Method and system for improving isolation in radio-frequency antennas

#The present invention relates generally to an antenna structure and, more particularly, to an antenna structure for use in a mobile terminal.

#In PCS band full duplex systems, the duplex separation of receiving and transmitting bands is so small that it sets very stringent requirements for the duplex filters. To meet these requirements, the overall volume of the duplexer is typically very large. Consequently, the losses in the signal path are very high. Moreover, the thickness of a mobile phone equipped with such a duplexer is not easily reduced.

#It is advantageous and desirable to provide an antenna structure that does not require a large volume, while the isolation in the antenna can be improved.

#FIG. 1 is an isometric view showing the relationship between the Tx/Rx antenna pair, the ground plane and the slot in the ground plane, according to the present invention.

#FIG. 2 is a schematic representation showing the top view of the antenna structure of FIG. 1.

#FIG. 3a is a schematic representation showing a third antenna disposed over the ground plane.

#The radiating characteristics of a planar inverted-F antenna (PIFA) depend on the ground plane as well as on the antenna element itself. Using a transmit antenna as an example, the signal power fed to the radiating element of the transmit antenna also appears as current in the ground plane that is used to short-circuit the radiating element. Thus, in an antenna system where separate antennas operating in substantially the same frequency are used for transmission and reception, and wherein the transmit antenna and the receive antenna use a common ground plane for grounding, the power fed to one antenna also appears in another antenna via the ground plane. In that case, the isolation between the transmit antenna and the receive antenna is effectively diminished.

#It is a primary object of the present invention to improve the isolation between antennas operating in substantially the same frequency band and disposed over a common ground plane for grounding. To achieve this object, the present invention uses a cut in the common ground plane to change the phase of the conducted power in the ground plane.

#As shown in FIG. 1, the antenna structure 1 comprises a ground plane 10, a first antenna 22 and a second antenna 24. The first antenna 22 and the second antenna 24 operate in substantially the same frequency band, so one can be used as a transmit antenna and the other can be used as a receive antenna in a code-division multiple access system. For example, in PCS band, the transmit frequency band is in the range of 1850-1910 MHz, and the receive frequency band is in the range of 1930-1990 MHz. Systems such as CDMA IS-95, GSM-1900 and WCDMA-1900 are operated in this particular frequency band. The first antenna 22 comprises a radiating element 30, a feed line 32, and a grounding strip 34 connecting the radiating element 30 to the ground plane 10 for grounding. The second antenna 24 comprises a radiating element 40, a feed line 42 and a grounding strip 44 connecting the radiating element 40 to the ground plane 10 for grounding. In order to improve the isolation between the first antenna 22 and the second antenna 24, a slot 20 is provided in the ground plane 10 such that the ground plane 10 is separated into a first section 12 and a second section 14, galvanically connected via a connecting section 16. The slot 20 has an effective length L, which is substantially equal to one quarter-wavelength of the center frequency of the radiating elements 30, 40 as shown in FIG. 2. As such, the conducted power in the receive antenna via the ground plane 10 is phase-shifted by 180 degrees as compared to the power radiated in air from the transmit antenna. Consequently, the radiated power and the conducted power compensate each other, and the isolation between the first antenna 22 and the second antenna 24 is improved. It should be noted that the effective length L can be realized by the connecting section 16, as shown in FIG. 2, or by a switch 62, as shown in FIG. 3C.