WO1998009340A1

WO1998009340A1 - Summing network and stub

Info

Publication number: WO1998009340A1
Application number: PCT/FI1997/000493
Authority: WO
Inventors: Veli-Matti SÄRKKÄ; Timo AHONPÄÄ
Original assignee: Nokia Telecommunications Oy
Priority date: 1996-08-29
Filing date: 1997-08-27
Publication date: 1998-03-05
Also published as: FI963376A; CN1228878A; FI963376A0; NO990930D0; JP2000517116A; FI103227B; AU4016397A; EP0922310A1; NO990930L; FI103227B1; AU724725B2

Abstract

The present invention relates to a stub for tuning a summing network of a base station, which stub (1) comprises: a connector (9) for connecting the stub to the summing network, an elongated centre conductor (7) whose first end is connected to a first pole of the connector (9), and a substantially tubular conductor (8) encompassing the centre conductor (7), the tubular conductor being arranged concentrically with the centre conductor, and whose first end is connected to a second pole of the connector (9). To achieve an easily adjustable stub, the stub (1) comprises a moveable section (11) which is manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor (7), and which is moveable in the longitudinal direction of the centre conductor (7) for adjusting the phase angle of a wave reflecting from the stub (1) of the summing network.

Description

SUMMING NETWORK AND STUB

The invention relates to a method for tuning a summing network of a base station, which summing network includes a stub comprising an elongated centre conductor and a substantially tubular conductor encompassing the centre conductor, the tubular conductor being arranged concentrically with the centre conductor, and in which method the summing network is tuned by means of the stub. The invention also relates to a summing network for combing radio frequency signals supplied by radio transmitters and for supplying the signals to a common antenna means, which summing network includes conductors, connector pieces and a stub comprising a connector for connecting the stub to the summing network, an elongated centre conductor whose first end is connected to a first pole of the connector, and a substantially tubular conductor encompassing the centre conductor, the tubular conductor being arranged concentrically with the centre conductor, and whose first end is connected to a second pole of the connector. The invention further relates to a stub for tuning a summing network of a base station, which stub comprises: a connector for connecting the stub to the summing network, an elongated centre conductor whose first end is connected to a first pole of the connector, and a substantially tubular conductor encompassing the centre conductor, the tubular conductor being arranged concentrically with the centre conductor, and whose first end is connected to a second pole of the connector.

The invention especially relates to a summing network of combiner filters of a base station in a cellular radio system. A combiner filter is a narrow- band filter which is in resonance (in tune) exactly on the carrier frequency of a transmitter coupled to it. In a base station of the cellular radio system, for example, the signals obtained from the outputs of combiners are summed together by a summing network of a transmitting antenna, which summing network usually consists of a coaxial cable leading to the base station antenna, to which coaxial cable the combiner filters are usually coupled by T-branches.

In order that as much as possible of the transmitting power of the base station transmitters can be forwarded to the antenna (and not be reflected back to the transmitter), the summing network should be tuned with regard to frequency channels used by the transmitters of the base station. The summing network is optimally tuned (is in resonance) when the electrical length of its cables corresponds to a multifold of half the wavelength of the carrier wave of the signal to be transmitted. Strictly speaking, a summing network is thereby tuned on one frequency only, but the mismatch does not at first grow very abruptly when the frequency changes away from the optimum. In practice, the summing network is usually optimized approximately to the centre of the frequency band of the base station, in which case the transmitting power of transmitters that operate at the edge of the frequency band can also be supplied to the base station antenna without significant losses.

A usable frequency band of the summing network, which typically has a width of about 2-10% ^* fo (fo being the centre frequency of a band), is in practice, however, too narrow so that the frequency channels of the base station transmitters could be changed very much without having to deal with the tuning of the summing network. Therefore a need has arisen for a fast and simple change of the tuning of the summing network.

There is a prior art solution for tuning a summing network, in which solution a stub is coupled to the summing network. Said stub is coupled to the last T-branch of the summing network in which case it connects to a connector which would otherwise be left open. The stub contains a short-circuit screw which short-circuits the outer conductor and the centre conductor of a coaxial cable. The physical position of the short-circuit screw can be shifted within a certain adjusting range. The position of the short-circuit screw determines the electrical length of the stub, and thus of the summing network which consists of a coaxial cable and connectors, that is, the frequency to which the summing network is tuned.

