It is important to realize that antennas radiate from currents. Design consists ofcontrolling currents to produce the desired radiation distribution, called its pattern.In many situations the problem is how to prevent radiation from currents, such as incircuits. Whenever a current becomes separated in distance from its return current, itradiates. Simply stated, we design to keep the two currents close together, to reduceradiation. Some discussions will ignore the current distribution and instead, considerderived quantities, such as fields in an aperture or magnetic currents in a slot or aroundthe edges of a microstrip patch.
An antenna converts bound circuit fields into propagating electromagnetic wavesand, by reciprocity, collects power from passing electromagnetic waves. Maxwell’sequations predict that any time-varying electric or magnetic field produces the oppositefield and forms an electromagnetic wave. The wave has its two fields orientedorthogonally, and it propagates in the direction normal to the plane defined by theperpendicular electric and magnetic fields. The electric field, the magnetic field, andthe direction of propagation form a right-handed coordinate system. The propagatingwave field intensity decreases by 1/R away from the source, whereas a static fielddrops off by 1/R2. Any circuit with time-varying fields has the capability of radiatingto some extent.
We consider only time-harmonic fields and use phasor notation with time dependenceejωt . An outward-propagating wave is given by e−j(kR−ωt), where k, the wavenumber, is given by 2π/λ. λ is the wavelength of the wave given by c/f, where c isthe velocity of light (3 × 108 m/s in free space) and f is the frequency. Increasing thedistance from the source decreases the phase of the wave.
Consider a two-wire transmission line with fields bound to it. The currents on asingle wire will radiate, but as long as the ground return path is near, its radiation willnearly cancel the other line’s radiation because the two are 180â—¦ out of phase and thewaves travel about the same distance. As the lines become farther and farther apart,in terms of wavelengths, the fields produced by the two currents will no longer cancelin all directions. In some directions the phase delay is different for radiation from thecurrent on each line, and power escapes from the line. We keep circuits from radiatingby providing close ground returns. Hence, high-speed logic requires ground planes toreduce radiation and its unwanted crosstalk. |
