48.50

Figure 8.9 A ψ -shape patch antenna modified from an E-shape patch antenna ([4a, 4b], copyrightC 2009 and 2007 IEEE).

It can be also said that the E-shaped patch antenna is attractive for use in modern wireless systems.

8.1.2.1.1.1.2 ψ -shaped microstrip antenna

An E-shape can be modified to a ψ -shape by rotating the E-shape 90 degrees and removing the bottom side out slightly, excepting the center part. A resultant ψ -shape patch is shown in Figure 8.9 [4a, 4b], where antenna geometry is illustrated; (a) top view and (b) side view, along with dimensions of slots in mm. With this structure, wider bandwidth over that of an E-shaped patch can be obtained as a consequence of improvement in the current distributions on the patch that attributes to removal of the bottom side conductor in the E-shaped patch.

The removed part at the bottom side of the patch acts in an important role for controlling the current distributions on the patch so that the bandwidth is increased. The ψ -shaped patch is etched on the substrate (εr = 2.50, tan δ = 0.002) of thickness h = 0.33 mm placed on the foam substrate (εr = 1.06, tan δ = 0.0002) of thickness h = 6.0 mm. Dimensions of the original rectangular patch are 48.5 mm in width and 26 mm in length, respectively, including two slots 6 mm wide and 19 mm long cut symmetrically around the patch center, and two conductor-removed parts at the bottom side of the patch, 6 mm long and 23 mm wide, resulting in a tail part. The ground plane size is 75 mm × 75 mm, which is determined by the parametric study.

The simulated and measured S11 are shown in Figure 8.10, by which the simulated bandwidth is observed to be 55.02% (4.15 to 7.30 GHz) and measured bandwidth is 53.60% (4.10 GHz to 7.10 GHz), except that an S11 of –9 dB is seen near 4.75 GHz.

These results show much wider bandwidth than that of an E-shaped patch antenna, introduced in the previous section as 32% [1]. A small discrepancy is observed between the simulated and measured results possibly due to fabrication error, mainly stemming

Frequency (GHz)

Figure 8.10 Simulated and measured S11 ([4a], copyright C 2009 IEEE).

from the difference in the foam thickness, increased from 6 mm to 6.4 mm, that is determined by the simulation, assuming somewhat different structures from the practical foam substrate. However, it can be said that they are in reasonable agreement, notwithstanding that discrepancy.

8.1.2.1.1.1.3 H-shaped microstrip antenna

For exciting a circular polarization (CP), another shape of patch antenna, an H-shaped one, is proposed [5]. The antenna geometry is shown in Figure 8.11 with top and side views. The antenna is a single H-shaped copper plate, 0.5 mm thick, suspended by using three nylon bolts over a very thin dielectric layer, having εr = 3.38, tan δ = 0.0021, and thickness 0.2 mm, which is identified as the feed layer, where a horizontal strip line is printed on this layer to feed the patch. The H-shape is formed from a square, on which two rectangular slots are cut away from the top and bottom sides of the square. The square has side S = 37 mm (0.30 λ at 2.45 GHz) and the rectangular slot, which forms an H-shape, has size a =13 and b = 14 mm, respectively. Generally, to excite a CP wave, a square patch is fed at a point on the diagonal line. With a small perturbation on the two sides of a square, CP can be excited, although with a somewhat narrow band of operation, whereas with a large rectangular perturbation a wideband CP operation can be produced.

To excite a left-hand-sense CP wave, the H-shape patch is fed through a probe placed at a point on the diagonal line of the square where a printed monopole is placed at 45 degrees with respect to the axis. The printed monopole is arranged toward the patch corner instead of the center, and used to compensate the feed inductance due to the long probe, thereby making better impedance matching easy. The patch is placed at height H = 32 mm (0.26λ) from the ground plane and the feed layer is placed at height Fv = 24 mm, which equals the length of the vertical probe. The width and length of the horizontal monopole are Fw = 2 and FH = 5 (in mm), respectively.

Measured return loss and boresight axial ratio (AR) are shown in Figure 8.12, where simulated results are also given for a comparison. It shows that bandwidth in terms of

10-dB return-loss is 2.22–2.64 GHz (17.2%) in the measurement and is 2.21–2.77 GHz (22.5%) in simulation. The 3 dB axial ratio bandwidth is 2.3–2.77 GHz (18.5%) in simulation and 2.28–2.77 GHz (19.8%) in the measurement. These wideband characteristics are attributed to the inclusion of a thick air substrate (0.26λ) in the design. This discrepancy between measured and simulated results is possibly attributed to the practical structures, which are not included in the simulation, and to fabrication tolerances. By using thick air substrate and low-loss mechanisms, the antenna can show the maximum gain of 5.7 dBic and high efficiency of 95% at 2.45 GHz.

8.1.2.1.1.1.4 S-shaped-slot patch antenna

A dual-band single-feed circularly polarized S-shape-slotted patch antenna is proposed in [6]. The antenna geometry is depicted in Figure 8.13, which shows (a) side view, (b) S-shape-slotted patch radiator, and (c) aperture-coupled feeding structure. An S-shaped slot is embedded at the center of the patch surface and fed by a microstrip line located underneath the center of the coupling aperture ground plane. The frequency ratio of the two frequencies can be controlled by adjusting the length of the S-shaped slot and attained finally to be 1.28. The measured 10-dB return loss bandwidth for the lower and higher bands are 16% (1.103–1.297 GHz) and 12.5% (1.568–1.577 GHz), respectively. Measured and simulated return loss are illustrated in Figure 8.14. The measured axial ratio bandwidth is 6.9% for the lower band (1.195–1.128 GHz) and 0.6% for the higher band (1.568–1.577 GHz). The measured gain is observed more than 5 dBc both lower and higher bands. Measured and simulated gain and axial ratio are shown in Figure 8.15(a) and (b), respectively. The overall antenna size is 0.46λ0 × 0.46λ0 × 0.086λ0 at f0 = 1.2 GHz.

The antenna exhibits good impedance matching, high gain, small dual-band frequency ratio, and wide CP (Circular Polarization) beamwidth. The antenna is suitable for GPS applications and array design.

8.1.2.1.1.2 Use of slot/notch (slit) on planar antenna surface

Embedding a slot/notch (slit) on a planar antenna is quite beneficial to implement a small, compact, low-profile, wideband antenna, yet with enhanced performances such as higher gain, circular polarization, and so forth. The shape, number, and location of slots or notches, are essential parameters in designing a wideband antenna, and in turn, the type, size, shape, and dimensions are the essential antenna parameters. The most popular types of antenna are Planar Inverted-L antenna (PILA), Planar Inverted-F antenna (PIFA), and various shapes of microstrip antenna (MSA) such as rectangle, triangle, circle, and so forth. Slots/notches may be embedded also on the ground plane as well as on the radiating surface of the patch to reduce the antenna size and generate multiband operation.

In a practical example, small mobile terminals employ slots/notches (slits) on a planar antenna (a PIFA is the most representative type of antenna) to enhance the bandwidth or increase the number of operating frequency bands without increasing the antenna dimensions, keeping the antenna structure compact. Modified PIFAs have been observed, on which slots or slits are embedded to the point where the structure is transformed to