Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1998-07-08
2002-01-01
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S562100, C455S575100
Reexamination Certificate
active
06336036
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to radiotelephones, and, more particularly, to retractable antenna systems for use with portable radiotelephones.
BACKGROUND OF THE INVENTION
Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link (including optionally both voice and data) to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems.
Cellular telephone systems are commonly employed to provide voice and data communications to a plurality of subscribers within a prescribed geographic area. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. Recently, digital cellular radiotelephone systems such as those designated IS-54B (and its successor IS-136) in North America and GSM in Europe have been introduced and are currently being deployed. These systems, and others, are described, for example, in the book entitled
Cellular Radio Systems,
by Balston, et al., published by Artech House, Norwood, Mass. (1993). In addition to the above systems, an evolving system referred to as Personal Communication Services (PCS) is being implemented. Examples of current PCS systems include those designated IS-95, PCS-1900, and PACS in North America, DCS-1800 and DECT in Europe, and PHS in Japan. These PCS systems operate at approximately the 2 gigahertz (GHz) band of the radio spectrum, and are typically being used for voice and high bit-rate data communications.
Many radiotelephones, and in particular handheld radiotelephones such as those typically used with cellular telephone systems, employ retractable antennas which may be extended out of, and retracted back into, the radiotelephone housing. Typically, such retractable antennas are electrically connected to a printed circuit board located within the housing of the radiotelephone that contains signal processing and other radio frequency circuitry. In order to maximize the transfer of power between the antenna and this radio frequency circuitry, the antenna and the radio frequency circuitry are typically interconnected such that the impedance of the antenna and the signal processing circuit are substantially matched. As many radiotelephones use 50 ohm impedance coaxial cable or microstrip transmission lines to connect the antenna to the radio frequency circuit, such matching typically comprises mechanically adjusting, electrically tuning or otherwise configuring the antenna so that it exhibits an impedance of approximately 50 ohms at its connection with the coaxial cable or microstrip transmission line.
Unfortunately, however, matching the impedance of a retractable antenna is more difficult, as the impedance exhibited by the antenna is generally dependent on the position of the antenna with respect to both the housing of the radiotelephone and the printed circuit board which contains the radio frequency circuitry. As these respective positions change when the antenna is moved between the extended and retracted positions, the antenna typically exhibits at least two different impedance states, both of which should be matched to the 50 ohm impedance of the feed from the printed circuit board. Accordingly, with retractable antennas, it is generally necessary to provide an impedance matching system that provides an acceptable impedance match between the antenna and the radio frequency circuitry both when the antenna is retracted and extended.
A number of different matching techniques are conventionally used with retractable antennas. For instance, many radiotelephones with retractable antennas employ dual impedance matching circuits, one of which is associated with the extended antenna position and the other with the retracted position. These matching systems typically comprise two or more resonant circuits and switches for switching between these circuits as a function of the position of the antenna. Other radiotelephones only provide a single matching circuit (which is switched in when the antenna is in the extended position), and operate without the benefit of any matching circuit when the antenna is in the retracted position. In other designs, a half-wavelength (&lgr;/2) antenna may be used so that the antenna radiates as a half-wavelength structure in the extended position and as a quarter-wavelength (&lgr;/4) antenna in the retracted position (as the retracted portion of the antenna does not radiate). With this arrangement, impedance matching is typically only required in the extended position, as the antenna may be designed to have a natural impedance reasonably close to 50 ohms in the retracted position. Still other radiotelephones use parasitic elements or printed transformer segments to match the impedance of the antenna to the radio frequency circuit board. However, each of the aforementioned techniques typically require some sort of matching means, which in turn requires space within the housing (or antenna) for matching components, and which additionally increases the overall cost of manufacturing the radiotelephone.
The aforementioned matching problems are further compounded in “dual-band” radiotelephones that are designed to transmit and receive signals in two or more widely separated frequency bands. In such phones, it is desirable to provide a single antenna structure that can operate in both bands. For example, a cellular telephone may operate in a conventional analog (AMPS) band at around 800 MHz and also in a PCS band at around 1900 MHz.
As the impedance seen at the base of the antenna is usually a function of frequency, antenna systems for such dual-band radiotelephones may provide separate matching networks for each of the two frequency bands of operation. Accordingly, if retractable antennas are used on such radiotelephones, it is often necessary to provide as many as four matching networks to ensure that an acceptable impedance match is achieved at each frequency of operation and each possible antenna position (i.e., extended or retracted).
Helix antennas provide an advantage over rod or monopole antennas in applications such as cellular telephones as they typically are shorter. This class of antenna refers to antennas which comprise a conducting member wound in a helical pattern. As the conducting member is wound about an axis, the axial length of a quarter-wavelength or half-wavelength helix antenna is considerably less than the length of a comparable quarter-wavelength monopole antenna, and thus helix antennas may often be employed where the length of a quarter-wavelength monopole antenna is prohibitive. Moreover, although a half-wavelength or a quarter-wavelength helix antenna is typically considerably shorter than its half-wavelength or quarter-wavelength monopole antenna counterpart, it may exhibit the same effective electrical length.
Several dual-band helix antenna systems have been proposed. For instance, U.S. Pat. No. 4,554,554 to Olesen et al. discusses a quadrifilar helix antenna which includes PIN diode switches along each of its elements to provide means for selectively resonating the antenna at one of two distinct frequencies by changing the electrical length of the elements.
Similarly, U.S. Pat. No. 4,494,122 to Garay et al. discusses an antenna system comprising an upper radiating element and a tank circuit which resonate at one frequency, and a helical element and associated sleeve member which resonate at a second frequency. While this apparatus is potentially shorter than a conventional sleeved dipole, it is still relatively large, and the usable operating bandwidth of the antenna about each resonant frequency is very small, such that this antenna system is not suitable for many potential dual-band applications such as cellular telephone.
U.S. Pat. No. 4,442,438 to Siwiak et al. discusses an antenna system com
Chow C.
Ericsson Inc.
Hunter Daniel
Myers Bigel & Sibley & Sajovec
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