An
antenna array is an antenna that is composed of more than one conductor. There
are two types of antenna arrays:
Driven
arrays – all elements in the antenna are fed RF from the transmitter
Parasitic
arrays – only one element is connected to the transmitter. The other elements
are coupled to the driven element through the electric
fields and magnetic
fields that
exist in the near field region of the driven element
There
are many types of driven arrays. The four most common types are:
Collinear
array
Broadside
array
Log Periodic Array
Yagi-Uda Array
COLLINEAR ARRAY
The
collinear array consists of l/2 dipoles oriented end-to-end. The center dipole is fed
by the transmitter and sections of shorted transmission line known as phasing
lines connect the ends of the dipoles as shown below.
The length of the phasing lines are adjusted so that the currents in all the dipole sections are in phase, as shown below.
The input impedance of a collinear array is approximately 300 ohms. The directivity of a collinear array slowly increases as the number of collinear sections is increased.
BROADSIDE ARRAY
A broadside array consists of an array of dipoles mounted one
above another as shown below. Each dipole has its own feed line and the lengths
of all feed lines are equal so that the currents in all the dipoles are in
phase.
Rows of broadside arrays can be combined to form a two dimensional array as shown below:
The
two-dimensional array is used in high performance radar systems. The amplitude
and phase of each input current is adjusted so that the antenna radiates its RF
in a narrow beam. By making changes to the input phase and amplitude, the beam
can be made to scan over a wide range of angles. Electronic scanning is much
faster than mechanical scanning (which uses a rotating antenna) and permits
rapid tracking of large numbers of targets.
A special type of phased array consisting of 2 or more vertical antennas is widely used in AM broadcasting. Consider an AM transmitter located in a coastal city such as Charleston, SC. It would make no sense to radiate a signal in all directions; there is only water to the east of city. Two or more antennas could be used to produce a directional pattern that would radiate most of the signal to the west.
The design and analysis of phased arrays is quite difficult and will not be covered further in this unit.
The log periodic dipole array (LPDA) is one antenna that almost everyone over 40 years old has seen. They were used for years as TV antennas. The chief advantage of an LPDA is that it is frequency-independent. Its input impedance and gain remain more or less constant over its operating bandwidth, which can be very large. Practical designs can have a bandwidth of an octave or more.
Although an LPDA contains a large number of dipole elements, only 2 or 3 are active at any given frequency in the operating range. The electromagnetic fields produced by these active elements add up to produce a unidirectional radiation pattern, in which maximum radiation is off the small end of the array. The radiation in the opposite direction is typically 15 - 20 dB below the maximum. The ratio of maximum forward to minimum rearward radiation is called the Front-to-Back (FB) ratio and is normally measured in dB.
Log-Periodic Dipole Array
The log periodic antenna is characterized by three interrelated parameters, a,s, and t.as well as the minimum and maximum operating frequencies, fMIN and fMAX. The diagram below shows the relationship between these parameters.
Unlike many antenna arrays, the design equations for the LPDA are relatively simple to work with. If you would like to experiment with LPDA designs, click on the link below. It will open an EXCEL spreadsheet that does LPDA design.
You can also learn more about LPDA's by checking out some of the manufacturers' web sites:
YAGI-UDA ARRAY (YAGI)
The Yagi-Uda array, named after the two Japanese physicists who invented it, is the most common antenna array in use today. In contrast to the other antenna arrays that we have already looked at, the Yagi has only a single element that is connected to the transmitter, called the driver or driven element. The remaining elements are coupled to the driven element through its electromagnetic field . The other elements absorb some of the electromagnetic energy radiated by the driver and re-radiate it. The fields of the driver and the remaining elements sum up to produce a unidirectional pattern. The diagram below shows the layout of elements in a typical Yagi.
Behind the driven element is a single element that is approximately 5% longer. This is the reflector. It prevents radiation off the back of the array. In front of the director are a series of elements that are shorter than the driven element. These are the directors. They help focus the radiation in the forward direction. Together the reflector and directors can reduce the radiation off the back of the antenna to 25 - 30 dB below the forward radiation. As more directors are added, the forward gain increases.
The table contains hyperlinks to manufacturers of Yagi antennas. You may want to check some of these links out to get a look at typical Yagi antennas.
The design and analysis of Yagi antennas is very involved and is best done using antenna modeling software. However, to get insight into the basic operation of theYagis, we will examine one with only three elements: a reflector, driver, and director.
Simple 3 Element Yagi-Uda Array
The reflector is 5% longer than the driver, and the director is 5% shorter. The spacing is the same between all three elements. The plots below show the radiation pattern of the Yagi in two perpendicular planes.
Notice that the pattern is unidirectional, and somewhat wider in the plane perpendicular to the elements. This is true in general for Yagis, regardless of the number of directors used. However, as more directors are added, the forward gain will increase, and the beamwidth will become narrower in both planes.
You may wonder what would happen if additional reflectors are added. The answer is that nothing happens. The first reflector reduces the power radiated rearward to approximately 1% of the forward value. The additional directors cannot couple strongly to the driver because the radiated field passing by them is so small. Only 1 reflector is necessary to reduce rearward radiation.
The operating bandwidth (the range of frequencies over which the gain and FB ratio stay within design criteria) for a Yagi is generally quite narrow and can be altered to some extent through careful adjustment of the length and spacing of the elements. The chart below shows how the gain and FB ratio of the 3 element Yagi depend on frequency.
Notice that the maximum gain and FB ratio do not occur at the same frequency. This is true in general for 3 element designs. By making the Yagi longer (adding more elements) and controlling the length and spacing of each new element, it is possible to bring the frequency of maximum gain and FB ratio closer together.
The operating bandwidth of a Yagi is often defined as the range of frequencies over which the FB ratio is greater than 20 dB. In the chart above, the range of frequencies over which the FB ratio is greater than 20 dB is 0.985 f0 to 1.01f0 or 2.5% of the design frequency. This is a typical bandwidth for a Yagi array. It is possible to widen the operating bandwidth by lengthening the array and adding elements, although this improvement normally comes at the expense of forward gain.
There are many variations on these basic designs, but our examination of array antennas will end here.