Fast Circuit Boards

5Radiation

5.1 Introduction

The number of circuit board applications that involve transmitting or radiating signals is growing steadily. Radio, television, and garage door openers held center stage for a long while. Today, global positioning, cordless phones, cell phones, remote monitors, automobile keys, and communications links of all types are taking over. Most devices require a circuit board that receives and/or transmits data. Many devices are battery operated, so efficiency is often an issue. It is interesting that the same technology that allows the rapid processing of digital data can also transmit and receive data in the form of modulated carriers.

The general radiation problem involves sine wave signals, active circuits, transmission lines, multiple ports, and antennas. A practical approach to handling rf design is to measure the reflection and transmission coefficients at each port. These coefficients are known as scattering or S parameters. This approach is practical because these parameters can be acquired without using short and open circuits. Just as a reminder, we are leaving the world of step functions and entering the world of sine waves. In most cases, we deal with single‐frequency phenomena where the language of circuit theory is appropriate.

On a circuit board, an antenna must couple energy into free space which has a characteristic impedance 377 ohms. A driver usually connects to a transmission line with a characteristic impedance of 50 ohms. At the antenna, ...