EP0375840B1 - Apparatus for transmitting high-frequency signals - Google Patents

Abstract

An apparatus for transmitting high-frequency signals with a coaxial cable provided with apertures in the outer conductor, where the apertures recur, seen in the longitudinal direction of the cable, in groups with a period length which is selected so that the groups of apertures radiate high-frequency signals from a desired lower limit frequency f0 and that the pole positions which are allocated to the multiples of the lower limit frequency f0 are eliminated or at least substantially attenuated in the frequency response of the coupling attenuation, is designed so that the number of apertures per period is greater than 30, the apertures are slits (1) which are as narrow as possible and the slits (1) are disposed essentially perpendicular in relation to the cable axis.

Description

The invention relates to an arrangement for transmitting high-frequency signals according to the preamble of claim 1 and a method for their production

• a) nicht abstrahlende offene Wellenleiter; hierzu gehören u.a. Kabel mit einem Außenleiter aus grobem Drahtgeflecht, Kabel mit durchgehendem Längsschlitz, und Kabel mit kleinen Öffnungen in kurzen Abständen und
• b) radial abstrahlende Kabel oder Leckkabel; diese werden hier näher betrachtet.

In order to maintain the radio connection to vehicles along a lane (road, railroad), it is sufficient to transmit the radio signal within a limited environment of this lane or to receive it from there. Open coaxial cables are suitable for radio operation in the 50 to 1000 MHz range, ie conductors consisting of an inner conductor and an electrically permeable outer conductor surrounding it. These cables enable a reliable radio connection even under unfavorable environmental conditions, such as in a tunnel. The radiated energy should fluctuate as little as possible in space and time in the largest possible frequency range. Coaxial cables with openings are available in many different designs that have been used for many years.

• a) non-radiating open waveguides; These include cables with an outer conductor made of coarse wire mesh, cables with a continuous longitudinal slot, and cables with small openings at short intervals and
• b) radially radiating cables or leakage cables; these are considered in more detail here.

From DE-OS 21 03 559 a slotted coaxial cable with an inner conductor and an outer conductor is known, the outer conductor having a series of slots that periodically at a fixed interval arranged in succession and their dimensions are changed in accordance with a sinusoidal source distribution. On the one hand, the angle of inclination of the slots is changed from slot to slot, on the other hand, the length of the slots, their curvature, or their geometry are modified in a certain way. The disadvantage of this solution is the limited useful frequency range and the small number of slots per period, their different shape and the complicated production method.

A high-frequency coaxial cable with an inner conductor and - insulated from this - an outer conductor provided with bores is known from European Patent 0 028 500. The bores and their mutual spacing are dimensioned such that the mutual spacing between adjacent bores decreases in the longitudinal direction, a maximum value of the spacings being provided at one end of the row and a minimum value at the other end of the row. The holes are designed as circular holes. Overall, a larger part of the outer conductor is occupied by holes. The disadvantage of this arrangement is that, due to the size of the holes, only a few holes have to be made on the outer conductor within a periodicity interval.

From DE-OS 22 30 280 calculation methods for suppressing unwanted pole points in the frequency response of the coupling loss are known, which include various suitable functions. Products from sine functions, cosine functions and their mixed products as well as aperiodic functions were examined primarily.

Disadvantages of the previously known solutions are, on the one hand, the limited bandwidth, which is only about 5 times the basic frequency, and, on the other hand, the field strength, which is not sufficiently constant for all frequencies within the transmitted frequency range. In the case of a cable with a periodic arrangement of openings, the frequency response of the coupling attenuation has maxima in addition to the fundamental frequency and also with an unlimited number of integral multiples of the fundamental frequency (Pole positions). If the poles are not sufficiently suppressed, interference will appear. The transmission along a transmission line, for example in a tunnel, is significantly affected. So far it has not been possible to suppress the higher order pole points in a broad frequency band and to achieve a reasonably constant frequency response.

The object of the invention is to design a leakage cable in such a way that the largest possible frequency range is transmitted with a field strength that is as large and constant as possible in this frequency range. This object is achieved according to the invention by the features listed in the characterizing part of claim 1; Further developments of the invention and a method for producing a leakage cable are described in the subclaims.

