Антенны, СВЧ / OC / Broadband microstrip antennas

Figure 7.11 An MSA on a ferrite substrate with an electromagnet.

turns of wire wound over it, which changes the effective permeability of the ferrite substrate. A rectangular patch of L = 4 cm and W = 1.12 cm on a ferrite substrate with h = 0.2 cm and er = 14.78 operated over the frequency range 200–500 MHz with a change in the biasing current of the electromagnet. However, there are disadvantages connected with these configurations, including very poor efficiency, which reduces the gain. In addition, the requirement of additional magnets makes the overall size of the antenna very large.

7.3 Dual-Band MSAs

There is a considerable amount of interest in the development of a dualband MSA because of its usefulness in various applications [21]. These dualband operations can be at fixed frequencies, or tunable at both or one of the frequencies. Figure 7.12 depicts the VSWR plots for the fixed and tunable categories. The solid lines show two fixed frequencies, and the dotted lines show the tunability. Either both or one of the frequencies of dual-band operation could be tunable based on the application. Several MSA configurations for obtaining dual-band characteristics are discussed in this section.

Figure 7.12 VSWR plots of two categories of dual-band operation: (a) fixed and (b) tunable.

7.3.1 Higher Order or Orthogonal Mode Dual-Band MSAs

An MSA operates at many frequencies corresponding to its various resonant modes, which makes it a natural choice for dual-frequency operation. However, characteristics such as the radiation pattern, polarization, and input impedance are not the same for the different modes. Therefore, all these modes are not useful for a given application. Also, for a given MSA geometry, all the modes are at a fixed resonance frequency. A single element can produce dual-band operation either with a single or dual feed as described below.

7.3.1.1 Single Feed Dual-Band MSA

An RMSA operating in the TM10 and TM30 modes has radiation patterns in the broadside direction with the same polarization at both the frequencies as described in Chapter 2. The resonance frequency of the TM30 mode is nearly three times of that of the fundamental TM10 mode of the RMSA. Consequently, these modes may be used for dual-band operation.

Similar to the RMSA, the dual-frequency operation may be achieved by using higher order modes of the CMSA and TMSA as discussed in Chapter 2. For the CMSA, the resonance frequencies corresponding to higher order modes are spaced at an interval given by the roots of the derivative of the Bessel function of order n . For ETMSA, the TM10, TM20, and TM21

268 Broadband Microstrip Antennas

modes yield broadside radiation with similar polarization. A single feed point yields good impedance matching for all these three modes. Thus, it is possible to utilize the ETMSA for dualor even triple-frequency operation.

The above configurations give dual-frequency operation with the same polarization. However, many applications require orthogonal polarization at the two frequencies. An RMSA may be used to operate at TM10 and TM01 modes. The two frequencies correspond to the length and the width of the RMSA. The ratio of resonance frequency of these two modes is approximately equal to the L /W ratio of the RMSA. Accordingly, for a desired frequency ratio, the length and the width can be appropriately chosen.

For an RMSA, both the orthogonal modes may be excited with good impedance matching at both the frequencies using a single feed as shown in Figure 7.13(a). The coaxial feed is displaced from the two principal axes of the patch to excite both the orthogonal modes [22]. The antenna is analyzed using IE3D for L = 3 cm and different values of W with er = 2.55, h = 0.159 cm, and tan d = 0.001. For W = 4 cm, the input impedance and VSWR plots for the feed at x = 0.7 cm and y = 0.5 cm are shown in Figure 7.13(b, c). For three values of W (3.5, 4.0, and 4.5 cm), the results are summarized in Table 7.6. As W increases from 3.5 cm to 4.5 cm, the higher resonance frequency f 2 decreases slightly due to an increase in ee , but the lower resonance frequency f 1 decreases from 2.594 GHz to 2.044 GHz. The ratio of the two frequencies for all these cases is approximately the same as that of the ratio of effective length and the effective width of the RMSA. The BW at the two frequencies is almost the same as that of the BW of the corresponding RMSA when only a single mode is excited.

