Briefly talk about the development of airborne radar antenna

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As the demand for war changes and advances in science and technology, the "gold content" of air combat is also getting higher and higher. The development of fighter airborne radar is a typical example. In terms of appearance, the most intuitive part of the "evolution" of radar equipment is the change of the antenna. The following briefly talks about the development of airborne radar antennas.

Airborne radar evolved from the early simple air-to-air search and ranging functions to now not only taking into account large-area search, tracking, and fire control guidance, but also mapping and sar imaging. In general, airborne radars require the antenna to have high gain (easy to increase the detection distance), narrow beam (which helps to increase the angle measurement accuracy), and low side lobes (anti-interference). And achieve high gain, narrow beam. The simplest is to use directional antennas such as Yagi antennas and parabolic (single reflector) antennas. Yagi antenna size is too large and the parabolic antenna is easier to implement than the Yagi antenna low side lobe, so early after the Cold War airborne radar commonly used in the form of a parabolic antenna.

Parabolic antenna (single reflector antenna). This antenna uses a large-sized paraboloid as the main surface, and a horn at the center of the main surface acts as a feed (positive feed), (the horn can also be off-center, called bias feed). Its working principle is similar to the parabolic mirror in optics. The working principle is that when the antenna is operating in the radiation mode, the spherical wave radiated by the horn hits the paraboloid, and the paraboloid transforms the spherical wave incident on the horn into a plane wave to radiate into the free space. When working and receiving modes, the main reflector converges the plane waves coming from the free space into a spherical wave and makes it “back” to the feedhorn. This type of antenna is not difficult to process in the x-band and below. The structure is simple and the cost is not high. However, the disadvantages are also obvious. Since the parabolic antenna usually has a high focal length ratio (a ratio of the distance from the horn to the reflective surface to the size of the main reflective surface), it is easy to realize high performance, and therefore the overall antenna section is high and the volume is large. In particular, when the entire antenna rotates and scans, it will greatly occupy the head space, so its scanning angle is also relatively limited. In order to solve these difficulties, a double reflector named "Cassegrain" came into being.

Parabolic antenna basic structure and operation diagram

Parabolic antenna basic structure and operation diagram

The antenna of the sl1 type radar (Imitate the Soviet type РП-1/5 type) equipped with 5 all-weather equipment

Cassegrain antenna. It is an improved antenna in the form of a single reflector antenna. Compared with a single-reflector antenna, an increased number of sub-reflectors can be used to preliminarily optimize the electromagnetic wave emitted by the horn and make it a more ideal distribution, reflecting back to the main reflector. The main reflector then changes the shaped spherical wave. Make a plane wave and let it radiate into free space. This has the advantage of improving the aperture efficiency of the antenna, increasing the gain, greatly reducing the focal length ratio, reducing the overall antenna profile and reducing the volume. After the receiver and the feeder have also become main surfaces, it is more conducive to the layout of the system and reduces system noise. However, the introduction of the subreflector also brings about an increase in the obstruction of the main surface, which in turn reduces the overall gain of the antenna and raises the sidelobe level.

Basic structure and working principle of Cassegrain antenna

Su-15 Fighter Aircraft's Eagle Radar (РП-11)

Su-15 Fighter Aircraft's Eagle Radar (РП-11)

