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Polar Diagram Predictions - Coupling Calculation - Near Field Patterns

PatternMaster - pattern and coupling prediction services, especially for antennas on masts, aircraft, vehicles, military platforms and other structures

PatternMaster

Introduction

The overall capability of a Radio Frequency (RF) system is ultimately dependent upon its ability to operate effectively in its environment. If an antenna is sited on or near a structure, the resulting antenna pattern can be modified by blockages and structural scattering.

PatternMaster consists of software packages for FanField's prediction of antenna patterns . It contains specially selected software tools capable of solving a comprehensive range of problems for a wide variety of RF systems. PatternMaster can be used from 15 kHz to 100 GHz, a very broad range of frequencies.

Special emphasis has been paid to analysing the effect of surrounding structure on the antenna pattern.

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Practical Implementation

PatternMaster has been successfully applied to simulate the scattering and predict installed antenna patterns for the following structures:

Computer Model of a Truck l Superstructure of a ship
l Broadcast antennas
l Body of a truck
l Body of a tank
l Fuselage, wings and stores of aircraft
l Helicopter
l Manpack antenna installations

Furthermore, the dielectric modelling capability allows simulation of the following:

l Radomes
l Windshields
l Composite materials
l Absorber-coated ground planes
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Aspects Covered by PatternMaster

Far Field Patterns

FanField can produce far field radiation patterns for an antenna on any structure or vehicle.  A computer model of the structure is produced and its effect on the antenna's free space pattern is calculated.

Near Field Patterns

Sometimes there is a requirement to know the pattern in the region between the antenna and the far field.  Using the same techniques as in the far field prediction, the antenna pattern or gain can be determined for any point in space.

Rad Haz and EMC

With any RF emitter there is a legal necessity to determine if there is a radiation hazard or an electromagnetic compatibility (EMC) problem at places where sensitive materials, equipment or people may be present. PatternMaster can determine the field strengths at such places or verify that the field strengths are within permitted levels.

Mutual Coupling & Interference

When antennas are collocated or sited close together, mutual interference is a potential problem.   PatternMaster can be used to quantify or minimise the effect.   The analysis can be performed at the frequency of the receiving or transmitting antenna or at defined harmonics.   The effect of other antennas on the radiation pattern can  also be determined .

Antenna Site Optimisation

The best  site for an antenna is rarely obvious. The problem is exacerbated when more than one antenna on a complex structure is included, where the best solution is often a compromise.   PatternMaster is used to perform predictions of the patterns for antennas at several potential sites.   Instead of making an arbitrary decision, the information obtained can be used to choose the best sites for antennas.

Complex Structure Analysis

Although first order effects on a structure on an antenna pattern can be estimated, a full analysis can only be performed by using mathematical modelling techniques. PatternMaster can explain a particular feature of an antenna pattern, such as the occurrence of a low gain region, in terms of the structure and relevant electromagnetic effects. This adds to the Clients' confidence in FanField's ability and can be used to improve the antenna pattern.

Multiple Emitters

If a system consists of more than one radiating antenna there will be interference to their radiation patterns. This aspect can be analysed using PatternMaster.

Multiple Antenna Layout

On new platforms, there may be a requirement for several, or even a few dozen, antennas. PatternMaster can determine the best compromise for the layout of antennas, maximising coverage and eliminating potential interference problems.

 
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Proven Techniques

The application packages are soundly based upon universally accepted theoretical techniques, including Method of Moments (MoM), Transmission Line Matrix and Uniform geometrical Theory of Diffraction (UTD). These theories complement each other in that the first is used to assess effects of structures which are small in terms of wavelength, the second is also for similar small structures, but can assess the effects of dielectric material, whilst the last theory is applicable to electrically large geometries.

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Electrically Small Structures

Antennas operating below 100 MHz and those above, but mounted on structures of only a few wavelengths, require solutions for electrically small structures. PatternMaster employs either Method of Moments (MoM) or Transmission Line Matrix (TLM) techniques. These solutions are complementary to Uniform geometric Theory of Diffraction (UTD) techniques, which are used for electrically large structures.

