Applying thin-film technology to the manufacture of capacitors has enabled the development of components where both electrical and physical properties can be tightly controlled. These devices can be used to improve the efficiency of antennas used in mobile handsets. Antenna efficiency quantifies the resistive loss of the antenna in terms of the proportion of power that is actually radiated versus the power that is first delivered to it. In order to increase efficiency, it is necessary to match the antenna to the source resistance. See the typical matching scheme in Figure 1.
Figure 1. Standard antenna matching scheme
Inverted-F antennas (Figure 2) are widely used in cell phones, WLAN hardware, and other small wireless devices. The performance is similar to a quarter-wave ground plane.
Figure 2. Inverted F-antenna (left) with capacitive load ( right )
It is found that the capacitive load reduces the resonance length from λ/4 to less than λ/8.
Capacitive load can be applied to different antenna types. For standard planar inverted F-antenna (PIFA), approximately 0.5 pF of loading capacitance is required. The tuning sensitivity to that capacitance is 250 MHz shift per 0.1 pF change.
Figure 3. Typical antenna resonance for different capacitive loads . Y axis is VSWR, x axis is frequency in GHz. F-Antenna Characteristics
We will show how the typical F-antenna characteristics depend on the capacitor value and accuracy.
The following charts display simulation results for a typical PIFA.
Figure 4. Antenna bandwidth at -6 dB level vs. capacitor tolerance for the 1.1 pF capacitor chosen for the load, simulation.
Figure 5. Antenna efficiency vs. capacitance, simulation
Another option to minimize the antenna size is to use an inductive load instead of the capacitive load . The coil inserts a series inductive reactance that cancels the capacitive antenna reactance, thus only the resistive part remains. The disadvantage of this method, compared with the capacitive load method, is a smaller bandwidth (~4%