9.6 Phased Array Antenna
A phased array antenna is an array antenna whose single radiators can be fed with different phase shifts. As a result, the common antenna pattern can be steered electronically. The electronic steering is much more flexible and requires less maintenance than the mechanical steering of the antenna.
9.6.1. Principle of Operation
The principle of this antenna is based on the effect of interference, that is, a phase-dependent superposition of two or (usually) several radiation sources. It can be observed that in-phase signals amplify each other and counter-phase signals cancel each other out. So if two radiators emit a signal in the same phase shift, a superposition is achieved – the signal is amplified in the main direction and attenuated in the secondary directions. Here in the left radiator group both radiators are fed with the same phase. The signal is amplified in the main direction.
On the right, the signal from the upper radiator is transmitted phase-shifted by 22 degrees or 45 degrees (i.e., slightly delayed) than from the lower radiator. Therefore, the main direction of the signal emitted in common is slightly steered upward.
If the signal to be transmitted is now routed through a phase-regulating module, the direction of radiation can be controlled electronically. However, this is not possible indefinitely, because the effectiveness of this antenna arrangement is greatest in a main direction perpendicular to the antenna field, while extreme tilting of the main direction increases the number and size of the unwanted sidelobes, while at the same time reducing the effective antenna area. The sine theorem can be used to calculate the ideal phase shift.
Any type of antenna can be used as a radiator in the phased array antenna. Significantly, the single radiators must be controlled with a variable phase shift and thus the main direction of the radiation can be changed continuously. To achieve high directivity, many radiators are used in the antenna field. Older antenna systems may consist of over 1,500 radiators whose received signal is combined with analog solutions. More modern multifunction radar sets use digital beamforming in their receivers.
Advantages and Disadvantages
Advantages:
- high antenna gain with large sidelobe attenuation
- very fast change of beam direction (microseconds)
- high beam agility
- arbitrary space scanning
- freely selectable dwell time
- multifunction operation by simultaneous generation of multiple beams
- failure of some components does not result in a complete system failure
Disadvantages:
- limited scanning range (up to max. 120° in azimuth and elevation)
- deformation of the antenna pattern during beam steering
- low-frequency agility
- very complex structure (computer, phase shifter, data bus to each radiator)
- high costs
9.6.2. Linear Array
These phased array antennas consist of lines, which are commonly controlled by a single phase shifter. (Only one phase shifter is needed per group of radiators in this line.) A number of linear arrays arranged vertically on top of each other form a flat antenna.
Advantage: simple arrangement
Disadvantage: beam steering only in a single plane
Examples given:
- PAR-80 (horizontal beam-deflection)
- FPS-117 (vertical beam-deflection)
- Large vertical aperture (LVA), an SSR antenna with fixed beam pattern
Linear array of a phased array antenna
9.6.3. Planar Array
These phased array antennas consist completely of single elements with a phase shifter per element. The elements are arranged like a matrix; the flat arrangement of all elements forms the entire antenna.
- Advantage: beam deflection possible in two planes
- Disadvantage: a large number of phase shifters
- Examples given: AN-FPS-85 and Thomson Master-A
Planar array of a phased array antenna
9.6.4. Frequency Scanning Array
The frequency scanning array is a special case of the phased array antenna, in which the beam steering is controlled by the transmitter’s frequency without the use of any phase shifter. The beam steering is a simple function of the frequency. This type of phased array antenna was often used in older radar sets.
A vertical antenna array is fed serially by a so-called snake feed. At the main frequency F1, all radiators get a part of the power of the same phase through structurally identical detours, which cause a phase shift of n · 360 degrees. All radiators therefore radiate with the same phase. The resulting beam is thus perpendicular to the antenna’s plane.
If the transmitter’s frequency is increased by a few percent, however, the constructively defined length of the detour lines is no longer correct. At a higher frequency, the wavelength decreases and the detour line is now a bit too long. There appears a phase shift from one radiator to the next radiator. The first radiator radiates this few percent earlier than the next neighboring radiator, and so on. The resulting beam for the F2 frequency is thus steered upward by the angle Θs.
Although this type of beam steering is very simple, it is limited to a few permanently installed frequencies. In addition to the susceptibility to interference, there are even more limitations to be accepted; for example, this radar set cannot use pulse compression because its bandwidth is too low.
Frequency Scanning Array
9.6.5. Phase Shifter
Phase shifters switching different detour lines are faster than regulators. In the picture a 4-bit switching phase shifter which is used in the radar unit is shown. Different detour lines are switched to the signal way. Therefore 16 different phase angles are created between 0 degrees and 337.5 degrees in steps with a distance of 22.5 degrees.
The inductivities (the thin meander wires as lowpass filters) also can be recognized in the switching voltage supplies for the 24 pin diodes.
Since this phase shifter module works both for the transmitting path and for the reception path, branching between these two paths are attached with pin diode switches on the ceramic strap at the entrance and exit of the module.
The same data word must be used for the reception time and for the transmitting moment. It is easy to understand: This one radiator, transmitting the latest phase shift, first receives the echo signal. Its phase shifter must have the largest detour line for diagrams forming in a decided direction. The same detour line is needed for a summation of the received energy.
The phase shifter routes the microwave signal that is supplied to each radiating element through cables of varying lengths. The cables delay the wave, thereby shifting the relative phase of the output. The illustration shows the three basic delays each phase shifter can introduce. The switches are fast pin diode switches. A central computer calculates the proper phase delay for each of the radiating elements and switches in the appropriate combination of phase-shifters pathways.