With Method of Moments (MoM), the structure is modelled using wire segments or surface patch elements, or a combination of both. The maximum segment or patch dimension is one tenth of a wavelength. Complex structures, such as aircraft or vehicles can be modelled up to UHF. Alternatively, simpler structures can be modelled at higher frequencies.

A model may include non-radiating networks and transmission lines connecting parts of the structure, lumped element loading and perfect or imperfect conductors. A structure may also be modelled over a finite or perfectly conducting ground plane.

Transmission Line Matrix (TLM) Modelling is a simple and unconditionally stable time-domain method for the modelling of wave propagation through a medium and can thus be used for structures of arbitrary shapes where dielectric materials play a significant role.

The space to be modelled is represented as a three dimensional Cartesian mesh of electrical transmission lines, which are joined where they intersect at ‘nodes’. Waves are represented by impulsive samples injected at appropriate points. At successive time steps, these impulses propagate along the branches to the next node, where they meet an impedance mismatch. They are scattered from this mismatch along the four branches connected to the node, and at the next iteration are incident on the surrounding four nodes and so on. Boundaries in the propagating medium are achieved by placing loads at the nodes.

Node spacing must generally be less than one tenth of a wavelength and thus the technique is generally limited to electrically small structures.

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Electrically Large Structures

UTD techniques are used for antennas mounted on or near electrically large complex structures. These solutions are complementary to Method of Moments (MoM) and Transmission Line Matrix (TLM) techniques, which are used for electrically small structures. Such large UTD structures are modelled on the computer by combining simple geometrical shapes such as elliptic cylinders, ellipsoids, cones, conical sections, frustums and multi-edged flat plates.

For example, an aircraft can be made up of a composite ellipsoid (two ellipsoids smoothly joined back to back at the antenna) to represent the fuselage, flat plates for wings, cylinders as stores and cones or frustums to represent the nose.

PatternMaster contains UTD radiation and scattering solutions for a wide range of geometrical shapes. UTD packages used by FanField can include dielectric slabs,  allowing the effects of dielectrics to be predicted. Virtually all antenna types from simple monopoles to phased arrays can be included in the UTD model.

The field strength at a given point is calculated by the summation of individual contributions from the different contributing rays.

The first ray considered is the direct wave.  The next most important term is the creeping wave. When an antenna is mounted on a curved metal surface, the current flowing over that surface generates a creeping wave, which is attenuated quite rapidly as the power radiates away. The antenna pattern is therefore very dependent on the curvature of the surface on which the antenna is mounted.

If a direct wave hits a structure, a reflected wave is formed. If a direct wave hits a discontinuity, that wave is diffracted. Likewise, if the creeping wave hits a discontinuity it is diffracted. Finally, the UTD models used by FanField include double terms of reflected-reflected and reflected-diffracted rays.

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Validation

 

Validation of all modelling is important. Fortunately, antenna pattern prediction software can be validated against measurements, trials and, to some extent, by comparison with other techniques. All packages within PatternMaster have been extensively validated by these methods. These packages, which have been written by the world leaders in their respective specialisations, are used extensively in the USA, and by FanField in the UK. They have had far greater validation than any UK or in-house developed software.

Being a member of the pertinent users group, FanField receives regular reports on software updates from other users throughout the world.

Comparison of Predicted and Measured Antenna Patterns
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Benefits of PatternMaster

PatternMaster has many benefits. They include the following:

 

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It is the most cost effective method of determining the effect of a structure on an antenna pattern.
 

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It is very flexible , allowing many what if analyses to examine the effect of different antennas, various antenna sites, or the addition or removal of sections of the structure.
 

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Coupling analyses and interference assessments can ensure that potential mutual interference problems are solved prior to installation.
 

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Unlike in-house developed packages, PatternMaster is extensively validated in the USA and UK.
 

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It uses quality packages produced by the world leaders in their respective specialisations.
 

Results have been extensively used to recommend improvements to existing or future antenna installations.
 

When used for antenna placement studies on new platforms, the best compromise for antenna layout can be obtained.
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FanField Ltd., Oxley House Cottage, Oxley Hill, Tolleshunt d'Arcy, Maldon, Essex, CM9 8EN
Tel: 01621 810095 Fax: 01621 810095
e-mail:
sales@fanfield.co.uk