The formulas for the radiation field for the EM mode and the Plusmu mode of a fem'te-based unnulur ring microstrip antenna are deiieloped. The radiation eficiency is found when the antenna is immersed in u plasma medium. The experimental findings reueul the enhancement of the radiution eficiency of u fem'te-based microstrip antenna inside a plasma medium. D 1996 John Wiley & Sons, Inc.
I. INTRODUCTIONIn space communications, demands for small-size, lightweight radiating systems have brought the ferrite-based microstrip antenna into prominence by virtue of its many advantages [ 1-31. Also, ferrite-based microstrip antennas are capable of low-profile mounting, as desired on the aft end of reentering vehicles. 11 has been established earlier [4, 51 that the generation of electroacoustic waves when an antenna is immersed in a plasma medium affects the radiation efficiency of the antenna, and the antenna radiation efficiency falls gradually from its frce-space value (without plasma medium) to a total cutoff with an increase in the ratio of plasma frequency (fJ in a plasma medium, which a space vehicle encounters in its voyage. In this article an attempt is being made to develop a formula for the radiation field of a ferrite-based annular ring microstrip antenna inside a plasma medium, and subsequently to study its radiation efficiency expcrimentally, which has not been dealt with so far.
THEORYThe geometry and coordinate system of the ferrite-based annular ring microstrip antenna is shown in Figure 1. Using the cavity model, this antenna can be considered as ring cavity resonator. For far-field calculation, the cavity can be replaced by a current sheet Jh, which in TE,, mode can be In warm plasma, in the absence of any applied static magnetic field, two different kinds of waves may exist, one depending solely on the existence of an electromagnetic source designated as an electromagnetic mode (EM), and the other on the cxistcnce of a pressure gradient and referred to as a plasma ( P ) mode. The plasma mode is longitudinal in nature and has no magnetic field, unlike the EM mode, which is transverse and has both electric and magnetic fields. It is known that these two modes are orthogonal in the sense that the total power flow is equal to the sum OP the power of the individual modes. Considering plasma as a fluid and using linearized hydrodynamic theory, the radiation fields of thc annular ring microstrip antenna on ferrite substrate for the electroacoustic (P) and electromagnetic (EM) modes are By integrating thc Poynting vector over a closed surface: in the upper half space, the expression for the radiation resistance for the electromagnetic mode is expressed as Using Eqs. (4) and (6) the radiation efficiency of the annular ring microstrip antenna on ferrite substrate in plasma medium can be calculated with the expression (7)
CONCLUSIONTheoretical and experimental surveys have been made on an annular ring microstrip antenna built on a typical ferrite substrate Ni,~,2Co,,,,Fe,~,,,0, with a = 1.2 cm, b = 2.02 cm,...
