Communications: radio wave antennas – Antennas – Slot type
Reexamination Certificate
2000-08-28
2002-10-15
Phan, T (Department: 2821)
Communications: radio wave antennas
Antennas
Slot type
C343S702000
Reexamination Certificate
active
06466176
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to antennas and, more particularly, to small and high efficiency antennas for mobile and handset communication devices.
BACKGROUND OF THE INVENTION
Mobile communication devices are becoming smaller as the technology is developed. For an antenna to operate properly, it should usually be about half a wavelength in size, except for monopole-like antennas (which normally operate above a ground plane), where a quarter wavelength is required. For advanced mobile communication devices, e.g., cellular handset units, such dimension are impractical since the overall handset dimension is smaller than half a wavelength of the appropriate frequency.
Using small antennas reduces their efficiency, and hence requires higher power to be supplied in order to operate the device. Higher power causes shorter battery cycles between charging and increases the radiation into the user's head/body. The level of power radiated into the human head is most significant, and serious limitations and specifications are prescribed in order to protect the users.
Operation of such devices adjacent to a human body also changes the field and/or current distribution along the antenna, and hence changes its radiation pattern, as well as the radiation efficiency. Practically speaking, the reduction in efficiency may be even in the range of 10-20 dB or more. The result is a requirement for higher power to operate the device with the consequent disadvantages described above. The use of external whip antennas, such the “STUBBY” or retractable antennas, is also inconvenient, as the antennas are often “caught up” inside the pocket. They also detract from the aesthetic appearance of the mobile communication device and most important—the radiation pattern is quasi-omni, so no enhancement is achieved in radiation at the user's head/body.
Internal antennas supplied by several companies are relatively inefficient as compared to external antennas. Furthermore, these known internal antennas generally do not decrease the radiation into the user's head/body, and in many cases even increases such radiation. The antenna gain is also generally poor (especially while used adjacent to the head/body), and the SAR (Specific Absorption Ratio) results are generally high.
Another problem in the known internal antenna is the narrow bandwidth of operation. In addition to the narrow bandwidth where the input impedance is matched the radiation efficiency is even further reduced. The latter is considered an even more difficult problem in cases where dual frequency bands or triple-band operations of the mobile communication devices are required, such as cellular GSM 900/1800, 900/1900, 900/1800/1900 MHz, etc.
Internal antennas for mobile communication devices are known that utilize a resonant radiation element as the main radiator. In particular, printed antennas, e.g. patches and slots, are very convenient to use because of their ease of manufacture, their low profile, and their low production cost. If such printed elements could be used in mobile communication devices with respect to efficiency, gain, impedance matching and reproducibility, it would be the best choice. Unfortunately, such elements, because of the small size of the mobile communication device, will show very low efficiency and hence low gain, and it will be difficult to match their impedance to that of the mobile communication device.
Generally, slots excited by a feed line (e.g., by microstrip or stripline structures) or by a coax cable, are usually narrow band. In order to achieve matching of the slot even over a narrow band, the excitation of the slot is generally made off-center, to reduce the input impedance of the slot, which is naturally very high. U.S. Pat. No. 5,068,670 by one of the inventors in this application and hereby incorporated by reference describes a broadband slot antenna achieved by adding matching networks at both sides of the slot. In the preferred embodiment, the feed lines are located off-center of the slot.
The direction of maximum radiation of an off-center excited slot is changed with frequency due to the asymmetrical electric and magnetic field distributions excited along the slot. While narrow bandwidth slots are not significantly affected by this phenomenon, broadband slots are indeed affected. The best solution is to excite the slot symmetrically by dual feed and load lines, which may be split from a single excitation feed. Each of the strip arms has a dual matching network in order to widen the bandwidth of the antenna. The length and width of each arm may be equal in order to achieve full symmetrical structure, but may also differ in order to maximize the bandwidth. If the arms art not identical, there will be some squint with frequency.
The slot may be a non-resonant one, by making it open at both ends (“open-ended”), or a resonant one, by making it closed at both ends (“short-ended”). The reaction efficiency depends on the field distribution—amplitude and phase, along the slot. The fields in short-ended slots mush vanish at both ends of the slot; and since they are continuous, their value at any point along the slots cannot reach the required level as with shorter slots. Therefore, short-ended slots are relatively large, usually in the range of half wavelength at the operation frequency.
The fields in open-ended slots may have finite value at their ends and should not vanish. It follows that a reasonable value of the field can be reached even for relatively short-length slots. The excitation point may then be optimized for single or dual feeds. It should be taken into consideration that radiation pattern will be different from the usual one. Further, the load type of the strip for open-ended slots would preferably be of the form of a short circuit, to eliminate a floating ground at the far end of the slot. As a result, this configuration is more complex to match by means of the relative part of the slot impedance. Furthermore, a floating ground would decrease the antennas efficiency.
EP 0924797 describes a slot antenna configuration in which the slot is curved along two axes, and is excited at its center point by a coax cable. There are a number of disadvantages of such configuration as suggested by this patent. Thus, the matching of such a slot is very difficult due to the centered excitation point (as described above and in U.S. Pat. No. 5,068,670). In addition, the part of the slot which contributes to the radiation in the desired direction is very small while, due to the folded arms of the slot which are parallel, the fields are opposite in polarization and hence cancel the radiation at most desired directions. Further, the excitation is complex and costly to implement. Finally, slots which are open-ended at one end are less efficient as compared to short-ended slots, and cause radiation in undesired directions. The radiation pattern will be asymmetrical due to the radiation from the open end of the slot, since the fields do not vanish, as above-mentioned.
U.S. Pat. Nos. 5,929,813 and 6,025,802 describe similar antennas. Such antennas are actually loop antennas where a “wired slot” generates a loop antenna. There are a number of disadvantages of such configuration as suggested by this patent. Thus, “wired slot” is open at the connecting points, is cut along the edge of the antenna and is also folded on the metal sheet, hence it causes radiation in undesired directions and with opposed (horizontal) polarization. The “wired slot” is excited by the antenna connector very close to the antenna (and telephone) edge; hence, radiation at the user's head is not reduced. Actually, because of the phone's PCB, which significantly contributes to the radiation at CDMA/TDMA/GSM frequencies (800 and 900 MHz), it would appear that the radiation at the user's head is even increased.
Further, in the embodiment of a dual frequency operation according to these referenced patents, the radiation pattern in the higher band has nulls, or at least significant reduction at
Kadichevitz Michael
Maoz Joseph
G. E. Ehrlich Ltd.
In4tel Ltd.
Phan T
LandOfFree
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