Pulsed electromagnetic field generation using timed array structures
A novel approach to radiated NEMP hardness validation testing for large naval platforms
N.H.W. Elshove (TU Delft - Electrical Engineering, Mathematics and Computer Science)
Alexander Yarovoy – Mentor (Microwave Sensing, Signals & Systems)
Peter Zwamborn – Mentor (TNO)
R.F. Remis – Graduation committee member (TU Delft - Electrical Engineering, Mathematics and Computer Science)
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Abstract
Bounded-wave NEMP simulators cannot accommodate a full-scale warship and the conventional radiating alternative, a large monocone, cannot deliver sufficient field strength at acceptable homogeneity over the testing surface. This thesis investigates an unconventional architecture for radiated NEMP hardness validation of large naval platforms: a timed array of directive, ultra-wideband radiators that deliberately trades full-platform illumination for local higher field strength and better homogeneity.
Starting from the NEMP standards, a set of field-strength, homogeneity and timing requirements is distilled, which may be interpreted as design specifications. The monocone baseline (EMIS-III-VPD) is characterised and shown to meet none of the three criteria, achieving only 1.28~kV/m peak field, roughly 10~dB amplitude inhomogeneity and 138~ns of peak-time spread across a testing surface of 200 m x 50 m at 100 m distance. \\
The design process is started by evaluating candidate antenna concepts that are all at least ultra-wideband and non-dispersive. It is revealed that directivity is the enabling factor for antennas in radiated NEMP testing, resulting in the reflector IRA to be selected as element in the array. Subsequently, this antenna is modelled at three scales in a full wave-solver, exposing the trade-off between aperture size and low-frequency operating limit. Using results from the full-wave solver, the array is synthesized and optimized in MATLAB using a multi-objective genetic algorithm. This was done over a far-field model anchored to the outcome from the full-wave solver, reducing runtime significantly. With peak field strength imposed as a minimum, the amplitude and timing homogeneity could be minimised simultaneously.
A single 12.2~m IRA fed at 500~kV radiates 4.5~kV/m. An optimised four-element array reaches 9.6~kV/m peak at 0.56~dB inhomogeneity and 1.06~ns peak-time spread over a 10$\times$10~m surface at 100 m distance. The array is therefore a peak-field multiplier rather than a means of improving homogeneity: the single antenna is the most uniform configuration of any studied, and optimisation only recovers near-single-antenna uniformity up to a ceiling of roughly 9~kV/m, beyond which it degrades steeply. The 50~kV/m of the standards is unreachable by this approach, since the feasible element counts and feed voltages required are impractical and the binding limits, aperture size and voltage breakdown, lie outside the array. This contribution is not a finished simulator but a quantified baseline, and bounds on what a radiated NEMP test of a large platform can realistically achieve, as well as a framework within which future work may proceed from.