Introduction
In antenna theory, a phased array usually refers to an antenna that can be electronically steered.
This is in contrast to a fixed antenna such as a parabolic dish (think DirecTV dish), which has a fixed beam pattern. To steer the beam of a fixed antenna, you would need to physically move the antenna to adjust its pointing. In a phased array, the beam pattern can be steered electronically by adjusting the phase or time delay of the signals fed to each antenna element.
Phased arrays are used in many applications, including radar, satellite communications, and cellular base stations.
Angle of Arrival
Let us start with a 1-dimensional array of N antennas. We'll assume that the antennas are spaced a distance apart from each other and the array is oriented along the x-axis.
For this initial exploration, we will consider a receiving array, although the general concepts apply in both directions, as the antenna elements and combiners are generally bi-directional. In practice, antenna elements, time delay blocks, passive combiners, and transmission lines are bi-directional. However, amplifiers and phase shifters are often unidirectional, meaning there will need to be separate feed structures for transmit and receive arrays. For now, we'll focus on the receive array, although if it is easier for you to think of this as a transmit array, that is fine too.
The angle of arrival of a wavefront is defined as the angle between the direction of the wavefront and the boresight direction (normal) to the array.
Each of the (initially horizontal) wave front lines arrives one after another.
Wave front arrival
Tilt the incoming plane wave and watch each element see a different phase.
Use the input slider to change θ, the angle of arrival of the wave front, and see how the change in θ changes the relative time of arrival of each wave front for each of the antenna elements.
Now that we are beginning to understand that the wave front may arrive at each antenna element with different delays, depending on the angle of arrival of the wave front, let's dig in further about what that time difference does to how the signals combine (and what we can do about it)!
Signal combining
As the angle of arrival of the wave fronts deviates from the boresight (θ = 0º) direction, the arrival time delay for each wave front line changes at each element.
One of the main goals of the circuits beyond the antenna elements is to combine (for receiving arrays) or split (for transmitting arrays) the signals from/to each array element back to a common point with a common phase for a specific frequency or frequency range (beam squint occurs when steering for a specific frequency range but setting delays for a frequency within the range).
In order to achieve this, the antenna controller needs to make adjustments to each path to compensate for the arrival time differences to allow the signals to have a common phase at the point of combining/splitting.
Since these antennas are used in communications systems, the common point often is the network modem, with amplifiers and mixers, in addition to the delay blocks, as appropriate for the frequency plan and system requirements assigned to the receiver or transmitter's portion of the link budget.
Delay Blocks
These paths of combiners or splitters ideally could have a block that compensates for the time delay differences for each antenna element.
These functional blocks are called time delay circuits. They are often implemented with switches to route the signal through or past a set of transmission lines of varying lengths. They are often paired with amplifiers to make up for the signal loss while traversing the transmission lines, depending on the frequency of the system. If you have digital control of the switches and a set of lengths of transmission lines that have a delay of 1, 2, 4, 8, 16, 32 times a base delay value, you can digitally toggle different amounts of delay, often picoseconds or nanoseconds, to each element (or a group of elements).
The above pattern is arbitrarily chosen to be 6 bits. Delay blocks can be designed to match the sets of arrival angles that need accurate compensation.
In the example below, the red and green sine waves represent signals from two antenna elements, and the blue sine wave is the reference arrival signal. The red and green signals are shifted relative to the blue signal, indicating different arrival times at their antenna elements.
Using the sliders below, adjust the delay of the red and green signals to see when the signals overlap with the blue signal.
Phase delay combiner
Slide the delayed elements until the sum snaps from cancellation to gain.
This illustrates adding time delay to the red and green signals to match the blue signal's arrival time. In practice, the signal amplitudes add together, so aligning the delay of all the signals achieves the desired constructive combination of all the signals.
This example is a simple case of adjusting only a few signals, but in practice, there are many more signals to combine or split in a 1- or 2-dimensional phased array.
Additionally, the delay unit in the example is degrees (of the sine wave, not to be confused with θ, the angle of arrival of the wave front discussed in the section above) to align the visual more clearly to the period of the sine waves.
In practice, the delay unit is often time, such as nanoseconds or picoseconds, depending on the frequency of the system.
Phase shifters are often used to emulate time delay blocks, as they are often smaller and easier to fit physically near each antenna element. Phase shifter units are degrees, and phase shifters are designed to operate at a specific frequency or frequency range (with some tolerable error for the latter case as you deviate from a nominal frequency).
Electronic steering
Let's put it all together.
The concept from the visual applies to the delay blocks for every element in the phased array.
In our wave front example, the delay blocks would be set to match the delay of the wave front arriving at each antenna element, depending on the angle of arrival of the wave front.
Since the arrival time differences vary with θ, the angle of arrival of the wavefront, the controller of the phased array needs to compensate (delay in time/phase) for arrival time differences of the wavefront at each antenna element. Doing so ensures that signals from each element are in phase at the point of combining/splitting for a given θ.
When the signals align in phase at the point of combining, they will constructively combine perfectly.
This intentional adjustment to combine signals constructively at a specific θ is called electronic steering, since the physical plane of the antenna array is not moving, but the signals from a specific θ are combined more effectively than other angles. In practical terms, the electronic steering moves the main beam of the antenna to a specific angle θ. The main beam is the direction of the antenna pattern with the highest gain.
This is the key to the flexibility of phased arrays: the ability to steer the beam of the antenna electronically, without moving the physical antenna.
Mechanical Steering
In contrast, mechanical steering is when someone or something simply moves the antenna. This can be done by hand or by some other method (electronic screws, gimbals, etc.).
Both electronic and mechanical steering attempt to point the main beam of the antenna toward the transmitter (if the antenna is acting as a receiver) or toward the receiver (if the antenna is acting as a transmitter).
Systems also can employ a combination of electronic and mechanical steering. This is often done to increase the range of angles that can be steered to while meeting certain performance requirements.
Closing
Common examples would be phased array antennas on the user terminals of satellite internet systems, such as Starlink's user terminals, as well as satellite transmitters/receivers and radar systems.
There are many parameters that differ between electronically and mechanically steered antennas: reliability, cost, beam pointing time, the number of beams supported, and the ability to null beams (destructive combining). These parameters are beyond the scope of this introduction, but serve as starting points for further learning.
Additionally, it is common for phase shifters to be used instead of time delay blocks. This is often because of the availability of phase shifters, in addition to the absence of transmission lines, which are often too long to be placed next to antenna elements. This is especially true when the frequency range of operation is higher, which leads to a smaller distance between antenna elements when targeting an element spacing near a half wavelength (which we have not discussed yet, but around 1/2 of a wavelength is a common spacing).
References
- Phased Array Antennas on Wikipedia.
- Phased Array Antenna Patterns—Part 1: Linear Array Beam Characteristics and Array Factor, by Peter Delos, Bob Broughton, and Jon Kraft on Analog Devices website.