Phased array

A phased array antenna ( from English- phased array, phased array ') is a phased array antenna with high directivity, which reached a concentration of the radiation energy by the arrangement and interconnection of individual radiators. If the individual radiators can be controlled differently, the antenna pattern of the antenna is electronically swivel. Phased array technology is widely used in radar systems in the military field.

  • 3.3.1 series feed
  • 3.3.2 Parallel feed
  • 3.3.3 radiation power

Use

In principle, all array antennas and panel antennas and array antennas, but they have an immutable same phases of all emitters to each other. They are used for satellite reception (weather and communication satellites), in mobile radio stations, radio stations (FM, VHF, UHF) and used for radar equipment.

The bundling is the stronger, the greater is the transverse dimension of the antenna. Therefore stacked antennas for mobile communications vertical, horizontal radar antennas, however. Targeted deviation of the phase angle of the directional pattern may be asymmetrically affected, to provide, for example, a surveillance radar with an even obliquely upward extended directional pattern, but to limit it down.

At radars often electronically steerable phased-array antennas are used. They allow fast and accurate tracking of a mobile target. Examples are air defense missile systems such as the American Patriot and the Russian SA -10 and SA- 15th A variation of phased array radar is the Active Electronically Scanned Array ( AESA ) radar, in which each transmit or receive element has an RF source. It is mainly used in fighter aircraft.

  • Beam scanning in the microsecond range
  • Very flexible antenna pattern ( directional spotlights, radiator fan, radiator sector )
  • Possible high directivity, the way to fix the transmission power to a point at any distance ( magnifying glass effect)
  • Satisfies any complex shape of the antenna needed a flat, rectangular surface
  • High level of redundancy due to high number of transmit and receive elements
  • Limited region of the directional angle (technical maximum / ​​- 60 °, economically < 30 °).
  • High precision requirements for the elements which are necessary for the phase shift.
  • Low bandwidth of the antenna, since the geometric distances of the transmitting elements together define the wavelength.

History

Grouped and phase-synchronously fed Yagi antennas were the first radar antennae, they served the broadcasting wide receiver and even today at radio amateurs spread (70 -centimeter band).

Consisting of a waveguide vertical slot antenna arrays were used to Soviet radars ( 1970s) at frequencies around 1 GHz in addition to the main antenna for reception of the friend - foe identification, as these could not be received from the heavily bundled, now further rotated main antenna due to their delay.

The first radar with electronic scanning was 1944, the German Remote paging system FuMG 41/42 Mammoth.

Technology

Function

The antenna array uses the phase shift of the transmission are arranged in a matrix of elements in order to achieve a concentration by interference. The transmission energy is reinforced in the desired direction, while the undesired directions are canceled by destructive interference. The individual transmission elements in this case do not require bundling facilities.

To move about the radiation or reception angle upward, only the phase angle of the lower elements must start earlier, the phase angle of the upper members accordingly later. Due to the time differences, the energy of the lower radiator no longer in the antenna center, but further up phase with the energy of the upper radiator. Thus, the wave front is angled relative to the antenna surface, and the beam angle tilts upwards. (Denoted in the graph as x) The phase difference between the radiators is constant between the antenna elements and any run-time differences in the supply line must to be considered.

If a different phase difference is adjusted by, for example, is introduced to the phase angle of the outer elements and the inner after, the concentration of the overall antenna is changed, that is, the antenna pattern shape changes. This method is used in a multi- mode radar to switch from a wider diagram for target searching on a very narrow chart for accurate target accompaniment.

For a very narrow antenna pattern of a large number of individual emitters are required, add the phase difference between the radiators to the edge of the antenna array back. The phase shifter must therefore achieve a phase shift of nearly 360 °, this phase shift has to be implemented very quickly. In practice, different phasing lines can be used which are switched to a connected with a 16 bit control word of the 4-bit phase-shift in increments of 22.5 ° in the feed line. ( This control word must also include information on addressing. )

The air defense radar RRP 117 uses a phased array antenna with 1584 individual radiators, which are summarized horizontally in groups, since this radar rotates its antenna and the antenna pattern must swing electronically only in the elevation angle.

The beam angle can indeed be theoretically near ± 90 °. In practice, however, only up to ± 60 ° is reached, as the bundling of the antenna pattern deteriorates rapidly with increasing angle. For a search in a full circle around the antenna three antenna groups at an angle of 120 ° can be distributed in practice. Effectively, four groups at an interval of 90 °, as it is used for example in the apar, a radar device of the Navy.

Arrangement options

Frequency-dependent beam steering

The frequency-dependent beam steering is a special case of phased array antenna in which the beam steering will be controlled without any phase shift by the transmission frequency. The beam deflection is a function of frequency.

