the slot layer. Feed for slot is isolated from patch by ground layer. Thickness and permittivity of the substrate is determined as a trade off between crosspolar radiation and bandwidth [2]. Bottom ground layer is required to avoid radiation in undesired direction as slot has the characteristic of bidirectional radiation. Complete assembly is sharing a common aperture with slot and patch radiating in presence of each other without affecting their individual radiation pattern.

2.1. Design of Patch Antenna At C-Band

Microstrip patch is selected as radiating element at C-band. In direct fed patch configuration, input impedance of the patch is very high as a result of which the width of the microstrip line feeding the patch becomes very small. Typically width of 200 ohms line is 0.167 mm which with relative permittivity of 2.2 is very difficult to print accurately. Therefore inset fed patch geometry is used. In this geometry input impedance of the patch can be controlled by moving the feed line inside the patch. Microstrip line is fed inside the patch at the point of 100 ohms input impedance

The various parameters have been optimized through analysis using Ansoft Ensemble software. The final dimension of the patch which yields the desired RF performance in the presence of the X-band slot cut in the ground plane of the patch is shown in Fig 1.

2.2. Design of Slot Antenna At X-Band

At X-band, printed slot is employed as radiating element. This slot is cut in the ground plane of C-band patch and it is fed by a microstrip line below a dielectric
of thickness 0.79 mm and relative permittivity of 2.2. Feed for this slot is microstrip line of 50 ohms impedance and width 2.3 mm. Below this microstrip line another ground plane at the separation of 5.0 mm foam is used to avoid radiation from slot in lower half plane. The RF performance of the slot has to be optimized in the presence of the C-band patch which acts as a parasitic element. Simulation has been carried out using rectangular as well as dog-bone slot to study the relative advantage and disadvantage of these two configurations of the slot.