In order to solve the problem of blocking the secondary reflector, an antenna called Inverted Cassegrain was proposed and widely used in airborne radar. Inverted Cassegrain, also known as the deformed Cassegrain antenna. On the basis of the Cassegrain antenna, it changes the position of the secondary reflector to a paraboloid of the polarization grid, and the position of the primary surface becomes a polarization torsion plate. (In fact, in the reversed-card antenna, the main and sub-surface positions are already significantly different from the ordinary Cassegrain.) The working principle is quite different from that of the Cassegrain antenna: the horn feed at the polarization-torsional version The horizontally-polarized electromagnetic wave emitted is almost totally reflected by the front polarization grid, and the spherical wave is changed into a plane wave, hitting the rear polarization plate, and the horizontally polarized wave is “twisted” into the vertical line. Electromagnetic waves are transmitted from the front polarization grid and radiate into free space. In short, although the beam emitted from the reversed card feed is also reflected twice, it is different from the ordinary Cassegrain antenna in that it has a polarization torsion process. The polarization grid located in front of the antenna shields only horizontally polarized electromagnetic waves, and has almost no effect on vertical polarized waves. Here, by the way, in order to combat ground clutter, airborne radar antennas are mostly vertical line polarized antennas. It performs beam scanning by appropriately rotating a polarised torsion plate. Therefore, the inverted card antenna solves the problem of shielding the secondary reflector, and it also can slightly offset the feed and the polarization grid, further reducing the overall antenna profile. Due to its unique advantages, the inverted card antenna is very popular in the second generation machine.

The working principle of the inverted card antenna is a polarizing grid at the front bar. A polarization torsion plate is arranged at an angle of 45 degrees behind

The inverted card antenna used by the Sapphire-21 Radar for MiG-21BIS is noticed that its polarization grid is not installed.

Inverted Cassegrain Antenna Used in Raging Fighter AI-24 Radar System

Mig25 Tornado A radar uses an inverted card antenna with its polarizing grid surface not installed

Su-27's n001 Radar uses an inverted card antenna. Also note that its polarizing grid is not installed.

Su-27's n001 Radar uses an inverted card antenna. Note that this is a complete state. It has a polarized grid surface.

The basic working principle of the above-mentioned antenna is based on the form of reflective surface, is nothing more than a single reflective surface or dual reflective surface, with or without polarization and other details of the difference. By controlling the distance of the horn from the main surface, adjusting the illumination cone, etc., it is easy to realize high gain and low side lobes. At the early stage, satisfactory performance can also be achieved. Despite this, antennas that operate in the form of reflective surfaces do not require high processing (X-band is still good, but even higher frequencies are more difficult) and the cost is acceptable. However, with the increase in the performance of airborne radars, new requirements have also been placed on the antenna part, such as larger scanning angles, lower side lobes, and profiled beams. The inherent flaws in the reversed-card antenna include that there will always be energy leakage (which will cause the reduction of aperture efficiency, loss of gain), and the beam distortion during scanning is also more serious (the main lobe gain decreases, the main beam becomes wider, and the side lobes rise ), and there is always a problem with the weight of the antenna. So everyone thinks that the three generations of aircraft that want to seize the right to air: mig29 and Su-27, all of which used inverted Cassegrain antennas in their early days.

We know that in order to meet the requirements of high-gain narrow-beam low side lobes of airborne radars, parabolic antennas are used earlier due to their simple structure. There is also an antenna that is slightly more complex in form, but with better performance, that is, a planar array antenna. A planar array antenna is a typical array antenna. From tens to hundreds, even thousands, of small cell antennas are uniformly arranged on the array plane in accordance with certain rules, spacings, and the like. A single unit antenna may have a wide beam and a low gain, but depending on the numerous unit antennas on the antenna array, a high gain, a narrow beam, and even an ultra low side lobe can be achieved by working together (later can even be upgraded to Plane phased array. Therefore, the planar array antenna quickly replaces the reflector antenna because of its excellent performance, and has become the mainstream in various airborne radar systems of three generations, three generations, and four generations. Current advanced airborne radars - phased array radars - are almost always used as planar array antennas.

Common planar array antennas include waveguide (plane) slot arrays, open waveguide arrays, dipole arrays, Vivaldi arrays, microstrip patch arrays, and the like.

The waveguide slot array is a common microwave transmission structure in which the waveguide surface is slit (slotted) so that the small slot becomes an antenna and radiates electromagnetic waves. It has the advantage of matching the feed structure and has a large power capacity. The carrier guide slot looks like it is on a flat panel, so it is sometimes called a flat slot array.

