Антенны, СВЧ / OC / Broadband microstrip antennas

larger f 2 /f 1 of 2.5 to 4.9, an ETMSA with a single shorting post may be designed.

7.3.4.5 Dual-Band Shorted Planar Inverted F-Antenna

Two modified dual-band inverted F-antenna configurations to cover the GSM and DCS 1800 systems are shown in Figure 7.28 [40]. The two frequencies are around 0.9 GHz and 1.8 GHz. The antenna may be excited either using dual or single coaxial feed as shown in Figure 7.28. The configuration with two feeds has a shorted rectangular element for the 1.8-GHz and a shorted L-shaped element for the 0.9-GHz operation. The dimensions of the antenna are shown in centimeters, and the substrate parameters are er = 1 and h = 0.9 cm. The shorting posts are placed near the gap between the two patches to reduce mutual coupling (less than −17 dB). The antenna is fed by two 50-V coaxial cables, which are located very close to the shorting posts with reasonably good match. The dual-frequency response is achieved at 0.9 GHz and 1.76 GHz with respective BWs of 63 MHz and 110 MHz. The radiation pattern is almost omni-directional at both the resonance frequencies.

Dual-band operation is also achieved with a single feed as shown in Figure 7.28(b). The substrate parameters are er = 2.2 and h = 0.1143 cm. The ratio between the two dual frequencies of 1.8 is achieved with return loss better than −17 dB at both the frequencies [40].

A patch could be loaded with a chip resistor instead of a short to yield dual-frequency operation [41]. With an increase in the resistor value from 0 to 2V, the loss in the chip resistor increases, which increases the BW but significantly decreases the efficiency.

Figure 7.28 Modified inverted F-antenna for dual-band operation using (a) dual feed and

(b) single feed.

7.3.5 Dual-Band MSA Using Lumped Element Loading

Instead of using shorting posts, two lumped capacitors may be used to obtain dual-band operation. The capacitor can be realized using varactor diodes with reverse-bias voltage. An RMSA with two varactor diodes is shown in Figure 7.29(a). It is fed at the edge by a microstrip line and double-stub impedance-matching network to improve the impedance characteristics. The microstrip patch length L is designed for the resonance frequency f 2 of 2.1 GHz for er = 2.17 and h = 0.079 cm. The varactor diodes are placed symmetrically at the positions 0.3L and 0.7L to reduce the effects of crosspolarization. Because of the loading of the patch with varactor diodes, a lower resonance frequency f 1 is obtained; this is tuned with the bias voltage of the varactor diodes as shown in Figure 7.29(b). A tuning range of 375–650 MHz is obtained for f 1 as the reverse-bias voltage increases from 2.5V to 16V. The measured BW at both frequencies is 1–2% [42].

7.3.6 Dual-Band MSAs Using Slots

Reactive loading can also be introduced by etching slots inside the patch to yield dual-band operation. A combination of slot and shorting posts is also used for tuning the dual-band operation.

7.3.6.1 Dual-Band RMSAs with Slots for Orthogonal Polarization

An RMSA with a square slot at the center is shown in Figure 7.30(a) [43]. The slot in the center lengthens the excited surface current path for both

Figure 7.29 (a) An RMSA with two varactor diodes for dual-band operation and (b) variation of lower resonance frequency with the reverse-bias voltage.

Figure 7.30 RMSA with (a) a square slot at the center and (b) a pair of bent slots.

the TM01 and TM10 modes and hence reduces the corresponding resonance frequency. A feed point along the diagonal generates a dual band with orthogonal polarization. The length and the width of the RMSA are 3.32 cm and 2.55 cm, respectively, with er = 4.4 and h = 0.16 cm. The variation of the two frequencies with slot length l is given in Table 7.11. The feed point location d p is optimized for different slot lengths, and it varies between points A and B. Point B is at the corner of the slot. With an increase in l from 0.0 to 0.9 cm, both the resonance frequencies decrease by about 9%. The frequency ratio f 2 /f 1 remains nearly the same because the two dimensions increase with the same proportion because of the square slot. The resonance frequencies decrease further with an increase in the slot length, but the input impedance is too large even for the feed point at the corner of the slot.

A square slot is replaced by a pair of bent slots in the center of the RMSA as shown in Figure 7.30(b) [44]. The configuration with L = 3.77 cm, W = 2.84 cm, s = 0.14 cm, and w = 0.1 cm is fabricated on the substrate having er = 4.4 and h = 0.16 cm. Tunable dual-frequency operation with

Table 7.11

Variation of Resonance Frequencies of RMSA with a Square Slot with Slot Length l (L = 3.32 cm, W = 2.55 cm, er = 4.4, and h = 0.16 cm)

orthogonal polarization is obtained by varying the length of the bent slots. When the length of the bent slot is increased from 0 to 2.6 cm, both the resonance frequencies decrease, thereby realizing a compact MSA. The percentage decrease in the lower frequency is slightly more than the second frequency and hence the frequency ratio increases slightly from 1.29 to 1.38. This configuration has the advantage that even for a large slot length, impedance matching is obtained at both the frequencies by varying the feed point between the points A and B. The radiation pattern for both the above configurations is in the broadside direction with orthogonal polarization at the two frequencies, and the cross-polar levels remain below 15–20 dB.

