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Điện thoại di động vô tuyến điện - Tuyên truyền Channel P2

Fundamentals of VHF and UHF Propagation Having established the suitability of the VHF and UHF bands for mobile communications and the need to characterise the radio channel, we can now develop some fundamental relationships between the transmitted and received power, distance (range) and carrier frequency. We begin with a few relevant de®nitions. At frequencies below 1 GHz, antennas normally consist of a wire or wires of a suitable length coupled to the transmitter via a transmission line. At these frequencies it is relatively easy to design an assembly of wire radiators which form an array, in order to beam the. | The Mobile Radio Propagation Channel. Second Edition. J. D. Parsons Copyright 2000 John Wiley Sons Ltd Print ISBN 0-471-98857-X Online ISBN 0-470-84152-4 Chapter 2 Fundamentals of VHF and UHF Propagation 2.1 INTRODUCTION Having established the suitability of the VHF and UHF bands for mobile communications and the need to characterise the radio channel we can now develop some fundamental relationships between the transmitted and received power distance range and carrier frequency. We begin with a few relevant definitions. At frequencies below 1 GHz antennas normally consist of a wire or wires of a suitable length coupled to the transmitter via a transmission line. At these frequencies it is relatively easy to design an assembly of wire radiators which form an array in order to beam the radiation in a particular direction. For distances large in comparison with the wavelength and the dimensions of the array the field strength in free space decreases with an increase in distance and a plot of the field strength as a function of spatial angle is known as the radiation pattern of the antenna. Antennas can be designed to have radiation patterns which are not omnidirectional and it is convenient to have a figure of merit to quantify the ability of the antenna to concentrate the radiated energy in a particular direction. The directivity D of an antenna is defined as power density at a distance d in the direction of maximum radiation mean power density at a distance d This is a measure of the extent to which the power density in the direction of maximum radiation exceeds the average power density at the same distance. Tire directivity involves knowing the power actually transmitted by the antenna and this differs from the power supplied at the terminals by the losses in the antenna itself. From the system designer s viewpoint it is more convenient to work in terms of terminal power and a power gain G can be defined as G power density at a distance d in the direction of .