Browsing by Author "Downing, B J"

Now showing 1 - 18 of 18
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    Broadband pin diode switching and phase shifting
    (1986) Hirson, Desmond; Downing, B J
    Investigation into the limitations on the bandwidth of PIN diode switches in their different configurations is performed. The Single Pole Double Throw switch constructed with shunt mounted PIN diodes is shown to be bandlimited due to the line lengths involved in the operation. Series mounted diode switches have a wider bandwidth but the diodes cannot perform as power devices due to their physical construction. Radiation due to discontinuities were found to degrade the isolation but this was overcome by an evanescent waveguide structure. The software program TOUCHSTONE (TM)/RF by EEsof is used extensively as a basic development tool. The characterisation of the PIN diodes which was used by the software, was carried out. A bias network had to be developed to allow for the biasing of the diodes before any test boards could be fabricated. A literature survey was done on the different types of phase shifters available. A circuit which enables shifts in 45 degree increments was developed. The system consists of Reflection Phase Shifters which are made up of Quadrature Couplers that have different terminations switched in on two of their ports, a Wilkinson power splitter and a power combiner in the form of a 3 dB Quadrature Coupler. It was found that this configuration works well in theory but is too demanding on the practical components to allow for low phase shift errors to occur.
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    Broadband, low-noise and power microwave amplifiers
    (1986) Hall, Andrew D; Downing, B J
    The design of Broadband, Low-Noise and Power Microwave Amplifiers using microstrip softboard technology is investigated. The software program TOUCHSTONE (TM) by EEsof is used extensively as a basic design tool. The characterisation of the GaAs Field Effect Transistors, used for the amplifiers, is carried out. These characterisations are then used by the program in its circuit analysis. A determination of the validity of using the manufacturer's data, for the designs, is determined by comparing it to the measured data. Source-Pull and Load-Pull measurements were performed for the Power GaAs FET characterisation. The noise-parameter device characterisation is carried out in a similar way to that for Load-Pull data. Each amplifier required final tuning adjustments in order to peak the performances. The Broadband Maximum Gain Amplifier had a 10 ± 1.5dB gain over a bandwidth from 2- to 6-GHz. The Low-Noise amplifier achieved 5dB Noise-Figure and 5.4 ± 1. 4dB gain over the 2- to 6-GHz band. The Power amplifier Output Power was 390mW over the 3.7- to 4.2-GHz band. Techniques of broadband matching are investigated, with Double-Stub matching producing the widest bandwidth. A literature survey is presented on aspects of broadband microwave amplifiers, as well as a survey on Computer-aided-design at microwave frequencies and techniques of Large-Signal Transistor characterisation.
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    Development of an automatic system to monitor the performance of a dense medium (mineral) separation process
    (1989) Alberts, P A; Downing, B J
    Dense Medium Separation (DMS) is a process used extensively in the minerals processing industry to separate dense from less dense material in a dense fluid. It may be considered to be a simple “sink-float” separation process. DMS is used on a large scale in South African coal, iron ore and diamond operations. There are, however, no commercially available systems that can determine the separation efficiency of a DMS process on-line. This presents severe problems to those operating DMS processes. The present study attempts to provide a measurement technique for on-line application.
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    The development of an ore pass level indicator
    (1990) Rix, Gary M; Downing, B J
    All open cast mines have a limited economic life which is usually dictated ·by the so called economic depth, where the costs of hauling the ore via large diesel trucks becomes prohibitive. A mine that is about to reach its .economic depth is the Finsch mine. In order to prolong its life mining will now take place underground [1.1]. Ore passes, vertical storage shafts, will form the backbone of the new mine. Safe and efficient mining calls for the depth of ore in these ore passes to be monitored accurately [1.2]. This thesis is concerned with the design and construction of such an instrument. The ore pass could in theory propagate a number of waveguide modes. However, with the particular frequency chosen for the instrument reported here, the mode of propagation was the same mode as free-space. A FMCW radar system was designed for this specific application but had the advantage that testing could take place in freespace. Two prototype systems were constructed and tested down the ore pass which proved the instrument concept satisfactorily. The instrument has definite commercial viability.
