The present invention relates generally to radio communication systems and, in particular, to built-in active integrated antennas incorporated into portable communication devices.
Portable communication devices, such as mobile telephone handsets and pagers, are operated in an environment that is power limited. Thus it is important for these devices to be power efficient. To that effect it is well known that most of the power dissipation occurs in the power amplifier used for transmission. Furthermore there is a commercial demand to reduce the size and profile of the portable communication devices.
The power amplifier used by a portable communication device is typically connected to an antenna by a network of lumped passive electrical elements, such as capacitors, resistors and inductors. The network of lumped passive electrical elements is referred to as the output-matching network. The network is used to provide an impedance match between the power amplifier and the antenna. However, the network typically restricts the operating bandwidth of the power amplifier to a narrow band of frequencies. This restriction leads to power amplifier designs that ire inherently narrow-band and specific to certain types of applications.
Therefore it would be a desirable to enable a direct connection between the power amplifier and the antenna that increases the power-added efficiency of the power amplifier, eliminates the lumped passive electrical elements and reduces the size and profile of the antenna.
The prior art describes using the active integrated antenna approach, whereby active devices, such as amplifiers and mixers, are directly connected to an antenna to minimize circuit size and increase the power-added efficiency of the active device. By adopting this approach there are a number of requirements that must be met by the design of the antenna for the resulting circuit to be operable. For example, once combined with a power amplifier, in addition to its original role as a radiating element, the antenna must also serve as a power-combiner and harmonically tuned load.
From hereinafter the antenna will be referred to as the radiating element, as now the power amplifier and radiating element together serve as an active antenna.
The typical load impedance for a power amplifier ranges between 10-25 xcexa9. By directly connecting the output terminal of the power amplifier to the radiating element, an output-matching network is eliminated. Thus the amplifier design is simplified because the classical 50 xcexa9 termination, realized by a network of lumped passive electrical elements, is not required. Furthermore, since there is no longer a need for lumped passive electrical elements connected to the output of the power amplifier, chosen for specific frequencies, the power amplifier itself becomes broadband.
Although there is some mismatch loss between the power amplifier and the radiating element, it is tolerated by tuning the radiating element to provide a class-F or inverse class-F load directly to the amplifier for high-efficiency and high-power operation.
To further elaborate on the efficiency problem, a signal upon entering a power amplifier is typically free of distortion. However, due to the typically non-linear operation of a power amplifier, distortion of the signal occurs within the power amplifier. The distortion manifests itself as harmonics of the fundamental (carrier) frequency f0 of the signal that can be easily identified in the frequency domain. The second (2f0) and third harmonics (3f0) of the fundamental frequency f0 of a signal typically consume the most power of all the harmonics generated; thus, these two harmonics are of primary concern as they lead to the largest reductions in power added efficiency within the power amplifier.
However the presence of harmonic frequency components alone do not lead to the greatest reductions of the power added efficiency of a power amplifier. It is only when the harmonic voltages and currents are substantially in phase with one another within the power amplifier resulting in heat dissipation will the power added efficiency of the power amplifier suffer substantial reductions. Furthermore it should be noted that at low input power levels there is little or no harmonic energy but the efficiency is typically quite low. This is due to the fact that the energy dissipated within the power amplifier is typically quite high.
Class-F and inverse class-F load impedances can be used to provide impedance matching at the output of a power amplifier. The class-F load provides an optimum power match for the power amplifier at the operating frequency f0, a short circuit at the second harmonic 2f0 and an open circuit at the third harmonic 3f0. The inverse class-F load provides an optimum power match for the power amplifier at the operating frequency f0, an open circuit at the second harmonic 2f0 and a short circuit at the third harmonic 3f0. Inherently these classes of impedances provide the desired harmonic loading for a power amplifier to reduce the amount of power transferred to the transmission of the second and third harmonics, thus raising the efficiency of the power amplifier. The short circuits and open circuits for the harmonics at the load cause the voltages and currents to be reflected away from the load. By generating harmonics and then reflecting them back from the load creates a situation where the voltage and currents at the output of the power amplifier are sufficiently out of phase, such that the power dissipation is minimized by effectively minimizing the overlap of voltages and currents of the harmonics.