As W increases from 3.5 cm to 4.5 cm, the BW at the lower resonance decreases from 37 MHz to 19 MHz, because f 1 decreases, which in turn decreases h /l0. Also at this mode, length L of the patch acts as width, so L /l 0 decreases with an increase in f 1; therefore, radiation resistance increases leading to larger resistance variation and hence a decrease in BW. However, BW at the higher resonance increases from 60 MHz to 69 MHz with an increase in W. This is because at this mode, f 2 remains nearly constant as L is constant, but W /l0 increases, resulting in increase in BW.

Instead of using a single coaxial feed for the dual-band RMSA, similar results are obtained by using an aperture coupled RMSA, in which an inclined slot is cut in the ground plane with respect to the microstrip feed line as shown in Figure 7.14 to give proper matching at both the frequencies [23].

A single feed elliptical MSA, shown in Figure 7.15(a), also yields dualfrequency operation with orthogonal polarization. The coaxial feed is placed at approximately 45° from the major axis. Its location is optimized to excite

Figure 7.13 (a) RMSA with a single feed for orthogonal dual-band operation and its (b) input impedance and (c) VSWR plots.

Table 7.6

Dual-Band Response of RMSA with Single Feed for Different Values of W (L = 3.0 cm, er = 2.55, h = 0.159 cm, and tan d = 0.001)

both the orthogonal modes with proper impedance matching. For a = 4 cm, b = 3 cm, er = 2.33, h = 0.159 cm, and tan d = 0.002, the input impedance and VSWR plots for the feed at x = 1.0 cm and y = 1.05 cm are shown in Figure 7.15(b, c). The lower and higher resonance frequencies are 1.430 GHz and 1.835 GHz with BWs of 13 MHz and 27 MHz, respectively. The ratio of the two resonance frequencies is approximately the same as that of the ratio of the effective radii in the two orthogonal planes.

7.3.1.2 Dual Feed Dual-Band MSAs

For a single feed dual-band MSA, if one frequency is used for transmission and the other is used for reception, a circulator or diplexer is required to isolate the receiver from the transmitter. The use of a circulator or diplexer may be avoided by feeding the RMSA at two orthogonal points as shown in Figure 7.16(a) [24]. Since these feed points are at null locations of the respective orthogonal modes, the loading of one feed point does not affect the input impedance at the other feed point. For the RMSA with W = 4 cm and other dimensions given in Table 7.6, the input impedance plots corresponding to the two ports and the magnitude of S-parameters (S11, S22, and S21) are shown in Figure 7.16(b, c). For the feed at x = 0.7 cm, the resonance frequency is 2.980 GHz, and for the feed at y = 0.5 cm, the resonance frequency is 2.287 GHz. The isolation between the two modes using orthogonal feeds is nearly 30 dB and 40 dB at the lower and higher resonance frequencies, respectively. These results are similar to those described in Section 2.2.3.

Similar results are obtained for an ellipse with two orthogonal feed points. The dimensions of the ellipse are the same as in Section 7.3.1.1. The two orthogonal feed points are at x = 1 cm and at y = 1.05 cm as shown in Figure 7.17(a). The isolation between the two frequencies is nearly 30 dB. Instead of using two coaxial feeds, orthogonal polarization could be excited using electromagnetic or aperture coupling. A variation in the ellipse is obtained by using only the intersecting portion of the two circles of same radius as shown in Figure 7.17(b). This configuration is fed with two orthogonal electromagnetically coupled microstrip lines [25]. As before, the frequency ratio of dual-band operation is approximately equal to the ratio of the orthogonal dimensions in the two planes. The isolation between the two ports is 27 dB. Another variation using a circular patch is shown in Figure 7.17(c). It is excited by two orthogonal microstrip lines through the two orthogonal slots cut in the ground plane. By changing the slot dimensions, the two orthogonal resonance frequencies can be changed [26].

Figure 7.16 (a) RMSA with two orthogonal feeds for dual-band operation and its (b) input impedance plots at two ports: ( —— ) port 1 and ( - - - ) port 2, and (c) S-parameter plots: ( —— ) S11, ( - - - ) S22, and ( – ? – ) S21.

