## 6 Tunable Wearable Body Area Network  
Small printed tunable antennas are crucial in the development of Wearable BAN, WBAN, systems. Tunable antennas consist of a radiating element and of a voltage-controlled diode, varactor. Varactor diodes are used in voltage-controlled oscillators, VCOs. Varactor diodes are semiconductor devices that are used in many microwave systems where a voltage-controlled variable capacitance is required. Varactor diodes function as a voltage-controlled variable capacitance. The radiating element may be a microstrip patch antenna, dipole, or loop antenna. The antenna resonant frequency may be adjusted by using a voltage-controlled diode to tune variations in the antenna resonant frequency at different environments. The varactor capacitance is given in Equation (1). The circuit frequency, \(f_{r}\), may be calculated by using Equation (2). \[C=\frac{A\varepsilon}{d}, \tag{1}\]  
where \(C\)--capacitance, \(A\)--plate area, and \(d\)--diode thickness. \[f_{r}=\frac{1}{2\pi\sqrt{LC}} \tag{2}\]  
Varactors are voltage-controlled diodes used to tune microwave devices. Low frequency varactors are manufactured on silicon substrates. High frequency varactors are manufactured on gallium arsenide substrate. Gallium arsenide diodes have higher Q values, in comparison to silicon diodes. Hyper-abrupt voltage-controlled diodes have nearly linear frequency variation as a function of the applied bias voltage. Abrupt varactors have inverse fourth root frequency variation as a function of the applied controlled voltage. For example, a varactor with 20 pF capacitance may tune a small wearable loop antenna to 433 MHz. A folded printed tunable antenna is presented in Figure 4c. The antenna volume is 70 mm \(\times\) 50 mm \(\times\) 0.016 mm. The folded dipole has a horizontal polarization. The folded slot antenna is vertically polarized. The combined tunable antenna has dual orthogonal polarizations. The antenna is printed on two dielectric substrates. The antenna feed network is printed on a RO3035 substrate that is 0.8 mm thick. The folded antenna is printed on a substrate with 2.2 dielectric constant that is 0.8 mm thick. The location and size of the coupling stubs, shown in Figure 4c, are optimized to achieve the best antenna VSWR. The antenna electrical performance was optimized by using ADS software [32]. The antenna resonant frequency may be tuned by connecting the small tuning bars located along the feed line to the antenna feed line. Varactors are connected to the antenna feed lines, as shown in Figure 4c, to tune the antenna resonant frequency. The varactor control voltage may be  
Figure 4: **(a) Active transmitting printed loop antenna layout; (b) Active transmitting loop antenna S21 parameter; (c) Tunable folded compact dual polarized antenna; (d) Resonant frequency of a tunable antenna as function of varactor capacitance. **varied automatically to tune the antenna center frequency at different locations on the patient body The antenna bandwidth is better than 10% for VSWR better than 2:1. The printed tunable antenna beam width is around 100\({}^{\circ}\). The tunable antenna resonant frequency as a function of the varactor capacitance for capacitances up to 2.5 pF is presented in Figure 4d. The antenna resonant frequency varies around \(\pm\)5% for capacitances up to 2.5 pF. The antenna gain may vary between 1 to 3 dB as a function of the antenna environment. Photos of small printed tunable antennas for WBAN applications are shown in Figure 5a. Figure 5b presents a photo of small dual polarized tunable antenna with a varactor connected to the antenna feed line. The varactor bias voltage was varied from 0 to 9 V. Figure 5c presents measured S\({}_{11}\) as function of varactor bias voltage. The antenna resonant frequency was shifted by 5% for bias voltage between 7 and 9 V. A voltage-controlled varactor may be used also to tune the loop antenna resonant frequency at different antenna locations on the body.