Transmitter that selectively polarizes a radio wave

Details Transmitter that selectively polarizes a radio wave Transmitter that selectively polarizes a radio wave

1997-10-20

2001-01-30

Hunter, Daniel S. (Department: 2749)

Telecommunications

Transmitter

Diversity

C455S101000, C455S102000, C455S103000, C343S853000

Reexamination Certificate

active

06181920

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to the field of transmitters, more particularly, to a transmitter that transmits selectively polarized radio waves.
BACKGROUND OF THE INVENTION
Radio terminals that communicate with orbiting satellites are gradually coming into commercial use. These terminals can communicate bi-directionally with direct broadcast TV and telephone satellite systems for providing communication services directly to households via dish antennas. In such communication systems, a satellite uses polarized radio waves (beams) to communicate with terminals located in a service area.
Satellite communication systems commonly use various types of radio wave polarization. A radio wave may be polarized linearly, for example, vertically or horizontally, or it may be polarized non-linearly, for example, elliptically or circularly. The polarization of a radio wave is defined by the direction in which electric vectors are aligned during at least one full cycle. Generally, both the magnitude and the direction of the electric vectors vary non-linearly during each cycle. Usually, such non-linearly varying electric vectors map out an ellipse on a plain normal to the direction of propagation at a point of observation. In this case, the non-linear polarization of the radio wave is said to be elliptical. The minor-to-major-axis ratio of the ellipse, which is expressed in decibels, is called the ellipticity of the radio wave. A linearly polarized radio wave has an ellipticity of infinity, that is, the minor-to-major-axis ratio is zero. A circularly polarized wave has an ellipticity of zero dB, that is, the minor-to-major axis ratio is unity. The linearly polarized wave is, therefore, defined as a transverse electro-magnetic wave whose field vector at a point in a homogenous isotropic medium at all times lies along a fixed line. The circularly polarized wave is similarly defined as a electro-magnetic wave for which the electric and/or magnetic field vector at a point describes a circle.
A circularly polarized wave may have a right-hand circular polarization or a left-hand circular polarization. A right-hand circular polarization occurs when, for an observer looking in the direction of wave propagation, the rotation of the electric field vector in a stationary transverse plain is clockwise. Conversely, the rotation is counter-clockwise for a left-hand polarization. A circularly polarized wave may be produced by a helical beam antenna having a corresponding right-hand or left-hand sense. The circularly polarized wave may also be produced by the coexistence of two linearly polarized waves, such as a vertical and a horizontally polarized waves, each having the same amplitude but a 90° phase difference between them. If the linearly polarized waves are not equal in amplitude or have a phase difference other than a 90° relationship, the resulting radio wave will be polarized non-linearly. If for example, the amplitude of the vertical polarized wave is zero, the resulting wave is linearly polarized with a horizontal orientation. Further, if the two waves have equal amplitude but zero degree phase difference, the resulting wave is linearly polarized with a 45° orientation.
In order to better use the limited frequency spectrum allocated for offering satellite services, two different terminals communicating with the same satellite may use the same radio wave, but different polarization. For example, the same radio wave may have a horizontal polarization for communicating a modulated signal with one terminal and a vertical polarization for communicating the same or another modulated signal with the other terminal. Thus, it becomes necessary to selectively control the wave polarization in a terminal transmitter that transmits the modulated signals according to an allocated polarization.
FIG. 1A
shows a conventional transmitter
10
for transmitting radio waves, which are selectively polarized either vertically or horizontally. The transmitter
10
includes a relatively high-power amplifier
12
for amplifying a modulated signal provided by a modulated signal generator
14
. A polarization switch
16
, which is controlled by a control signal on line
18
, selectively connects the output of the power amplifier
12
to either a vertical input
20
or a horizontal input
22
of an antenna feed
24
, which radiates a polarized wave with a selected vertical polarization or horizontal polarization. Because the high-power amplifier
12
must amplify the modulated signal to a full transmit power level, for example, 8 watts, the polarization switch
16
must be selected to withstand the full brunt of such a high power. A high-power active switch with a low loss, however, is expensive. Lower cost switches, on the other hand, introduce substantial loses of up to 20% of the amplifier's power. Consequently, an amplifier with higher power, i.e., 10 watt, must be used to accommodate the loss caused by the low cost switch. Alternatively, a mechanical switch may be used to perform the switching function. In addition to being bulky, however, such switches are subject to mechanical failures.
With the success of modern processing technologies in reducing the cost of power amplifiers more than the cost of high-power switches, another conventional transmitter
26
, shown in
FIG. 1B
, uses two separate power amplifiers
28
A and
28
B, instead of the single switch and power-amplifier arrangement of
FIG. 1
, to eliminate the losses caused by a switching arrangement. Under the arrangement of
FIG. 1B
, however, only one power amplifier is enabled, via a controller
29
, at a time. Because one power amplifier is idle half the time, the transmitter of
FIG. 1B
wastes the cost of one of the high-power amplifiers at any instance of time.
Therefore, there exists a need for a low-cost transmitter that can selectively transmit differently polarized modulated signals, without any loss ar waste of transmission resources.
SUMMARY OF THE INVENTION
Briefly, the present invention is exemplified in a transmitter that couples non-linearly polarized waves, which correspond to amplified modulated signals having a selectable phase relationship, to produce a linearly polarized wave with an orientation that corresponds to a selected phase relationship. In another embodiment, linearly polarized waves maybe coupled to produce circular polarization of either sense.
A transmitter according to an exemplary embodiment of the present invention includes a modulated signal generator, a first power amplifier, a second power amplifier, a first antenna feed, and a second antenna feed. The modulated signal generator generates a first modulated signal and a second modulated signal and is responsive to a control signal for selectively providing a predefined phase relationship between the first and second modulated signals. The first and second modulated signals are applied to the first and second power amplifiers, respectively. The first power amplifier, which has a first gain, amplifies the modulated signal and applies it to the first antenna feed. Similarly, the second power amplifier, which has a second gain, preferably, equal to the first gain, amplifies the second modulated signal and applies it to the second antenna feed. According to the present invention, the first antenna feed produces a first polarized wave having a first non-linear polarization. Similarly, the second antenna feed produces a second polarized wave having a second non-linear polarization. The polarization of the first polarized wave and the second polarized wave are selected such that, when jointly radiated they produce a third polarized wave having a linear polarization with an orientation that corresponds to the predefined phase relationship of the first and second modulated signals, as selected by the control signal.
In a second embodiment, a transmitter for producing either right hand or left hand circular polarization comprises a first transmit power amplifier driven at its input by a first modulated signal and coupled at its output to a first ante

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