Fast Circuit Boards

2Transmission Lines—Part 1

• Waves on transmission lines imply energy activity.
• They can indicate that electromagnetic energy is being transported.
• They can indicate that electric field energy is being deposited as magnetic energy.
• They can indicate that magnetic field energy is being deposited as electric field energy.
• They can indicate that electric field energy is being converted to magnetic field energy.
• They can indicate that magnetic field energy is being converted to electric field energy.
• Several of these activities can occur in parallel.

To understand which activity is occurring is not obvious. Measuring voltages is just a part of the story. The explanations that follow will take two full chapters.

2.1 Introduction

Chapter 1 introduced the electric and magnetic fields. These fundamental fields exist around static and moving charges. These fields are basic to all electrical activity including radiation. It was shown that it takes work to move fields into a volume of space. Without conductors, the fields we have been discussing simply radiate into space. To store and move energy in a circuit, fields must be confined in a conductor geometry that has capacitance and inductance. On a transmission line, energy is stored, converted, or moved by wave action. These transitions take place when sine waves are involved but it is difficult to present these mechanisms graphically. Step functions provide an opportunity to appreciate how energy is stored, moved, and converted.