Top-load

Open-sleeve element

Driven monopole

Ground

Figure 7.176 Geometry of the cross-T line loaded antenna with sleeves ([85], copyright C 2006 IEEE).

Frequency (GHz)

Figure 7.177 Return-loss characteristics ([85], copyright C 2006 IEEE).

and spherical-cap dipole, respectively, for the resonance frequency of near 300 MHz. Antennas were matched to the load of 50 by using a shunt stub at the feed point.

7.2.3.1.2 Cross-T-wire top-loaded monopole with four open sleeves

The antenna configuration is illustrated in Figure 7.176 [85]. The ground plane is assumed to be infinite. The geometrical dimensions of the antenna are: length of driven element L0 = 0.13λ0, length of the top-load element L1 = 0.035λ0, length of sleeve L2 = 0.1λ0, wire diameter a = 0.015λ0, and distance between the driven element and the sleeve R = 0.049λ0, where λ0 is the wavelength at the resonance that is 100 MHz. Return loss is depicted in Figure 7.177 and its variation for different length L2 of the sleeves is shown in Figure 7.178, where L2/λ0 is varied from 0.110 (case 1-1), 0.0105 (case 1-2), 0.095 (case 1-3), and 0.09 (case 1-4). The figure shows that decreasing the length L2 of the sleeves increases the bandwidth.

202Design and practice of small antennas I

the antenna is reduced in size by half, and further reduction is accomplished by bending the radiating slot line. The antenna geometry along with dimensions is illustrated in Figure 7.179(a). The antenna occupies an area of about 0.15λ0 × 0.13λ0.

Since this antenna exhibits very narrow bandwidth, less than 1%, another parasitic antenna with the same configuration is placed in the remaining area in order to increase the bandwidth without significantly increasing the overall PCB (printed circuit board) size, as Figure 7.179(b) illustrates [87]. One of these two antennas is fed by a microstrip line, leaving the other one as a parasitic antenna. The parasitic antenna is coupled with the radiating slot at the elbow section, where the electric field is large. The magnetic currents on each antenna are in phase, so the radiation is enhanced. The two antennas are designed to resonate at the same frequency fr1 = fr2 = f0, where fr1 and fr2 are the resonance frequencies of each antenna, and f0 is the center frequency. In this antenna system, S11, spectral response of the two coupled antennas, exhibits two nulls, as the coupling is adjusted strong enough to increase the bandwidth compared with that of a single slot antenna. The separation of these two frequencies is a function of the separation s and distance d of the overlapped elbow section of these two antennas. The coupling kt between these two antennas is defined as,

where fu and fl, respectively, are the frequencies of the upper and lower nulls in S11. The kt can be adjusted by varying s and d, increasing with decrease in s and increase in d. The designed resonance frequency fr1 = fr2 = 850 MHz; however, slightly different frequencies can be used to achieve a higher degree of control for tuning response. The input impedance of this antenna, for a given slot width, depends on the location of the microstrip line feed relative to one end of the slot and varies from zero at the short circuited end to a high resistance at the center. The optimum feed position can be observed in Figure 7.179(b), which shows the feed line, consisting of a 50transmission line connected to an open-circuited 75transmission line, which crosses the slot. The 75line is extended by 0.33λm beyond the strip-slot crossing to couple the maximum energy to the slot and also to compensate for the imaginary part of the input impedance (λm: wavelength of the wave in the strip line).

Antennas, two single-element antennas (SEA1 and SEA2) and a double-element antenna (DEA) (Figure 7.179(a) and (b)), are fabricated on a substrate of thickness 500 µm, having a dielectric constant of εr = 3.5 and a loss tangent of tan δ = 0.003, with a copper ground plane of 33.5 mm × 23 mm. The SEA1 is the constitutive element of the DEA and the SEA2 is an SEA having the same topology as the SEA1. Both calculated and measured S11 of the DEA and the SEA2 are shown in Figure 7.180, which indicates bandwidth of 21.6 MHz (2.54%) in the DEA, being wider than 8 MHz (0.9%) of the SEA1 and 11.7 MHz (1.31%) of the SEA2. The measured gain of the DEA is 1.7 dB at 852 MHz, which is greater than that of the SEA1 of approximately 0.8 dB at 850 MHz. The antenna size of the SEA1 is 0.133λ0 × 0.154λ0, while that of the SEA2 and the DEA is 0.165λ0 × 0.157λ0.

By adding series inductive elements to a slot antenna, the size of the antenna can be further reduced. A dual-band small antenna is also developed by adjusting the coupling factor kt so as to create two separate frequencies in the S11 response [87].