Antenna For Strip Transmission Lines

Abstract

An antenna for strip transmission lines has slots of different lengths in the planar outer conductors. The slots are situated so that they intersect the planar inner conductor and each slot is matched for transmission or reception of signals with a certain frequency by the length of the slot and the distance from the slot to the end of the planar inner conductor. In one embodiment of the antenna the end of the planar inner conductor is formed to match the distances to the individual slots. In another embodiment of the antenna the slots merge to form only one cut-out in the planar outer conductor.

Description

The present invention relates to an antenna for matching signals from a transmission line to waves in the free space and vice versa and comprising one end of a strip transmission line with a planar inner conductor and one or two planar outer conductors, whereby the planar inner conductor and the planar outer conductors are insulated from each other by means of a dielectric.

In the following description consideration will only be given to the case when the transmission line consists of a planar inner conductor and two planar outer conductors, one on each side and situated in parallel with the planar inner conductor. An antenna according to the invention is, however, in fact also implemented when the transmission line only has one planar outer conductor.

The characteristic impedance of a strip transmission line is determined by the breadth of the planar inner conductor and the distance between the planar outer conductors and the dielectric constant of the dielectric insulation material. A theoretically ideal strip transmission line has infinitely large planar conductors and an infinitely thin planar inner conductor. In practice the planar inner conductor consists of for example a thin copper foil. The width of the planar outer conductors are chosen so that the planar inner conductor can have the same width as the planar inner conductor in an ideal transmission line. The width of the planar outer conductors are also chosen so that an extension of undesired waveguide modes is made impossible. Waveguide modes exist as soon as the symmetry is interrupted, which is caused for example by a slot in the planar outer conductor. In order to simplify the dimensioning of the transmission line it can often be suitable to surround it by a metal shell, for example of aluminum.

It is known to produce an antenna by making a slot in the one planar outer conductor near the end of the planar inner conductor at right angle to the planar inner conductor and symmetrically thereto. Such an antenna is poorly matched to the transmission line. It has moreover low efficiency and is moreover narrow-banded. It is furthermore known to combine a number of slots of different length according to the logarithmic periodic principle in order to obtain a wide banded antenna. Such antennae become, however, proportionately large as a great number of slots are required in order to obtain a sufficient good impedance matching. This means in its turn that the use of this antenna as an elementary antenna in an antenna array with this application is either difficult if not impossible when these applications require that the distance between adjacent elementary antennae be about half a wavelength. It is an object of the invention to provide a slotted antenna which does not have the above-mentioned drawbacks. The characteristics of such an antenna are defined in the appended claims.

The invention will be described in greater detail by means of the accompanying drawing which shows different embodiments of an antenna according to the invention.

FIG. 1 shows an exploded view of an execution of an antenna according to the invention.

FIGS. 2 and 3 show several embodiments of a detail of an antenna according to the invention, and in particular the shape of existing slot outline.

FIG. 4 shows an embodiment of another detail of an antenna according to the invention, viz. the end of the planar inner conductor.

In FIG. 1 a planar inner conductor, denoted by 1, has a terminating end denoted by 8. This planar, inner conductor 1 is fastened on a rectangular plate 2 of a dielectric material, for example plastic. This plate 2 is provided with two outgoing arms 3 which laterally extend in opposite directions. A further similar plate 4 with outgoing arms 5 is produced of the same material as the plate 2. These two plates are fastened to each other so that the planar inner conductor 1 is between the same. The surface on each plate remote from and parallel with the planar inner conductor 1 is covered with a layer of conductive material to form planar outer conductors 6 and 11 respectively. Thus, inner conductor 1 and outer conductors 6 and 11 form a strip transmission line.

In the one or both of these planar outer conductors 6, 11 there are slots 7a-7f positioned to intersect the planar inner conductor 1. Each such slot, for example slot 7a is designed for transmission or reception of signals with a certain frequency by making the length of the slot 7a a multiple of half the wavelength and locating the slot at a distance from the end line 8 of the planar inner conductor which is an odd multiple of a quarter of the wavelength, the wavelength being calculated from the frequency of the signal with reference to the present transmission medium. The distance to the end of the planar inner conductor is measured from the point where the symmetry centerline of the slot crosses the symmetry line of the conductor and in parallel with the conductor.

A slot made in the planar outer conductor of an antenna in the above described manner can be adjusted to a certain frequency, but it will send or receive signals also within a narrow band about this frequency. Thus an antenna with such a slot is narrow-banded. By making several slots in the planar outer conductor, each adjusted to a certain frequency, and if the frequencies to which the slots are adjusted, are so close to each other that the band width of the slots in part overlap each other, a wide banded antenna is obtained. This case is shown in FIG. 1 with further slots 7b-7f.

FIG. 2a shows how several slots together form only one cut-out with two opposite step shaped arms. The cut-out has been formed by laying a number of slots in parallel so close to each other that there is no outer conductor material between the slots i.e., the slots merge. The different slots can clearly be interpreted as they have different lengths and their ends give a step form to two of the opposite sides of the cut-out. In FIG. 2a a cut-out is shown consisting of four slots 12a, 12b, 12c and 12d, where the ends of the longest slot have been denoted reference numeral 12a and the ends of the shortest slot have been denoted reference numeral 12d. FIG. 2b and 2c further show two conceivable designs of the cut-out. These executions are often suitable for mass-produced antennas when separate slots otherwise will lie very close together.

