The present invention relates to an antenna system having helical feeders.

A helical feeder antenna consists of a single conductor or multiple conductors wound into a helical shape. Beside some other possible modes a helical antenna is normally used in a so-called axial mode or in a normal mode. The axial mode pro¬ vides maximum radiation along the helix axis, which occurs when the helix ci rcum erence is of the order of one wave¬ length. The normal mode which yields radiation broadside to the helix axis, occurs when the helix diameter is small with respect to a wavelength. For the application according to the present invention the axial mode is of special interest.

The use of helical antennas for antenna systems are widely known. For example US-patent 3 184 747 presents a coaxial feed helical antenna which has a di rector disk between feed and helix producing endfi re radiation towards the disk. In this US-patent the dimensions of the helix for such an anten¬ na system are given.

US 4 742 359 presents an antenna system using a helical anten¬ na with two ends where the first end is linked to a feeder line. For the purpose of the following explanation it is understood that the said feeder line is aligned with the axis of the said helical antenna. Such a helical antenna may be bui lt as a so-called endfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said fi rst end is in the same direction as the received radiation. Such a helical antenna can also be bui lt as a so-called backfire helical antenna, where under maximum received power conditions the direction of the signal power flow at the said fi rst end is in the opposite di rection to the received radiation.

In said US patent an antenna system is presented, which com¬ prises a reflector, a primary helical antenna having a co l with a pai r of ends, said coi l located at the focal point of said reflector so that the axis of the helical antenna co¬ incides essentially with the axis of said reflector. A feeder line couples the antenna system with an external circuit, so that said primary helical antenna represents a backfire helical antenna coupled with said feeder line at the nearer end from said reflector and the other end of the helical antenna is free standing, and said feeder line is a coaxial cable.

It is further known from the international publication WO 92/13373 to use one or more helical feeders together with a dielectric lens. Thereby signals from several di rections can be received simultaneously.

In the axial mode a helix ound like a right-hand screw re¬ ceives right-hand ci rcular polarization, whi le a helix wound like a left-hand screw receives left-hand polarization.

If both polarization di rections are to be received simultane¬ ously there must be provided at least two helices. If these helices are part of an antenna system using focussing means, it is impossible to have at the same time the two feeders in the focal point of the focussing means. Thereby inacceptable gain degradation is involved for at least one of the polarization di rections. Additionally it is possible that cross-talk occurs due to inevitable defocussing and/or strong coupling between the helices if placed too close to each other.

It is an ob ect of the present invention to present an anten¬ na system with focussing means and helical feeders where at least two different circular poarized radiations can be re¬ ceived si ultaneously with no gain degradation compared to kno n systems.

According to the invention there is not just one helical feeder pro ided for each polarization direction, but two or more. These helical feeders work preferably in the axial backfi re mode.

The following ideas have led to the principle of the present invention.

Means for focussing, e.g. a parabolic reflector, a dielectric lens, like a Luneburg-type Lens, or thelike, have a focal point in which they focus an incoming radiation. If radia¬ tions with two opposite polarizations are to be received two helical feeders are to be provided near the focal point. That means that not the two helices can be located together at the focal point. To compensate the according gain degradation there are two or more helices for each polarization di rection provided according to the present invention.

Further characteristics, advantages and detai ls of the inven¬ tion are explained in the following embodiments with the aid of the drawings. Therein

Fig. 1 shows a side view of a preferred embodiment; Fig. 2 shows a front view of the preferred embodiment Fig. 3 shows a preferred embodiment of a power combin e r .

In Fig. 1 a parabolic reflector 10 acts as focus means and has a reflector axis 10a and a focal point 11. Around this focal point 11 there are two helical feeders 12a, 12b provid¬ ed which have the same winding sense and receive mainly radia¬ tion with the same circular polarization. Each of the helical feeders 12 has a di rector disk 13a, 13b respectively. Signals received by the feeders 1 are led via feeder lines 14a, 14b respectively, which may be e.g. coaxial cables, semi-rigid cables or theli ke, to ci rcuit boards which are included in a housing 15 and which cannot be seen in Fig. 1.

Additionally there is a second pair of helical feeders 22a, 22b provided with an oppositional winding sense compared to the feeders 12. These feeders 22 cannot be seen in Fig. 1 but can be seen in Fig. 2, which gives a front view of this embod¬ iment. Each of the feeders 22 has a director disk 23a, 23b respectively.

The diameter d1 of the director disks 13, 23 is about

0, 25 * La, wherein La is the wavelength of the radiation to be received.

For the preferred embodiment the diameter d2 of the helices 12, 22 is about

0,3 * La, the distance d3 between the centers of two helices 12a, 12b or 22a, 22b respectively of the same pair is about

0,7 * La.

In this embodiment the housing 15 is shaped like a tube ith a round basic form having a diameter d4 of about

0,8 * La.

For each pairs the helices 12 or 22 respectively are placed symmetrically on either side of the focal point 11 in such way that the center of the segment F-F ¹ or f-f respectively is coincident with the focal point 11 of the concentration means 10.

The input powers of each sens of polarization are added in¬ side the housing 15 using an according power combiner. A preferred embodiment of such a power combiner is shown just for one polarization sense in fig. 3. There the inner conduc¬ tors of the feeder lines 14 are led to microstrip lines 16a, 16b respectively which have a common junction point 17. A resulting outline 18 is led to further stages ⁽ not shown) of a low noise converter (LNC). If the signal to be received is

a television-broadcast signal, the information of the signals can be presented by an according TV-set.

For the other polarization sense another power combiner is provided which may be of the same type as shown in fig. 3.

For processing the signals received by the feeders 12, 22 two LNC ci rcuit boards can be provided which could be orthogonal to each other, e.g. such that they bui ld a cross, a "T" or thelike, and they are enclosed in the housing 15 which may be shaped like a tube, with a round, a triangular, a quadrangular basic form or thelike.

Versions of the presented embodiments may include at least one of the following variations: it is possible to provide more than two helical feeders for the reception of each polarization di rection. In such a case the phase centers F, f bui ld a triangle, a quadrangle or thelike. It is preferred to place the helical feeders such that the center of the triangle, the quadrangle or the like is coincident with the focal poi nt 11 ; instead ^* of the reflector 10 any other focal means can be used which work by reflection, refraction and/or diffraction. Another preferred focal means is a Luneburg-type lens, which can be spherical, hemi-spheri- cal, quater-spheri ca I or thelike; by connecting helical feeders of the fi rst pair (12) with those of the second pair (22) linear polarized signals can be received. The linear polarization direc¬ tion can be selected by according phase shifter means; the antenna system can be used for the reception of broadcast signals, like television signals, audio-broad¬ cast signals or thelike, which can be transmitted di rect¬ ly or not di rectly from a satellite. The antenna system can also be used for the reception of any other radiofrequency signals with different polarizations.