Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
2003-03-03
2004-12-28
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
Reexamination Certificate
active
06836257
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is that of wideband antennas with hemispherical or near-hemispherical radiation patterns. More specifically, the invention relates to helical antennas of this type.
The antenna of the invention is found especially in applications of satellite mobile communications between fixed and/or mobile users of all types, for example aeronautical, maritime or terrestrial communications. In this field, several satellite communications systems are implemented or are now being developed (these include the INMARSAT, INMARSAT-M, GLOBALSTAR, and other systems). These antennas are also valuable in the deployment of personal communications systems (PCS) using geostationary satellites.
The systems are designed to provide terrestrial users with new communications services (multimedia, telephony and other services) through satellites. By means of geostationary or orbiting satellites, they provide global terrestrial coverage. They have to be similar to terrestrial cellular systems in terms of cost, performance a size. Thus, the antenna located in the user's terminal is a key factor in size reduction.
Systems of this kind are described especially in Howard Feldman, D. V. Ramana: <<An introduction to Inmarsat's new mobile multimedia service>>, Sixth International Mobile Satellite Conference, Ottawa, June 1999, and J. V. Evans: <<Satellite systems for personal communications>>, IEEE A-P Magazine, Vol. 39, No. 3, June 1997.
For all these systems, which provide links with geostationary satellites, the very different values of incidence of the signals received or sent require that the antennas should possess a radiation pattern with hemispherical or near-hemispherical coverage. Furthermore, the polarization must be circular (left-hand or right-hand) with a ratio below 5 dB in the useful band.
More generally, the invention can be applied in all systems requiring a small-sized antenna, the use of a very wide band and circular polarization.
In these different fields of application, the antennas must often have the above characteristics either in a very large bandwidth of about 10% or in two neighboring sub-bands corresponding respectively to reception and to transmission. It is also essential that the size and weight should be reduced to the greatest possible extent.
The invention can be applied especially to quadrifilar antennas.
A quadrifilar antenna is formed by four radiating strands. An exemplary quadrifilar antenna is described in detail in A. Sharaiha and C. Terret, “Analysis of quadrifilar resonant helical antenna for mobile communications” (IEE Proceedings H, vol. 140, no 4, August 1993).
According to this embodiment, the radiating strands are printed on a thin dielectric substrate and then wound about an RF-transparent cylindrical support. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end.
This antenna requires a power circuit that excites the different antenna strands by signals having the same amplitude in phase quadrature. This function may be performed by means of structures comprising 3 dB−90° couplers and a hybrid ring. This assembly can be made in printed circuit form and placed at the base of the antennas. Thus, a simple but bulky power supply system is obtained.
As mentioned further above, it is desirable for the antenna (including its supply) to be as small-sized and lightweight as possible.
Several solutions have been proposed to this end.
For the power supply system, a solution has been proposed based on the making of three hybrid couplers designed as semi-localized elements and printed in the prolongation of the antenna. This technique is described especially in the patent FR-96 03698, filed on behalf of the present applicant.
The antenna itself has three known improvements in particular.
A first approach is described by B. Desplanches, A. Sharaiha, C. Terret in <<Parametrical study of printed quadrifilar helical antennas with central dielectric rods>> (Microwave and Opt. Technol. Letters, Vol. 20, No 4, Feb. 20, 1999). This solution of miniaturization augments the permittivity of the cylindrical support around which the substrate is wound.
This technique reduces the height by about 30 percent. It is also very simple to make. However, it has the drawback of reducing the bandwidth. Furthermore, it is costly.
According to a second solution, the height of the antenna may be reduced by cutting each strand into two distinct parts having a length of about &lgr;/4 with a symmetry with respect to the middle of each strand. This technique is described especially in the article by D. F. Filipovic, M. Ali Tassoudji, E. Ozaki: <<A coupled-segment quadrifilar helical antenna>> (MTT-S Symposium on technologies for wireless applications, Vancouver, Canada, 1997).
Again, this gives a satisfactory reduction in height (by 28.4% in the example given), without any modification in the radiation pattern and the ratio of ellipticity. Furthermore, the structure proves to be simple.
By contrast, the bandwidth is reduced to 3% for a SWR value<2. Furthermore, an antenna of this kind requires difficult adjustments of the coupling between the active strands and the passive strands.
A third proposal for reducing the height of the printed quadrifilar helix (PQH) antenna is to wind each strand of the helix according to a non-linear equation as described in M. E. Ermutlu: <<Modified quadrifilar helix antennas for mobile satellite communication>> (IEEE APS Conference on antennas and propagation for wireless communications, Piscataway, N.J., 1998). This approach can give a size reduction of 14%.
However, this technique introduces a deterioration of the ratio of ellipticity throughout the coverage.
In other words, the known techniques used to reduce the height of the antenna show major defects in terms of characteristics. The operation of reduction leads to the deterioration of the bandwidth and/or of the ratio of ellipticity.
Furthermore, as mentioned further above, it is often desirable to have a large bandwidth and/or bandwidths corresponding to transmission and reception respectively.
The patent FR-89 14952 filed on behalf of the present applicant describes a type of antenna particularly suited to such applications.
This antenna, known as a printed quadrifilar helix (PQH) antenna, possesses characteristics similar to those laid down by the criteria set forth, in a frequency band generally limited to 6% or 8% for an SWR of less than two. A wider band operation can be obtained by using two-layer PQH antennas. These antennas are formed by the concentric “nesting” of two electromagnetically coupled coaxial, resonant quadrifilar helixes. The assembly works like two coupled resonant circuits whose coupling separates the resonant frequencies. Thus, a two-layer, resonant, quadrifilar helix antenna, according to the technique described in FR-89 14952, is obtained.
This technique has the advantage of requiring only one power supply system and of enabling dual-band and wideband operation.
However, it has the drawback of requiring the manufacture of two printed and nested circuits and of offering only a small bandwidth in each sub-band.
A quadrifilar antenna is formed by four radiating strands. An exemplary embodiment is described in detail in A. Sharaiha and C. Terret, “Analysis of quadrifilar resonant helical antenna for mobile communications,” (IEE—Proceedings H, vol. 140, No. 4, August 1993).
According to this embodiment, the radiating strands are printed on a thin dielectric substrate and then wound about an RF-transparent cylindrical support. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end.
This antenna requires a power circuit that excites the different antenna strands by means of signals having the same amplitude in phase quadrature. This function may be performed by means of structures comprising 3 dB−90° couplers and a hybrid ring. This assembly can be ma
Blot Jean-Pierre
Louvigne Jean-Christophe
Sharaiha Ala
Dieu Minh
France Telecom
Kinney & Lange , P.A.
Wong Don
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