Thesis (MEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2002. / An efficient radiating strucllire was needed for borehole pulsed radar applications in the 10-100
MHz frequency band. Both resistively loaded and insulated wire antennas were investigated and
an active antenna is proposed as a fmal solution.
The study proceeded from the characterization of the origin of radiation on a conductive dipole
wire antenna when excited with a transient. Different radiation mechanisms were identified when
the antenna was excited with a current or voltage source.
The wire antenna in insulated surroundings was modelled using transmission line theory to
simulate the antenna in the borehole environment. The transmission line model proved to be
useful for investigating conducting and resistively loaded antennas for the dimensions associated
with borehole surveys.
From the modelling results, it became apparent that the asymmetric resistively loaded antenna
might provide the best practical solution. This antenna displays reasonably stable input
impedance and low far-field variations for different theta angles across the desired frequency
band. Different percentage Wu-King resistive profiles were studied to show that a 50% reduction
in the normal Wu-King resistor values will add only a little ringing but have better amplitude
response than the 100% Wu-King loading.
The asymmetric resistively loaded antenna has better sensitivity to receiving transients when
combined with a high impedance source load than when symmetrically loading the antenna. An
active antenna incorporating the asymmetric resistively loaded antenna and a low noise current
feedback front end amplifier was built and measured in an air environment. The results show that
the active antenna has a flat transfer function and reacts as a wide band electric field probe with
better resolution than in the conventional 50 Q loaded case
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2521 |
| Date | 12 1900 |
| Creators | Van Wyk, M. D. |
| Contributors | Palmer, K. D., University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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