The invention is preferably suitable for transmitting messages between mobile and / or fixed radio systems, for example in rail or road traffic, as well as in tunnels, shaded areas or underground.

Because of the strong radiation with low wave damping leakage cables according to the invention, these are particularly suitable for broadband communication along traffic routes. Traffic management systems (e.g. along motorways) are particularly important here. The cables are suitable for both sending and receiving signals.

The invention is based on the knowledge that it is not primarily the hole size or shape that is decisive for the radiation intensity, but rather the number of openings and their expansion perpendicular to the cable axis.

In contrast to the known arrangements, the spacing of the slots within the period length does not follow a simple law. The essence of the invention is that the slots are as narrow as possible and, due to their special arrangement, the pole points which are to be emitted in the frequency range to be radiated are extinguished or at least strongly damped are. Narrow slots are not only easy to produce, but more of these slots can be accommodated within a given period than of any other opening shape.

The slot spacing for suppressing pole positions is calculated using suitable functions using the Fourier transformation. In this, a frequency function is calculated from a frequency function, which according to the invention is realized by narrow slots perpendicular to the cable axis.

The first stage of this procedure is explained below. By making openings that are at the same distance from one another in pairs, one causes for a certain fundamental frequency f _o , as well as for all integer multiples of f _o pole positions in the frequency response of the radiation. The 2nd, 3rd, 4th, nth-order poles should be suppressed as far as possible. This is achieved by successively multiplying the output spectrum by cosine functions F₁, F₂, F₃, F₄, F₅, ... (ie F = F₁ · F₂ · F₃, ...). The functions have their maximum at frequency f = 0 (ie amplitude 1). At 2f _o the function F₁ must go through 0 so that the 2nd order pole is suppressed. With 4f _o this function has the amplitude -1. The period of oscillation is 8f _o . The Fourier transform of the product from the output spectrum - with all pole positions - and this cosine function of the oscillation period 8f _o is the convolution of the periodic individual opening with a pair of openings with a distance 2/8 of the period length; ie one opening per period becomes two.

By doubling the number of openings per period with the distance calculated from the Fourier transformation, the 2nd, 6th, 10th,. 14th, 18th, ... pole point deleted. The following amplitudes can be expected at the remaining pole positions:

The next step in the frequency domain is multiplied by F₂, the cosine of the oscillation period 12f _o . In the same way as for the first step, the 3rd, 9th, 15th, ... pole positions are deleted.

In the local area, each of the two openings per period is replaced by a double opening; The two newly created openings are offset 1/12 of the period length to the right or left. The amplitudes are analogous to those above:

In the next step, the remaining spectrum is multiplied by F₃, a cosine of the oscillation period 16f _o ; this also eliminates the 4th, 12th, 20th, ... pole positions. The four openings per period so far have become eight.

· $$\frac{\text{π}}{\text{2}}$$

) in Ansatz gebracht und das Produkt aller Cosinusfunktionen fouriertransformiert werden. Dieses Verfahren führt bei Leckkabeln für eine niedrige Grundfrequenz f_o und einen breiten Übertragungsfrequenzbereich rasch zu einer sehr großen Zahl von Schlitzen innerhalb der Periodenlänge und damit zu teilweise sehr kleinen Schlitzabständen.To the pole P in the frequency response of the coupling attenuation _n suppress must for each pole frequency f _n = n * f _o a corresponding cosine function F _n = cos ^m ( $$\frac{\text{1}}{\text{n + 1}}$$

· $$\frac{\text{π}}{\text{2nd}}$$

) and the product of all cosine functions is Fourier transformed. With leakage cables for a low fundamental frequency f _o and a wide transmission frequency range, this method quickly leads to a very large number of slots within the period length and thus to very small slot spacings in some cases.

The pattern of the openings has a period length of, for example, 2.2 m and 64 openings per period - which corresponds to the realization of the function F = F₁ · F₂ · F₃ · F₄ · F₅ · F₆ - the form shown in Fig. 1.

A preferred embodiment of the invention therefore consists in determining the zeros of the cosine functions in such a way that further pole positions are greatly weakened with a cosine function. This will the position of the zeros is not chosen exactly on the pole frequencies, but in such a way that the product of all cosine functions - for the pole frequencies as an argument - gives values <5 · 10⁻². In this way, the number of cosine functions required for smoothing the frequency response is reduced, so that the number and the minimum spacings of the slots to be accommodated in one period length can be realized technically.

