Kee Choon Yeong

A Simple Method for Calculating Attenuation in Waveguides

We present a simple technique to compute the attenuation of electromagnetic waves propagating in waveguides with imperfect conducting walls. The propagation constant can be computed by numerically solving the transcendental equations, derived from the surface impedance of the walls and substituting the results into the dispersion relation. Good agreements were found between the results obtained using our method and that using StrattonÌs rigorous equation and Kohler and BayerÌs approximate equation, for both circular and rectangular waveguides, respectively. Index Terms – Attenuation, waveguides, propagation constant, surface impedance

Full Wave Analysis of Normal and Superconducting Microstrip Transmission Lines

We present a full wave analysis to compute the propagation constant of electromagnetic waves traveling in a normal and superconducting microstrip transmission line. The transverse wavenumber in the dielectric substrate is obtained as the root of a set of transcendental equation, derived by matching the tangential fields at the dielectric-conductor and dielectric-air interfaces. The propagation constant can then be obtained by substituting the transverse wavenumber into the dispersion relation. For normal microstrip lines, we found good agreement between our results and those obtained using the quasi-static methods. As compared to some of the available techniques used to calculate loss in superconducting microstrip lines, such as MatickÌs and Yassin-WithingtonÌs method, our method gives higher loss especially in the regime of millimeter and submillimeter wavelengths. Since our method takes into account the superposition of TE and TM modes, we attribute the differences as due to the fringing fieldÌs effect and the existence of the longitudinal field components in our formulation. Index Terms – propagation constant, superconducting microstrip, dispersion relation, submillimeter wavelengths, fringing fieldÌs effect.

Propagation in Lossy Rectangular Waveguides

)In millimeter and submillimeter radio astronomy, waveguide heterodyne receivers are often used in signal mixing. Wave guiding structures such as circular and rectangular waveguides are widely used in such receiver systems to direct and couple extraterrestrial signals at millimeter and submillimeter wavelengths to a mixer circuit (Carter et al., 2004; Boifot et al., 1990; Withington et al., 2003). To illustrate in detail the applications of waveguides in receiver systems, a functional block diagram of a typical heterodyne receiver in radio telescopes is shown in Fig. 1 (Chattopadhyay et al., 2002). The electromagnetic signal (RF signal) from the antenna is directed down to the front end of the receiver system via mirrors and beam waveguides (Paine et al., 1994). At the front end of the receiver system, such as the sideband separating receiver designed for the ALMA band 7 cartridge (Vassilev and Belitsky, 2001a; Vassilev and Belitsky, 2001b; Vassilev et al., 2004), the RF signal is channelled from the aperture of the horn through a circular and subsequently a rectangular waveguide, before being coupled to the mixer. In the mixer circuit, a local oscillator (LO) signal which is generally of lower frequency is then mixed with the RF signal, to down convert the RF signal to a lower intermediate frequency (IF) signal. Here, a superconductor-insulator-superconductor (SIS) heterodyne mixer is commonly implemented for the process of down conversion. At the back end of the system, the IF signal goes through multiple stages of amplification and is, eventually, fed to a data analysis system such as an acousto-optic spectrometer. The data analysis system will then be able to perform Fourier transformation and record spectral information about the input signal. The front-end receiver noise temperature TR is determined by a number of factors. These include the mixer noise temperature TM, the conversion loss CLoss, the noise temperature of the first IF amplifier TIF, and the coupling efficiency between the IF port of the junction and the input port of the first IF amplifier IF  . A comparison of the performance of different SIS waveguide receivers is listed in Table 1 (Walker et al., 1992). It can be seen that the value of TR for the 230 GHz system is a factor of 3 to 4 less than that achieved with the 492 GHz system. The decrease in system performance at 492 GHz is due to the increase of CLoss and TM by a factor of approximately 3.

Prioritising Redundant Network Component for HOWBAN Survivability Using FMEA

Deploying redundant component is the ubiquitous approach to improve the reliability and survivability of a hybrid optical wireless broadband access network (HOWBAN). Much work has been done to study the cost and impact of deploying redundant component in the network but no formal tools have been used to enable the evaluation and decision to prioritise the deployment of redundant facilities in the network. In this paper we show how FMEA (Failure Mode Effect and Analysis) technique can be adapted to identify the critical segment in the network and prioritise the redundant component to be deployed to ensure network survivability. Our result showed that priority must be given to redundancy to mitigate grid power outage particularly in less developed countries which is poised for rapid expansion in broadband services.