The orthogonal polarization at the two frequencies is not acceptable for applications requiring the same polarization. The following subsections describe various configurations to yield dual-frequency operation with the same polarization.

7.3.6.2RMSAs with Slots Near the Nonradiating Edge for the Same Polarization

A dual-band RMSA with a pair of step slots embedded close to its nonradiating edges is shown in Figure 7.31(a). The dimensions given in Figure 7.31 are in centimeters, and the substrate parameters are er = 4.4 and h = 0.16 cm. Two slots are chosen to be identical for symmetry, but different widths are selected in one-half of the length to obtain impedance matching at the two frequencies using a single feed as shown in Figure 7.31(a). These slots are resonant at one frequency, and the rectangular patch is resonant at the other frequency. Since the polarization of the slots and the patch is same, this antenna yields dual-frequency operation with the same polarization. By increasing the larger width w of the slot, tunable dual-band operation is obtained as shown in Table 7.12. The optimum feed-point location x and the BWs at the two frequencies are also given in Table 7.12. As w increases from 0.2 cm to 1.0 cm, the effective length of the slot increases, and hence its resonance frequency f 1 decreases from 1.878 GHz to 1.480 GHz. With an increase in w, the resonance frequency f 2 corresponding to the patch decreases initially because of the increase in the path length, but later, the effect of the decrease in ee becomes dominant, which increases f 2. Therefore, f 2 /f 1 increases from 1.23 to 1.63 as w increases from 0.2 cm to 1.0 cm [45].

Instead of using step slots, angular bent slots can be used as shown in Figure 7.31(b) [46]. The effect of an increase in the bent angle is similar to that of increasing the step width discussed above. The tuning of both the frequencies is obtained by changing the angle of the bent slot, which

Figure 7.31 RMSA with a pair of (a) step slots and (b) angular bent slots.

Table 7.12

Effect of the Slot Width w on the Performance of RMSA with a Pair of Step Slots

increases the total slot length and hence decreases its resonance frequency. As the slant angle increases from 15° to 30°, the f 2 /f 1 ratio increases from 1.29 to 1.60. The feed-point location has to be moved toward the center when the slant angle is increased. The radiation pattern is in the broadside direction with the same polarization at the two frequencies for these two configurations.

In the above cases, dual-band operation is due to the resonance of the slot and the patch. In the following subsection, the nonresonant slot is used to change the effective current path length for the TM10 and TM30 modes of an RMSA for which the radiation is in the broadside direction with the same polarization. The parameters and locations of the slots are chosen in such a way that they change the current path lengths for the two modes in a different ratio to obtain the desired frequency ratio of dual-band operation.

7.3.6.3 Slotted RMSA for Higher Order Modes

An RMSA with two narrow slots etched inside the patch, which are close to and parallel to the radiating edges is shown in Figure 7.32(a) [47, 48]. The dual-band operation of the slotted RMSA can be explained as arising from the perturbation of the two modes (i.e., the TM10 and TM30 modes of the RMSA), as shown in Figure 7.32(b, c). These narrow slots are placed close to the radiating edges, where the current is nearly minimum for the TM10 mode. Consequently, the current distribution for this mode is only slightly perturbed. Hence, its resonance frequency is only slightly different from that of the RMSA without a slot. For the TM30 mode, slots are located where the current of the unperturbed TM30 mode is significant, leading to the strong modification of the current distribution. As a result, the resonance frequency decreases due to the increase in the current path length introduced by the slot. For er = 2.2 and h = 0.08 cm, dual-frequency operation is obtained at 2.22 and 3.48 GHz corresponding to the modified TM10 and TM30 modes of the RMSA. The frequency ratio is 1.57, which is much smaller than the ratio of 3 for the RMSA without slots. The ratio of two frequencies can be changed by varying the dimensions and location of the slots. The radiation pattern remains in the broadside direction at both the frequencies.

The size of the antenna can be reduced by short-circuiting along the vertical central axis, which corresponds to the zero-potential plane for the TM10 and TM30 modes, and by using only one-half of the antenna [49]. The two resonance frequencies corresponding to the modified TM10 and TM30 modes are further reduced by decreasing the shorting width. This leads to a large reduction in the antenna size as compared to the regular slot-

Figure 7.32 (a) RMSA with two slots near the radiating edge and current distributions for the (b) TM10 and (c) TM30 modes.

loaded RMSA. This configuration, along with its dimension in centimeters, is shown in Figure 7.33(a). The antenna is fabricated on a foam substrate of h = 0.3 cm. The input impedance matching is obtained for both the modes with a single feed point. The return loss plot for various shorting width ratios ws /W is shown in Figure 7.33(b). As ws /W increases from 0.1 to 1, the f 2 /f 1 ratio decreases from 2.2 to 1.6. For ws /W = 0.1, the lower resonance frequency is 1.535 GHz, which is 0.29 times the resonance frequency of 5.35 GHz for a conventional RMSA of the same size.

