Browsing by Author "Musyoki, Stephen"

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Analysis of Electromagnetic Field Radiation from a Rectangular Cavity-Backed Slot Antenna Using ADI-FDTD Method
(2012) Musyoki, Stephen; Ouma, Heywood A.; K’Onditi, Dominic B.O.; Nyaory, George M.
In this paper, a rectangular Cavity Backed Slot Antenna (CBSA) Model excited by a probe is investigated. The analysis is carried out using the Alternating Direction Implicit - Finite Difference Time Domain (ADI-FDTD) Method which is applied to investigate its characteristics in terms of radiation patterns and power. This is because the method is capable of providing a more accurate definition of the electromagnetic fields within the rectangular apertures, while eliminating the Courant-Friedrich-Levy (CFL) stability condition which is present in the regular Finite Difference Time Domain (FDTD) method. A cavity-backed slot antenna structure with dimensions of 14cm×22cm×30cm is analyzed with the slot and aperture measurements done at 3GHz. Results showing current distribution on the material surrounding the apertures are presented and a discussion on the physical aspects of the aperture radiation phenomenon is also presented.
Auto-resonant eniotron oscillator using a magnetron type cavity
(1991) Musyoki, Stephen; Yokoo, Kuniyoshi; Sato, Nobuyuki; Ono, Shoichi
In an ideal auto-resonant eniotron oscillator (ARPO), all the electrons are able to give almost all their kinetic energy to an electromagnetic wave, resulting in a conversion efficiency of nearly 100%. However, the electric field strength required to drive a highly efficient interaction is difficult to achieve in an ordinary rectangular or circular waveguide cavity because of the low interaction coupling between electrons and the electromagnetic wave in the cavity. In this aper, we describe our investigation of an ARPO with a magnetron type cavity in lace of that above. In this cavity, the coupling becomes sufficiently strong to confirm experimentally the romising features of an ARPO. Additionally, the experiments should be useful for developing highly efficient medium-power oscillators in the microwave region. Here we resent our rocedure for designing such an experimental tube and our computer simulation results for its operational characteristics
Auto-resonant peniotron amplifier with a down tapered DC magnetic field
(Taylor & Francis, 1990-02) Musyoki, Stephen; Yokoo, Kuniyoshi; Sato, Nabuyi; Ono, Shoichi
High efficiency operation of an auto-resonant peniotron amplifier operating with a down tapered DC magnetic field is discussed and the basic design concepts for a 200 GHz experimental prototype tube are described.
Bit Error Rate Performance Analysis of a Wideband Code Division Multiple Access System Model in an Indoor Environment using Convolution Coding over Additive White Gaussian Channel
(Volume 3, Issue 5 October 2014, 2014) Musyoki, Stephen; Kibet, Philip; Ombongi, Filbert
The research entails the development of a WCDMA system model in an indoor environment that transmits data over an Additive White Gaussian Noise channel. In this environment the data rate is taken as 2Mbps. The performance of this model is enhanced by employing convolution coding scheme which reduced the error rate encountered in the system. The results show that the performance of the system improves when convolution coding is implemented. The performance of QPSK is found to be better that that of 16-QAM which means that QPSK is an efficient modulation scheme but its throughput is less that of 16-QAM. Therefore, there must be a trade-off between the modulation format to be applied in a given system and the error rate generates at the receiver by that format. The convolution coding improves the power efficiency of the system when it is incorporated into the system.
