Experiment-supported study on the bipolar disk microstrip antenna.

Morariu, Gheorghe ; Alexandru, Marian ; Miron, Mihai 等

1. INTRODUCTION

The operation of the microstrip antenna is based on the resonance principle at electromagnetic wave frequency from the exposure field. This type of antenna can replace the voluminous and expensive antennas. The planar structure of the microstrip antenna permits the implementation of a variety of surfaces of different shapes (Targonski et al., 1998).

2. ANTENNA STRUCTURE. FIELD STRUCTURE AND RADIATION PROCESS

From the physical point of view, the microstrip antenna contains an active plane made from resonant elements, dielectrically separated by a ground conductor plane.

The frontier radiation coefficient of the electromagnetic field is proportional to the relative permittivity ([[epsilon].sub.T]) of the dielectric layer.

In Figure 1 an element of a bipolar antenna with disk shape dipoles and symmetrically parallel slots is presented (Pozar & Schaubert, 1996; Kobayashi et al., 2007).

The main source of radiation is the frontier electric field disposed around the active elements and the frontier electric field of the two slots. The Figure 2 shows the electric field distribution (E) generated by the radiant element.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The dynamics of the electric field on the frontier of the two slots results from the electromagnetic wave equation of the plane resonant cavity (Figure 3) (Balanis, 2005; Voskresensky & Ovchinnikova, 2003).

The real wavelength in the active element is:

[[lambda].sub.r] = c/[square root of [[epsilon].sub.r] x f] (1)

Where, c represents the speed of light, [[epsilon].sub.r] is the relative permittivity and f is the frequency.

The electromagnetic wave equation of the plane resonant cavity is:

[[partial derivative].sup.2] [H.sub.z]/[partial derivative][x.sup.2] + [[partial derivative].sup.2][H.sub.z]/[partial derivative][y.sup.2] + [mu][epsilon][[omega].sup.2] [H.sub.z] = 0 (2)

Considering the following conditions:

[E.sub.z] = 0, x = 0 and x = [lambda]/16;

[E.sub.z] = 0, y = 0 and y = [lambda]/4,

the value for [H.sub.z] can be calculated:

[H.sub.z] = [H.sub.0] cos(n[pi] 16x/[lambda])cos(m[pi] 4y/[lambda]) (3)

for 0 [less than or equal to] x [less than or equal to] [lambda]/16 and 0 [less than or equal to] y [less than or equal to] [lambda]/4

From equation (3), results the resonant specific pulsations of the different wave propagation mode:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Because OY axe is parallel to the diameter of the slots alignment, [H.sub.01] mode is dominant. Thus, n = 0 and m = 1.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

For [[epsilon].sub.r] [congruent to] 3,3, the resonant frequency is:

[f.sub.r] = 2c/[[lambda].sub.r][square root of [[epsilon].sub.r]] [congruent to] 1,1GHz (6)

The resonant frequency of a disk shape resonator is obtained by solving the following equation:

[J.sub.0] ([omega] x r/c) = 0 (7)

Where, [omega] is the pulsation, r is radius of the disk and [J.sub.0] is the zero order Bessel polynomial.

The solutions of the polynomial are x [congruent to] 2,402 for the fundamental mode, and x [congruent to] 5,52 for the first order harmonic.

The calculus of the disk radius:

X = [omega] x r/c = 2,402 (8)

r [congruent to] 1,12 [[lambda].sub.r]/4 (9)

The Figures 4, 5 and 6 present the experimental results regading the disk microstrip antenna construction.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The change of the polarization planes is obtained by switching the radiant dipoles, (horizontal--vertical polarization) using two high frequency switches.

The separation of the two polarization planes is about 30dB, because the antenna is sensitive to circular polarization.

3. CONCLUSION

The dimensioning of the antenna was based on the general equations for electromagnetic waves, imposing limit conditions specific to the surface resonance.

The presented microstrip antenna has a lot of advantages, such as: a smaller size, low cost of production, easy to put in position, linear and circular polarization, and feeder coupling simplicity.

Some drawbacks are: power losses in dielectric, limited gain, reduced directivity.

4. REFERENCES

Balanis, C. (2005): Antenna Theory, Wiley, 0-471 66782-X

Kobayashi, H.; Sasamori, T.; Tobana, T. & Abe, K.: A Study on Miniaturization of Printed Disc Monopole Antenna for UWB Applications Using Notched Ground Plane, IEICE Transactions on Communications, Vol E90-B(9), 2007, pp. 2239-2245, 0916-8516

Pozar, D.M. & Schaubert, D.H. (1996): The Analysis and Design of Microstrip Antennas and Arrays, IEEE Press, 978-0-7803-1078-0, New York

Targonski, S.D.; Waterhouse, R.B. & Pozar, D.M.: Design of wide-band aperture-stacked patch microstrip antennas, IEEE Transactions on Antennas and Propagation, Vol 46, Issue 9, 1998, pp. 1245-1251

Voskresensky, D.I. & Ovchinnikova, E.V.. : Broadband phase array with wide-angle scanning, 4th International Conference on Antenna Theory and Techniques, Vol 1, 2003, pp. 77-80, 0-7803-7881-4