The antenna units of communication terminals, particularly antenna units having planar antennas, react to an alteration in their environment by altering their impedance. A discrepancy in the impedance of an antenna unit from a prescribed characteristic impedance value, for example, 50 Ω, in a signal path between the antenna unit and a transmission/reception unit results in the possibility of power losses occurring as a result of reflection of some of the radio-frequency signals propagating in the signal path. Hence, only a portion of the signal power of transmission signals reaches the antenna unit for radiation into free space. In addition, only a portion of the signal power of received signals reaches the receiver.
Component parts and functional units which are electrically arranged in the signal path between the transmission/reception unit and the antenna unit also have separate frequency-dependent impedance and can cause reflections of propagating radio-frequency signals if their impedance differs from the prescribed impedance value. By way of example, an apparatus for wireless information transmission has not only a transmission/reception unit and an antenna unit but also a front end module having at least one duplexer, which is electrically connected in the signal path between the transmission/reception unit and the antenna unit. The duplexer is a necessary functional unit which serves as a frequency filter or filter for propagating radio-frequency signals—possibly at different frequencies—and separates received signals propagating between the antenna unit and the transmission/reception unit, in particular, from transmission signals propagating between the transmission/reception unit and the antenna unit. The duplexer is constructed from electronic components and therefore has a separate frequency-dependent impedance. For different frequency ranges, it is also possible for a plurality of duplexers to be combined in one complex front end module, with the individual duplexers being selected by means of switches, for example.
In order to avoid reflections and hence power losses in the signal path, it is therefore desirable to attain the most uniform impedance possible that has the prescribed impedance value within the signal path. This is achieved by matching the impedances of the antenna unit and of the transmission/reception unit to one another. This can be accomplished by what are known as matching circuits or networks, which connect tunable impedance elements in order to alter the impedance of the signal path and tune it to the prescribed impedance value.
There are already solutions for sensing the signal power of propagating radio-frequency signals in the signal path and using this to determine a power loss which occurs as a result of reflection. This power loss is finally a measure of the mismatch between the antenna unit and the transmission/reception unit and is used for impedance or power matching in the signal path. This means that on the basis of the measured power loss the impedance in the signal path is altered by a matching circuit until the power loss is reduced.
The possible influences of component parts and functional units on the total impedance in the signal path, and hence on the possible reflection of propagating radio-frequency signals, as a result of the separate frequency-dependent impedance of said component parts and functional units mean that precise and meaningful sensing of power losses and hence adequate impedance matching are very difficult.