FIG. 3a shows a variant of a cut-out according to FIG. 2. The cut-out in FIG. 3 has two sides situated just opposite each other which are symmetrically decreasing in step form from the central line and out towards the edges of the sides. In this case the cut-out can be thought to be formed by a long slot, the ends of which in FIG. 3a are denoted by 13a, and of two equilength shorter slots 13b and 13c, one on each side of the long slot 13a . There are furthermore two even shorter slots 13d and 13e, one on each side of the other slots. The shortest slots 13d and 13e also have the same length. Further slots can in the same manner be situated on each side of the other slots. FIG. 3b, 3c and 3d show further three conceivable embodiments of the cut-out. The cut-out can also be designed so that only one end agrees with any of the figures while the other end is cut, as is shown in FIG. 3d.

In connection with FIG. 3a it can be pointed out that starting from this point of the form of the cut out a configuration can be built up of several separated slots, where the longest slot is situated in the middle and where shorter slots are situated on each side of this longest slot. Whereby several slots will with it exterior outline together remind about the cut-out in FIG. 3a.

FIG. 4 shows an example of how the edge of the planar inner conductor 1 can be designed with a symmetrical decreasing step form. With such a conductor the distances of the slots to the end of the conductor are determined so that the first step 9 is counted as the end of the conductor when the position of the slot is to be determined which is adapted to the longest wave length. Consequently the exterior step 10 on the planar inner conductor is the starting-point for calculation of the distance to the slot which is adapted to the shortest wavelength. By designing the end of the conductor in this manner a longer distance is obtained between the slots with maintained band width. The figure shows a conductor with three steps, which is adapted to three slots.

Besides the design shown in FIG. 4 the planar inner conductor can, as is shown in FIG. 1, be squarely cut or it can have a design analogous to the one end of the cut-out in FIG. 3b, 3c and 3d. A further alternative is that the planar inner conductor is increased in step form so that it gradually or continuously becomes broader towards its end.

An antenna according to the invention is impedance matched to the transmission line by rotating the slots with respect to the conductor or by moving them at right angle to the conductor. However, the slots must not be displaced so much that they do not intersect the conductor and they must not be rotated to the point that they are in parallel with the conductor as the antenna then is quite choked. By combining the methods of rotation and displacement of the slots a good match between the antenna and the transmission line is obtained in an easy manner.

An advantage in the construction of an antenna according to the invention is that it is possible to mass produce the same by photo engraving techniques. One can start with two plates of a dielectric material, which plates on both sides are covered with a thin copper foil. The copper is removed from one side of one plate while on one side of the other plate the planar inner conductor is engraved. The plates are put together, so that the sides which are still completely covered by copper will be directed outwards. These outer sides constitute the planar outer conductors and in the one or both of these planar outer conductors the slots are engraved. Of course one can start with one plate with both sides covered with copper and one plate with only one side covered with copper.

As an example of the dimensioning of an antenna consider one built up according to FIG. 1, but provided with 11 slots. In such an antenna the distance from the central 6 slot to the end of the planar inner conductor is about 15 millimeters, the length of the longest slot about 12.5 millimeters and the length of the shortest slot about 10.5 millimeters. The width of the slots is about 0.1 millimeters and the distance between them is 0.2 millimeters. Such an antenna has, at a standing wave ratio of better than 2, a frequency range covering 9.5 - 12 GHz.

Claims

We claim:

1. A wideband antenna comprising a strip transmission line having a planar inner conductor and at least one planar outer conductor separated from but electromagnetically coupled to each other by a dielectric medium, said planar inner conductor having a terminating end and said planar outer conductor being provided with a plurality of slots whose projections intersect said planar inner conductor, each of said slots having a length which is a multiple of one half the wavelength of a different particular signal frequency and the distance from each slot to the terminating end of said inner conductor being an odd multiple of a quarter of the wavelength of the associated particular signal frequency in such a way that the slots are displaced from the terminating end of said inner conductor in the order of increasing length with the shortest slot closest to said terminating end.

2. The antenna according to claim 1 wherein the width of the exterior part of said planar inner conductor decreases symmetrically with respect to its central line.

3. A wideband antenna comprising a strip transmission line having a planar inner conductor and at least one planar outer conductor separated from but electro-magnetically coupled to each other by a dielectric medium, said planar inner conductor having a terminating end and said planar outer conductor being provided with a cut out formed from a plurality of merged subslots whose projections intersect said planar inner conductor; each of said subslots having a length related to a different particular signal frequency and the distance from each subslot to the terminating end of said inner conductor being related to an odd multiple of a quarter of the wavelength of its associated particular signal frequency in such a way that the subslots are displaced from the terminating end of said inner conductor in the order of increasing length with the shortest slot closest to said terminating end.

4. The antenna according to claim 3 in which the ends of adjacent subslots merge smoothly into the next to form at each end a continuum of subslot ends.

5. The antenna according to claim 3 in which the adjacent sides of adjacent slots are of different lengths to give at each end of the subslots a stepped profile of slot ends.