K.D. Palmer

Optimized bow-tie antenna for pulsed low-frequency ground-penetrating radar

The bowtie antenna provides a solution to the requirements of a broadband non-dispersive antenna for low frequency pulsed ground penetration radar (GPR) applications. This paper reports on a bowtie antenna numerically optimized with respect to bandwidth and radiation performance into the ground over the 10-100 MHz band. Rounding the edges of the bowtie was found to stabilize the antenna performance by reducing the internal angle dependence of the 1St resonant frequency. Half hemisphere radiation was achieved using a nonabsorptive cylindrical cavity placed over the rounded geometry bowtie. Late time ringing due to reflections was minimized by the use of edge termination resistors connected between the cavity wall and the bowtie. The antenna match to the ground was improved using an optimized low permittivity polypropylene slab. This restored the radiation pattern distortion due to the presence of the ground and stabilized the input impedance for fluctuations in ground parameters. This also permitted the antenna to be wheel mounted at various heights above the ground with minimal influence on the antenna performance.

A guarded cylindrical capacitor for the non-destructive measurement of hard rock core samples

The novel design of a flexible guarded capacitor which conforms to a core samples cylindrical surface is presented. The proposed device allows non-destructive measurements of hard rock samples produced by diamond drilling, and has numerous advantages over previous methods. No material is lost to sample preparation and a detailed characterization of the entire core, including inclusions and transitions between rock types, may be performed. A detailed methodology for the rapid and cost-effective construction of a robust capacitor is given. Guidelines for its operation to achieve repeatable and accurate measurements of the complex dielectric constant of samples of varying homogeneity in the 1–25 MHz frequency range are presented.

A practical 50∶1 bandwidth PCB-LPDA antenna for MeerKAT RFI studies

Design details of a 50:1 bandwidth printed circuit board log periodic dipole array antenna for practical radio frequency interference measurement use are discussed. A feeding tab design optimization is incorporated, which obviates the need for a dummy cable on the opposite board to balance the feed cable structure. Computer simulated and measured realized gain and reflection coefficient results are shown from 50 MHz to 8 GHz. Although the antenna works well over its intended bandwidth of 150 MHz to 7.5 GHz, it can be used down to 70 MHz.

The thin-slot and thin-arm planar spiral antenna operated with and without a ground-plane

Most planar spiral antennas are made as complementary structures, i.e. with equal slot and arm widths. This paper is a study of the properties of the thin-slot and thin-arm spiral where the slot is respectively narrower or greater than the spiral arm, and includes results for free space and in the presence of a ground plane. A numerical study is conducted for a range of arm-to-slot ratios, from 5% to 95% where 50% is the complementary case. The results show that changing the arm-to-slot ratio can be used to vary the input impedance, but that the thin-arm case is of limited practical value. Results are compared to theoretical expectations and independent numerical results. As an introduction, a discussion on the band theory and a physical insight into the power flow near the antenna is presented.

Borehole radar imaging in tactical support of miners working in very narrow stopes

Hitherto, Tx→Rx cross-talk has inhibited the profiling of shorter tactical probe-holes, drilled from stopes at shallow angles below reef. Now that recent component advances have shrunk bistatic radars, raising their close-in response without compromising long range performance, BHR designers are moving towards mapping face & hanging wall fractures in stope automatically at one hundredth the scale of reconnaissance, in part with proximity coupled blast & bolt drillrod guided radars.

Design of a wideband orthomode transducer

The design of an offset ridge quad-ridged waveguide (QRWG) orthomode transducer (OMT) is presented. By offsetting one of the ridges in the direction of propagation, a much wider single mode operating bandwidth can be achieved than that achievable in a traditional symmetric QRWG OMTs. This wide single mode operating bandwidth ensures that no higher order modes are excited at the feeds of the OMT, which in turn eliminates unwanted resonances often observed in traditional QRWG OMTs. The performance of the OMT is similar to that of traditional types, with reflections below −20 dB and cross coupling below −40 dB across most of the desired 1.7∶1 bandwidth.

A Defocused Rotman Lens With Reduced Conjugate Port Coupling

The Roman lens is suitable as an antenna beam-forming network, but performance suffers from high coupling between symmetrically opposed (or conjugate) ports. A practical technique to reduce this coupling is presented and experimentally validated. It is proposed that the curvature of the lens contour is modified to be sub-optimal in terms of phase errors, but is done in such a way that the defocusing spreads the power and leads to reduced conjugate port coupling. The phase errors thus generated are shown to be of the same order as those found in conventional Rotman lenses. This new, defocused lens is compared to the conventional lens showing improved coupling performance without degradation of the phase performance. A lens is implemented in waveguide and measured results agree well with simulations.

Antenna design, simulation and testing at the University of Stellenbosch

The authors summarizes the antenna design, simulation, and testing activities in the Department of Electrical and Electronic Engineering at the University of Stellenbosch, South Africa. The purpose of this work is to train engineers at the Masters and Doctoral levels for careers in telecommunications; to develop technology which may be useful for the South Africa industry; and to conduct good research. The experimental facilities span the spectrum from below 1 MHz to about 20 GHz. The following activities are reviewed here: microstrip antenna analysis and design, wire grid modeling on parallel computers, penetrable body of evolution antennas, and near-field metrology.