6 - Radar Antennas: Fundamental Concepts and Advanced Technologies

The radar antenna is a critical subsystem where electromagnetic energy departs from the radar and propagates into free space, and where echoes returning from targets are collected. The IEEE defines an antenna as a "means for radiating and receiving radio waves." In the radar equation, antenna characteristics appear very strongly: the signal-to-noise ratio is proportional to the square of the antenna gain. In this article, we will present a comprehensive review from basic antenna concepts to advanced phased array technologies.

Electromagnetic Waves and Antenna Fundamentals

An electromagnetic wave consists of electric and magnetic fields that satisfy the equations developed by Maxwell in the late 19th century. The antenna is the transition structure between the transmitter and free space—it transfers energy from a cable or waveguide into free space.

In a simple horn antenna, the electromagnetic wave propagates along the waveguide, the electric field vectors bend in the region where the horn opens, and the energy is pushed outward. Upon reaching the vacuum, the electromagnetic field lines must close upon themselves. This process provides a certain directivity depending on the antenna design.

Antenna Functions

The radar antenna performs several critical functions:

1. Transition point: Acts as a transducer between the transmitter and free space

2. Beamforming: Focuses the transmitter energy into a collimated beam

3. Selective reception: Efficiently collects energy arriving from a specific angular direction during reception

4. Angle measurement: Determines target position in azimuth and elevation with a well-defined beam

5. Target separation: Distinguishes closely spaced targets angularly

6. Scan control: Determines how quickly space can be scanned

Antenna Gain

An isotropic antenna radiates equal energy in all directions. Directional antennas, on the other hand, concentrate energy in a preferred direction. Gain is the ratio of the radiation intensity of a directional antenna in a given direction to that of an isotropic antenna.

The gain of a dipole antenna is 1.6 in natural units or 2.15 dBi (dBi: decibels over isotropic). For a parabolic reflector, the gain is much higher:

G = 4πA_eff/λ² = (πD/λ)²

Here, D is the antenna diameter and λ is the wavelength. Gain increases dramatically as the electrical size of the aperture (D/λ) increases. For example, a parabolic antenna with a diameter of 5 meters and a wavelength of 1 meter (300 MHz) provides 24 dBi gain—about 250 times more intensity than an isotropic antenna.

Beamwidth and Sidelobes

The half-power beamwidth (HPBW) is the angular width between the points where the antenna gain drops by 3 dB from its peak value. Typically:

θ_3dB ≈ λ/D (radian) or θ_3dB ≈ 57.3λ/D (degree)

As the beamwidth decreases, angular resolution increases.

Sidelobes are unwanted maxima that cause energy radiation in directions outside the main beam. In a well-designed antenna, the sidelobe level is 20–40 dB below the main peak value. Sidelobes can result in the reception of clutter and interfering signals from undesired directions.

Parabolic Reflector Antennas

Parabolic reflector antennas are the most commonly used high-gain antenna type in radar systems. Due to the geometric property of the parabolic surface, energy radiated from the focal point emerges as parallel beams after reflection, and the path length of all beams is equal.

The Cassegrain feed arrangement uses a hyperbolic subreflector placed at the focal point with the main parabolic reflector. This design:

• Reduces blockage caused by the feed structure

• Minimizes waveguide losses

• Provides a more compact structure

At high frequencies (e.g., 35 GHz), a 100-foot waveguide can cause 7–8 dB loss, making the Cassegrain design preferable.

Polarization

Polarization defines the orientation of the electric field vector during propagation. There are three basic types of polarization:

1. Vertical linear polarization: Electric field is perpendicular to the ground

2. Horizontal linear polarization: Electric field is parallel to the ground

3. Circular polarization: Electric field vector rotates

Circular polarization is used to reduce rain clutter. Raindrops (being spherical) reflect the component of the incoming circular polarization that rotates in the opposite direction. An antenna tuned to receive the given polarization significantly suppresses rain backscatter. Targets usually reflect mixed polarization, so part of the target signal is preserved.

Phased Array Antennas

Phased array antennas are formed by combining a large number of elements with controlled phase. By electronically adjusting the phase of each element, the beam direction can be changed within milliseconds—thousands of times faster than mechanical scanning.

Basic principle: the emissions of the elements interfere constructively and destructively. With appropriate phase settings, constructive interference is achieved at a specific angle, forming a narrow main beam. An element spacing of half a wavelength (λ/2) or less prevents grating lobes.

Phased array advantages:

• Beam agility: direction change within microseconds

• Multifunction: simultaneous scanning and tracking

• Reliability: failure of a single element does not disable the system

Disadvantages:

• High cost

• Complex design

• Gain decreases as scan angle increases (cos θ factor)

• Beamwidth increases (1/cos θ factor)

Mutual Coupling

Phased array elements do not behave independently. Due to near-field interactions, currents in one element induce currents in neighboring elements. This mutual coupling:

• Affects input impedance

• Varies with scan angle

• Can cause "scan blindness" in some cases

Mutual coupling is a complex issue that expert antenna designers are still working on.

Reflector and Phased Array Comparison

Reflector antennas:

• Low cost

• Simple design

• Ideal for single target tracking

• Mechanical scanning – slow

Phased array antennas:

• Beam agility and flexibility

• Multifunction capability

• 3–4 faces required for 360° coverage

• Much higher cost

• Longer design time

Conclusion

Radar antennas are critical determinants of system performance. Parameters such as gain, beamwidth, sidelobe level, and polarization directly affect the radar's detection, measurement, and resolution capabilities. While parabolic reflectors offer high-gain and low-cost solutions, phased array antennas provide superior beam agility and multifunction capability. Selecting the right antenna technology according to application requirements forms the basis of a successful radar system design.