Method of feeding electromagnetic power from an antenna element

Abstract: A method of feeding out field power from a circularly polarized antenna (2), mounted on a conductive aircraft surface (1). All power is normally fed out in circular polarization to the receiver, irrespective of the elevation angle (ϑ) and the azimuth angle (α). For increasing the antenna amplification at elevation angles ϑ greater than or approximately equal to 60 o , where substantially only the vertical polarization can be seen, it is proposed in accordance with the method that all power is fed out in linear polarization with the aid of a polarization switch (4).

Description: The present invention relates to a method of feeding out electromagnetic power in an antenna element or an antenna array including a plurality of antenna elements. The method is primarily intended to be utilized in antenna elements mounted on the surface of an airborne vechicle satellite. Communication from an aircraft to a satellite or between satellites requires circularly polarized antennas, i.e. antennas which transit circularly polarized radiation, and which have a very wide covering area. If the antenna must be mounted on the surface of the aircraft or the satellite, due to aerodynamic requirements, only limited coverage can be achieved by circular polarization, as described, e.g., by R. J. Mailloux "Phased array aircraft antennas for satellite communications", Microwave Journal Oct. 1977, p. 38. The reason is that circular polarization can be regarded as a combination of a vertical and a horizontall polarization with 90° phase shift. If the antenna is mounted on the surface of the vehicle, the horizontal polarization component of the field, which is thus parallel to the surface of the vehicle, will be short-circuited while the vertical polarization component at right angles to the surface is only decreased or attenuated by a certain amount (approximately 3.2 dB). Hereinafter, a horizontal and a vertical polarization component are respectively defined as components parallel and perpendicular to an electrically conductive surface (the surface of the vehicle). The loss in a circular-polarized antenna outside the vehicle will be a further 6 dB, however, of which 3 dB is because only vertical polarization can be seen, and a further 3 dB in the feed network, since both polarization components are fed. The object of the present invention is to increase the transmitting power of an antenna mounted on the surface of an airborne vehicle which is fed with circular polarization and for different reception angles in the elevation direction. This is achieved in accordance with the proposed method by changing the polarization in the field that is fed out from the antenna in response to the direction the receiver is in, in relation to the feed plane (the surface of the vehicle) of the antenna. The method is characterised as disclosed in the characterising portion of claim 1. The invention will now be described in more detail with reference to the accompanying drawings, where Figure 1 illustrates part of an aircraft surface with an antenna element, Figure 2 is a simplified depiction of the field from a feed polarization for the antenna element in Figure 1, using linear polarization, Figure 3 illustrates how two (linear) polarizations are divided into their components in circular feed polarization, Figure 4 is a simplified block diagram of an antenna feed carrying out the method in accordance with the invention, Figure 5 is a graph of received power when the proposed method is utilized. In Figure 1, there is illustrated an aircraft surface 1, on which an antenna element is disposed. The antenna element can receive or transmit a field with two feed polarizations, the components of which are denoted M1 and M2, where M1 is perpendicular to M2, although both are in the same horizontal plane. The feed field from the antenna waveguide is circularly polarized in this case, and the planes of both components are in the same plane as that of the aircraft surface 1. Figure 2 is a depiction of the field about a feed polarization component M1. This gives rise to a field about the antenna element 4 which contains a vertical polarization V1 and a horizontal polarization H1. The field is here linearly polarized. Figure 3 illustrates the two feed polarizations M1 and M2, which according to Figure 2 each can be divided into a vertical and a horizontal polarization component. A circularly polarized feed field can thus be regarded conventionally as two orthogonal polarizations V1, H1 and V2, H2, where the H component is phase-shifted 90° in relation to the V component. Each of the polarizations M1 and M2 can resolve into linearly vertical or horizontal polarization depending on from what azimuth angle α they are observed. The angle of elevation for transmitting to different receivers is denoted by ϑ in Figure 1. It is obvious that for large elevation angles ϑ the components H1 and H2 will be short-circuited in the conductive aircraft surface 1. In accordance with the invention, it is therefore proposed that all power is fed out solely in linear polarization V1, H1 or V2, H2 when the receiver is in elevation angles ϑ greater than a given value ϑ₀, while for ϑ < ϑ₀, the feed-out takes place in circular polarization. The value of ϑ₀ is selected as will be apparent from the graph according to Figure 5. Since, according to the above, the vertical or the horizontal component will dominate, in response to which azimuth angle α is observed, the selection of vertical or horizontal polarization will be dependent on the value of α. Figure 4 is simplified block diagram of an antenna feed for carrying out the method in accordance with the invention. It comprises a switch means 4, which receives an incoming microwave signal, which is to be fed out to the antenna element 2 and be transmitted to a given receiver. The switch means 4 is controlled by a signal giving the values of the angles ϑ, α applying to the receiver in question, and according to the conditions set out above. The switch means 4 may comprise, for example, a circular wave conductor, two switches and a power divider. The circular wave conductor is provided with two probes which are inserted in the wave conductor wall, one probe being displaced at 90° to the the other. The power divider can divide the incoming microwave signal into two waves of equal power when it is switched into the circuit. If ϑ < ϑ₀, the power divider is switched in and both components M1, M2 are fed out, but with the phase difference 90°, which gives a circularly polarized field. If ϑ > ϑ₀, the power divider is switched out of the circuit and the input signal is either connected to one or the other probes depending on the value of the azimuth angle α, which applies to the receiver in question (as will be seen from below). Either M1 or M2 is fed out in response to the azimuth angle α, and a lineary polarized field is obtained. The waveguide 5 can comprise, for example, an extension of the circular waveguide included in the switch means 4. The following table states within which azimuth angle interval the different feeds are used: The above values of α are, of course, repeated every 360°. Figure 5 is a simplified directivity graph for the circularly polarized field, graph 1, and for five different linearly polarized fields, graphs 2,3,4,5 and 6, where the latter are dependent on ten different values of the azimuth angle α, according to the following: Graph 1: Coverage by circular polarization irrespective of the value of α, Graph 2: Coverage with linear polarization for α = 0, α = 90°, Graph 3: Coverage with linear polarization for α = 10°, α = 80°, Graph 4: Coverage with linear polarization for α = 20°, α = 70°, Graph 5: Coverage with linear polarization for α = 30°, α = 60°, Graph 6: Coverage with linear polarization for α = 40°, α= 50°. From the graphs according to Figure 5, it will be seen that the graph 1 intersects the graphs 2-6 at certain points where ϑ = ϑ₀ and for different values of the azimuth angle α. Directivity gains can be obtained at these points if there is a change from circular to linear polarization. When a receiver is at an elevation angle ϑ < ϑ₀(α), the antenna power is fed out with circular polarization, i.e. When ϑ = ϑ₀(α) switching over takes place as described above in connection with Figure 4, and all power is fed in linear polarization, i.e. M1=0 or M2=0. In this way, antenna amplification can be increased by up to 3 dB for receivers in elevation angles close to the horizon, (ϑ = 90°). According to Figure 5, the greatest gain is obtained when ϑ = 90°, α = 0 or 90°, namely 3 dB. For other ϑ- and α- angles, when ϑ ≧ or approximately equal to 65°, the directivity gain varies between 0 and 3 dB according to Figure 5.

