Wireless communications apparatus often include a transmitter, a receiver, and an antenna. Information signals can be encoded and amplified by the transmitter and transferred to the antenna for radiation to the environment. Information signals received by the antenna can be transferred to the receiver where they can be decoded and amplified. Impedance mismatches between the transmitter and the antenna or between the antenna and receiver can cause reflection of transmitted signals or received signals, respectively, and an associated loss of power in the transferred signals. Efficient power transfer of transmission signals can occur when the impedance presented between the transmitter and the antenna are about equal (i.e., matched). Similarly, efficient power transfer of received signals can occur when the impedance between the antenna and the receiver are about equal.
Conventional solutions for providing efficient power transfer include matching the impedance of the transmitter and antenna and/or matching the impedance of the receiver and antenna during manufacturing of the communications apparatus. During use, however, the impedances of the antenna, transmitter, and receiver may change, resulting in impedance mismatches.
Impedance mismatches may be caused, for example, by motion of the communications apparatus and/or the presence of reflective or shielding objects in its operating environment. When impedances are matched for one frequency band, they can become mismatched during operation in other frequency bands. Such a mismatch can become particularly apparent for communications apparatus that operate in multiple frequency bands, such as those provided for Global System for Mobile Communication (GSM), Digital Communications System (DCS), Advanced Mobile Phone Services (AMPS), Personal Communication Services (PCS), or wireless local area network (WLAN). Moreover, miniaturization of communications apparatus and the use of small antennas, such as patch antennas, can increase the variation of the impedance of the antenna, transmitter, and receiver as a function of frequency.
When the transmitter and receiver share an antenna, adjusting the impedance match between the transmitter and the antenna can affect the impedance match between the receiver and the antenna and vice versa.
For such apparatus, impedance mismatch may cause unacceptable power loss in signals transmitted or received as the apparatus operates between transmit and receive frequency ranges and between frequency bands.