Helical antenna system

A helical antenna system for receiving simultaneous signals with different polarizations, with the aid of helical feeders and without gain degradation. Two or more helical feeders are provided for each polarization direction.

BACKGROUND 
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 provides maximum radiation along the helix axis, 
which occurs when the helix circumference is of the order of one 
wavelength. 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 U.S. Pat. No. 3,184,747 presents a coaxial feed helical antenna 
which has a director disk between feed and helix producing endfire 
radiation towards the disk. In this US-patent the dimensions of the helix 
for such an antenna system are given. 
U.S. Pat. No. 4,742,359 presents an antenna system using a helical antenna 
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 feeder line 
is aligned with the axis of the helical antenna. Such a helical antenna 
may be built as a so-called endfire helical antenna, where under maximum 
received power conditions the direction of the signal power flow at the 
first end is in the sane direction as the received radiation. Such a 
helical antenna can also be built as a so-called backfire helical antenna, 
where under maximum received power conditions the direction of the signal 
power flow at the first end is in the opposite direction to the received 
radiation. 
In said US patent an antenna system is presented, which comprises a 
reflector, a primary helical antenna having a coil with a pair of ends, 
said coil located at the focal point of said reflector so that the axis of 
the helical antenna coincides 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 and 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 so use 
one or more helical feeders together with a dielectric lens. Thereby 
signals from several directions can be received simultaneously. 
In the axial mode a helix wound like a right-hand screw receives right-hand 
circular polarization, while a helix wound like a left-hand screw receives 
left-hand polarization. 
If both polarization directions are to be received simultaneously 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 directions. 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 object of the present invention to present an antenna system with 
focussing means and helical feeders where at least two different circular 
polarized radiations can be received simultaneously with no gain 
degradation compared to known systems. 
According to the invention there is not just one helical feeder provided 
for each polarization direction, but two or more. These helical feeders 
work preferably in the axial backfire 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 the like, have a focal point in which they focus an 
incoming radiation. If radiations with two opposite polarizations are to 
be received two helical feeders are to be provided near the focal point. 
That means that the two helices cannot be located together at the focal 
point. To compensate the according gain degradation there are two or more 
helices for each polarization direction provided according to the present 
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 provided which have the same winding sense and 
receive mainly radiation with the same circular polarization. Each of the 
helical feeders 12a, 12b has a director disk 13a, 13b respectively. 
Signals received by the feeders 12a, 12b are led via feeder lines 14a, 14b 
respectively, which may be e.g. coaxial cables, semi-rigid cables or the 
like, to circuit 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 12a, 12b. These 
feeders 22a, 22b cannot be seen in FIG. 1 but can be seen in FIG. 2, which 
gives a front view of this embodiment. Each of the feeders 22a, 22b has a 
director disk 23a, 23b respectively. 
The diameter d1 of the director disks 13a, 13b, 23a, 23b is about 
EQU 0.25*La, 
wherein La is the wavelength of the radiation to be received. 
For the preferred embodiment the diameter d2 of the helices 12a, 12b 22a, 
22b is about 
EQU 0.3*La, 
the distance d3 between the centers of two helices 12a, 12b or 22a, 22b 
respectively of the same pair is about 
EQU 0.7*La. 
In this embodiment the housing 15 is shaped like a tube with a round basic 
form having a diameter d4 of about 
EQU 0.8* La. 
For each pairs the helices 12a, 12b or 22a, 22b respectively are placed 
symmetrically on either side of the focal point 11 in such way that the 
center of the segment F-F' or f-f' respectively is coincident with the 
focal point 11 of the concentration means 10. 
The input powers of each sense of polarization are added inside 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 conductors of the feeder lines 14 a, 14b 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 12a, 12b 22a, 22b two 
LNC circuit boards can be provided which could be orthogonal to each 
other, e.g. such that they build a cross, a "T" or the like, 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 the like. 
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 direction. In such a case the phase centers F, f 
build a triangle, a quadrangle or the like. 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 point 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-spherical, 
quarter-spherical or the like; 
by connecting helical feeders of the first pair (12a, 12b) with those of 
the second pair (22a, 22b) linear polarized signals can be received. The 
linear polarization direction can be selected by according phase shifter 
means; 
the antenna system can be used for the reception of broadcast signals, like 
television signals, audio-broadcast signals or the like, which can be 
transmitted directly or not directly from a satellite. The antenna system 
can also be used for the reception of any other radiofrequency signals 
with different polarizations.