Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 8.38 Sequential array of four CP elements with a rotation of (a) 0° and 90° and

(b) 0°, 90°, 180°, and 270°.

The concept of a sequentially rotated array can be extended to larger arrays. An example of 2 × 4 sequentially rotated array is shown in Figure 8.39. The elements are rotated and fed with equal amplitude but with a phase difference equal to that of the angle of rotation. The angle of rotation of each element is also shown in Figure 8.39(b). For comparison, a conventional array of 2 × 4 elements is shown in Figure 8.39(a). In the conventional array, there is no rotation of the CP elements and all elements are fed with equal amplitude and 0° phase difference. The measured AR and VSWR plots of these two arrays are compared in Figure 8.39(c, d). The BW for AR ≤ 3 dB of the sequentially rotated array is 14% at the center frequency of 2.3 GHz, which is 15 times of that of the conventional array. The BW for VSWR ≤ 1.5 of the sequentially rotated array is 13.7%, which is twice of that of the conventional array [50].

The BW of the sequentially rotated array can be increased further by using broadband CP elements instead of narrow band CP elements. Only stringent requirement for this wideband sequential rotation array is the requirement of a wideband phase shifter [51]. A sequentially rotated array could also be excited indirectly using electromagnetic or aperture coupling instead of direct feeds such as microstrip line or a coaxial line [52].

8.6.4.2 Sequentially Rotated Array Using Linearly Polarized Elements

Broadband CP is also obtained by using linearly polarized elements in a sequentially rotated array. Two single-feed linearly polarized rectangular patches that are orthogonal to each other and fed with a 90° phase shift as shown in Figure 8.40(a) yield CP [53]. Similarly, a 2 × 2 array configuration

Figure 8.39 Circular patches with two notches are arranged in (a) conventional and (b) sequentially rotated 2 × 4 array configuration. Measured (c) AR and (d) VSWR plots [( - - - ) conventional and ( —— ) sequentially rotated.]

of single-feed rectangular elements with orthogonal orientation and phase difference of 90° between them yields broadband CP. The orientation and phase difference of the four elements could be arranged in 0°, 90°, 0°, and 90° or in 0°, 90°, 180°, and 270° manner. The latter configuration shown in Figure 8.40(b), gives better performance due to the cancellation of the undesired reflected and radiated fields from the 0° element with the 180° element, and from the 90° element with the 270° element, respectively.

The gain of the circularly polarized antenna array can be increased by using N linearly polarized elements arranged in a circular ring geometry as shown in Figure 8.41 [54]. The N elements are placed at an angle of 360/N degrees with respect to each other and are sequentially fed to generate CP response. The feed phase of each individual element is arranged in steps of wn = 2p(n − 1)/N, where n = 1, 2, . . . . N. The initial phase a0 could

Figure 8.40 (a) Two linearly polarized rectangular patches with 90° rotation and phase difference. (b) A 2 × 2 array with rotation and phase difference of 0°, 90°, 180°, and 270°.

Figure 8.41 Array of N linearly polarized elements arranged in a circular ring geometry.

be given to the first element. For the single-ring configuration with four elements, the maximum gain is approximately 11.5 dB. The two-ring configuration with four elements in the inner ring and eight elements in the outer ring yielded a maximum measured gain of 15.4 dB. Similarly, a three-ring

examples are rampart line, chain antenna, square loop line, and crank line as shown in Figure 8.43. All the bends in the microstrip lines are chamfered to reduce the discontinuity susceptance [1, 49]. All these arrays are fed at one end and terminated in matched load at the other end, so that the reflected power does not degrade the CP response. When the antenna is fed from the left side, the field rotates in the counterclockwise direction yielding RHCP. Similarly, LHCP is obtained when the antenna is fed from the right side. For all these cases, the length of the unit cell, which is repeated periodically, is 2l. In the unit cell, the polarity of the total radiated field rotates and completes one full cycle in the time period T = 1/f 0 to produce CP. The lengths of each section within the unit cell are adjusted such that the orthogonal field components have equal magnitude with a 90° phase difference.

All these traveling-wave CP configurations give wide AR and VSWR BWs, but the direction of the main beam is off broadside and the beam scan is frequency dependent. The problem of the beam scan with frequency is reduced by forming a line structure into the cross configuration [59].

Figure 8.43 Microstrip line circularly polarized arrays: (a) rampart line, (b) chain antenna,

(c) square loop line, and (d) crank line.

to the 50-V coaxial feed. The main limitation of the antenna is that its beam shape and peak directions vary with frequency.

The performance of the antenna is improved by using a two-turn sixbranch cross as shown in Figure 8.44(c). The dimensions are scaled up to operate the antenna around 1.55 GHz. After matching with the transformer at the input, the BW for VSWR ≤ 2 is more than 15%. The measured gain of the antenna increases to 15 dB at 1.552 GHz. The AR remains below 1.5 dB over a 7% BW with stable radiation pattern [59].

8.8 Summary

CP is obtained when two orthogonal modes of an antenna are excited with an equal amplitude and a 90° phase difference. The CP response of an antenna is defined in terms of its AR. Several dual-feed and single-feed MSA configurations are described to yield CP. For a dual-fed patch, excited at two orthogonal feeds with an equal amplitude and a 90° phase difference, the BW is limited by its VSWR variation and not by its AR variation. When a thicker substrate is used to increase the VSWR BW, its AR degrades. A patch fed with four orthogonal feeds having an equal amplitude with a relative phase difference of 0°, 90°, 180°, and 270° yields an excellent AR response with a BW of more than 30% for VSWR ≤ 2.

Several single-feed modified square MSA configurations are discussed to obtain CP, including the diagonally fed nearly square, the corner-chopped square, the dual-stub loaded square, and the square with a diagonal slot. Similarly, CP is obtained using modified circular and equilateral triangular patches. These single-feed CP elements are limited by their AR variation and not by their VSWR variation. A typical AR BW is on the order of 1%, and hence these single-feed configurations are suitable for narrowband applications.

Compact CP configurations are realized by cutting slots inside the patch. Nearly square ring and square ring with the notches in the outer or the inner square are some examples. Similar compact configurations are realized using circular and triangular patches.

Broadband CP response is obtained by extending the conventional methods to increase the BW of linearly polarized MSAs. These are dualfeed gap-coupled patches, direct-coupled patches, stacked patches, aperturecoupled patches with cross-slots of equal or unequal arms, or inclined or V-loaded inclined slots. These configurations yield an AR BW of up to 15% and a VSWR BW of around 30%. The multiresonator planar configurations

give increased AR BW with higher gain. The AR BW of the single-feed CP configurations is increased significantly by forming their array in sequentially rotated manner. In the sequentially rotated array, each element or a group of elements are rotated and fed with a differential phase, which is same as the rotational angle with respect to each other. The elements of the array could be either circularly or linearly polarized. The gain of these configurations is higher than a single element since many elements are used in an array configuration.

Traveling-wave antennas arrays such as sectoral, spiral, and linear microstrip line arrays also yield CP. A maximum AR BW of up to 40% is obtained using these line geometries. However, the direction of the beam maximum varies with frequency. This problem is reduced by using cross antennas.

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