HARDWARE DESCRIPTION

This section describes the details of the three main components of the receiver system used in design viz. software defined radio comprising of a PC and RTL-SDR USB dongle, Antenna and LNA block.

RECEIVER ANTENNA

Two antennas- Quarter-wave monopole and Quadrifilar Helical Antenna (QHA) are selected such that the combined beam pattern provides optimum coverage in both low and high elevation angles. Design parameters such as return loss and beam pattern of both the antennas are first simulated and optimized using software called 4NEC2 which can be obtained from website http://www.qsl.net/4nec2/. This software is based on Numerical Electromagnetic Code version two (NEC-2). The Numerical Electromagnetics code (NEC-2) is a computer code for analyzing the electromagnetic response of an arbitrary structure consisting of wires and surfaces in free space or over a ground plane. The analysis is accomplished by the numerical solution of integral equations for induced currents[6]. 4NEC2 is a simple yet powerful tool which comes with an optimizer which can be used to optimize various parameters of the antenna under design.
Qurter-wave monopole Antenna
Using the Image theory monopole antenna can be modeled as a half the size of dipole antenna. It is always mounted above some ground plane. The ground plane prevents the radiation below the ground plane and therefore its radiation pattern is equivalent to the radiation pattern of a dipole in upper hemispere[7][8][9]. Four numbers of radials having length of the order of λ/4 are used to realize the ground plane. Futher the dimension of radiator and radials are optimized to obtain the beam pattern and refelction coefficients as shown in Figure 2. To achive the impedance matching with 50Ω co-axial feeder, radials are bent about 45° from the horizontal plane. Theoritical dimenstions, dimenstions after optimisation and actual dimentions of radiator and radials are tabulated in Table 2.
Table 2: Monopole antenna dimenstions