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Installed PerformanceThe performance of an antenna when installed on a structure will be affected by that structure. Whilst high gain directive antennas should not, if well sited, `see' much of a support structure, low gain antennas will, by their very nature, have wide angle coverage and will illuminate parts of the structure. Examples are omnidirectional bicones on lattice masts and communication antennas on ships and aircraft. Part of the `craft' of installing antennas on a structure for minimum degradation is knowing what aspects of the structure will cause most degradation and being able to apply the relevant modelling tools to predict the final performance. Computation of the performance of antennas on aircraft or spacecraft has considerable advantages. An examination of the antenna performance can be carried out at a early stage in the life of a design before the drawings have finalised. It allows the investigation of alternative sites for the antennas and can identify potential problems at an early stage. Later, when a new system has to be added, the effect of the proposed new systems on existing systems can be examined. There are two types of computer code which may be used to examine installed performance - Method of Moments (MoM) and the Geometrical Theory of Diffraction (GTD). Work on installed performance has included
Method of MomentsMoM [1] may be used when the structure is small in terms of wavelengths. In this technique, the conducting surface of the structure is modelled as a grid of wires. The radius must be such that the total surface area of the wires is the same as the total surface area of the true structure. Assumptions are made about the form the currents take on each wire which might be, for example, a polynomial with several unknown coefficients. In order to avoid ambiguity, the length of each wire must be restricted to less than 0.25 wavelengths. In practice, this length should be less than 0.1 wavelength. The solution for the coefficients on each wire is the core of the technique. Since a matrix inversion is required to obtain all the unknown coefficients, the maximum permissible number of segments of about 15000 is about the limit. This method has been used widely for HF antennas on aircraft, antenna farms on ships, tanks and many other vehicles . The restrictions imposed by memory and runtime mean that this method is limited to less than 100 MHz for antennas on structures although design of antennas and small structures are not so limited. The radiation patterns of the central notch of an array of three notches all mounted on a square ground plane of side two wavelengths are shown in Figure 5 and the wire-grid geometry is shown in Figure 4. Further MoM examples are available. Diffraction TheoryDiffraction theory [2] may be used to calculate the radiation patterns of an antenna in the neighbourhood of a conducting structure and has the advantage that runtime and complexity is not dependent on the frequency of investigation since GTD uses ray tracing. However the theory is only applicable to structures which are more than one or two wavelengths in dimensions. Table 1 shows the frequency ranges at which the two methods are usable. For the Method of Moments, the upper frequency is determined by setting a limit of 5000 segments which would require a large computer. The best a desktop computer can do is around 2000 segments which would reduce the upper frequency limit by a factor of 0.6. If GTD is used, the elements of the structure must be a minimum size in wavelengths for the theory to be valid and the shape of the structure is important. For instance, a small helicopter is much larger than a saloon car but has rotors only a few centimetres in width. These rotors must be included in any structure model of a helicopter. There may be problems if the frequency of interest is below 1 GHz where the rotor blades are half-wavelength or less in width. The saloon car, on the other hand, is box shaped with few excrescences. In the application of diffraction theory, there is therefore a lower frequency limit to the frequency below which the theory is invalid. LearjetThe installed performance (Figure 2 and Figure 3) of a Circularly Polarised antenna built by EASAT Antennas Limited for use in C-band was computed on a Learjet 45 (Figure 1) using the diffraction program, ALDAS. The antenna was designed to have good coverage in the upper hemisphere. A considerable amount of work has been carried out on high-gain antennas mounted on structures, including reflectors. MAAS ExpertiseMAAS is an expert in the computation in installed performance. The US standard program for Method of Moments in its latest version is used. MAAS is responsible for the writing and validation of a large GTD program (ALDAS) which is widely used in the aerospace industry. A presentation made at Queen Mary and Westfield College, London in April 1998 on installed performance for Low Earth Orbit and Medium Earth Orbit satellites is available here. References
Table 1: FREQUENCY LIMITS Structure MoM GTD Saloon car < 500 MHz > 1 GHz Small Helicopter < 250 MHz > 1 GHz Large Helicopter < 150 MHz > 500 MHz Fighter aircraft < 100 MHz > 500 MHz Large Civil aircraft < 30 MHz >100 MHz e.g, BOEING 747 Transmitter mast < 30 MHz > 600 MHz FIGURE 1 Input Geometry of a LEARJET 45
FIGURE 2 Contour Plot of Installed Performance on LEARJET 45 - Upper Hemisphere Note the disturbance due to the tail
FIGURE 3 Contour Plot of Installed Performance on LEARJET 45 - Lower Port Rear Quadrant
FIGURE 4 Wire Grid Geometry of Notch on Square Ground Plane 2 wavelengths on a side. There are Stop Notches between Adjacent Elements
FIGURE 5 Radiation Patterns of Central Notch in an Array of Three on a Square Ground Plane
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