which includes a varactor diode and its biasing network and a rectangular‐shaped cutout portion on the left‐hand side on the central feed arm (Figure 1.11). The varactor capacitance variation alters the electrical length of the meandered portion of the antenna, which in turn varies the resonance frequency of the antenna. In addition to the tunable meandered portion of the antenna, the design also includes a rectangular‐shaped cutout geometry for realizing higher band. Thus, this antenna follows a two‐step design approach.
In the first design step, the meandered line was chosen as the location for placement of the varactor diode. Since the meandered line enables the antenna to resonate at the lower frequency band, adding a varactor to this section allows the antenna to become tunable. To provide a wider tunable band between 690 and 970 MHz, the meandered line's electrical length is essentially increased by adding the varactor diode (SMV1281). After optimizing the antenna with the varactor diode set in place, Figure 1.12 shows the meandered line length as ML = 137.5 mm, which is slightly below λ/2 at 970 MHz and the width of the meandered line is MW = 1.2 mm.
Figure 1.11 Antenna geometry with varactor diode and biasing network.
Source: Damman et al. [5].
Figure 1.12 Design parameters for the low (right) and high bands (left).
Source: Damman et al. [5].
The second design step requires adding a higher band antenna structure to the design such that, while the tuning of the lower band is obtained, the higher band is consistently present. The higher band antenna was chosen as a simple rectangular‐shaped structure with a rectangular‐shaped slot which was cutout within the middle of the structure (Figure 1.12). Adding this cutout improved the matching of the higher band and subsequently lowered the effect of frequency tuning in the higher band when the lower band was tuned. In such an antenna, current distributions on the two portions of the antenna should be as independent as possible, which is shown in Figure 1.13a and b for the lower and upper bands, respectively, for capacitance value of 1.6 pF. When the lower band meandering structure is radiating, it has little effect on the upper band antenna. The outer rectangular length and width are RL = 25 mm and RW = 22.25 mm. The rectangular slot cutout (slot length of SL = 15 mm and a slot width of SW = 5.125 mm) dimensions are noted. The centerline feed has a length and width of FL = 19.5 mm and FW = 3 mm. Photograph of this antenna is shown in Figure 1.14 with an overall substrate size of length L1 = 80 mm and width W1 = 60 mm. The material used was FR‐4 (εr = 4.4) with a thickness of t = 0.762 mm. Response of this antenna can be found in [5] and hence is not repeated here.
As a reconfigurable antenna, the lower band meandered antenna also includes a biasing network. The DC biasing lines are placed on the same side as the ground plane (Figure 1.12), so as to limit the effect of coupling. The DC biasing lines include a RF choke inductor to block any high‐frequency signal from entering the DC power supply. A current limiting resistor is also added to the positive voltage terminal to protect the varactor diode. On the top side of the antenna where the radiating structure is located, a single DC blocking capacitor is placed above the varactor diode such that the potentially damaging DC supply voltage does not enter the RF signal. For the varactor diode, the reverse biasing voltage is varied from 0 to 20 V which varies the capacitance from 13.3 to 0.69 pF, respectively.
Figure 1.13 Surface current distribution with capacitance of 1.6 pF: (a) 810 MHz and (b) 1.68 GHz.
Source: Damman et al. [5].
Figure 1.14 Photograph of the fabricated single‐feed dual band antenna: (a) top view and (b) bottom view.
Source: Damman et al. [5].
Although mechanisms to achieve frequency reconfigurable and frequency agile/tunable antenna are different, for our discussion, we refer both antennas under the “Reconfigurable Antenna,” category.
1.8 Antenna Measurements
Antenna performance parameters measurement is important for verifying computation, simulation, and analysis results. For measuring circuit parameters such as scattering parameters (Sii/Sij), where Sii and Sij refer to self‐port reflection coefficient and coupling port transmission coefficient, vector network analyzers are preferred after proper calibration [6]. For example, Figure 1.15 shows photographs of vector network analyzers from Anritsu and Keysight, both of which are available at the Antenna and Microwave Laboratory (AML), San Diego State University.
For measuring radiation patterns, we can use far‐field, near‐field, and compact antenna test range (CATR) chambers. Figure 1.16 shows photographs of the far‐field and CATR anechoic chambers at the AML, San Diego State University. The first anechoic chamber is shown in Figure 1.16a, which is capable of far‐field radiation measurements. It can cover a frequency range from 800 to 40 GHz. The chamber comes with ORBIT/FR 959 acquisition measurement software and provides measurement results for 2D/3D radiation pattern, realized gain, and polarization with sense of rotation.
The Mini‐Compact Antenna Test Range (M‐CATR) from Microwave Vision Group (MVG) for millimeter‐wave antenna measurement covers frequencies between 26.5 and 110 GHz, as shown in Figure 1.16b. Keysight N5225A Power Network Analyzer (PNA) serves its signal power generator that ranges from 10 to 50 GHz. The frequency is extended up to 110 GHz using proper external frequency extenders: V‐band (50–75 GHz) and W‐band (75–110 GHz). This chamber is capable of measuring realized gain, 2D and 3D radiation patterns, and polarization of the antenna with the sense of rotation using the ORBIT/FR 959 acquisition measurement software.
Interested readers should review text books on the theory behind antenna radiation pattern measurements such as [1].
