Антенны, СВЧ / OC / Broadband microstrip antennas

Table 7.3

Effect of the Position x s of Stub Along the Nonradiating Edge of an RMSA

(L = 3 cm, W = 4 cm, l = 1 cm, w = 0.4 cm, x = 0.7 cm, er = 2.55, h = 0.159 cm, and tan d = 0.001)

Figure 7.4 (a) CMSA with a single stub. (b) VSWR plots for various stub lengths l (i) 0, (ii) 1.2, (iii) 1.85, and (iv) 2.2 cm: ( - - - ) theoretical and ( —— ) measured.

stub lengths l (0 to 2.2 cm), the results are given in Table 7.4. The theoretical and measured VSWR curves for four different stub lengths are shown in Figure 7.4(b).

As l increases from 0.0 to 2.2 cm, the measured resonance frequency f 0 decreases from 1.868 GHz to 1.693 GHz, which yields a tuning range of nearly 9%. The increase in l increases the effective area of the CMSA, which decreases its resonance frequency. With an increase in the stub length, the effective center of the antenna shifts toward the feed point from the physical center. Therefore, the real part of the measured input impedance Z in decreases from 53.4V to 30.0V, and hence the BW decreases from 29 MHz to 9 MHz. The decrease in the BW is also due to the decrease of

258

7.2.2 Tunable MSAs Using Shorting Posts

When a shorting post is placed between the patch and the ground plane, it changes the field distribution and provides inductive loading to the patch and hence changes its resonance frequency. The resonance frequency can be tuned by changing the position and the number of shorting posts in the patch. As discussed in Chapter 6, the conventional MSAs, such as the RMSA, CMSA, and ETMSA, are made compact by shorting the zero potential field lines and using part of the configuration. It is also described that by reducing the shorting width or number of shorting posts, the resonance frequency decreases, thereby achieving tuning.

An RMSA of length L = 1.2 cm and W = 1.2 cm with shorting posts placed along one of its radiating edges is shown in Figure 7.6(a) [10]. The feed point is at x = 0.155 cm, and the substrate parameters are er = 2.55 and h = 0.12 cm. The variation of normalized frequency f /f 0 with respect to the shorting ratio ws /W (ratio of width of the shorted portion to the width of the patch) is shown in Figure 7.6(b). As the shorting ratio decreases from 1.0 to 0.1, the normalized resonance frequency decreases from 1.0 to 0.65, which is similar to that described in Section 6.2.

Another configuration for tuning the resonance frequency of the RMSA by using two shorting posts inside the patch is shown in Figure 7.7(a) [10, 11]. The dimensions of the RMSA are L = 6.2 cm and W = 9.0 cm, and the substrate parameters are er = 2.55 and h = 0.16 cm. For the feed at x = 1.6 cm along the nonradiating edge, the variation of the frequency with s ′/L

Figure 7.6 (a) RMSA with shorting posts placed along one of its radiating edges and (b) its normalized frequency f /f 0 variation with the shorting width ratio w s /W : ( - - - ) theoretical and ( —— ) measured.

Figure 7.7 (a) RMSA with two shorting posts and (b) its frequency variation with s ′/L : ( - - - ) theoretical and ( —— ) measured.

ratio (separation of the two shorting posts to the length of the patch) is shown in Figure 7.7(b). As s ′/L increases (i.e., the two shorting posts are shifted away from the center to the edge of the patch), the measured resonance frequency increases and then decreases slightly, and a tuning range of approximately 18% is obtained. The radiation pattern remains in the broadside direction.

7.2.3 Tunable MSAs Using Varactor Diodes

Instead of using two shorting posts, which provide inductive loading to the patch for tuning its resonance frequency, two varactor diodes are connected at the two radiating edges of the patch to the ground plane, as shown in Figure 7.8(a). The dimensions of the patch are L = 4.65 cm and W = 3.0 cm and the feed point is located at x = 1.7 cm. Frequency tuning is achieved by varying the reverse-bias voltage applied to the varactor diodes. The plot of measured resonance frequency versus reverse-bias voltage (V) is shown in Figure 7.8(b). With an increase in the reverse-bias voltage, the diode capacitance decreases, which decreases the overall capacitance of the patch; hence the resonance frequency of the antenna increases. Due to the nonlinear behavior of the varactor capacitance with bias voltage, frequency variation is not linear with bias voltage. As the bias voltage is increased from 0 to 30V, the frequency increases from 1.40 GHz to 1.81 GHz, giving a tuning range of about 30% [12].

where f 0 is the resonance frequency of the patch without any air gap and eeq is the effective dielectric constant of the two layers given by (2.25). The above equation is valid for small values of D and any patch shape.

7.2.6 Tunable MSAs Using a Ferrite Substrate

A ferrite substrate has a large permeability mr and high dielectric constant er . When an RMSA is fabricated on a ferrite substrate, its resonance frequency is given by

where me is the effective permeability.

Because of its large value in the lower UHF band, the size of the patch on a ferrite substrate reduces drastically for a given frequency. A fully shorted quarter-wave RMSA with L = 4 cm, W = 1.12 cm, and a feed point at x = 1.12 cm from the open end is fabricated on a ferrite substrate, whose parameters are h = 0.2 cm, mr = 14.74, er = 15, dielectric loss tangent factor = 0.0476, and magnetic loss tangent factor = 0.0476. The measured BW of the antenna is 7 MHz (3.2%) at 219 MHz for VSWR ≤ 1.5 [17]. When a CMSA of radius a = 1 cm and x = 0.5 cm is fabricated on the above ferrite substrate, it has a resonance frequency of 661 MHz. The antenna yielded wider BW of 12.4% for VSWR ≤ 1.5 because of increase in h /l0 [18].

When a ferrite substrate experiences a varying magnetic field, its permeability changes, which yields frequency tuning. An RMSA of L = 1.4 cm and W = 1.8 cm is fabricated on a ferrite substrate with h = 0.127 cm, er = 15 and 4p Ms = 1,720 Gauss [19]. The RMSA is fed at the middle of one of the radiating edges for the TM10 mode. The tuning of the resonance frequency is achieved by varying the dc magnetic bias field, which is realized by placing a permanent magnet behind the ground plane of the MSA. The strength of the magnetic field is changed by adjustable spacers. The tuning range of 40% is achieved at 4.6 GHz for VSWR ≤ 2. The radiation pattern remains in the broadside direction with cross-polar level less than −10 dB in the complete tuning range.

The continuous tuning of the resonance frequency of the MSA over the large frequency range is also possible by changing the biasing current of the electromagnet placed below its ground plane as shown in Figure 7.11 [20]. The electromagnet is biased by setting the current through the fixed