Embodiments of the invention are explained below with reference to the drawing; 1 shows the result of the transformation of the "ideal" excitation function F into the spatial area, FIG. 2 shows the same for an optimized excitation function, and FIG. 3 shows the schematic representation of a manufacturing process for a cable.

The slot arrangement shown in FIG. 1 shows a special feature of some groups of closely adjacent slots, which are separated from the next slots by more or less large gaps. The space without slots is particularly noticeable in the right part of FIG. The slot arrangement is based on the "ideal" excitation function F = F₁ · F₂ · F₃ · F₄ · F₅ · F₆, where F₁ = cos (nπ / 2 · 2); F₂ = cos (nπ / 2 · 3); F₃ = cos (nπ / 2 · 4); F₄ = cos (nπ / 2 · 5); F₅ = cos (nπ / 2 · 7); F₆ = cos (nπ / 2 · 8). N = the order of the pole.

The shift of the zeros of the cosine functions already mentioned results, for example, in a slot configuration as shown in FIG. 2. The associated optimized cosine functions are: F₁ = cos (nπ / 2 · 2.02); F₂ = (nπ / 2 x 3.06); F₃ = cos (nπ / 2 x 4.41); F₄ = cos (nπ / 2 x 5.94); F₅ = cos (nπ / 2 · 8.48) and F₆ = cos (nπ / 2 · 12.63).

Although the zeros of the F _i functions differ significantly from the "ideal" excitation function in the optimized excitation function, the slot arrangement is similar in both cases.

At a basic frequency of 63 MHz, such a leakage cable can effectively suppress pole positions up to the 15th order. The smallest slot spacing is 8 mm.

When multiplying several cosine functions under the secondary condition that they should also cause a predetermined damping of the pole positions up to a certain order n for a predetermined number, solutions with different slot lengths can also result. For manufacturing reasons, however, predetermined, identical slot lengths are preferable. The parameters to be used for the optimization task are then the spacing of the slots, which is the minimum possible, or their number. The desired attenuation of the pole points is, for example, the value 25 db, since then there are no more interfering interferences. In order not to weaken the outer conductor mechanically more than necessary, it can be advantageous to divide the slots by crossbars.

The method for producing an arrangement according to the invention is explained in more detail with reference to FIG. 3. It shows a schematic picture of the cable production. The conductor strip 3 is provided with a certain slot sequence, which is repeated with the period length p, by means of mechanical or electrical methods. The period length is preferably 2.2 m. The slotted strip, preferably a copper strip, passes through the two rollers 5 simultaneously with a tensile plastic strip 6, which is laminated onto the strip 3 by pressure or heat. The band 6 covers the slots in such a way that the slots are mechanically secured against expansion when subjected to tensile stress. This also prevents the slots from being compressed or widened when the laminate is subsequently bent. The inner conductor 8 is surrounded by the dielectric 4 made of insulating material. This arrangement is enveloped by the laminate or by the copper strip, which is welded together at the interface by a welding device 7 to form a tube. The coaxial cable is finished by extruding a sheath. In a variant of this method, the slots are cut through sealed an adhesive. The adhesive has such a short hardening or setting time that the slots are mechanically secured before further deformation in the further processing to the cable can cause damage.

The slots can be produced by spark erosion or cut into the conductor strip 3 by a laser. An alternative would be to produce the slots using rotating saw blades, which already have the correct slot spacing by means of spacers. Since these slots repeat with the period length p, it is possible in this way to produce the slots for one period length in one operation with one set of saw blades. For continuous production, the copper strip 3 is only to be shifted exactly by the period length p before the next group of slots is produced.

In a preferred type of production, the pattern of the slots is applied as raised webs on the circumference of a roller, the circumference of which corresponds to the period length of the slot arrangement. The roller is constantly coated with a non-stick agent so that the webs can apply this substance to a belt. The tape is made of polyester, for example, and is coated with non-stick agent, for example graphite powder, after printing. The slots remain free. The strip is then copper-plated. The coated tape is then formed into the outer conductor of a coaxial cable.

