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