Type of Document Master's Thesis Author Caswell, Eric D. Author's Email Address ecaswell@vt.edu URN etd-82598-202822 Title Analysis of a Helix Antenna Using a Moment Method Approach With Curved Basis and Testing Functions Degree Master of Science Department Electrical and Computer Engineering Advisory Committee

Advisor Name Title Davis, William A. Committee Chair Brown, Gary S. Committee Member Stutzman, Warren L. Committee Member Keywords

- Helix
- Method of Moments
- Curved Segments
Date of Defense 1998-09-09 Availability unrestricted AbstractAnalysis of a Helix Antenna Using a Moment MethodApproach With Curved Basis and Testing Functions

Eric D. Caswell

(ABSTRACT)

Typically wire antenna structures are modeled by approximating curved structures

with straight wire segments. The straight wire approximation yields accurate results, but

often requires a large number of segments to adequately approximate the antenna

geometry. The large number of straight wire segments or unknowns requires a large

amount of memory and time to solve for the currents on the antenna. By using curved

segments which exactly describe the contour of the antenna geometry the number of

unknowns can be reduced, thus allowing for bigger problems to be solved accurately.

This thesis focuses on the analysis of a helix antenna. The Method of Moments is

used to solve for the currents on the antenna, and both the triangle basis and pulse testing

functions exactly follow the contour of the helix antenna. The thin wire approximation is

used throughout the analysis. The helix is assumed to be oriented along the z-axis with

an optional perfect electric conductor (PEC) ground plane in the x-y plane. For

simplicity, a delta gap source model is used. Straight feed wires may also be added to the

helix, and are modeled similarly to the helix by the Method of Moments with triangular

basis and pulse testing functions.

The primary validation of the curved wire approach is through a comparison with

MININEC and NEC of the convergence properties of the input impedance of the antenna

versus the number of unknowns. The convergence tests show that significantly fewer

unknowns are needed to accurately predict the input impedance of the helix, particularly

for the normal mode helix. This approach is also useful in the analysis of the axial mode

helix where the current changes significantly around one turn. Because of the varying

current distribution, the improvement of impedance convergence with curved segments is

not as significant for the axial mode helix. However, radiation pattern convergence

improvement is found. Multiple feed structures for the axial mode helix are also

investigated. In general, the many straight wire segments, and thus unknowns, that are

needed to accurately approximate the current around one turn can be greatly reduced by

the using the curved segment method.

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