Attenuation in Superconducting Rectangular Waveguides

Abstract We present an accurate analysis on the attenuation of waves, propagating in rectangular waveguides with superconducting walls. The wavenumbers kx and ky in the x and y directions, respectively, are first obtained as roots of a set of transcendental equations developed by matching the tangential fields at the surface of the wall with the electrical properties of the wall material. The complex conductivity of the superconducting waveguide is obtained from the extended Mattis–Bardeen theory. The propagation constant kz is found by substituting the values of kx and ky into the dispersion relation. We have computed and compared the loss in the waveguides below and above the critical temperature. At frequencies above the cutoff frequency fc but below the gap frequency fg, the loss in the superconducting waveguide is significantly lower than that in a normal conducting waveguide. Above the gap frequency, however, the result indicates that the attenuation in the waveguide below the critical temperature is higher than that at room temperature. We attribute the higher loss as due to the higher surface resistance and field penetration for superconducting waveguides operating above the gap frequency.

Attenuation of the dominant mode in a lossy rectangular waveguide

We present a fundamental and accurate approach to compute the attenuation of electromagnetic waves propagating in a rectangular waveguide with imperfect conducting walls. The wavenumbers in the x and y directions, i.e. kx and ky, respectively, are first computed as roots of the transcendental equations developed by matching the tangential fields with the constitutive properties of the wall material at the boundary of the waveguide. The propagation constant kz is solved by substituting kx and ky into the dispersion relation. We have compared the attenuation constant of the dominant TE10 mode computed using our technique with that using the approximate perturbation method. The results from the two methods agree very well at a reasonable range of frequencies above the cutoff. The two curves, however, deviate below cutoff and at millimeter wave frequencies. We attribute the discrepancies to the dispersive effect and the presence of the longitudinal electric field in a lossy waveguide. The presence of the longitudinal field is not accounted for in the perturbation method.

Impact of Backup Power in Optimizing Deployment Cost of Hybrid Optical Wireless Broadband Access Network (HOWBAN) with Survivability

The dependence on broadband to enhance the economy and environment sustainability is imposing immense urgency to provide ubiquitous wireless broadband access which is a key enabler to a greater spread of society. One of the key barriers to the network connectivity and survivability identified is the lack of or unreliable infrastructure. In our previous studies we have identified that the most unreliable component in a hybrid optical wireless broadband access network infrastructure is the power grid in remote areas and least developed countries. In this paper we extend our studies to evaluate the effect of the costs of backup power on sustaining survivability at the front end of two types of network configurations in a hybrid optical wireless broadband access network. The configurations investigated are full survivability and alternative path survivability networks. Our results show that the provision of backup power to all the nodes in a network configuration with one alternative path is able to reduce the cost of deployment compared to that of a full survivability network configuration without significantly affecting the network survivability.

Innovations to improve wireless mesh network performance: A survey

The close correlation between the Gross Domestic Product (GDP) of a country and the availability of the broadband services has intensified the growth of the broadband access in all the continents in the world. This has put further pressure on the frequency spectrum available to support the rising demand. Optical fiber broadband technology which has the capacity to carry high volume of traffic at extremely high speed is the obvious choice to fulfill the demand. However its implementation cost and time have inhibited the widespread installation especially at the front end of the broadband access network. Wireless mesh network with its relatively short set up time and comparatively lower cost has been identified as a suitable technology to support the fiber backbone in providing front end connections to the mobile clients. The number of clients that can be served by the wireless segment of this hybrid communication system is inundated by spectrum limitation, unpredictable weather conditions, radio interferences and transmission power regulated by the authority. Many innovations have been used to extend the limited resources so as to maximise the number of mobile clients that can be supported by the wireless mesh network. This paper surveys the innovations with respect to radio channel, network topology, routing strategy and channel assignment that have been proposed to improve the performance of the wireless mesh network and suggest some areas for further work.

Temperature tuning of band edge resonant modes in one-dimensional photonic crystals

The transmission characteristics of Si-based 1D photonic crystals was investigated using transfer matrix method. As the temperature and wavelength dependence of Si refractive index induces band edge resonant modes that are temperature tunable, the average wavelength shift per Kelvin of the resonant mode with the largest group index was determined in the vicinity of the band edge. A general linear chirp structure consisting of a constant and chirp component was implemented. Using the simulated annealing algorithm, the structural parameters were optimized and it was found that the average wavelength shift per Kelvin of the band edge resonant mode can be enhanced by 28.9% with high transmission. It was also found that spatial chirping does not affect the wavelength shift. The enhancement was achieved by a uniform increase of PC layer widths. Tunable band edge resonant modes may be exploited in designing optical thermal sensing devices.

Full wave analysis of a superconducting stripline with large width-to-separation ratio

This paper presents a full-wave analysis on the wave propagating in a superconducting stripline with a large width-to-separation ratio. A set of transcendental equation is derived, by matching the ratio of the tangential electric and magnetic fields, respectively, with the surface impedance of the strip. The propagation constant can be obtained by substituting the complex conductivity of the superconducting strip into the equation and numerically solving for the root. We have computed the attenuation constant and phase velocity using our method and compared with the analytical equation formulated by Matick using the general transmission line model, which is applied by Kautz in a superconducting stripline. The results for both methods agree exceedingly well.