This program computes the Far-Field Angular Gain Spectrum (also known as the "Gain Spectrum") of an group of antennas and presents those results in a two-dimensional graphical format. The spectrum is a result of constructive/destructive interference among the various antennas and depends sensitively on the physical arrangement of those antennas, the direction in which they are pointed, and their individual electromagnetic characteristics.
The data calculated with this program is presented in the form of a two-dimensional density or raster graph. The data value representing the power gain at an altitude-azimuth angle pair is "mapped" into a color to create an image. The graph is encoded as a PNG image file which is easily rendered by v4.0 and later Web browsers. The colors in the color mapping table or color lookup table have been selected to approximate a rainbow with red representing the highest gain through shades orange, yellow, green, blue, violet, and finally white representing the lowest gain. It should be noted that this gain scale is logarithmic rather than linear with each color change representing a 3dB (decibel) step. The graph data is normalized to the highest gain value in the data set which is presented in the table to the left of the graph.
It is possible to trace the path of a celestial object as it traverses the sky (and hopefully the antenna array's sensitive region). It the "Enable Object Path" check box is checked then several additional parameters are displayed. At this time only the "Observing Latitude", the "Object Declination", and "HA Display Step" need be supplied. A series of black "+" marks will be plotted over the antenna gain distribution spaced at arbitrary intervals. In a future version the "Observing Longitude" and "Object Right Ascension" parameters will also be used to place the marks at specific and identifiable locations.
The characteristics of each antenna are specified individually on a single horizonal row of the antenna table. Additional antennas can be added by using the "Add Antenna" button. Any antenna can be removed using its corresponding "Delete" button. There is no program limit to the number of antennas which can be specified; computing time does provide a practical limit (see below) and it is also possible to specify enough antennas to crash the program. At least one antenna must be defined at all times. When an new antenna is added to the list of antennas, it is created with the characteristics of the last antenna in the list.
The North-South and East-West positions of each antenna are specified in units of observing wavelength with reference to an arbitrary origin. The choice of origin does not change the calculated Angular Spectrum unless it is so far away from the group of antennas so as to induce computation and round-off errors.
The electrical phase and gain of each antenna are also specified relative to an arbitrary reference. The gain parameter is intended to allow for differential gain among an antenna array. The phase parameter is provided to allow for differental phase shift among the array caused either intentionally or not by differential cable lengths or other means.
Three individual antenna gain distribution patterns are available: isotropic, dipolar, and anisotropic. An isotropic antenna has the same gain in every direction and is mostly useful for understanding interference patterns. An isotropic antenna requires no additional specification, therefore the remaining columns are blanked out. A dipolar antenna displays a cylindrically symmetric pattern with gain maximum and minimum perpendicular and parallel to the long axis of the antenna, respectively. A dipolar antenna requires specification of its orientation, which is the direction of the long axis of the antenna. Please note that this calculation assumes "free space" conditions for these two antennas types where the antennas are far removed from any perturbing influences such as ground.
An anisotropic antenna requires specification of its pointing direction which refers to the direction of maximal gain and its gain pattern. The shape of the gain pattern is characterized in this program using three different (full) beamwidth values at -3, -6, and -10dBc (decibels below the central value) and is assumed to be circularly symmetric. These values depend upon the diameter of the antenna in wavelengths (assuming uniform illumination by the feed). Examples for a few different antenna diameters follow:
| Full Width | 1 | 3 | 10 | 30 | 100 |
|---|---|---|---|---|---|
| -3dB | 62° | 20° | 5.9° | 1.96° | 0.60° |
| -6dB | 89° | 27° | 8.1° | 2.69° | 0.81° |
| -10dB | 120° | 33° | 9.9° | 3.32° | 1.00° |
The time taken to complete a calculation depends on the number and type of antennas, the state of the ground computation, and the load on the computation server. As of this writing, there is a fixed twenty second overhead and each isotropic, dipolar, and anisotropic antenna takes three, four, and eight seconds to calculate, respectively. Enabling the object path display adds some additional time that has not yet been quantified. Methods for displaying elapsed and/or remaining computation time and for reducing the computation time are being examined.
Wouldn't you like to know. ;) Well, be patient and you will because I plan to fill this section with real information rather than smart aleck remarks.
This feature simulates a perfectly conducting, lossless, and planar ground at zero elevation. It works by internally "mirroring" the antenna configuration with another set, identical in antenna count, antenna type, horizontal position, pointing direction, antenna onfiguration, and antenna gain but with each mirror antenna possessing a phase 180° offset from its original. Future versions of this program can be expected to include "imperfect" ground where the mirror antennas' gains will be a fraction of the corresponding originals and the phase offsets will be adjustable from 180°.
This program is currently in development and this page is a prototype. Some (!?!) bugs in execution remain and are also being tracked down. Send comments or questions as described in the footer of this document.