The APY1 radar used by E3 uses a waveguide slot antenna array

The outer edge of the flat slot array used by the F15 APG63 radar highlights the IFF antenna (array)

The outer edge of the flat slot array used by the F15 APG63 radar highlights the IFF antenna (array)

Planar slot array for APG66 radar of F16A/B

Similar to this is the open waveguide array antenna. The open waveguide also utilizes a waveguide structure, but instead of a slotted slot, electromagnetic waves are directly radiated from the waveguide aperture face. Because this type of open waveguide is slightly larger in profile and heavier in weight, it is more common in land-based or shipborne radars, and airborne radars are less commonly used.

The above structure using the waveguide for feeding and radiating facilitates matching of the antenna, and a large power capacity is a significant advantage thereof. However, the antenna bandwidth is often limited and the weight is difficult to control. Subsequent many novel designs were gradually used in airborne radar systems.

In fact, by the end of the Cold War, engineers used phased array antennas for airborne radars in order to allow airborne radars to obtain faster scanning speeds and more powerful performance (such as the simultaneous search and tracking of fire control to ground detection). It is still regarded as advanced technology. The phased array antenna is not very different from the common planar array antenna in appearance, and can even be simply understood as being obtained by modifying the feed structure on the basis of an ordinary sweeping planar array antenna (back-end transmission/receiver and signal processing Of course, the algorithm will change a lot.) A phased array antenna (PESA) can be obtained by adding a phase shifter to the rear end of the antenna unit, and an active phased array antenna (AESA) can be obtained by adding a TR component to the phase shifter. For antenna engineers, the same antenna array can be AESA or PESA. Therefore, for the planar array antenna, there is a great potential for upgrading to a phased array antenna.

For radars that have been upgraded to AESA antennas, they have higher transmission power, farther detection distances, more sensitive beam scanning, and more powerful beam shaping functions. It is also easier to obtain an average sidelobes of -50 and -60dB.

Take F22 as an example. The AESA radar of this type of new generation stealth fighter likes to use a dipole array (the picture shows the umbrella dipole array used by the apg77 radar of F22). The unit uses a dipole antenna, which has wideband characteristics, a wide unit pattern, and easy implementation of large-angle scanning

F22's APG77 Radar Antenna

The frontmost bulge can easily be mistaken for a TR component. Strictly speaking, it is the surface of the antenna. The TR component is connected behind the antenna (although the TR component is generally integrated with the antenna, it is still distinguished here.) )

For the RBE2 AESA radar used in the new gusts (shown below), the Vivaldi antenna array was used. This antenna unit is characterized by a particularly wide bandwidth, so the bandwidth of the entire antenna array can be extended.

RBE2 AESA

RBE2 AESA

Of course, there are some special ones. For example, the APY9 radar used in the E2D early warning aircraft using the Yagi antenna as a unit. Because the wavelength of the electromagnetic wave in the uhf band used by it is long, its antenna size is very limited. In order to obtain a more ideal narrow beam and high gain, its antenna unit beam has to be narrow, so the Yagi antenna has become an ideal choice. The high-gain Yagi antenna acts as a unit and can significantly increase the gain of the array. However, things are always relative. The narrow unit beam of the Yagi antenna greatly limits the wide-angle scanning capability of the array. When the antenna scan angle deviates from the normal, the gain reduction and waveform distortion will be very significant. Therefore, APY9 uses a combination of electromechanical scanning to make up for the shortcomings.

2*9 unit Yagi antenna array for E2D APY9 radar

2*9 unit Yagi antenna array for E2D APY9 radar

2*9 unit Yagi antenna array for E2D APY9 radar

It is the development of phased array technology that makes the airborne antenna enter a new stage.

The development of the antenna is a microcosm of the overall radar technology development. Although we cannot determine the radar performance based on the appearance of the antenna and measure the overall performance of the radar, engineering often focuses on the coordination and balance between the systems. If the overall performance of a radar system is advanced, the antenna naturally cannot be worse. In the future, engineers will continue to overcome problems such as phased array antenna large-angle scanning (expanding the scope of scanning at this stage, typically ±60 degrees), ultra-wideband, common aperture, conformal, etc., to promote the development of airborne radar systems.