A vertical antenna array is serially fed. On the fundamental frequency radiators all get a benefit of the same phase by constructively same phasing lines that cause a phase shift of n × 360 °. So all radiators radiate with the same phase at the same time. The resulting beam is thus perpendicular to the antenna plane.

If the transmit frequency is increased by a few percent, but the constructive fixed length of phasing lines is no longer true. The bypass line is now a bit too long. There occurs a phase shift of emitters, emitter. The first radiator can radiate a few percent more than the next adjacent radiator and so on. The resultant beam is thus pivoted about the angle.

This type of beam steering is very simple, but is limited to a few fixed transmission frequencies. In addition to the vulnerability of even more restrictions are to be accepted, for example, can not use this pulse compression radar.

Linear arrays

Linear phased array antennas are composed of lines, which are controlled jointly via a phase shifter. A plurality of vertically superposed linear arrays form a planar antenna.

  • Advantage: simple arrangement
  • Disadvantage: beam steering possible only in one plane

Planar arrays

Planar phased array antennas consist entirely of separate elements each with a phase shift per element. The elements are arranged as in a matrix, the flat arrangement of all the elements forming the whole antenna.

  • Advantage: beam steering possible in two planes
  • Disadvantages: complicated arrangement and more electronically controlled phase shifter

Feeding of the phased array antenna

Phased array antennas may be line powered, then the energy is guided by coaxial cable or waveguide series or in parallel to the antenna elements. Alternatively, power can be supplied by a central radiator, ie with an energy radiated done: The antenna is then called " radiation- fed ."

Series feed

In the series feed of the phased - array antenna, the radiating elements are successively supplied to the transmission power. The thereby increasing phase shift due to the longer lead must be considered in the setting of the phase shifter. A frequency change is not readily possible for a series power supply. However, if a frequency change be made, the computer must also recalculate the phase shift (or mostly in the programming practice: use a different phase angle table).

Examples:

Parallel feed

In the parallel feed of the phased - array antenna, the transmission power of each node is distributed in phase. Thus, each radiating element has an equally long cable and is therefore supplied in phase. This has the advantage that the computer can ignore the length of the feed lines in the calculation of the phase shift and the phase shift is not also frequency dependent.

Examples:

Radiation power

Wherein the radiation power, the transmission power is distributed from a central primary antenna to an antenna array. This can happen on the one hand from behind the antenna array with phase shifters directs the energy through ( transmission type ). Alternatively, the radiation may also be done from the front feeding, the energy received by the elements of the antenna array is delayed with the phase shifter is reflected at a mismatch, and emitted again (reflection type).

Examples:

  • Air defense missile system MIM -104 Patriot ( transmission type )
  • Air defense missile system SA -10 Grumble (reflection type)

Radars with phased array antennas

Phased array antennas are a wide range of applications mainly for military radars. This has historically arise because initially the cost of a phased array antenna were extremely high and only military users could afford these costs. The advantages of not equipped with mechanical rotary antenna elements played less a role, as many phased array antennas are nevertheless mechanically rotatable. The greatest advantage from a military point of view is the great speed of the possible beam pivotings, which can affect the time budget of a pulse radar favorable. By using the technologies of the digital beam steering, it is even possible to focus the antenna during reception time simultaneously in multiple beam directions. This results in a General-purpose multi-function radar, which replaces several older highly specialized radars for both air surveillance, navigation and target monitoring / tracking. The disadvantage of a limited observation sector is effectively compensated by using multiple antennas. A so-called 3 antenna which can cover all directions within a hemisphere around the antenna at the same time (2 for 360 ° in azimuth and another for the 180 ° in elevation angle ), was from the Fraunhofer Institute for High Frequency Physics and Radar Techniques under the name Crows Nest antenna developed and patented.

Various phased array antennas with different distinct degree of agility of beam scanning are now widely used. It will be electronically rigid antennas ( LVA antenna) in air traffic control uses a special antenna pattern to generate. A form of frequency-dependent beam steering is applied with slot antennas and the FMCW radar methods in security technology as a barrier Radar. Electronically tilted in one direction only linear arrays (for example, PAR -80, RRP 117) still use a rotating antenna platform for the entire antenna. Planar arrays, some of which are completely dispense with a mechanical movement, for example, the APAR, AN/MPQ-53 and used in the Cobra Dane. ALs particularly beneficial affects that planar arrays a possible pitch, yaw and roll of an airborne or maritime antenna carrier can compensate electronically. The distribution of the individual radiating elements of a phased array antenna has to be made not only into a flat surface. There antennas are already built, for example, adapt to their geometrical shape of the aerodynamic cross-section of the leading edge of an aircraft wing exactly.

Gallery

Reconnaissance and ballistic missile of the Russian anti-aircraft missile complex S- 300PMU2

Installation of a phased array radar

The APAR on the frigate "Sachsen"

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