Digital Circuit Boards: Mach 1 GHz

3.4 Radiation from Step Function Waves

In logic structure, the waves are step functions and not sinusoids. A spectrum of frequencies with different amplitudes makes up this wave, and the radiation at each frequency is different. The higher frequency components are more efficient radiators. It is difficult to relate this type of signal to wave impedance. We can assume that the fields that are generated by loops will have a high H field content. It is correct to say that loops generate fields that have a low wave impedance character.

N.B.

The E field has units of volts per meter, the H field has units of amperes per meter.

The ratio of E/H has units of volts per ampere or simply ohms. There is no available way to measure wave impedance directly.

It is interesting to note that characteristic impedance and wave impedance are both measured as the ratio of field intensities. In transmission lines, characteristic impedances are usually below 300 ohm. Near dipoles, where the H field is very small, the wave impedance can be well over 5000 ohm.

At a distance from a sinusoidal radiator, the ratio of E field to H field intensity in space is 377 ohm. These waves are referred to as plane waves. The distance from a sinusoidal radiator where the wave is considered to be a plane wave is λ/2π, where λ is the wavelength of the sine wave signal. This length is called the near-field/far-field interface distance. Beyond this distance, both fields attenuate proportional to the distance, and the ...