Design of a high power, 10 GHz auto-resonant peniotron amplifier
(1992) Musyoki, Stephen; Sakamoto, Keishi; Watanabe, Akihiko
The autoresonant peniotron amplifier is a suitable source of high power RF radiation because of its high gain, high power, high frequency and high efficiency operation features. In this report we present our simulation results of a 10 GHz, 2.2 GW autoresonant peniotron amplifier with an electron energy conversion efficiency of 72.5 % and a gain of about 58 dB
Design of a high power, 2.75 GHz relativistic peniotron oscillator
(1992-10) Musyoki, Stephen; Sakamoto, Keishi; Watanabe, Akihiko
In the peniotron oscillator, the kinetic energy of relativistic electrons can be efficiently converted to an electromagnetic wave. This feature makes the peniotron a very attractive source of high power microwaves, which is required in heating of fusion plasma in tokamaks and high gradient particle accelerators. In this report we present the design of a 2.75 GHz relativistic peniotron oscillator which is capable of generating a microwave radiation of 30 megawatt with an efficiency of about 60 %. The experimental test results of the designed cavity are also presented
The Effects of a Human Hand on a Wireless Mouse Antenna
(2015) Musyoki, Stephen; Konditi, B. O.; Njoroge, Dominic; Rainsford, K
In this paper, Finite Difference Time Domain (FDTD) method is used to analyze the effect of a human hand on a wireless mouse antenna. The need for this analysis is based on the fact that human tissues are dielectric in nature and they can, therefore, be treated as electromagnetic absorbers. In the first part, hand effect on reflection coefficient, input resistance, bandwidth and radiation efficiency are studied. In the second part, the variation of radiation efficiency with the position of the hand from the antenna feed point is estimated. The Method of Moments is used to validate the results.
Emittance measurement of high-brightness microbeams
(Japanese Journal of Applied Physics, 1994) Musyoki, Stephen; Ishizuka, Hiroshi; Nakahara, Yuriko; Kawasaki, Sunao; Shimizu, Hiroshi; Watanabe, Akihiko; Shiho, Makoto
Arrays of microtriodes have recently become available due to the development of microfabricated field-emission electron sources. Computer simulation has shown that the brightness of beams emitted by them is significantly higher than that of the common microbeams, and possible application of the accelerated beam to free electron lasers has been discussed. Experimentation on beam generation has started, but methods for diagnosing the beam have not yet been established. Difficulty is predicted, because of the high brightness, in applying the conventional methods of emittance measurement. In this paper we propose a new method that determines the emittance without using apertures. The cross section of a converging beam is elongated by a quadrupole lens, and parameters of the emittance ellipse are obtained from the beam size on a screen when changing either the strength or the axial position of the quadrupole lens.
Improving the Bandwidth of a Circular Microstrip Patch Antenna through Shape Modification
(2014) Langat, Benard Kipkorir; Langat, Kibet P; Musyoki, Stephen
An antenna is used in a communication system to radiate or receive radio waves. The most desired antenna is one which is small in size, light in weight, cheap and can easily fit to the surface attached. All these features are inherently possessed by microstrip patch antennas. However, no antenna is perfect. Microstrip patch antennas like all other types of antennas do have their associated disadvantages. One of the major disadvantages of this type of antenna is narrow bandwidth. In this study, the narrow bandwidth of a circular microstrip patch antenna was improved through shape modification. The bandwidth of the antenna was optimized by adding some parts to and removing some parts from the initial circular patch. Although the main aim was to improve the bandwidth, it had to be ensured that other important parameters of the antenna such as radiation efficiency, impedance matching and gain are not degraded in the process. Microstrip line feeding technique was utilized in the design. HFSS 13.0 full wave simulator based on Finite Element Method (FEM) was used to simulate the antenna. Simulation results were then presented. Simulation results of return loss showed that the antenna achieved a bandwidth of 13.16% at 1.755 GHz and a very broad bandwidth from 3.315-20 GHz. This is a great improvement when compared with the bandwidth of a conventional microstrip patch antenna of less than 3%. Simulation results of Z11 parameters showed that the antenna achieved satisfactory impedance matching with a 50Ω transmission line at 1.755 GHz, 3.65 GHz, 4.668 GHz and 7.031 GHz and with a 75Ω transmission line at 9.366 GHz, 14.634 GHz, 16.461 GHz, 17.304 GHz and 18.265 GHz. The antenna recorded satisfactory gains and excellent radiation efficiencies in all these frequencies. The antenna can be used for a wide range of applications because of its wider bandwidth.