Claims (20)

1. Arrangement for transmitting high-frequency signals with a coaxial cable (2) which is provided with openings (1) in the outer conductor, the openings (1) being repeated, viewed in the longitudinal direction of the cable, in groups with a period length which is selected in such a way that the groups of opening radiate high-frequency signals starting from a desired lower cut-off frequency f₀, and that the pole points assigned to the multiples of the lower cut-off frequency f₀ are cancelled out, or at least strongly attenuated, in the frequency response of the coupling attenuation, characterized in that the number of openings per period is greater than 30, in that the openings are slots (1) which are as narrow as possible, and in that the slots (1) are essentially arranged perpendicular to the axis of the cable.
2. Arrangement according to Claim 1, characterized in that the slot intervals are selected to be such that, in order to attenuate more than 15 pole points, they do not drop below a specific minimum interval which is at least twice as large as the width of the slot.
3. Arrangement according to Claim 1 or 2, characterized in that an excitation function F is selected which, with a prescribed number of slots, has zero points at the largest possible number of pole points of the frequency response or in their vicinity and the function F is defined in that it produces an arrangement of slots during the transformation from the frequency domain into the space domain.
4. Arrangement according to one of Claims 1 to 3, characterized in that the slots (1) are subdivided by webs which are arranged in the longitudinal direction of the slots.
5. Arrangement according to one of Claims 1 to 4, characterized in that the slots (1) are of different lengths.
6. Arrangement according to one of Claims 1 to 5, characterized in that the slot intervals (1) form a non-equidistant and non-periodic sequence within the period.
7. Arrangement according to one of Claims 1 to 6, characterized in that the width of the slots is approximately one thousandth of the period length.
8. Arrangement according to one of Claims 1 to 7, characterized in that, in order to attenuate the pole points, a function F is provided which is a product of a plurality of functions F_i in the frequency domain.
9. Arrangement according to one of Claims 1 to 8, characterized in that the attenuation of pole points takes place up to approximately 15 times the lower cut-off frequency by functions F_i which are optimized according to amplitudes and/or frequencies.
10. Arrangement according to one of Claims 1 to 9, characterized in that the optimization is selected in such a way that a prescribed attenuation of the pole points by at least 20 dB can be achieved with a smallest possible number of functions F_i.
11. Arrangement according to one of Claims 1 to 10, characterized in that the function F is a product of cosine functions with different arguments.
12. Method for producing a coaxial cable (2) according to Claim 1, with openings (1) which are arranged in the outer conductor and are repeated in groups with a period length, characterized in that in a conductor strip (3) the groups of openings (1) are produced cyclically in such a way that the number of openings per period is greater than 30, that the openings are slots (1) which are as narrow as possible and that the slots (1) are essentially arranged perpendicular to the axis of the cable and the conductor strip (3) is moved on after each cycle by the period length and is shaped to the cylindrical outer conductor of the coaxial cable.
13. Method according to Claim 12, characterized in that the slots (1) are cut into the conductor strip (3) by a laser.
14. Method according to Claim 12, characterized in that the slots (1) are produced for a period length by means of a set of synchronously rotating sawblades at fixed intervals.
15. Method according to Claim 12, characterized in that the slots (1) are produced by punching.
16. Method according to Claim 12, characterized in that the slots (1) are produced by photolithography and etching.
17. Method according to Claim 12, characterized in that the slots (1) are produced by spark erosion.
18. Method according to Claim 12, characterized in that the slots are produced by coating a plastic strip by at least one pattern of the slots for one period length being applied to the circumference of a pressure roller and the different surface properties of the pattern on the roller being used in such a way that, after the pressure roller rolls on the plastic strip, a conductive layer is applied to the plastic strip outside the periodically arranged slots.
19. Method according to Claim 18, characterized in that after a first conductive layer is applied the said conductive layer is reinforced by electrical deposition of a metal layer which is a good conductor.
20. Method according to one of Claims 12 to 17, characterized in that the conductor strip (3) which is provided with slots (1) is laminated with a tension-resistant plastic strip which covers the slots, and in that the edges of the conductor strip (3) are left free for the subsequent joining process.

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