Section 7.3.1 explained that by feeding the RMSA or square MSA at two orthogonal feed points, orthogonal polarization is obtained. This concept is extended to two pairs of orthogonal slots cut in a square MSA to yield a dual-band (corresponding to the modified TM10 and TM30 modes), dualpolarized response using two coaxial feeds as shown in Figure 7.34(a). The slotted square patch antenna exhibits a smaller range of frequency ratio than

Figure 7.33 (a) Partially shorted RMSA with a slot near its radiating edge and (b) its return loss plot for various values of ws /W : ( —— ) 1.0, ( ? ? ? ) 0.5, ( – ? – ) 0.25, and ( - - - ) 0.1.

Figure 7.34 Dual-feed (a) square MSA and (b) cross MSA with two pairs of orthogonal slots.

the unslotted patch. At the lower and upper frequencies, an isolation of 35 dB and 40 dB is obtained between the two ports, respectively [21].

In the above configuration, when all the four corners of the square are removed, it becomes a slotted cross patch antenna as shown in Figure 7.34(b). By chopping off the four corners, the isolation between the two orthogonal ports at the lower frequency is improved by 5–6 dB [21]. Also, it occupies less space and has reduced cross-polar components.

A compact dual-band antenna is realized when multiple slits are cut along the length of the slot loaded RMSA [50]. The geometry looks like a

meandered structure as shown in Figure 7.35. With the slits on the nonradiating edges, the surface current path for the two modes TM10 and TM30 increases, leading to a reduction in the resonance frequency. For the dimensions shown in Figure 7.35 in centimeters, the variation of resonance frequencies with slit length l is given in Table 7.13.

As l increases from 0 to 1.3 cm, the lower resonance frequency f 1 decreases from 1.915 GHz to 1.096 GHz, and the higher resonance frequency f 2 reduces from 3.620 GHz to 2.590 GHz, thereby giving variation of f 2 /f 1 ratio from 1.89 to 2.36. The feed point is shifted toward the center to obtain impedance matching, because with an increase in l, resonance frequency decreases, which increases the radiation resistance. The radiation pattern is in the broadside direction at both the frequencies. At the lower frequency, the cross-polarization is less than 20 dB, whereas at a higher frequency, it increases slightly.

Figure 7.35 RMSA with multiple slits for dual-band operation.

Table 7.13

Variation of Two Frequencies of RMSA with Multiple Slits for Different Slit Length t

7.3.7 Multiresonator Planar MSAs for Dual-Frequency Operation

Dual-frequency operation can also be obtained by using multiresonators, similar to that of the broadband MSA configurations described in Chapters 3 and 4. These resonators could be either coplanar or stacked in two or more layers. If the individual resonators are compact, then the composite dual-band configuration is also compact.

7.3.7.1 Dual-Band Gap-Coupled RMSA

When two rectangular patches are gap-coupled, a broad BW is obtained as described in Section 3.3. A two gap-coupled RMSA configuration is shown in Figure 7.36(a). The dimensions of the antenna are L = 3 cm, W = 4 cm, L 1 = 2.9 cm, x = 1.1 cm, er = 2.55, h = 0.159 cm, and tan d = 0.001. For three values of the gap s, the input impedance and VSWR plots are shown in Figure 7.36(b, c). As the gap decreases from 0.1 cm to 0.01 cm, the coupling increases and hence the loop size increases. Accordingly, the broadband operation becomes dual-band operation, and the separation between the two resonance frequencies increases. The upper resonance frequency corresponds to the parasitic patch, and it increases slightly because with a decrease in the gap, the extension along the coupled side decreases, which reduces its effective length. The lower resonance frequency corresponds to the fed patch, and it decreases with a decrease in the gap because of the increase in the value of the series gap capacitance. Thus, for given fed and parasitic patch dimensions, the tunable dual-frequency operation is obtained by simply varying the gap between the two patches. The radiation is in the broadside direction at the lower frequency only, and it is shifted away from the broadside at a higher frequency, because of the phase delay experienced by the parasitic patch as described in Section 3.3.

Instead of using l /2 RMSA in a two gap-coupled configuration, if a compact RMSA with a single short (described in Section 6.2.2) is used, a compact dual-band MSA is realized. The tunability of the two frequencies can be obtained either by reducing the air gap or by changing the width of the shorting strip across the gap between the two patches as described below.

7.3.7.2 Hybrid-Coupled Compact Shorted RMSA

A radiating edge hybrid-coupled shorted RMSA is shown in Figure 7.37(a). Both the patches have a single shorting post at the center of the extreme radiating edges. The dimensions of the antenna are L = 2.3 cm, W = 4.6 cm, s = 0.1 cm, x = 0.65 cm, er = 4.3, h = 0.16 cm, and tan d = 0.02. For gap coupling between the fed and the parasitic patches, the loop size is within the VSWR = 2 circle leading to the broadband response. To increase the