7.2.1.2Extension of current path

This section describes antennas other than wire types introduced in the previous sections. Typical examples are antennas which have one or more slots or notches inserted in the current path on the surface of the antenna element. A term “slit” is often used instead of “notch.” Slots embedded on the surface of a planar antenna element force the current into a roundabout geometry as shown in Figure 7.72, lengthening the current path so that the resonance frequency is lowered. Notches or slits embedded on the edge of a rectangular surface also lengthen the current path (Figure 7.73). Slits embedded alternately on both sides of a rectangular surface make the current path meandering and the resonance frequency lower (Figure 7.74). Another example is a pair of triangular (bow-tie) patches (Figure 7.75(a)), which is constituted by expanding the mouth of a notch to the end of the planar surface (Figure 7.75(b)). The current flows in this case spread toward the end of the triangular surface taking paths with different lengths so that the resonance occurs at not only one frequency, but also at two or more. Current flows on the surface of these antennas are shown in each of their corresponding figures with arrows. Operating at lower resonant frequency in a given antenna size implies an equivalent downsizing of the antenna, as normally an antenna of lower resonance frequency should have larger size.

Slots or notches are not only employed for the purpose of making antennas small, but also for attaining such operations as dual band, multiband, wideband, and circular polarization by arranging the size, shape, and the number of slots, and the places for embedding them. A slot may have an L-shape (Figure 7.76(a)), a folded shape (Figure 7.76(b)), or

Figure 7.72 Current flow on the surface of planar element with a slot.

Figure 7.73 Current flow on the surface of planar element with a notch.

be combined with a notch (Figure 7.77(a)) or with a bow tie (Figure 7.77(b)). Various other examples are illustrated in Figure 7.78, showing (a) cross slots, (b) two bent slots,

(c) four bent slots, (d) four notches, (e) a square slot, (f) four square slots, (g) a circular slot, and (h) an offset circular slot [29].

Probe feed

Probe feed

Air substrate

L-shaped slit

Folded slit

Shorting plate

Ground plane

Figure 7.76 Variation of slit on the surface of patch; (a) L-shape and (b) folded shape.

Embedded slot

Meandering slits

Figure 7.77 Variation of slits and slots on a patch: (a) meandered slits and two slots on a rectangular patch and (b) two slots on triangle patch [29].

Mostly, these antennas have been developed not merely to create small antennas, but also to enhance antenna performance such as wide bandwidth, multiband operation, circular polarization, and so forth. Then, most such antennas are treated as FSA (Functionally Small Antenna) rather than ESA, and designed to satisfy demands for achieving small or compact antennas and yet having improved performance without increasing