Let's continue our discussion on antennas by looking at some more antennas .
An array antenna is a special arrangement of basic antenna components involving new factors and concepts. Before you begin studying about arrays, you need to study some new terminology.
DEFINITION OF TERMS
An array antenna is made up of more than one ELEMENT, but the basic element is generally the dipole. Sometimes the basic element is made longer or shorter than a half-wave, but the deviation usually is not great.
A DRIVEN element is similar to the dipole you have been studying and is connected directly to the transmission line. It obtains its power directly from the transmitter or, as a receiving antenna, it delivers the received energy directly to the receiver. A PARASITIC ELEMENT is located near the driven element from which it gets its power. It is placed close enough to the driven element to permit coupling.
If all of the elements in an array are driven, the array is referred to as a DRIVEN ARRAY (sometimes as a CONNECTED ARRAY). If one or more elements are parasitic, the entire system usually is considered to be a PARASITIC ARRAY.
MULTIELEMENT ARRAYS in antennas frequently are classified according to their directivity. A BIDIRECTIONAL ARRAY radiates in opposite directions along the line of maximum radiation. A UNIDIRECTIONAL ARRAY radiates in only one general direction.
Arrays in antennas can be described with respect to their radiation patterns and the types of elements of which they are made. However, you will find it useful to identify them by the physical placement of the elements and the direction of radiation with respect to these elements. Generally speaking, the term BROADSIDE ARRAY designates an array in which the direction of maximum radiation is perpendicular to the plane containing these elements. In actual practice, this term is confined to those arrays in which the elements themselves are also broadside, or parallel, with respect to each other.
A COLLINEAR ARRAY is one in which all the elements lie in a straight line with no radiation at the ends of the array. The direction of maximum radiation is perpendicular to the axis of the elements.
An END-FIRE ARRAY is one in which the principal direction of radiation is along the plane of the array and perpendicular to the elements. Radiation is from the end of the array, which is the reason this arrangement is referred to as an end-fire array.
Sometimes antennas systems use the characteristics of more than one of the three types mentioned. For instance, some of the elements may be collinear while others may be parallel. Such an arrangement is often referred to as a COMBINATION ARRAY or an ARRAY OF ARRAYS. Since maximum radiation occurs at right angles to the plane of the array, the term broadside array is also used.
The FRONT-TO-BACK RATIO is the ratio of
the energy radiated in the principal direction compared to the energy
radiated in the opposite direction for a given antenna.
Various reflected and refracted
components of the propagated wave create effects of reinforcement and
cancellation. At certain distant points from the transmitter, some of
the wave components meet in space. Reception at these points is either
impaired or improved. If the different components arrive at a given
point in the same phase, they add, making a stronger signal available.
If they arrive out of phase, they cancel, reducing the signal strength.
Radiation-pattern and stub phasing
Direcivity, interference, and major and minor lobes
Directional-arrays, collinear arrays, radiation pattern, and length and phasing
Broadside-Arrays, radiation pattern, and gain and directivity
End-fire-arrays, radiation pattern, and gain and directivity
Multielement parasitic arrays, construction and Yagi antennas
Special-antennas, beverage, V, long-wire, and rhombic antennas
Advantages and Disadvantages, radiation patterns, terminating resistors, and the turnstyle antenna
The Ground Plane antenna and the corner reflector