Overview
Based on physical structure, radar antennas are divided into two main types: parabolic reflector antennas and lens antennas.
Parabolic Reflector Antennas
Principle of operation
A parabolic curve is defined as the locus of points for which the sum of the distance to a fixed point (the focus) and the distance to a fixed line is constant. For a parabolic reflector, point F is the focus (where the feed is located) and V is the vertex. The line connecting F and V is the axis of symmetry. Incoming rays reflect off the parabolic surface so that reflected wavefronts become collimated plane waves.
The distance from F to any reflecting point on the parabola relative to the incident wave is constant, so reflected waves form a collimated wavefront. The ratio of focal length to aperture diameter, known as the f/D ratio, is an important parameter of a parabolic reflector.
The law of reflection states that the angle of incidence equals the angle of reflection. Applied to a parabola, this law causes waves originating at the focus to reflect and travel parallel to the axis, which enables beam focusing by the reflector.
Properties of the parabola
Key properties relevant to antennas:
- All waves originating from the focus reflect and travel parallel to the parabola axis, so waves across the aperture are in phase.
- Because the waves are in phase, the radiated beam along the axis is strong and concentrated.
- Parabolic reflectors produce high directivity with narrow beamwidths, which increases with aperture size relative to wavelength.
Construction and operation
For transmission, the feed (usually a dipole or horn antenna) is located at the focus and illuminates the parabolic surface. The waves from the feed reflect off the parabolic surface and are converted into a collimated wavefront for transmission. The same antenna geometry can be used for reception: incoming plane waves reflect off the parabola and converge at the feed, where the dipole or horn converts the energy to an electrical signal for the receiver front end.
Gain of a parabolic reflector is a function of its aperture size relative to the wavelength (D/λ) and the aperture efficiency. Effective radiated power is the product of input power and the antenna power gain. Waveguide horn antennas are commonly used as feeds for parabolic reflectors, but other feed arrangements are also used.
Feed types
Common feed configurations include:
- Cassegrain feed: A convex hyperboloidal subreflector is placed near the vertex of the main paraboloid. One focus of the hyperboloid coincides with the main reflector focus, resulting in two reflections before the wave exits the antenna.
Gregorian feed: A concave ellipsoidal subreflector is used instead of a hyperboloid. Gregorian systems and Cassegrain systems are two common dual-reflection feed configurations. In practice, simple parabolic reflectors and Cassegrain-fed parabolic reflectors are among the most widely used reflector antennas.
Lens Antennas
Lens antennas use refractive surfaces to transmit and receive electromagnetic waves. These antennas are typically made from dielectric materials and operate by the same focusing and diverging principles as optical lenses. Lens antennas are used starting from around 1 GHz, with more common application at higher frequencies such as 3 GHz and above.
Principle of operation
A point source located at the focal point of a lens will produce collimated plane waves after refraction through the lens. Rays passing through the center of the lens are refracted less than those passing near the edge, so the lens converts spherical wavefronts to approximately planar wavefronts. The reverse operation applies for reception.
This reciprocity makes lenses suitable for use as antennas for both transmit and receive functions. To achieve the desired focusing at microwave frequencies, materials with appropriate refractive index are chosen. E-plane and H-plane lenses are designed to adjust the phase of wavefronts differently for polarization control and phase compensation.
Matched Filters in Radar Receivers
A matched filter maximizes the ratio of peak output signal power to average noise power in the filter output for a given input signal. This is an important criterion in radar receiver design. The frequency response of a matched filter is proportional to the complex conjugate of the input signal spectrum.