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    Electronically tuned 23 GHz Gunn oscillators for a microwave datalink
    (1988) Kratzenstein, L; Downing, B J
    A market has been identified for 23 GHz, short-haul, low-capacity, digital radio. The dissertation presents the development of the varactor controlled Gunn oscillators that constitute the crystal locked microwave sources of the radio. An accurate description of a design procedure for Gunn oscillators at 23 GHz is presented. With reference to advanced modulation methods which require constant modulation indices, a method of linearising the voltage/frequency characteristic of the varactor controlled Gunn oscillator is described, which allows direct modulation of the source at 23 GHz. Due to the wide operating temperature of the radio a technique to temperature compensate the oscillator is presented. The dissertation ends with an investigation how the semiconductor device's spread affects the oscillator characteristics and an evaluation of the noise performance of the Gunn oscillator.
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    Gallium arsenide field effect transistors microstrip integrated circuit dielectric resonator oscillators
    (1988) Crouch, David Andrew; Downing, B J
    This thesis is concerned with Gallium Arsenide Metal Semiconductor Field Effect Transistor Microstrip Integrated Circuit Dielectric Resonator Oscillators (GaAs MESFET MIC DROs) - the different types, their design and their performance compared to other high Q factor (ie narrowband) microwave oscillators. The thesis has three major objectives. The first is to collate the information required to build microwave DROs. The second is to present the practical results obtained from Dielectric Resonator Bandreject and Bandpass filters (DR BRFs and DR BPFs). The last is to present and compare results from a DR stabilised microstrip oscillator and three types of series feedback DROs. Narrowband oscillators are usually evaluated in terms of their frequency stability, reliability, size, cost, efficiency and output power characteristics. In terms of these parameters DROs outperform Gunn cavity oscillators and are only bettered by crystal locked sources in terms of frequency temperature stability and long-term stability. The components of a GaAs MESFET MIC DRO possess ideal properties for the construction of a narrowband source with the exception of the long term stability of the GaAs MESFET. GaAs MESFET•DROs have the best published DRO results for efficiency, output power, power temperature stability and external Q factor. Basic oscillator theory derived by Kurokawa can be applied to both negative resistance and feedback oscillators. Impedance locus, device-line and operating point concepts provide a convenient framework for understanding hysteresis in microwave oscillators. The work by Kurokawa can also be translated into the S-parameter domain which has proved convenient for the design of microwave oscillators.
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    An investigation of minerals using microwave measurement of complex permittivity
    (1990) Pendock, Graeme; Downing, B J
    Microwave measurement techniques have found many industrial and commercial applications. This measurement potential of microwaves, together with observations that different minerals show different microwave heating characteristics, suggests the possibility of applying microwave techniques to various forms of mineral analysis. Simple, low cost, on-line mineral analysis techniques are of interest to the mining industry. The objectives of this research project were to cover the background theory of microwave interaction with minerals and to investigate different microwave measurement techniques that could possibly be applied to mineral measurement. Measurements were then to be performed on selected minerals in order to observe any differences between them. Finally, to comment on the feasibility of using microwave measurement techniques for the differentiation, identification and analysis of minerals.
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    Low loss microwave power combiners/dividers
    (1986) Waardenburg, Thijs; Downing, B J
    As many current applications require microwave solid state sources with output powers greater than that available from a single device a need for combining the power from these devices is required. N-way combiners/dividers may be used to achieve this. For high output powers these combiners/dividers must have a high combining efficiency. This dissertation describes various power combining techniques and essentially a power combiner/divider that is both planar and low loss is required. The planar structure is a requirement if efficient heat dissipation is to be achieved. Cylindrical resonant cavity structures give very high combining efficiencies, however, they are non planar and have narrow bandwidths. N-way planar combiners/dividers fabricated on microstrip provide the desired planar structure but due to the characteristics of microstrip are lossy. As a culmination of this work a low loss 8-way planar stripline combiner/divider was constructed that gave a peak combining efficiency of 94 percent which approaches the combining efficiency of 98 percent that was obtained with the cylindrical resonant cavity combiner. The former offers its broader bandwidth and planar structure as advantages.
