BACKGROUND OF THE INVENTION A quadrifilar helix antenna typically consists of four symmetrically positioned helix shaped metallic wire of strip elements. The four helices are fed in phase quadrature, i. e. with equal amplitude and with the phase relation 0°, 90°, 180° and 270°. The quadrifilar helix antenna can receive and transmit circular polarised signals over a large angular region. Its radiation characteristics is determined mainly by the shape of the helices, i. e. the number of turns, pitch angle, antenna height and antenna diameter, and in the case of conical shaped helices also the cone angle.

The phase quadrature feeding of the four helices can be accomplished in different manners. One possibility is to have a separate feeding network that generates the phase quadrature. Alternatively a balun system can be used combined with a separate 90°-hybrid or with a self-phasing helix antenna.

A difficulty with the traditional quadrifilar helix antenna is its relatively strong frequency dependent input impedance. This makes it difficult to design broad band matched or dual-frequency matched antennas. However, this problem can be solved to some extent by having a double tuned quadrifilar helix antenna.

Dual frequency quadrifilar helix antennas are frequently requested for many applications, commonly for the purpose of having separate frequency bands for receiving signals and for transmitting signals.

For mobile satellite communication systems, dual-frequency circularly polarised antennas are requested for the use on hand held terminals. These antennas are designed to operate at L-or S-band with a coverage over a cone with a half angle between 40° up to 90° depending on the system.

One object of the invention is to provide a novel compact dual-frequency quadrifilar helix design that has the potential of low cost mass production. A second object is to provide a dual-frequency quadrifilar helix antenna design that makes a simple mechanical design possible and suitable for space applications.

SUMMARY OF THE INVENTION The present invention is a mechanically simple dual-frequency (or wide band) quadrifilar helix antenna. It includes four helix shaped radiating elements where each helix element consists of two or more parallel helices of different lengths that are in galvanic contact at, or close to, the feeding point. The four feeding points of the helix elements are located at the bottom of the helix, meaning that the feedings of the helix elements are located at the end of the helix pointing in the direction opposite to the direction of its main radiation.

The present invention also includes a compact dual-frequency (or wide band) quadrifilar design with an integrated feeding network (power distribution network).

In this case the four feeding points of the helix elements are connected via small matching sections to a distributed series feeding network consisting of transmission lines that serves for the phase quadrature feeding of the four helix elements, yielding a single input feeding point for the complete antenna assembly. The matching section and the series feeding network is preferably realised in stripline or microstrip techniques.

By providing a quadrifilar helix antenna of the suggested design it becomes a very attractive candidate for use in mobile satellite communication systems as an example, but it requires a compact dual-frequency design with an integrated feeding network that is simple from a manufacturing point of view.

Further, in mobile satellite communication systems a dual-frequency design is very attractive as it is simple from a manufacturing point of view. Very often a simple mechanical design means a safe design for space applications.

Quadrifilar helix antennas can also be used in applications as transmission and/or receiving antennas on board satellites.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view of a conventional cylindrical quadrifilar helix antenna.

Figure 2 is a perspective view of a dual frequency quadrifilar helix antenna, feeding network excluded, in accordance with one aspect of the present invention.

Figure 3 is a Smith chart showing the active input impedance of a conventional cylindrical quadrifilar helix antenna.

Figure 4 is a Smith chart showing the active input impedance of a cylindrical quadrifilar helix antenna in accordance with the teaching of the present invention.

Figure 5 is a block diagram showing a hybrid feed network with four output ports feeding a dual frequency quadrifilar helix antenna in phase quadrature via four matching sections, yielding a single input feed point for the complete antenna assembly with the other hybrid ports being terminated with resistive loads.

Figure 6 is a schematic view of a distributed series feed network consisting of transmission lines with four output ports and one input port, yielding four output

signals with equal amplitude and with a relative phase relation of 0°, 90°, 180° and 270°, when feeding the input connector.

Figure 7 is a partial sectional view of a dual-frequency quadrifilar helix antenna with an integrated feed network in accordance with the teaching of the present invention.

Figure 8 is a plan view of a substrate containing printed pattern of four double tuned helix elements, four matching sections, and a distributed serial feed network, in accordance with the teaching of the present invention.

EMBODIMENTS OF THE INVENTION Figure 1 is a side view of a cylindrical quadrifilar helix antenna constructed in accordance with conventional teachings of the prior art. The four helices can be fed in phase quadrature, i. e. with equal amplitude and with the phase relation 0°, 90°, 180° and 270°, either at the bottom or at the top of the quadrifilar helix. Where the helices are fed and how the phase quadrature feedings is accomplished is not shown in the figure.

