Effect of warm ionized plasma medium on radiation properties of four elements microstrip antenna array printed on ferrite substrate

Effect of warm ionized plasma medium on radiation properties of four elements microstrip antenna array printed on ferrite substrate

Ayman Al Sawalha; Inas Al Mubarak

Physics Department, Faculty of Science, King Faisal University, P.O.Box 400, Al-Hassa, Saudi Arabia

ABSTRACT

This paper describes theoretically the radiation properties of four element microstrip antenna array printed upon a typical ferrite substrate Ni_1.062Co_0.o2Fe_1.948O₄ in the presence of normal dc magnetic bias field. In loss-less isotropic warm plasma, this array antenna geometry excites both electromagnetic (EM) and electroacoustic plasma (P) waves in addition to nonradiating surface waves. In the absence of an external magnetic field, the EM - mode and P-mode can be decoupled into two independent modes, the electoacoustic mode is longitudinal while the electromagnetic mode is transverse. Far zone electromagnetic mode and plasma mode radiation fields are derived using vector wave function technique and pattern multiplication approaches. The results are obtained in both plasma medium and free space .Some important antenna parameters such as radiation patterns, radiation conductance and directivity are plotted for different values of plasma to source frequency.

Keywords: Microstrip Antenna Array, Ferrite, Plasma Medium

Introduction

Microstrip antennas are proved useful for application on spacecrafts and mobile handsets due to their lightweight, small size, better aerodynamic properties and compatibility to get integrated with host objects. In past years, extensive research on different geometries of microstrip antennas under different conditions has been carried out to improve their inherent low bandwidth and low directive gain [1].

Ferrite and other magnetic materials have been extensively used in several microwave devices such as phase shifters, isolators, circulators, tunable filters, delay lines etc. Ferrite materials basically have a significant amount of anisotropy at microwave frequencies [2].This anisotropy induces on applying external dc magnetic field and brings about nonreciprocal behavior in them. Availability of low cost commercial ferrite substrates and recent advances in thin film technology has attracted noticeable attention of scientific community in the development of microstrip antennas on ferrite substrates [3-4].

The high dielectric constant of the ferrite substrate reduces the antenna dimensions and when biased with DC magnetic field, the antenna exhibits a number of novel properties. These include frequency tuning agility, the generation of circular polarization, reduction of surface waves and radar cross-section control. Microstrip antenna mounted on aerospace vehicles encounter plasma medium during their voyage in space, as a result of which radiation properties are altered significantly. This change is caused due to the generation of electroacoustic waves in addition to electromagnetic waves [5-7].

This paper reports theoretical work carried out to investigate the radiation properties of four element microstrip antenna printed upon a typical ferrite substrate Ni_1.062Co_0.o2Fe_1.948O₄ by considering the presence of dc magnetic bias field normal to the direction of propagation of electromagnetic waves. Several radiation characteristics of four element microstrip antenna array are analyzed theoretically by applying cavity modal based modal expansion technique. Design requirement and substrate characteristics considered for this theoretical analysis are listed in table 1.

Theoretical Considerations

Microstrip antennas printed on ordinary non-magnetic dielectric substrate with a single feed point have a single linearity polarized resonance mode. By applying two orthogonal feeds with 90 phase difference, a microstrip antenna will again have a single dominant resonance mode but can radiate circularly polarized waves. The use of magnetized ferrite substrate in place of dielectric substrate however leads to quite different results, since it can support various guided modes in different conditions. When direction of magnetization is normal to the direction of propagation of electromagnetic waves, two plane wave modes namely ordinary (O) and extraordinary (E) modes exist. The propagation constant in this case is still given by

The ordinary wave is similar to plane wave in a dielectric slab polarized transversally to the biasing direction with phase constant [8].

However extraordinary wave is a transverse electric mode polarized parallel to the biasing direction with phase constant:

The effective permeability of a magnetized substrate material (µ_eff) is given by [9]:

With

Here ω_o and ω_m are the precession and forced precession frequencies respectively and are defined as:

ω_o = µ_oγH_o, ω_m = µ₀γM_s and ω = 2πf

Applied dc magnetic bias field (H_o) and saturation magnetization (µ_oM_s) are considered in Amper/meter (A/m) and Tesla (T) units, respectively.

When µ_eff is negative, the extraordinary wave is decaying even if the material is loss-less. The frequency range for negative < ω < (ω_o + ω_m).

The geometry and coordinate system of four element microstrip antenna array of rectangular microstrip patch antenna are shown if figure (1).

In microstrip antenna array each patch is excited in a same phase and amplitude by corporate microstrip line feed connected to the edge of the radiating slot [10].

Radiation Field Expressions

Using hydrodynamic theory and vector wave function technique [11-12], the total far field of electromagnetic mode and plasma mode of the linear array are obtained as:

EM mode:

And in plasma mode:

Where

β_e, β_p are the propagation constants in electromagnetic mode and plasma mode respectively and A is the plasma parameter.

Radiated Power

The radiated power in the EM mode is obtained by integrating the Poynting vector over a large sphere. In our case study:

Where

Similarly P_p is obtained as

Where

The EM mode radiation conductance is given by:

And the plasma mode radiation conductance is given by:

The directive gain of the antenna in plasma medium can be expressed as:

Where, U(θ,) is the radiation intensity in (θ,) direction. The expression for directive gain of four element array

Where

Using equations (8) and (9), the radiation efficiency of a ferrite-based four element microstrip antenna array inside plasma medium is expressed as:

The value of radiation conductance , directive gain and efficiency of four element microstrip antenna array is computed for different values of plasma parameter with H_o = 6.37 × 10⁵ A/m and plotted in figures (2, ³, ⁴ and ⁵) respectively.

Conclusion

The radiation properties of four element linear microstrip antenna array printed upon a typical ferrite substrate Ni_1.062Co_0.o2Fe_1.948O₄ have been studied by considering the presence of dc magnetic bias field normal to the direction of propagation of electromagnetic waves It is observed that the radiation conductance (G_e) of four element array is more than that of single element for all the plasma frequency. It is maxima in free space and decreases on increasing plasma frequency. It is found that when microstrip antenna is biased the directivity and the efficiency are improved as compared to an unbiased case. Our results are consistent with the results reported by Yang [2]. Finally, it is concluded that the four element microstrip antenna array has unique radiation properties and can be employed in applications where high gain and narrow beam-width are required. The results of the present study are useful, particularly for space vehicles because such type of linear array can be mounted on the flat surface as well as on the curved surface of the space vehicles.

Acknowledgments

This research project (N0.90081) has been financially supported by Deanship of Scientific Research, King Faisal University, Saudi Arabia

(Received on 21 July, 2009)

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Publication Dates

• Publication in this collection
  22 Apr 2010
• Date of issue
  Mar 2010

History

• Received
  21 July 2009