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    The microwave digestion of solids
    (1990) Broomhall, G A; Downing, B J
    Microwave ovens have been available for domestic cooking since 1956. This method of heating is very efficient compared to conventional cooking methods. As a result many applications have arisen particularly in the field of analytical chemistry. Most routine analysis of biological and chemical samples by atomic absorption spectroscopy (AAS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) involves the time consuming operation of digesting the samples in acid on a hot plate. However, much time saving has been demonstrated by the use of microwave digestion, and successful digestions have been achieved in 10% of the time required for traditional methods. The first digestions using microwave heating were performed in open vessels. However many problems were identified.
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    A microwave doppler technique for vehicle speed determination
    (1987) Martin, Neil Cameron; Downing, B J
    Conventional Microwave Doppler Speed Measurement systems have long been considered unusable in many situations. In this dissertation a novel technique for speed measurement is presented which eliminates some of the problems associated with conventional systems, in particular the problem of vehicle identification. It is shown how a 'capture area' may be defined. Only the speeds of vehicles within the capture area are measured. A prototype speed measuring instrument was designed and constructed. The instrument was shown to work as predicted and the the notion of the 'capture area' was proved.
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    Microwave sterilization of breast milk infected with the AIDS virus
    (1990) Michelson, S C; Downing, B J
    The AIDS virus has been identified in breast milk which is donated by expectant and recently delivered mothers in maternity hospitals). In maternity hospitals, where the facilities exist for the treatment of pre-term infants, breast milk is collected from mothers premature even those who have not delivered premature babies) and it is used in its raw state to feed the children. The possibility of some of these mothers being infected with the AIDS virus is high in Third World countries. The premature infants have to be fed with breast milk as they are at risk of being infected if fed with commercial milk formulas. Commercially available milk sterilizers are available in Europe but they are very costly.
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    NeXtRAD antenna design: X-Band dual polarised conical horn antenna
    (2016) Cheng, Po-Kai; Murgu, Alexandru; Downing, B J; Inggs, Michael
    The purpose of the dissertation is to investigate a design and implementation of a dual polarised X-band antenna. The desired specifications include a centre frequency of 8.5 GHzand a 10_ azimuth-half power beam width in both vertical and horizontal polarisations. Other requirements include peak power handling of 400 W over a 50 MHz instantaneous bandwidth and the facility to be mounted on a standard tripod. The dissertation begins by introducing NeXtRAD, a radar project developed jointly by UCL and UCT, outlines the objective of the study and lists all the requirements to be achieved. After considering several types of antenna, horn antenna has been the most practical when trying to meet project requirements. Two of the horn antennas examined here have been the square and conical horn antennas using square and circular waveguides respectively. Each of these waveguides support dual polarisation as they provide a symmetrical structure in two orthogonal planes of polarisation. The square waveguide is flared into a square horn whereas the circular waveguide is flared into a conical horn to increase the gain and reduce the half-power beamwidth to the required angle.
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    On-line moisture measurement of rocks, using microwave techniques
    (1989) Celliers, Scarre; Downing, B J
    On-line measurement of the moisture content of process streams in a mineral processing plant is vital for their efficient operation. Although many industrial moisture meters are available, most of these were designed to determine the moisture content of low density, fine grained, uniformly surfaced and low resistance materials. Consequently these meters operate poorly on mineral treatment plants as the measured material, rock, is dense, uneven and consists of mixtures of various minerals. A literature survey revealed that of the many methods available, only three techniques could be used for on-line moisture determination in mineral processing operations.These were microwave attenuation, phase and frequency techniques.
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    Online microwave measurement of complex dielectric constant
    (1990) Mercer, Sean R; Downing, B J
    This dissertation examines the problem of on-line measurement of complex dielectric constant for the purpose of dielectric discrimination or product evaluation using microwave techniques. Various methods of signal/sample interaction were studied and consideration was given to the problem of sorting irregularly shaped discrete samples. The use of microwave transmission and reflection measurements was evaluated. The signal reflection methods were deemed to be best suited to applications with constant geometry feed presentation ( ie. a continuous, homogeneous product stream with little variation in surface geometry).
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    Ore level detector
    (1990) Broek, Michael; Downing, B J
    It was the objective of this project to design and build a transmitter/receiver unit that indicates when the ore level in a vertical ore chute falls below the level of the signal path. The design was commissioned by De Beers Consolidated Mines to be implemented in South Africa's largest diamond producing mine, the Finsch Mine at Limeacres in the north-western Cape. The system requirements laid out'by de Beers stressed that the unit is to be used as a backup system, and thus its absolute reliability was of highest importance.
