Introduction

Usually the radiation pattern of a single element antenna is relatively wide and each element provides low values of directivity. In many applications it is necessary to design antennas with very directive characteristics (very high gains) to meet the demands of long distance communications. This can only be accomplished by increasing the electrical size of the antenna.

Enlarging the dimensions of single elements leads to more directive characteristics, but at the expense of increased size. Another way to enlarge the dimensions of the antenna, without necessarily increasing the size of the individual elements, is to form an assembly of radiating elements in an electrical and geometrical configuration. This particular type of antenna is referred to as an array. In most cases the elements of an array are identical. This is not necessary, but it is convenient, simpler and more practical. The individual elements may be of any form (wires, rods, apertures, etc.)

Neglecting coupling between elements, the total field of the array is determined by the vector addition of the fields radiated by the individual elements. To provide very directive patterns, it is necessary that the fields from the elements of the array interfere constructively (add) to give a maximum field in the desired directions and interfere destructively (cancel each other) in the remaining space. Ideally this can be accomplished, but practically it is only approached. In an array of identical elements, there are five controls that can be used to shape the overall pattern of the antenna. These are:

1. The geometrical configuration of the overall array (linear, circular, rectangular, spherical, etc.)
2. the relative displacement between the elements,
3. the excitation amplitude of the individual elements,
4. the excitation phase of the individual elements,
5. the relative pattern of the individual elements.

Further discussion on the development of the equation used in this applet can found in the document Linear Arrays.

The applet

This applet displays the radiation electric field magnitude of an 'n-element' uniform linear array.

The scrollbars allow you to select the number of radiating elements, the distance between each element in wavelengths, and the phase shift, in degrees, between each element as shown in the figure below.

The polar pattern of the radiation field is graduated in 10 degree sectors and the inner circle is 0.7071 the radius of the outer circle, allowing the 3dB beamwidth of any part of the radiation pattern to be found. The right hand side of the applet display shows the number of elements chosen in the linear array form. The cross-section of the antenna on the right hand side of the display is keyed to the xy-plane indicating the positioning of the polar pattern on the antenna. By selecting the "Fill?" or "Unfill pattern?" the radiation pattern will be filled with yellow (as shown in Figure 1) or unfilled, where only a red line outlines the polar pattern.

Once the element spacing or number of elements has been chosen, the mouse must enter the unfilled space to allow the antenna element diagram to be updated. If the fill or unfill pattern is selected, the mouse should select the vertical bar in any of the scroll bar fields for the applet to be updated. The No. of elements scrollbar vertical bar being the easiest to select as it is the widest.

The basic source code for this applet was written and obtained, with thanks, from Karl L. Barrus klbarrus@nwlink.com. His

original applet may be found at: http://www.nwlink.com/~klbarrus/antenna/.

The Applet

Tony Townsend , tonyart@ieee.org