A dual-band operation is also obtained by loading the MSA with a reactive load. The reactive load modifies the field configuration of the patch and gives a dual resonant behavior. The reactance could be in the form of a single stub or double stubs, a lumped capacitor, shorting posts, or a slot in the patch, among others. In the reactive loaded MSA, both the resonance frequencies of the dual-band operation can be tuned by changing the value of the reactance. These configurations are discussed one by one in the following subsections.

7.3.2 Stub-Loaded Dual-Band MSAs

For dual-band response, the concept of loading an RMSA with a shortcircuited coaxial line stub as shown in Figure 7.18(a) was first demonstrated

Figure 7.17 Various dual-band MSA configurations: (a) elliptical MSA with two orthogonal feeds (b) common portion of two intersecting circles, and (c) CMSA with two orthogonal slots.

experimentally [27]. The tuning of the two frequencies was obtained by changing the length of the short-circuited coaxial line. Instead of shortcircuited coaxial line, a l /2 short-circuited microstrip line is used as shown in Figure 7.18(b), which can be etched on the same substrate along with the patch [28]. The separation between the two resonance frequencies is varied by either changing the short-circuited length of the microstrip line or by introducing an inset in the patch, as shown in Figure 7.18(c). A compact version is obtained by using an open-circuited l/4 stub (which is equivalent to a short-circuited l/2 stub) as shown in Figure 7.18(d). For frequency tuning, the length of the stub is varied slightly. A single or double open-circuited l /4 stub is connected to the RMSA or CMSA to obtain the dual-band operation as described below [5, 29].

7.3.2.1 Single Stub-Loaded RMSA

The tuning of the resonance frequency of the antenna with a small openended stub is discussed in Section 7.2. When the length of the stub is comparable to l /4, a dual-frequency operation is obtained. At the lower frequency, the open stub presents a capacitive load, whereas at the higher frequency, it presents an inductive load.

Figure 7.18 Various dual-band stub-loaded RMSA configurations: (a) short-circuited coaxial line, (b) short-circuited l /2 stub, (c) short-circuited l /2 stub with inset, and (d) open-circuited l /4 stub.

For the single stub-loaded RMSA shown in Figure 7.18(d), the dimensions of the patch are taken as L = 3 cm and W = 4 cm, and the substrate parameters are er = 2.55, h = 0.159 cm, and tan d = 0.001. For the feed at x = 0.8 cm and the stub dimensions l = 1.5 cm and w = 0.4 cm, the theoretical input impedance and VSWR plots are shown in Figure 7.19. A dualfrequency response is obtained at f 1 = 2.434 GHz and f 2 = 3.377 GHz with the corresponding BWs of 26 MHz and 35 MHz, respectively. The radiation pattern remains in the broadside direction at both f 1 and f 2 with cross-polar levels less than 24 dB and 14 dB, respectively. The stub length is varied for tuning both the frequencies, and the results are summarized in Table 7.7. As the length of the stub is increased from 1.4 cm to 2.0 cm, both the resonance frequencies decrease, and the ratio f 2 /f 1 changes from 1.37 to 1.54.

Figure 7.19 (a) Input impedance and (b) VSWR plots of single stub-loaded RMSA for dual-band operation.

Table 7.7

Resonance Frequencies of Single Stub-Loaded RMSA for Various Values of Stub Length l

(L = 3 cm, W = 4 cm, x = 0.8 cm, er = 2.55, h = 0.159 cm, and tan d = 0.001)

7.3.2.2 Double Stub-Loaded RMSA

An RMSA with a single stub has slightly higher cross-polar level because it is asymmetrical configuration. To make the configuration symmetrical, a l/4 open-circuited stub is placed along both the radiating edges of the rectangular patch as shown in Figure 7.2(a). For l = 1.5 cm and x = 0.6 cm, the input impedance and VSWR plots are shown in Figure 7.20. Dualfrequency operation is obtained at 2.243 GHz and 3.603 GHz with BWs of 23 MHz and 26 MHz, respectively. The radiation pattern is in the broadside direction at both frequencies. The ratio of the two resonance frequencies is 1.6, which can be varied by changing the length of the stubs.