AN EXPERIMENTAL STUDY OF CURVED RECTANGULAR MICROSTRIP ANTENNA IN SIMULATED PLASMA MEDIUM

The effect of plasma on the radiation characteristics of curved rectangular microstrip antenna is studied by means of a new plasma simulation technique. Unlike previous techniques [1,2], a relative index of refraction less than unity is obtained by representing free space with a high dielectric constant sodium chloride powder and plasma by a medium of lower dielectric constant (air). A wide range of dielectric constants of simulated plasma could be possible with this technique using solid dielectrics instead of liquids. It is observed that the resonance frequency is not affected by the curvature of the antenna. However radiation patterns are significantly affected.


INTRODUCTION
In many applications, antennas conforming to a non-planar surface are required, viz. in satellite, missiles, or space craft.Such antennas are flush mounted on these curved surfaces to reduce aerodynamic drag.Microstrip antennas are in this category since they are easily conformed to surfaces on which they are mounted.The experimental measurements of a curved rectangular microstrip antenna are described here in simulated plasma conditions.

MICROSTRIP ANTENNA
The geometry and coordinate system of a rectangular, curved patch antenna is shown in fig 1.The dimension of the straight edge is 2b while that of the curved edge is 2(a + h) 0 where 'a' is the radius of the cylindrical ground plane and 20 is the angle subtended by the curved edge.The resonant frequency of the structure is given as For TM modes, we have the following parameters in designing this antenna.
Resonant frequency 8.2 GHz (X-band) Dielectric substrate RT Duroid The antenna is fed by a coaxial feed.

EXPERIMENTAL SETUP
The details of the experimental setup are shown in fig. 2. Unlike previous simulation techniques [1,2], the relative index of refractive index less than unity is obtained by using two contiguous real dielectric media.Free space is represented by a high dielectric constant material (sodium chloride powder Er 5.9 at 8.2 MHz and a loss tangent tan 5 10-4) and plasma by a medium of lower dielectric constant (air).Since closely packed powder retains its position, the use of an acrylic box as a separator between the two media is not required.The value of the simulated plasma dielectric constant is given by (air) 1 (simulated plasma) .17 The horn antenna is fed by a EC Sweep generator (Model UM-400) and is used as a transmitting antenna whereas the curved MSA is receiving antenna, the 2D distance between than being greater then .The transmitting antenna is fixed in position and the curved MSA is rotated in an azimuthal plane.To avoid reflections from the plasma simulation chamber, microwave absorbers are used all around the inner surface of box.

PATTERN AND VSWR MEASUREMENTS
An EC Sweep generator model UM 400 was used as a microwave signal source.The desired frequency range is obtained from its RF plug in units XA-400 A. The far field radiation patterns measurements of curved MSA were done for free space and for plasma medium taking plasma parameter A 1 and A .41.The plasma parameter A is given as A (1 wP-) x/2 where wp and w are plasma frequency W 2 and source frequency respectively.The pattern factor in the EM mode is computed for different values of plasma parameters, i.e., A 1 for free space and A .41 for plasma.The results are plotted in figs.3 and 4 for 0 90 and b 90 in TM modes.
The VSWR of the curved rectangular MSA was measured in freespace as well in plasma medium.The values of VSWR in freespace and plasma medium are given in Table-1.It is seen that the minimum value of VSWR is 1.2 at 8.25 GHz, which is near the design frequency of 8.20 GHz.

CONCLUSIONS
From the experimental study of curved rectangular MSA in simulated plasma medium, it is observed that resonant frequency is not affected by the curvature of antenna.However, radiation patterns are significantly affected.In the H plane, beamwidth decreases as plasma parameter increases whereas in the E plane, beamwidth increases as the plasma parameter decreases.These results tally well with the theoretical results available for curved rectangular MSA [3,4] in plasma and freespace.

FIGURE 1
FIGURE 1 Geometry of curved rectangular microstrip antenna.

FIGURE 2
FIGURE 2 Experimental set-up for pattern measurement of curved MSA in simulated plasma media.

FIGURE 3
FIGURE 3 EM mode field pattern factor (H plane).

FIGURE 4
FIGURE 4 EM mode field pattern factor (E plane).

TABLE Variation of
VSWR with frequency in freespace & plasma for rectangular curvedMSA