Claims: Method of feeding an antenna element (2), which is disposed on and coplanar with the surface of a substantially planar electrically conductive material (1), with first and second orthogonal and linearly polarised electromagnetic field components (M1, M2) which are parallel to said planar conductive material (1), said antenna element (2) transmitting said fed electromagnetic energy to a remote target having a receiver, the elevation angle ϑ to said target being defined as the angle between the normal to the surface of said planar conductive material (1) and the target direction as seen from the antenna element (2), characterized in that a) said antenna element (2) is fed with both said first and second orthogonal and linearly polarised electromagnetic field components (M1, M2) phase shifted 90° to each other, i.e. circular polarisation, if the target is at an elevational angle ϑ less than an angle ϑ₀ and in that b) said antenna element (2) is fed with only one of said first and second orthogonal and linearly polarised electromagnetic field components (M1, M2), i.e. linear polarisation, if the target is at an elevational angle ϑ equal or greater than said angle ϑ₀, where said angle ϑ₀ is at least 60°. Method as claimed in claim 1, characterized in that said first component (M1) is selected for first and third angular intervals in azimuth and said second component (M2) for second and fourth intervals when linear polarisation is chosen according to method step b) of claim 1, said intervals being sucessive parts of a complete revolution around said antenna element. Antenna feed arrangement for carrying out the method according to claim 1, where said antenna element (2) is disposed on and coplanar with the surface of a substantially planar electrically conductive material (1) and said antenna (2) is to be fed with first and second orthogonal and linearly polarised electromagnetic field components (M1, M2) which are parallel to said planar conductive material (1), said antenna element (2) transmitting said fed electromagnetic energy to a remote target having a receiver, the elevation angle ϑ to said target being defined as the angle between the normal to the surface of said planar conductive material (1) and the target direction as seen from the antenna element (2), said feed arrangement comprising controllable switching means (4) including a wavequide, a pair of microwave switches and a power divider, characterized in that a) said switching means (4) is so controlled that said antenna element (2) is fed with both said first and second orthogonal and linearly polarised electromagnetic field components (M1, M2) phase shifted 90° to each other, i.e. circular polarisation, if the target is at an elevational angle ϑ less than an angle ϑ₀ and in that b) said switching means (4) is so controlled that said antenna element (2) is fed with only one of said first and second orthogonal and linearly polarised electromagnetic field components (M1, M2), i.e. linear polarisation, if the target is at an elevational angle ϑ equal or greater than said angle ϑ₀, where said angle ϑ₀ is at least 60°. Method as claimed in claim 3, characterized in that said first component (M1) is selected for first and third angular intervals in azimuth and said second component (M2) for second and fourth intervals when linear polarisation is chosen according to method step b) of claim 1, said intervals being successive parts of a complete revolution around said antenna element.

Inventors: ANDERSSON MATS RUSTAN, ANDERSSON, MATS RUSTAN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1989
Main classification: H01Q3
IPC codes: H01Q3, H01Q21