A new method of emittance measurement for electron beams from the Micro-emitter
(1994) Ishizuka, Hiroshi; Nakahara, Yuriko; Kawasaki, Sunao; Musyoki, Stephen; Shimizu, Hiroshi; Watanabe, Akihiko; Shiho, Makoto
Recently a new type of cathode called Micor-emitter has been put in progress. This cathode is a microfabricated field emitter having the characteristics of very low emittance and high brightness. We cannot measure the emittance of the cathode with conventional methods like the pepper-pot method. The reasons are: (1) the angle between the electron orbit and the axis is very small and (2) we cannot focus the electron beam in the vacuum or on the surface of the material since the current density of the cathode is extremely high. For the emittance measurement for such low emittance and high brightness cathode, we need to expand the beam, and measure the beam's cross section without any slits or apertures. We studied and proposed a new emittance measurement method for the Micro-emitter.
Performance Analysis of a WCDMA System Model in a Low Mobility and Indoor Environment with Channel Coding over Additive White Gaussian Noise Channel
(2015) Musyoki, Stephen; Ombongi, Filbert; Kibet, Philip L.
This paper has analyzed the performance a Wireless Division Multiple Access (WCDMA) system model at a data rate of 384kbps and 2Mbps over an Additive White Gaussian Noise (AWGN) channel. The signal was modulated by Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) with modulation order, M=16. The performance of the system was enhanced by implementing convolution coding scheme. This study was important as it formed a basis through which the performance analysis can be extended to Long Term Evolution (LTE) networks which have data rates starting from 1Mbps to as high as 100Mbps.The performance of the WCDMA at these data rates was seen to improve when convolutional coding scheme was implemented. Since the Shannon capacity formula depends on the BER of a system then this improvement means an additional capacity in the channel and this can accommodate more users in the channel. The results have further shown that the choice of a modulation technique depending on the throughput required affects the BER performance of the system. Therefore, there must be a trade-off between the throughput required, the modulation format to be used and the pulse shaping filter parameters.
Radar Waveform Generation and Optimization based on Rossler Chaotic System
(2012) Musyoki, Stephen; Nyakoe, George; Obadha, Joseph
The concept of Multiple-Input Multiple-Output (MIMO) radars has drawn considerable attention recently. Unlike the traditional Single-Input Multiple-Output (SIMO) radar which emits coherent waveforms to form a focused beam, the MIMO radar can transmit orthogonal (incoherent) waveforms. These waveforms can be used to increase the system spatial resolution. The challenge is on how to generate the large set of incoherent waveforms. Contemporary research has focused on using chaotic systems to generate these waveforms. With Chaotic waveforms obtained from a dynamical system, different radar waveforms can be generated from a single dynamical system; one only needs to change the control parameters and the initial conditions of the system. This scheme for radar waveform generation reduces the need for a comprehensive library of waveforms in a radar system and generates waveforms with good properties for both secure communications and high spatial resolution. This paper proposes the use of Rossler system– a type of a dynamical system to generate radar waveforms. Through Matlab/Simulink Simulations, it is shown that the Rossler waveforms, which are characterized by control variables and initial conditions are comparable to the Linear Frequency Modulated (LFM) waveforms, the most commonly used class of radar waveforms in terms of the ambiguity diagram and the frequency components and yet versatile enough to generate a large number of independent waveforms. An ambiguity diagram is a plot of an ambiguity function of a transmitted waveform and is a metric that characterizes the compromise between range and Doppler resolutions. It is a major tool for analyzing and studying radar waveforms. Impulsive synchronization theory is used to develop the ambiguity diagram.
Simulation of the influence of secondary electrons on the performance of potential depressed collectors for TWTs
(2000) Musyoki, Stephen; Takahashi, M; Uchikawa, T
Significant differences have been found between measured and simulated data for collectors designed with codes which disregard the influence of secondary electrons. To solve this problem, various codes have been developed. These codes assume that secondary electrons are only emitted in a single direction and ignore their spatial distribution. We have developed a code in which secondary electron yield is calculated from an empirical formula and the emitted secondary electrons are assumed to have cosine spatial distribution. To represent this distribution, for each primary beam point of impact, the emitted secondary beam is split into 3 beams; one beam normal to the impact surface and the other two at an angle to the normal. The normal beam carries half of the current and the rest of the current is divided equally between the side beams. The code uses a two dimensional, axis symmetric Finite Element Method (FEM) for modeling. FEM has been chosen because of its ability to model accurately arbitrary shaped structures. We designed a ku-band (14.25 GHz), 2-stage TWT by using the developed code.