AN EXAMPLE OF THE USE OF ALDAS

Introduction

ALDAS stands for `Analysis of Low Directivity Antennas on Structures'. It has been used for the prediction of installed antenna performance on a wide variety of structures, for example

  1. Spacecraft such RADARSAT, modern TT&C installations for spacecraft and antennas on small spacecraft
  2. Military Aircraft such as Harrier, TORNADO, EFA, F14, F1-11, NIMROD, SENTRY, HERCULES
  3. Commercial Aircraft such as BOEING 747, 707, ISLANDER, CESSNA, P40, BA146, L:EARJET 45
  4. Ships ranging from fast patrol-boats and submarines to aircraft carriers
  5. Ground vehicles such as vans, passenger cars, tracked vehicles
  6. Other structures such as toll-booths, mobile comms masts, toll-booth installations

The example discussed here has been created specially for the purpose of demonstrating how to use ALDAS.

The example is that of a spiral antenna with free space radiation patterns which have been measured over a limited angular range at 6 GHz. This antenna is placed on the Port wing tip of a fighter aircraft with the antenna boresight set to Azimuth +45.0 degrees. The aircraft has a full complement of stores. Radiation patterns are required in the farfield over Azimuth 0 to 90 degrees and Elevation +/- 80.0 degrees.

Required Input

ALDAS requires input for the antenna type and its position and the aircraft structure geometry. We must create the following files

  1. A file to describe the antenna. Here are screenshots showing a feed being set up with measured radiation pattern.
  2. A file to describe the antenna position. Here is a screenshot of the setting up of an antenna position. There is only one position being used in this case; in general, many positions can be set up.
  3. A file to describe the aircraft geometry. Here are screenshots for setting up the geometry.

When these basic data files have been completed, examine, checked and saved, we need to create a specification for the radiation patterns we want. Here is a screenshot. After running the patterns, these can be displayed in a variety of ways.

  1. Several radiation patterns can be displayed at once for comparison either as polar plots, or rectangular.
  2. Contour plots can be shown .

Comments

The results are limited by diffraction theory . These limitations state that the theory is not valid unless each item in the structure has dimensions greater than lambda/2*pi. There is also an assumption that the wavefront from the antenna striking the structure is well-formed, which precludes high gain antennas. This restriction can be overcome. Other limitations are that the structure is a perfect conductor.