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PRINCIPLES OF TRANSMISSION LINES

LEARNING OBJECTIVES

Upon completion of this chapter, you will be able to:

  • State what a transmission line is and how transmission lines are used.
  • Explain the operating principles of transmission lines.
  • Describe the five types of transmission lines.
  • State the length of a transmission line.
  • Explain the theory of the transmission line.
  • Define the term LUMPED CONSTANTS in relation to a transmission line.
  • Define the term DISTRIBUTED CONSTANTS in relation to a transmission line.
  • Define LEAKAGE CURRENT.
  • Describe how the electromagnetic lines of force around a transmission line are affected by the distributed constants.
  • Define the term CHARACTERISTIC IMPEDANCE and explain how it affects the transfer of energy along a transmission line.
  • State how the energy transfer along a transmission line is affected by characteristic impedance and the infinite line.
  • Identify the cause of and describe the characteristics of reflections on a transmission line.
  • Define the term STANDING WAVES as applied to a transmission line.
  • Describe how standing waves are produced on a transmission line and identify the types of terminations.
  • Describe the types of standing-wave ratios.

INTRODUCTION TO TRANSMISSION LINES

A TRANSMISSION LINE is a device designed to guide electrical energy from one point to another. It is used, for example, to transfer the output rf energy of a transmitter to an antenna. This energy will not travel through normal electrical wire without great losses. Although the antenna can be connected directly to the transmitter, the antenna is usually located some distance away from the transmitter. On board ship, the transmitter is located inside a radio room and its associated antenna is mounted on a mast. A transmission line is used to connect the transmitter and the antenna.

The transmission line has a single purpose for both the transmitter and the antenna. This purpose is to transfer the energy output of the transmitter to the antenna with the least possible power loss. How well this is done depends on the special physical and electrical characteristics (impedance and resistance) of the transmission line.

TERMINOLOGY

All transmission lines have two ends (see figure 3-1). The end of a two-wire transmission line connected to a source is ordinarily called the INPUT END or the GENERATOR END. Other names given to this end are TRANSMITTER END, SENDING END, and SOURCE. The other end of the line is called the OUTPUT END or RECEIVING END. Other names given to the output end are LOAD END and SINK.

Figure 3-1. - Basic transmission line.

NTX3-1.GIF (3152 bytes)

You can describe a transmission line in terms of its impedance. The ratio of voltage to current (Ein/Iin) at the input end is known as the INPUT IMPEDANCE (Zin). This is the impedance presented to the transmitter by the transmission line and its load, the antenna. The ratio of voltage to current at the output (E out/Iout) end is known as the OUTPUT IMPEDANCE (Zout). This is the impedance presented to the load by the transmission line and its source. If an infinitely long transmission line could be used, the ratio of voltage to current at any point on that transmission line would be some particular value of impedance. This impedance is known as the CHARACTERISTIC IMPEDANCE.

Q.1 What connecting link is used to transfer energy from a radio transmitter to its antenna located on the mast of a ship? answer.gif (214 bytes)
Q.2 What term is used for the end of the transmission line that is connected to a transmitter? answer.gif (214 bytes)
Q.3 What term is used for the end of the transmission line that is connected to an antenna? answer.gif (214 bytes)




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