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Online Radio & Electronics Course - 38 - Transmission_Lines

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Online Radio & Electronics Course - 38 - Transmission_Lines | Reading 38 Ron Bertrand VK2DQ http www.radioelectronicschool.com TRANSMISSION LINES A transmission line connects between a transmitter and an antenna and its purpose is to deliver all the signal power to the antenna. A perfect transmission line does not radiate any energy and does not have any losses. d Figure 1 Figure 1 transmission line examples a Parallel line - open wire - air dielectric. b Parallel line - solid dielectric - typically 300 Ohms. c d Parallel line - solid dielectric - round. e Parallel line - solid dielectric - with shield. f Coaxial line - spiral polythene dielectric low loss - typically 50 Ohms. g Parallel line - typical plastic dielectric - twisted pair - telephone line - typically 600 Ohms. WHAT IS MEANT BY CHARACTERISTIC IMPEDANCE OF A TRANSMISSION LINE When an electromagnetic wave travels through free space the current and voltage distribution of the wave settles into a particular ratio. For free space the current and Page 1 voltage distribution of an electromagnetic wave settles into the ratio E I equal to 120n or 377 Q. So we say the characteristic impedance Zo of free space is 377 Q. Similarly when a wave travels along an antenna or in a transmission line of infinite length the current and voltage distribution of the wave will settle to a particular ratio of E I and this is called the characteristic impedance Zo of that line. Now the reason why I said a line of infinite length is to eliminate what is connected to the end of the line that is the load. We will come back to this later and connect a load. So for a particular transmission line of infinite length and no losses by the way we could transmit a wave into it and literally measure the voltage across the line at any point and the current through the line at that point and the ratio of E I would give us the characteristic impedance of the line in ohms. WHAT IS A TRANSMITTER S LOAD When you connect an antenna directly to a transmitter the load for the transmitter is the antenna. When .