Design of an Active-Passive Integrated Antenna for Broadband Frequency Monitoring Applications

Wireless communication systems have stringent requirements on the miniaturization and broadband operation of the antennas. Generally, low-frequency antennas are more difficult to achieve a compact size, due to the constraint of electrical size [1]. In addition, high-frequency antennas are small in size, but it is also difficult to cover lower frequency range simultaneously. In principle, the Foster’s reactance theorem illustrates the narrow-band characteristics of an electrically small antenna [2], and the Bode-Fano criterion verifies that a passive antenna can achieve reflection-free matching at only certain frequency points. It is very difficult to achieve a reflection-free matching over the entire frequency band. In addition, a bounded relationship between the reflection coefficient and frequency band is quantitatively established by the Bode-Fano criterion [3]. Therefore, it is a challenge task to design a miniaturized antenna that covers a very wide frequency range from a few kHz to GHz. Such antennas are very useful for applications like the frequency monitoring [4].