IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006 195

Radiation Performance of SwitchableFerrite Microstrip Array Antenna

Rajeev Pourush, Student Member, IEEE, G. S. Tyagi, G. P. Srivastava, and P. K. S. Pourush

Abstract—This letter reports radiation performance of switch-able microstrip planar array of 6 6 element circular patch an-tenna printed on Ni–Al ferrite substrate with a normal magneticbias field. The extra degree of freedom offered by the biased ferritecan be used to obtain a number of novel characteristics includingswitchable and tunable radiation from a microstrip antenna with asingle feed point. Under certain bias conditions, most of the poweris converted into magnetostatic waves and little radiates into air.Under such circumstances, the antennas are highly inefficient, inthe sense that they are effectively absent as radiators. The bias de-pendent properties of a switchable antenna in S band (3.5 GHz)are investigated.

Index Terms—Biased ferrite antenna, circular patch, phasedarray.

I. INTRODUCTION

I N recent years the use of ferrites in aperture antennas hasbeen reported by several authors, where the purpose was to

scan the main beam by changing the bias on the ferrite materialor to reduce the radar cross section of the antennas [1]–[6]. Anattractive feature of biased ferrite is that the material charac-teristic is nonreciprocal and electronically tunable. Microstrippatches printed on a ferrite substrate have a number of novelproperties not found with normal dielectric substrates. The highpermittivity of the unbiased ferrite reduces the patch dimensionsallowing miniaturization, whereas the biased ferrite providesbeam steering, frequency agility, gain and bandwidth enhance-ment, radar cross-section reduction, switchability, and circularpolarization. These inherent characteristics of ferrite-based an-tennas have potential applications in radar and satellite commu-nication system [7], [8].

This letter presents a switchable microstrip array antennaprinted on Ni–Al biased ferrite in S band (3.5 GHz).

II. ANALYSIS AND FORMULATION

The array geometry and its coordinate system is shown inFig. 1. It consists of 36 identical elements of radius printedon Ni–Al ferrite substrate with , ,

, and thickness . Each patchcan be excited by a microstrip line connected to the edge or bya coaxial line from the back at the plane . The bias fieldis taken normal to the substrate plane.

Manuscript received January 7, 2005; revised February 7, 2006.R. Pourush, G. S. Tyagi, and G. P. Srivastava are with the Department of

Physics and Computer Science, Dayalbagh Educational Institute, DayalbaghAgra-282005, India (e-mail: rpourush@rediffmail.com).

P. K. S. Pourush is with the Department of Physics, Agra College, Agra-282002, India (e-mail: ppourush@yahoo.co.in).

Digital Object Identifier 10.1109/LAWP.2006.873954

Fig. 1. Array geometry and coordinate system of 6� 6 element planar arrayof circular patch microstrip antenna.

To describe the concept of a switchable antenna, it is consid-ered here that the bias field is applied normal to the patch andthe substrate in the -direction. In the case of the extraordinarywave, the propagation constant dependence on basic parametersis given as

(1)

It is seen that when is negative, the extraordinary wave isdecaying even if the material is loss-less. The frequency rangefor negative is . Be-tween these limits there is no possibility of microwave propa-gation through the ferrite, i.e., the antenna behaves as if it is notexcited in this region and radiates very little. On the other hand,it radiates when is positive and the ordinary wave is propa-gating. Thus, by proper bias selection, the properties of the ex-traordinary wave can be used to design an externally controlledswitchable antenna. The dispersion curve for grounded Ni–Alferrite slab at is shown in Fig. 2.

The dependence of the antenna resonant frequency on the ap-plied magnetic field above saturation is shown in Fig. 3. Overa bias range from 0.7 to 2.4 kOe, the resonant frequency variesfrom 3.297 to 3.412 GHz. Thus, by proper selection of biasing,a desired resonant frequency can be achieved. It is pertinent to

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196 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

Fig. 2. Dispersion curve for grounded Ni–Al ferrite slab at H = 1000 Oe.

Fig. 3. Variation in resonant frequency with applied magnetic field strengthH for Ni–Al ferrite.

