Phased Array Antennas: the basics

Phase array antennas alter the phase between different driven elements within the antenna to give directivity, gain, for simple and very complex antenna systems

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Phased array antennas includes:
Phased array antenna basics Collinear antenna Coaxial collinear Beamforming & beamsteering antennas

The concept of phased array antennas has been known for very many years and it has been used as the basis for very many antennas over the years.

Phased array antenna designs can range from the basic two element antennas used for applications like amateur radio, TV reception, etc, right up to multi-element steerable arrays that are used for 5G mobile communications, various forms of radar and even Wi-Fi now as well as many more commercial applications.

Whatever the application and level of complexity, phased array antennas provide gain, and directivity, and they give the possibility of being able to steer the beam either mechanically or electronically. The electronic beam-steering is of particular interest for many commercial applications because it can be achieved easily, effectively and accurately.

Accordingly phased array antennas are becoming increasingly important in many areas of radio communications and general connectivity.

Early development of phased arrays

The concept of phased array antennas has been known since the early days of radio technology.

The first experiments were undertaken in 1905 by Karl Braun who was able to use phasing techniques to enhance the radiation from an antenna in one direction.

Further developments were made in WW2 when Germany built their Mammut long range radar systems where the antenna used phasing techniques to provide an antenna that could be electronically steered over an arc of about 100°.

Also phased antenna array techniques were used for a ground approach system developed under the guidance of Luis Alvarez.

The techniques for phased array antennas were improved in the following years, but it was not until 2004 when Caltech demonstrated the first integrated silicon-based phased array antenna for reception at 24 GHz.

This paved the way for later developments where integrated circuit techniques were used as the basis of highly steerable small integrated antennas for use with many forms of wireless communication.

On top of this the use of more traditional antennas based on phasing techniques grew over the years to the degree that antennas based upon a series of driven antenna elements where the phased is controlled in one way or another form a large proportion of all the antennas used.

Phased array broad categories and uses

Phased arrays are used by many different sectors of the industry and for a wide number of different types of system.

As a result there could be thought to be two main types of phased array antennas, or two camps into which they can generally be grouped.

Passive antenna arrays: For passive phased antenna arrays, the antennas are all connected to a single transmitter and the phasing between the different elements of the antenna is undertaken in a passive manner, often with feed line lengths, etc.

These antennas tend to follow more traditional lines. They may be in the form of a beam antenna and look similar to a Yagi style antenna, but use phasing techniques where two or more active elements are driven. They may be in the form of a vertical antenna such as a colinear antenna where several elements are stacked vertically above each other to direct the beam. They could also be in the form of a waveguide where slots are cut to give a very narrow beam width, as in the case of antennas used on radar systems. There are of course many other types of passive phased arrays.
Active phased array antennas: With the need for highly flexible, electronically steerable antennas for everything from mobile communications to Wi-Fi and many other professional communications applications, there is a growing technology based around these highly advanced antennas that not only use phasing techniques, but electronically controlled steering, and normally under processor control.

In addition to these groupings or classifications for phased arrays, other classifications and types may also be seen defined. Other sub-classifications are given in some of the additional pages.

Whatever the category the antenna falls into, the basic concept of the way in which it works is the same - namely by controlling the phase of the signal to the different elements, it is possible to change its directional pattern and the direction in which the main beam is directed.

Both types of antenna are very important for their various radio communications applications and they are able to provide excellent performance.

Basic concept of phased array antennas

Phased array antennas, of whatever type work because they superimpose two or more phase dependent radiated signals.

When the signals are in-phase, they combine together to form a signal where the amplitude has been added, but if they are in the opposite phase, then the signals cancel each other out.

Phased array antennas include two or more emitters and the phase of these is controlled to give the reinforcement and cancellation in the required directions.

With only two emitters, such as two dipoles with a phase delay line between them, the control is relatively rudimentary, but one of these antennas can still provide a useful level of gain and provide the basis of an antenna with similar characteristics to a Yagi directional antenna.

With antennas like this that typically use a physical feed line that is cut to length to give the required phase difference, the antenna would need to be physically rotated to alter the direction in which the main lobe of the antenna fires.

