Notch Antennas

Reflector Systems

Passive Components

Multiband Systems

Wideband Systems

Installed Performance

High-power Components

Method of Moments

In the technique of Method of Moments, the object is modelled as a structure of wires of differing radii. Conducting surfaces are 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 practise, 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 10000 is about the limit. This method has been used widely for HF antennas on aircraft, antenna farms on ships, tanks and many other vehicles. Figure 1 shows part of a van with an extending mast carrying a bicone. Details of the bicone and its current flow when excited at the centre are shown in Figure 2 and Figure 3.

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. MoM codes may be used to

• design and analyse antennas. Examples from MAAS previous work are wideband dipoles, shunt-fed monopoles, TEM and ridged horns, LPDAs, helices
• optimise antenna performance. For example, the geometry of a Yagi antenna was varied to minimise the backlobe radiation over a specified bandwidth. This optimised antenna has been successfully manufactured and meets the design requirements.
• compute the impedance, nearfield and farfield patterns of an antenna on a structure. MAAS work has included monopoles and arrays on aircraft, bicones, LPDAs and dipoles mounted on vans, antennas on lattice masts.

MAAS Expertise

MAAS is an expert in the use of MoM codes and has worked in this field for more than 10 years. The standard US program for Method of Moments (NEC) in its latest version is used. For optimisation, the program NEC-OPT is used. For pre and post-processing, an in-house program, NECPLT, is used (See Figure 1 and Figure 2). NEC-WIN Professional is also used.

MAAS has contributed to the pool of knowledge on NEC by publishing papers on validation where NEC results have been compared with analytical approaches, measurements and the results from other programs such as ALDAS (a GTD program which computes the installed performance on a structure).

Figure 1 MoM model segments for a frequency of 250 MHz

Figure 2 Bicone of Figure 1 showing currents at 50 MHz

Figure 3 Bicone of Figure 1 showing currents at 150 MHz and also the segmentation