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    Ore sorting using microwave irradiation
    (1989) Booi, Mlamli; Downing, B J
    Automatic ore sorting machines are in common use in the mining industry. These machines generally separate valuable mineral-bearing rocks from waste rocks. At the Premier Diamond Mine in South Africa kimberlite, a diamond-bearing rock is separated from gabbro which is a waste rock. Work had been conducted previously in the Department of Electrical Engineering at the University of Cape Town to find a viable method for discriminating between gabbro and kimberlite. A technique using microwave irradiation attenuation was successful when using parallel-sided smooth-surfaced rocks. This technique used linearly polarized square antennas at 35GHz. Problems were experienced, however, with irregu1arly shaped rock s. The aim of the present study was therefore to deve1op a technique which will sort irregularly shaped rocks and eliminate the problems associated with them.
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    Rock differentiation using microwave irradiation
    (1987) Mercer, Sean R; Downing, B J
    This project arose as a result of inefficiencies in the diamond recovery process at Premier Mine. A considerable amount of barren waste rock, gabbro, is mined along with the diamond bearing kimberlite. No automated method exists for separating the kimberlite from the waste rock and a device was required to effect ore sorting on a rock by rock basis. Experimentation with a microwave oven indicated that samples of kimberlite were more attenuative than samples of gabbro. The possibility of using microwave heating for rock differentiation was investigated but was impractical to implement. A study of low power microwave attenuation and reflection measurements was undertaken. Reflection measurements were found to be impractical due to the similar amounts of reflected signal from the different rock types. Microwave signal attenuation through rock samples was studied over a broad frequency spectrum. A detectable difference in signal attenuation was found through the gabbro and kimberlite. The difference in signal attenuation increased with increasing frequency. Different techniques to implement signal attenuation measurements through rock samples were investigated. The passing of rock samples through waveguide structures was found to be impractical in this application. Microwave signal attenuation measurements were successful when rock samples were placed between a transmitting and a receiving antenna. Equipment was designed and constructed with an operating frequency of 35GHz chosen due to the small antenna aperture area and the large attenuation difference at this frequency. Static measurements with this equipment revealed the problems with signal scattering and reflection from some irregularly shaped samples of low loss gabbro. The importance of these phenomenon could only be gauged from dynamic measurements. Dynamic measurements were performed using a laboratory test system with a conveyor belt capable of moving at speeds of up to 5 m/s. It was found that 93% of the kimberlite could be correctly detected whilst rejecting 67% of the gabbro. The system functioned satisfactorily and led to the filing of several patents.
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    An ultra high resolution FMCW radar
    (1992) Bas, Alon; Downing, B J
    There is a great need for real-time non-intrusive measurements in industry. A short-range radar system can be used to make these measurements. A standard requirement for these type of applications is high resolution. This is a standard problem in radar. Using classical signal processing techniques, the range resolution is proportional to the bandwidth of the transmitted signal. This poses a serious problem in radar as very large bandwidths are required - typically lSOGHz for 1 mm range resolution. Alternative techniques have been sought which do not rely on large transmitted bandwidths, but which rely on large signal-to-noise ratio (SNR). Such techniques exist in modem spectral analysis eg. auto-regressive techniques. These techniques model the data. In other words, they assume a priori information. Linear frequency-modulated continuous-wave (FMCW) radar was utilized, since a pulsed radar would require very precise time measurements due to the short range (a few ns). The FMCW radar would have to be very linear for the modelling process to work properly. The frequency domain measurement of the received system data would then be proportional to range. An FMCW radar system was built and tested. The modem signal processing techniques were found to work well when injected with sinusoidal signals from signal generators. The hardware was also found to perform satisfactorily. However, amplitude modulation was observed in the mixing process and subsequently, the modelling process did not perform satisfactorily when interfaced to the hardware. Due to the amplitude modulation problem, two closely-spaced targets disrupted the high resolution properties of the modelling process. Nevertheless, a single target could be resolved within a resolution bin of better than 1 cm. A solution is proposed in chapter eight, however, it is out of the scope of this thesis.