1. Field of the Invention
The present invention relates to the field of tropospheric scatter radio links and more particularly to a radiating system with angular diversity comprising a main reflector, a subreflector, a transmitting horn and at least two receiving horns.
2. Description of the Prior Art
It is known that to establish microwave radio links beyond the horizon it is possible to use radiating systems which utilize the scattering of electromagnetic waves by the troposphere.
It is also known that the troposphere displays irregularities generally considered as bubbles or layers which vary continuously in number, form and position with resulting variation of the refraction index and diffusion angle. When such irregularities are illuminated by a beam of electromagnetic waves from a transmitting antenna they scatter the electromagnetic energy in all directions but predominantly within a cone having as its axis the direction of transmission.
It is clear that with such links path attenuation is much higher than that found in links with antennas which remain in a field of mutual visibility since the propagation mechanism is different. In addition, in troposcatter radio links there are met sudden deep fadings of the intensity of the signal received due mainly to random movements of the irregularities of the troposphere.
Diversity techniques are known which are used to avoid the aformentioned problems with tropospheric propagation, i.e. spatial, frequency and angular diversity. Diversity can also be simple or multiple. In case of multiple diversity suitable combinations of the different diversity techniques have been achieved.
Spatial diversity consists of transmitting the same signal with two antennas appropriately spaced and directed and in using two other antennas similarly arranged for reception. The basic assumption on which this technique is based is that fadings of signal intensity which appear on the two beams are poorly correlated.
Frequency diversity differs from spatial diversity in that the signal is radiated on a single beam but with two carriers appropriately spaced as to frequency so as to make intensity fadings of the two signals received uncorrelated.
Angular diversity consists of radiating electromagnetic power in a single beam and in equipping the receiving antenna with two receiving horns appropriately spaced from each other in such a manner that the single transmitted beam is received in two different directions forming a certain angle called diversity angle and giving rise to two signals as independent as possible from the point of view of tropospheric propagation. It is thus possible to effect in reception a combination of the two signals received such that the combination signal intensity or the signal-to-noise ratio of the combination is always kept sufficiently high.
It is also known that with angular diversity systems there is the problem of optimizing the diversity angle which, as mentioned above, depends on the distance between the receiving horns. As the diversity angle increases so does the statistical independence between the intensity fadings which appear on the two received signals, with a resulting system improvement. But antenna gain is simultaneously reduced because of defocusing.
It is also known that radiating systems in general and those with angular diversity in particular accomplish the transmitting part and the receiving part on the same antenna and bring about decoupling of the transmitting signals from the receiving signals by using different frequencies or by means of polarizations on the orthogonal planes or with a combination of these decoupling criteria. As concerns polarization, there are radiating systems with single polarization and radiating systems with double polarization.
Radiating systems with double-polarization angular diversity possess a first horn generally placed in the focus of the antenna parabola used for both transmitting and receiving and a second horn arranged parallel to the first used only for receiving.
The drawbacks of systems of this type are due mainly to the complexity of antenna horns. In general they include for effecting decoupling or discrimination between the two orthogonal polarizations many elements which lead to considerable occupied space with the resulting reduction of efficiency of the antenna compared with theoretical efficiency.
An example of a known tropospheric radiating systems with single-polarization angular diversity is British Pat. No. 1,178,782 granted Jan. 21, 1970 to the Marconi Company Limited which utilizes a parallel-conductor screen to separate the reception polarization from the transmission polarization, which are orthogonal to each other.
The above system described in the aforementioned British patent makes use of an offset paraboloid to permit the beam leaving the transmitting horn placed in the focus of said paraboloid to reach the surface of the antenna, avoiding blocking effects by the receiving horns which are outside the field of illumination.
The drawbacks of the angular-diversity radiating system described are due mainly to the fact that in such a system the primary illumination axis forms an offset angle with the orthogonal optical axis at the antenna aperture plane. As is known, offset systems provide performance generally poorer than symmetrical systems and in particular have less efficiency in crossed polarization because as is known for efficiency diminishes as antenna curvature increases, i.e. for smaller focus-to-diameter ratios and especially for geometrical dissymetries of the optical system.
The drawbacks mentioned hereinbefore are all the more serious in prior art systems which, as described hereinbefore, use a parallel-conductor screen to separate the two linear polarizations which are othogonal to each other. As a result of less efficiency under crossed polarization, a part of the electromagnetic power of the transmitted beam leaves the antenna with a polarization orthogonal to that which it should have. This part of the power, after reaching the receiving antenna, passes through the parallel-conductor screen and reaches the transmitting horn while it should be reflected from the screen toward the receiving horns.
Accordingly the primary object of the present invention is to overcome the aforementioned drawbacks of the prior art and provide an angular-diversity radiating system which is symmetrical, permits the use of antenna horns which are easy to fabricate, and has good efficiency under crossed polarization, and permits adjustment of the distance between the receiving horns to optimize the diversity angle.