TABLE IRESONANT FREQUENCY (f ), CUTOFF LIMIT (f ). RESONANCE LIMIT (f ), AND

EFFECTIVE PERMEABILITY (� ) OF THE ARRAY GEOMETRY AT 3.5 GHz

mention here that the resonant frequency in the case of an un-biased ferrite (i.e., ) is 3.429 GHz which is shown inFig. 3.

The cutoff and resonant limits ( and , respectively) forthree values of biasing (i.e., , 1350, and 2000 Oe)along with its corresponding resonant frequency and effectivepermeability are given in Table I.

From this table, it is clear that within the range of and ,the antenna essentially does not radiate. Therefore, by properly

POURUSH et al.: RADIATION PERFORMANCE OF SWITCHABLE FERRITE MICROSTRIP ARRAY ANTENNA 197

Fig. 4. Effective permeability as a function of applied magnetic field strengthH at different frequencies for Ni–Al ferrite.

selecting and changing the bias field value the antenna can beused as switchable antenna.

The effect of biasing on the effective permeability (viz 1.5,3.5, and 5 GHz) is shown in Fig. 4 for three different operatingfrequencies.

From the previous figure, it is clear that when no magneticfield is applied, tends to unity at microwave frequencies.A further increase in the field leads to a negative effective per-meability. In this region, the antenna is effectively off as a ra-diator and electromagnetic propagation is not possible throughthe ferrite. Further, on increasing biasing, the ferrite resonatessince is maximum (tends to ). After that the permeabilitybecomes large and positive, finally decreasing toward unity atvery high bias fields. Fig. 4 shows that at the operating fre-quencies of 1.5 and 3.5 GHz, is negative, since it is belowthe lowest allowable operating frequency for Ni–Al ferrite of

. A much slower variation inwith increasing magnetic field is found at 5 GHz. Also, the re-gion for which is negative becomes large on increasing op-erating frequency.

The values of directive gain, radiation conductance, ef-ficiency, and bandwidth have been calculated for the arraygeometry using references given in [9]–[11] for biased (threevalues , 1350, and 2000 Oe) ferrite material bytaking the same input parameters for two values of progressivephase excitation difference between the antenna elements, i.e.,

and and are plotted in Figs. 5–7.It is observed from the plots that there is a significant change

in the values of antenna parameters on applying magnetic fieldto the ferrite substrate as well as changing progressive phaseexcitation difference.

III. RESULT AND DISCUSSION

In the present communication, a concept of switchable an-tenna printed on Ni–Al biased ferrite substrate has been realized.For this, the extraordinary wave effects on the radiation charac-teristics of microstrip antenna are taken into account. Materialparameters for the case study are: ferrite substrate (Ni–Al fer-rite), , , ,

, . The directive gain, efficiency, andbandwidth are computed and plotted for different values of bi-asing and for two values of progressive phase excitation dif-ference, i.e., and and are given in Figs. 5–7.

Fig. 5. Directive gain of 6� 6 element planar array of circular patch microstripantenna as a function of applied magnetic field strength H .

Fig. 6. Efficiency of 6� 6 element planer array of circular patch microstripantenna as a function of applied magnetic field strength H .

Fig. 7. Band width of 6� 6 element planar array of circular patch microstripantenna as a function of applied magnetic field strength H .

The effect of the state of applied magnetic field strength onthe ferrite substrate for directive gain, efficiency, and bandwidth

198 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

are also investigated. Since on changing the biasing, the reso-nant frequency altered, therefore, we have taken into accountthis effect while computing the antenna parameters. The varia-tion of directive gain, efficiency, and bandwidth of array geom-etry for mode (resonant mode) with the applied magneticfield strength for propagation perpendicular to are shown inFigs. 5–7, respectively.

The variation in directive gain above saturation (from 0.7 to1.7 kOe) is random in nature. At 1.8 kOe, the gain attains highestvalue, i.e., 13.5 dB and then it remains virtually constant overthe bias range from 1.8 to 2.4 kOe. An almost similar behaviorof variation in efficiency and bandwidth of the array geometryover the bias range (0.7–2.4 kOe) has also been found. We haveobserved maximum value of efficiency, i.e., 86.5% and band-width, i.e., 74.9% at 2.2 kOe. These results are also supportedwith the values of radiation parameters in case of unbiased fer-rite (i.e., ). This nature of variation can be explained byconsidering the following phenomena:

1) Initially the resonant frequency falls with applied mag-netic field strength, the electrical size of the patch there-fore decreases, which results in lower directive gain, effi-ciency, and bandwidth (at 0.7 kOe). Thereafter at 0.8 kOethe electrical patch size increases and consequently valuesof gain, efficiency and bandwidth increase again.

