| 摘要 |
It is highly desired both for mobile communication industry and public safet y systems, that a whip antenna can work in a very wide VHF band with good impedance matching, for example from 138MHz to 174MHz with VSWR=1.5:1. However, seldom products are available in the market. In this invention, the broadband is obtained by the matching circuit integrated on two PCBs without modifying the whips. The PCBs are sma ll enough to be mounted inside a protective housing as small as regular mobile antenna base. Two different matching circuit topologies are proposed for different whip length s, i.e., for quarter wavelength antennas and half wavelength antennas. The first one is composed of one air coil and three capacitors in series, and one air coil and one capacitor in shunt. The whip is serially connected to a capacitor. Then an air coil and a capacitor are connected to the circuit in shunt to extend the bandwidth. A series capacitor and a shunt capacitor are then added to improve the impedance matching. A series resonant network composed of an air coil and a capacitor is used to further improve the bandwidth. It is known that as the whip length is increased up to around half wavelength, the antenna becom es more difficult to be matched in a wide band. In the second topology, a T-type transforming network composed of two series air coils and one shunt capacitor is used to obtain impedance match around the center frequency. Then a shunt resonant network composed of one air coil and one capacitor in parallel, and a series resonant network composed of one air coil and one capacitor in series are used to extend the bandwidth. A shunt capacitor is used to further improve the impedance matching. Although the present invention is described to VHF band antennas working in 138~174MHz, it is straightforward to apply the present invention to other frequency band by adjusting the whip length and circuit element values.
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