Figure 3 shows a Smith chart of a typical active input impedance as a function of frequency for a conventional cylindrical quadrifilar helix antenna. Assuming that the antenna is to operate at two separate frequency bands, where one frequency band is between marker 1 and 2 and the other between marker 3 and 4 in Figure 3, it follows that the active input impedance is very different between the two frequency bands. This will make it extremely difficult to obtain a good and simple impedance matching between the quadrature helix antenna and its feed network.

Figure 2 shows a perspective view of a dual frequency quadrifilar helix antenna 1, a feed network for feeding the antenna excluded, in accordance with the teaching of the present invention. The antenna consists of four helix shaped radiating elements 2-5, where in contrast to the conventional quadrafilar helix antenna, each helix element consists of two

parallel helices 2a, 2b, 3a, 3b, 4a, 4b, Sa, 5b of different lengths that are in galvanic contact close to its feed point. The four feed points 2c-5c of the helix elements 2-5 are located at the bottom 6 of the helix, meaning that the feedings of the helix elements 2-5 are located at the end of the helix pointing in the direction opposite to the direction of its main radiation.

Having the feed points 2-5 located at the bottom 6 of the helix makes it possible to provide a mechanically simple design, where a feed network can easily be added below the radiating helix part. The four helix elements 2-5 in Figure 2 are open circuited in the top of the helix, but an alternative is to have them short circuited. However, with open circuited helix elements the design becomes much simpler from a manufacturing point of view.

Figure 4 shows a Smith chart of a typical active input impedance as a function of frequency for a quadrifilar helix antenna in accordance with one aspect of the present invention. The effect of letting each helix element 2-5 consist of two parallel helices 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b of different lengths that are in galvanic contact close to its feed points 2c-5c is that we can now have the active input impedance to basically be the same for two separate frequency bands, one frequency band is between markers Al and A2 and the other between markers A3 and A4 as shown in figure 4. This makes a much simpler design possible for the impedance matching between the quadrifilar helix antenna 1 and its feed network 12.

Figure 5 shows a block diagram of a hybrid feed network 8 with four output ports 9a-9d feeding a dual frequency quadrifilar helix antenna 1 in phase quadrature via four matching sections 1 a-11 d, yielding a single input feed point 10 for the complete antenna assembly with the other hybrid ports being terminated with resistive loads. The four matching sections 1 a-11 d can be excluded or replaced by transmission lines if appropriate. The hybrid feed network 8 can be realised in either stripline or microstrip techniques or in a combination. The feed network 8 and the matching sections 1 la-1 ld can be placed in a separate box located, for instance, below the quadrifilar helix.

Figure 6 shows a schematic view of a distributed series feed network 12 consisting of transmission lines 13a-13d with four output ports 14a-14d and one input port 15, yielding four output signals with equal amplitude and with a relative phase relation of 0°, 90°, 180°, 270° when feeding the input port 15. In the figure L corresponds to the length of the

transmission lines 13a-13d in vawelengths. RA is the input impedance from a helix and Z is the characteristic impedance of transmission lines 13a-13d.

Figure 7 shows a partial sectional view of a dual-frequency quadrifilar helix antenna 1 with an integrated feed network 12 in accordance with the teaching of the present invention. In the antenna design of Figure 7, the four feed points 2c-5c of the helix elements 2-5 are connected via small matching sections 16 to a distributed series feed network 12 consisting of transmission lines. The matching sections 16 and the series feed network 12 is realised in stripline technique. Due to the double tuned helix design the matching between the feed network 12 and the radiating quadrifilar helix antenna 1 is easily obtained for both frequency bands using simple matching sections 16. The distributed series feed network 12 is of the type schematically viewed in Figure 6.

One advantage of the antenna shown in Figure 7 is that it is mechanically simple containing few parts. As an example, the four double tuned helix elements 2-5, the four matching sections 16 and the distributed series feed network 12 can be printed or etched on a single dielectric tube.

Figure 8 shows a plan view of a dielectric substrate 17 containing a printed or etched pattern including the four double tuned helix elements 2-5, the matching sections 16 and distributed series feed networkl2. Basically, the complete antenna design of Figure 7 can be obtained by rolling the dielectric substrate 17 to a tube. The matching sections 16 and the feed network 12 is thereafter coated with an inner dielectrica 18, an inner groundplane 19, an outer dielectrica 20 and finally an outer groundplane 21 in the described order.