Communications: directive radio wave systems and devices (e.g. – Directive – Including polarized signal communication transmitter or...
Reexamination Certificate
1999-12-01
2001-10-09
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including polarized signal communication transmitter or...
C342S368000
Reexamination Certificate
active
06300900
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an antenna for transmitting and/or receiving linearly polarized signals.
In the field of telecommunications, a particular carrier frequency is often used to transmit signals having different polarizations. This provides two channels transmitting different information on the same carrier. If the polarization is circular, one channel is allocated to right-hand circularly polarized signals and the other channel is allocated the signals having a polarization in the opposite direction, i.e. left-hand circular polarization. If the polarization is linear, one channel transmits with “parallel” (or “horizontal”) polarization and the other channel transmits signals with a perpendicular (or “vertical”) polarization.
Antennas, and in particular ground antennas, are generally designed to receive and/or to transmit signals in the same direction and with a particular polarization. Thus an antenna must both be pointed correctly at the source of transmission and, in the case of linear polarization, it must also be oriented about its axis to maximize reception of the linearly polarized signal.
When an antenna is designed to receive signals from a geostationary satellite, for example, the axis of the antenna must be pointed towards the satellite and its orientation about its axis must maximize the received signal.
An antenna for linearly polarized signals must be set up with particular care. In particular, if the setup is defective, the antenna transmits both with the required polarization and with the perpendicular polarization, which causes interference signals to be transmitted, which signals can then be received by other receivers designed for the perpendicular polarization. Standards generally require an antenna transmitting with linear polarization in a particular direction to be able to transmit with the perpendicular linear polarization only at a level that does not exceed some limit; this limit level is reached if the antenna's angular orientation departs only approximately 1° from its optimum orientation.
Until now, the polarization of the antenna has been adjusted manually.
It has been found that the result of such adjustment is not always satisfactory. Furthermore, it can vary with time.
Also, the adjustment of the antenna is verified only by confirming that the received signal has the expected qualities. When a television signal is received, for example, the adjustment is deemed to be correct when the quality of the picture is satisfactory. This type of verification is sometimes insufficient, however, especially in the case of a transmit antenna.
SUMMARY OF THE INVENTION
The invention remedies these drawbacks. It provides an antenna provided with means for simple, and preferably automatic, adjustment of its orientation with regard to the linear polarization that it is intended to receive and/or transmit.
The antenna in accordance with the invention is characterized in that it includes at least a first radiating element circularly polarized in one direction and at least a second radiating element circularly polarized in the other direction, the signals with substantially equal amplitudes supplied by the radiating elements circularly polarized in opposite directions being combined to supply a linearly polarized signal, the direction of the linear polarization being a function of a variable phase-shift between the combined signals.
The invention is based on the observation that the combination of two signals with the same amplitude and circularly polarized in opposite directions provides a linearly polarized signal inclined at an angle &thgr; to the axis of the antenna which is equal to half the phase difference between the signals. This property is explained below.
The invention therefore provides means of adjusting the orientation of the antenna, as a function of the direction of linear polarization of the signal to be received or transmitted, in a manner which is simpler and more reliable than mechanical adjustment.
To carry out the adjustment, a perpendicular polarization (or cross polarization) signal is detected and the phase shift is chosen that produces a minimum signal level for the cross polarization. An adjustment based on minimizing the cross polarization is much more accurate than an adjustment based on maximizing the required polarization.
The adjustment can easily be automated.
An adjustment using received signals is equally valid for transmission provided that the frequency band of the transmitted signals is near the band of the received signals. For example, the receive adjustment will be equally valid for transmission in the case of reception at a carrier frequency of 4 gigahertz and transmission at a carrier frequency of 6 gigahertz.
The adjustment is carried out using either a reference signal from the transmitter station, for example a geostationary satellite, or the received payload signal. In the latter case, the adjustment can be carried out automatically and continuously, for example at the start of each call.
In one embodiment of the invention, means are provided for indicating that correct adjustment of the orientation of the antenna with regard to the direction of linear polarization cannot be obtained. For example, if the minimum of the received cross polarization signal exceeds a predetermined threshold, an alarm signal is transmitted. It is preferable for the user to be advised of this situation because under these circumstances there is a risk of transmitting a polluting signal. As an alternative to this feature, or complementing it, means can be provided which automatically prohibit transmission when the minimum cross polarization signal exceeds a threshold.
The number of radiating elements with left-hand or right-hand circular polarization is not limited to one. It can be greater. The only constraints are that the number of elements with left-hand circular polarization must be equal to the number of elements with right-hand circular polarization and that the signals applied to the elements with polarization in opposite directions must have substantially the same amplitude.
When the number of circularly polarized elements in each direction is not less than two, these elements can be used as an active antenna for fine adjustment of pointing, i.e. of the direction of the axis of the antenna.
Pointing can be adjusted either by seeking a signal maximum on the axis of the antenna or a minimum on the axis of the antenna.
For example, to seek a minimum, in particular a null signal, the four radiating elements are fed with signals of the same amplitude but with regularly spaced phases, for example 0°, 90°, 180° and 270°.
The preferred embodiment relates to a reflector antenna, the radiating elements being at the focus of the reflector.
REFERENCES:
patent: 2953781 (1959-11-01), Donnellan et al.
patent: 3373433 (1968-03-01), Blaisdell
patent: 3523294 (1970-08-01), Soji Okamura et al.
patent: 3772701 (1973-11-01), Wilkinson
patent: 5027124 (1991-06-01), Fitzsimmons et al.
patent: 0 437 190 A2 (1991-07-01), None
Bleret Jean
Bourgeois Alain
Alcatel
Mull Fred H
Tarcza Thomas H.
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