Among the antennas that can facilitate this type of design, the planar inverted-F antenna (PIFA) is one of the most promising. The planar inverted-F antenna can be tuned to provide both class-F and inverse class-F load impedances. The planar inverted-F antenna serving as the radiating element also provides an attractive radiation pattern that provides a null towards the user, thus reducing potential biological interaction, and has a cross polarization pattern that is desirable for the urban multipath environment.
A planar inverted-F antenna of the prior art consists of a planar radiating element, a feed pin, a ground plane and a shorting plate of narrower width than that of the shortened side of the planar radiating element, The degree of freedom used to design and tune planar inverted-F antennas is the width of the short circuit plate. As such the prior art lacks features that make it flexibly tunable. In particular, the prior art is characterized by a difficulty in utilizing the classic rectangular planar inverted-F antenna structure, having only a single narrow shorting plate, to realize class-F and inverse class-F impedances over a wide range of real input impedances.
The present invention overcomes the above-identified deficiencies in the art by providing a low-profile, scalable radiating element which enables the power amplifier to be operable at a plurality of frequency bands, thus making the power amplifier effectively broadband. Furthermore, this invention relates to the tuning of planar radiating elements that may be used to provide optimal impedance matching between a power amplifier and free space, so that the power-added efficiency of the power amplifier is substantially increased.
An aspect of the invention is to provide a symmetrical planar radiating element structure defined by at least one line of symmetry along the planar radiating element surface and a method of tuning said symmetrical planar radiating element structure to realize either a class-F or inverse class-F load impedance, such that the input terminal of the radiating element can be directly connected to the output terminal of a power amplifier via a length of transmission line.
Another aspect of this invention is to provide a structure and method of tuning a rectangular planar radiating element to realize either a class-F or inverse class-F load impedance, such that the input terminal of the radiating element can be directly connected to the output terminal of a power amplifier via a length of transmission line.
The present invention also provides a means for harmonic tuning of the output of the power amplifier, in addition to providing either class-F or inverse class-F load impedances.
More specifically, the present invention provides a structure of an active planar inverted-F antenna that makes use of two shorting pins and a feed pin to realize inverse class-F impedances and to provide harmonic tuning for a power amplifier. This invention provides a planar inverted-F antenna structure that can realize class-F and inverse class-F impedances over a wide range of real input impedances. The elements of the invention combine with classic planar inverted-F antenna structure to provide a radiating element that can be tuned to realize class-F and inverse class-F impedances over a wide range of real input impedances. The second shorting pin allows the response of the radiating element to be tuned at the second and third harmonic. The short section of transmission line allows further fine-tuning at the fundamental frequency and its harmonics.
Yet another aspect of the invention is to provide a method of tuning a planar inverted-F antenna, for either class-F or inverse class-F impedances, to provide optimal matching at a single frequency.
Yet another aspect of the present invention is to provide a method of tuning the planar inverted-F antenna to operate at different transmission frequencies once it has been optimized for a single transmission frequency.
The invention utilizes two shorting pins, instead of a single shorting plate, connected between the top plate and the ground plane to tune the radiating element to either class-F or inverse class-F impedances over a wide range of frequencies. In doing so, the present invention also provides for a radiating element with co-polarized electromagnetic field components and cross-polarized electromagnetic field components.
Another aspect of the invention is to provide a method of tuning an offset top loaded monopole, for inverse class-F impedances, to provide optimal matching at a single frequency.
Yet another aspect of the present invention is to provide a method of tuning the offset top loaded monopole to operate at different transmission frequencies once it has been optimized for a single transmission frequency.
Other aspects and features of the present invention will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific embodiments of the invention.