Multiple antenna or emitter phased arrays where the numbers of emitters is above two, or three typically form quite complex antennas. The number of radiating elements or emitters can range up to the region of thousands and in many of these it is possible to electrically steer the main beam of the antenna by electrically altering the phase of the radiating elements.

The process of electrically altering the beam direction of the antenna is called beam-steering and it is a particularly important element of many modern communications systems and standards.

How antenna phasing works

Whatever the complexity of the phased array antenna, whether it has just two elements, or a thousand, the basic principle is the same. As has already been mentioned it is based around the fact that when signals are in the same phase, they reinforce each other and when that are in the opposite phase they cancel out. In this way they will produce an enhanced level of signal radiation in a given direction.

Also for the case where signals are out of phase, they cancel each other out. If they are of the same amplitude, then they will completely cancel each other out.

To see how this works take the case where signals from two elements are in phase - they add together and reinforce each other. The sum of two signals, assuming they are of equal strength, is twice that of the individual one, and as a result there is an increase in signal.

In the case where there is a 180° phase shift between two signals, then they cancel each other out, and assuming they are totally out of phase and of equal strength. Accordingly no signal will be heard in this direction.

At angles between 0° and 180° then there will be a transition from nothing to the maximum. It must be said that in reality there is never complete cancellation as it is never possible to fully achieve the ideal in real life.

This means that for a two element beam using phasing techniques like these the radiation pattern might be something like that shown below.

Passive antenna arrays

As mentioned there are many different types of phased array antennas that are used in every application from general radio communications to amateur radio and much more.

There are several different designs and techniques that can be used.

Collinear antenna: A collinear antenna is a form of vertical phased array antenna where several elements, often dipole elements are mounted above each other where they are parallel and collinear, i.e. sharing a common axis. By feeding them in the required phase it is possible to obtain a narrower beam, and hence achieve gain.

Typically, collinear antennas are designed to provide maximum gain at right angles to the antenna and as the antenna is usually vertical, this means the maximum power is radiated parallel to the ground and this gives the best coverage. It is possible to change the phasing of the feed to different radiation patterns which may be required from time to time.

A phased antenna array collinear antenna consisting of four dipoles in a vertical arrangement

This type of antenna is often used for mobile radio communications, where the collinear antenna is used at the base station - these antennas can be much larger than quarter wave or similar vertical antennas that are more likely to be used on the mobile stations.
ZL special antenna: The ZL special is a two element phased array antenna that is popular with radio amateurs. It was introduced in 1949 and was developed by George Prichard (ZL3MH, who later held the callsign ZL2OQ). It was Fred Judd, callsign G2BCX who further investigated the antenna and dubbed it the ZL- special because it was designed by a New Zealander holding a "ZL" callsign.
HB9CV antenna: The HB9CV antenna is a 2 element phased array antenna popular with radio amateurs and it uses two driven elements. The antenna was introduced by Rudolf Baumgartner, HB9CV, in the 1950's.

Active phased antenna arrays

Antennas using phased array techniques with large numbers of elements are now in widespread use, for applications ranging from Wi-Fi to mobile communications as well as antennas for advanced radar systems.

Beamforming: Beam forming is a technique often used with beam steering, where a beam is formed, typically using active elements to create a pattern required for a particular situation.
Beamsteering: The basis techniques involve the use of multiple elements (sometimes large numbers of elements) and which are often controlled by processor driven phase shifters to vary the phase and signal amplitude to the different antennas. In this way the beam can be controlled and steered where it is required.

Application of antenna beam-forming as used with mobile or cellular telecommunications

Read more about . . . . Beamforming & beamsteering.

There are many different aspects to active phased antenna arrays. With the increased use of highly targeted RF signals needed for the high density and high performance mobile communications and other wireless systems, the development of these antennas continues apace and new concepts and ideas are incorporated as the technology becomes available. This are of antenna design is only going to move forwards rapidly in view of the increasing requirements on all types of wireless connectivity.

Phased array antennas are a particularly important form of antenna. In some instances they provide a very neat and effective solution for small radio communications links for professional use and for radio amateurs. However they are also very important for many agile forms of radio communication and mobile communication links, wireless links and the like, where the capability to be able to programme the phase shifts enables highly steerable and effective antennas to be made to compliment the other technology used with these systems.