Communications: radio wave antennas – Antennas – With coupling network or impedance in the leadin
Reexamination Certificate
1997-10-16
2001-01-09
Weng, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With coupling network or impedance in the leadin
C343S713000, C343S715000, C333S109000
Reexamination Certificate
active
06172651
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the transmission of radio frequency signals through a dielectric wall (e.g. a vehicle window) and is illustrated in the context of a dual-band, glass mount mobile antenna system.
Window mounted antennas have been welcome for many years in mobile radio links, especially in 800 MHz cellular telephone service (sometimes known by the acronym “AMPS”) due to their obvious advantages to the consumer. These advantages include the ease of installation and the fact that it is not necessary to drill a hole in the vehicle, which would detract from its value. Others include enhancing the signal strength for better communication quality, and moving radiation outside the vehicle. Much effort has been devoted to designing effective window mounted antenna systems for mobile radio links.
A new type of cellular service, known in the United States as PCS, is growing in popularity. This service occupies frequencies between 1500 and 2000 MHz. (In the United States, the PCS band is at 1900 MHz. In Europe this service (termed PCN) is at 1800 MHz. In Japan this service (termed PHS) is at 1500 MHz.). This alternate cellular service creates a potential compatibility problem with the existing, well-established 800 MHz cellular infrastructure. Many effort have been made to address these comparability issues. The most effective solution seems to be the emergence of multi-mode, multi-band handsets that automatically adapt to the service available in a given area. For example, Qualcomm offers a dual-band, dual mode phone known as the QCP-2700, which provides service over both the 800 MHz AMPS band and the 1900 MHz CDMA PCS band. Ericsson has similar offerings, such as its models PD 328 and PD 398, which each provides both AMPS and PCS service.
These dual-band handsets pose a significant engineering challenge, namely the design of a single antenna that provides good performance at both the AMPS and PCS bands. This challenge is compounded when the antenna is vehicle-mounted and fed through-glass. The through-glass coupling system must provide high efficiency coupling (and in some instances antenna matching) at both AMPS and PCS frequencies. Moreover, the bandwidth required at each band is large (e.g. up to 11% in the PCS bands), posing a further engineering obstacle.
A variety of through-glass feed techniques are known, as illustrated by the cited patents. Many are capacitively-coupled systems. Examples include U.S. Pat. No. 4,089,817 (Kirkendall, 1978), U.S. Pat. No. 4,839,660 (Hadzoglou, 1989), U.S. Pat. No. 4,992,800 (Parfitt), U.S. Pat. No. 4,857,939 (Shimazaki) and U.S. Pat. No. 4,785,305 (Shyu). In addition to capacitive coupling, these systems also generally employ LC impedance matching networks.
There are several problems with the foregoing designs. First the capacitive coupling patches cannot be large in comparison with the operating wavelength. Therefore; high impedance coupling (several hundred ohms) cannot be avoided. This leads to high loss due to the leakage of electrical field at high frequencies. Also, at high frequency bands like PCN/PCS, even a small patch no longer behaves as a lumped capacitor element. Due to the thickness of vehicle glass and various stray capacitances, such capacitive coupling circuits can bypass the signal and make it more difficult to match the (typically) high impedance of the antenna to a 50 ohm system. Additionally, the high impedance coupling creates a moisture sensitive structure. U.S. Pat. No. 4,764,773 (Larsen, 1988) describes a better coupling structure to improve performance in the presence of moisture, but it is still subject to the patch size limitation.
Design of a vehicle-mounted radiator also poses difficulties at PCS frequencies. Collinear array whips are desirable for mobile service due to their gain in the vertical plane. However, such whips do not have uniform current distribution. The lower section of the array has the highest current and produces the strongest radiation. But in most vehicle mounting arrangements the lower section of the whip is blocked by the vehicle roof, causing severe pattern distortion and deep nulls. This situation becomes worse at the 1.5-2 GHz PCN/PCS bands simply because the length of radiator is only half that at the 800 Mhz hand due to the doubling of the frequency.
Elevated-feed whips are sometimes employed to avoid the pattern distortion caused by vehicle roof blockage of radiation. But elevated-feed antennas are not readily matched for broadband operation (i.e. 11% for DCS-1800). Moreover, many such antennas, employing decoupling sleeve or slots, have low impedance feeds (e.g. 50 ohms). High impedance capacitive-feed systems thus pose large impedance transitions. Impedance transformation at PCS frequencies by use of conventional LC circuits is very inefficient due to the high loss of such circuits at these high frequencies.
U.S. Pat. No. Re.33,743 (Blaese) proposes a capacitively coupled antenna system for coupling a coaxial cable through glass to a low impedance quarter-wave whip. But in the PCS bands, the suggested antenna is only 1.7″ long. Again, this is completely below the roof line of vehicle, causing severe pattern distortion and deep nulls.
To avoid some of the problems associated with capacitive coupling, a coupling arrangement employing resonant cavities has been proposed. U.S. Pat. No. 4,939,484 (Harada), for example, discloses a through-glass coupler employing a pair of tuned helix cavities. Unfortunately, the liarada cavity aperture must be sized to satisfy a ⅓ object frequency criterion, as described in the patent. That is, for 800 MHz, the helix should be designed for 266 MHz. The resulting cavity has a Q of over 1000 and sufficient coupling aperture. But at the 1.8 GHz band, the helix must be designed for 600 MHz. A 600 MHz helix cavity has a small aperture which is nearly half of the cellular band. A significant drop of unloaded Q is unavoidable due to the thin helix, and the coupling coefficient is not sufficient to provide an 11% bandwidth. Other drawbacks of such helix cavity couplers including highly critical tuning characteristics, and difficulties in mass production due to their complex 3D structure. Impedance matching is also difficult to implement in the cavity context.
In my U.S. Pat. No. 5,471,222, a pair of TE
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high dielectric, constant-Q Ba-Bd-Ti oxide (ceramic) resonators were employed to overcome various problems of prior art PCS band through-glass couplers. This approach proved to be highly efficient, with insertion losses of only 0.5 dB through 5 mm automobile glass at 1.8 GHz. However, this arrangement proved sensitive to de-tuning in the field. Additionally, it suffered from a high manufacturing cost.
In my U.S. Pat. No. 5,451,966, a rectangular slot coupling scheme was employed to replace the expensive Ba-Nd-Ti Oxide ceramic. This arrangement built on the concept of dual-cavity coupling, where coupling is through an aperture.
The idea of slot coupling on an MSA (microstrip antenna) originated by Pozar. It provides a means to overcome the narrow band nature generally associated with MSAs. A “doggie bone”-shaped slot suggested by Pozar significantly increases the magnetic polarisability on the slot. This allows a short slot to achieve the necessary coupling while at the same time keeping backward emissions low.
Pozar and other researchers' work was basically limited to numerical solutions of the slot-fed microstrip antenna and multilayer arrays on a ground plane. But the bandwidth advantages of this type of MSA can be used to enhance the performance of the planar slot-cavity coupler.
In my above-referenced pending application, an annular ring aperture is employed for through-glass coupling. It is understood that in the rectangular slot design, the requirement for a tight coupling coefficient leads to an increase in slot length, which increases the level of backwards radiation. A major advantage of the annular ring aperture coupler over rectangular slot coupling is that it provides an increa
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Larsen Electronics, Inc.
Malos Jennifer H.
Weng Don
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