies, the patch sizes of this antenna need to be care-fully designed. While all the mesh lines were set to be 1 mmwide, the capacitively coupled meshed patch was designed to be30 30mm , and the inductively coupledmeshed patch was setto 30.5 30.5 mm . This generates an LHCP. The impedancematching was obtained by tuning the design parameters: thegaps ( and in Fig. 4) between the patches and the 50-feedline; and the distances of the patches from the tip of thefeedline ( and in Fig. 4).An RHCP can be designed from the method explained in

Section II-B. Alternatively, one can simply flip the LHCP con-figuration to obtain an RHCP antenna as shown in Fig. 6.The prototyped LHCP antenna is shown in Fig. 7. It has a

transparency of 70% and was fabricated with a circuit boardmilling machine. The lower transparency was chosen for theease of fabrication. The LHCP meshed antenna was measuredusing an Agilent’s VNA 8510C and NSI near-field antennarange in an anechoic chamber. The measured parameter incomparison to its HFSS simulated result is presented in Fig. 8.It is seen that the two curves almost overlap with each other.The 10-dB impedance bandwidth is about 2%, a typical value

Fig. 7. Prototype of LHCP meshed antenna under test.

Fig. 8. of LHCP meshed antenna.

Fig. 9. AR of LHCP meshed antenna versus frequency in the normal direction.

for patch antennas. Fig. 9 presents the variation of the antenna’sAR with frequency, both the simulation and measurement, inthe normal direction of the antenna (Z in Fig. 7). It can beseen that the minimum AR (1.3 dB) appears at 2.48 GHz inthe simulation and at 2.47 GHz in the measurement, whichis a very good agreement. The 3-dB AR bandwidth is about15 MHz (2.461–2.476 GHz) in the measurement, which fallsinto the 10-dB impedance bandwidth range (Fig. 8). These

1060 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 10. Spatial AR of LHCP meshed antenna.

Fig. 11. Radiation of LHCP meshed antenna: (a) plane; (b)plane.

results indicate that the antenna can function efficiently with agood AR.The orientation of the antenna under test with respect to

the Cartesian coordinate system is illustrated in Fig. 7. Thevariations of the antenna’s AR with respect to and at2.47 GHz were measured and plotted in Fig. 10, where the3-dB AR beamwidth is about 20 on the plane of andabout 80 on the plane. This can also be observedfrom the measured radiation patterns for the minimum AR,which are plotted in Fig. 11. These AR beamwidth values aresufficient for a CubeSat link budget. It can also be seen fromFig. 11 that the measured radiation pattern agrees well with thesimulation, and the cross-polarization level is below 22 dB inthe broadside direction at the center frequency. The gain of theantenna was measured to be 5.15 dB at the center frequency2.47 GHz. Also, this measured gain agrees very well with thesimulation of 5.96 dB.

IV. CONCLUSION

In this letter, a design of an optically transparent antenna withcircular polarization has been presented after a detailed studyof a coplanar proximity feed scheme. The design is suitablefor small satellite applications as it can be conveniently inte-grated with solar panels of small satellites to save surface realestate. The proposed antenna consists of two square meshed

patches fed with a straight microstrip line, which generates twolinear polarizations that are orthogonal in space and quadra-ture in phase, leading to a circular polarization. The measuredAR is reasonable and lies within the bandwidth of the reso-nant frequency. The measured cross polarization level is below22 dB in the normal direction. The realized gain is more than

5 dB, which is reasonable because there is some loss due tomeshing [15].The proposed antenna design was validated at a transparency

of 70% on a nontransparent circuit board substrate. However,the transparency of the meshed antenna can be increased by re-fining the mesh lines, and the same design method can be usedto produce CP meshed antennas on the cover glass of solar cells.There may be some challenges in prototyping this antenna

because a good CP depends on the coupling of the patches withthe feedline, which is sensitive to the coupling gaps. However, acareful fabrication with a regular circuit board milling machinecan still yield a satisfactory result. Therefore, further refiningmesh lines [15] with a more accurate manufacturing process isbeneficial in achieving an optimal transparent, efficient antennawith a good CP character.

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