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3.2.2.4 Antenna Systems
For most ATC applications, the antenna is required to produce a fan beam in the vertical
cross section. This shape of beam ensures that optimum distribution of energy occurs
with the upper limit of coverage set by the maximum altitude required and the lower limit
set to minimise the energy transmitted into the ground. As the antenna rotates, it
describes a coverage volume with the characteristics of a flat cylinder with the antenna at
the centre. As already noted, the horizontal beamwidth is set to maximise angular
resolution of targets. Typical beam widths are in the range 1 to 2 degrees which gives an
indication of the directivity. Such beam widths equate to an antenna gain in the range 30
to 35 dB
This type of radar is only capable of defining target positions in two dimensions – range
and bearing. Using modern phased arrays it is possible to “steer” the beam under
computer control in two or three dimensions. Such techniques are very costly and have
not been generally applied to ATC surveillance radars to date.
Generally ATC radar antennas are horizontally polarised (i.e. with the electric field in the
horizontal plane) which gives maximum response from aircraft targets. Circular
polarisation is used as an option to improve the detection of targets in rain.
3.2.2.5 Transmitters
Radar transmitters are divided into two main types: driven and self-oscillating. This
terminology refers to the nature of the output device. In a self-oscillating system, the
output tube is simply switched on and off at intervals corresponding to the radar pulse
width. The transmission frequency is determined by the tube itself. In a driven system the
output device is a high power amplifier with the pulse pattern and transmission
frequencies determined at low level by the drive circuits. Driven systems provide better
control of the transmitted spectrum.
The only self oscillating tube widely used in ATC radar is the magnetron. The magnetron
is physically small and uses relatively lower voltages (typically 15 to 25kV). It is used
where compact dimensions and low costs are at a premium. Typical applications include
low cost airport radars and in aircraft where space is a problem. It is also widely used in
marine radar applications. However, as a self oscillating device, it has poor spectral
characteristics and stability.
The klystron is widely used in ATC radars. It is used in high power applications and
requires high voltages (typically 80 to 100kV). It therefore requires X ray shielding and
numerous safety interlocks.
The travelling wave tube can provide a wide bandwidth, which is necessary to transmit
short pulses or permit frequency diversity operation. The cost of TWT’s is high, but this is
usually offset by a long in-service life.
Solid state devices operate with low voltages and have a long life expectancy. They are
well suited to feeding conventional rotating antennas as well as planar array applications
where they provide a distributed power source. They are also much better suited to pulse
compression applications and generate pulses with good spectral characteristics. In
applications where they are used to drive a conventional antenna, a power combining
network must be provided.
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