2) The second reason may be that the resonant frequency in-creases, but the ferrite cutoff region shifts upwards withapplied magnetic field. This combined effect is thereforeresponsible for the rise and fall of given radiation param-eters. After 1.8 kOe the gain become virtually constant atabout 13 dB.

IV. CONCLUSION

This letter has described the switchable characteristics of amicrostrip planar array of 6 6 element circular patch antennaprinted on biased Ni–Al ferrite substrate with propagation per-pendicular to applied magnetic field strength. The followingconclusions are drawn from this study:

1) It is evident from the ferrite dispersion properties thatthere should be a radiating and nonradiating region foran antenna. There is a frequency range bounded by limitsnamely cutoff limit and resonance limit. In this region,

where is negative, the extraordinary wave is highlyattenuated and, therefore, the antenna is effectively not agood radiator [6].

2) Thus, by proper selection of applied magnetic fieldstrength, the switchable antenna can be designed and theoptimum performance of antenna can be achieved.

3) Further, by proper selection of applied magnetic fieldstrength, the array geometry can be tuned at a desiredresonant frequency [6].

4) The size of the patch is considerably reduced whendesigned on ferrite substrate. This reduction wouldcertainly have wide utility to create miniaturization ofantenna system [1].

5) When antenna is biased, the directive gain, efficiency, andbandwidth are improved as compared to an unbiased case.Our results are consistent with the results reported byYang [6], and Batchelor and Langley [2].

REFERENCES

[1] K. K. Tsang and R. J. Langley, “Design of circular patch antennas onferrite substrates,” Proc. Inst. Elect. Eng., Microwaves Antennas Propa-gation, vol. 145, no. 1, pp. 49–55, Feb. 1998.

[2] J. C. Batchelor and R. J. Langley, “Beam scanning using microstrip lineon biased ferrite,” Electron. Lett., vol. 33, no. 8, pp. 645–646, 1997.

[3] A. D. Brown, J. L. Volakis, L. C. Kempel, and Y. Y. Botros, “Patch an-tennas on ferromagnetic substrate,” IEEE Trans. Antennas Propag., vol.47, no. 1, pp. 26–32, Jan. 1999.

[4] H. Y. D. Yang, J. A. Castaneda, and N. G. Alexopoulos, “The RCS of amicrostrip patch on an arbitrarily biased ferrite substrate,” IEEE. Trans.Antennas Propag., vol. 41, no. 12, pp. 1610–1614, Dec. 1993.

[5] B. Lee and F. J. Harackiewicz, “The RCS of a microstrip antenna on anin-blane biased ferrite substrate,” IEEE Trans. Antennas Propag., vol.44, no. 2, pp. 208–210, Feb. 1996.

[6] H. Y. D. Yang, “Characteristics of switchable ferrite microstrip an-tennas,” IEEE Trans. Antennas Propag., vol. 44, no. 8, pp. 1127–1132,Aug. 1996.

[7] M. S. Sodha and N. C. Srivastava, Microwave Propagation in Ferrimag-natics. New York: Plenum, 1981.

[8] I. J. Bahl and P. Bhartia, Microstrip Antennas. Norwood, MA: ArtechHouse, 1980.

[9] C. A. Balanis, Antenna Theory: Analysis and Design. New York:Wiley, 1982.

[10] P. Bhartia, K. V. S. Rao, and R. S. Tomar, Millimeter Wave Microstripand Printed Circuit Antennas. Norwood, MA: Artech House, 1991.

[11] R. Staraj, E. Cambiaggio, and A. Papiernik, “Infinite phased arrays ofmicrostrip antennas with parasitic elements: Application to bandwidthenhancement,” IEEE Trans. Antennas Propag., vol. 42, no. 5, pp.742–746, May 1994.