The most serious weakness of the above-mentioned prior art stub is the difficulty in adjusting it. The stub has to be adjusted manually by removing the short-circuit screw first and by fixing it after this to a new place. The measure in question requires a visit by a service person on the site, which in turn takes a lot of time and increases costs. Especially the implementation of new automatically tuned combiner filters has set new demands for tuning the summing network because it would be otherwise possible to carry out a frequency change automatically (by remote control).

It is also previously known to have a stub where the extension of the centre conductor of the coaxial cable in the summing network is arranged to be pushed inside a grounded metal tube or the like. In this solution, a metal tube which can be moved by means of an electric motor in the axial direction of the extension of the centre conductor is provided with slide contacts protruding from the inner surface of the metal tube towards the extension of the centre conductor so that they are in contact with the extension. This contact point thus forms a grounding point for the centre conductor. When the metal tube is then moved by means of the electric motor in the axial direction of the extension of the centre tube, the slide contacts will slide along the surface of the extension of the centre conductor in such a manner that said grounding point will move, in which case the electrical length of the coaxial cable will change and the summing network can be tuned to a desired frequency.

The most significant weakness of this prior art solution is that it requires the use of mechanical slide contacts which makes it sensitive to disturbances because of wear and corrosion, for example. This solution is not either suitable to be used on high frequencies, that is, in practice, this solution can be used only up to a frequency of about 1 GHz.

The object of the present invention is to solve the above-mentioned problem, and to provide a solution for making the tuning of a summing network easier such that the summing network can be tuned by remote control, when needed, or that the tuning of the summing network can be even totally automatized. The object of the invention is also to provide a solution enabling linear adjustment for tuning the summing network which can be applied to a greater frequency range than previously, that is, also on frequencies of over 1 GHz. A further object of the invention is to accomplish a solution for tuning the summing network where mechanically wearing parts are not needed. These and the other objects of the invention will be attained with the method of the invention that is characterized by arranging to the stub a moveable section manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor, and adjusting the phase angle of a wave reflecting from the stub of the summing network for accomplishing a phase transfer and for tuning the summing network by moving said moveable section in the longitudinal direction of the centre conductor. The object of the invention is also to accomplish a summing network where the method of the invention can be realized. The summing network of the invention is characterized in that the stub comprises a moveable section which is manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor, and which is moveable in the longitudinal direction of the centre conductor for adjusting the phase angle of a wave reflecting from the stub of the summing network for tuning the summing network of the base station.

A further object of the invention is to accomplish a stub by which the method of the invention can be realized, and which is suitable to be used in the summing network of the invention. The stub of the invention is characterized in that the stub comprises a moveable section which is manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor, and which is moveable in the longitudinal direction of the centre conductor for adjusting the phase angle of a wave reflecting from the stub of the summing network.

The invention is based on the idea that by adjusting the phase angle of a wave reflecting from the stub of the summing network by means of an air- insulated coaxial construction where a moveable section manufactured of a ferrimagnetic material or dielectric material, such as ceramic or Teflon encompasses the centre conductor, in which case a more accurate, reliable and uncomplicated solution fortuning the summing network will be attained.

By moving the moveable section, through a longitudinal slot in the outer sheath of the coaxial construction, for example, the phase angle of a wave reflecting from the stub will change. One end of the stub can be short-circuited or alternatively open, and a coaxial connector is arranged preferably to its other end. When the other end of the stub is short-circuited (or open), an RF signal coming from the summing network to the stub is reflected back, whereby a standing wave will be formed in the construction. The energy distribution of the fields will change in the formed standing wave when moving along the coaxial construction as a function of place in such a manner that the maximum of the magnetic field and the minimum of the electric field are located at the short- circuit point. When moving away from the short-circuit point, the energy distribution of the fields will change so that when one fourth of the wave length from the short-circuit point will be reached, the energy of the electric field will be at the maximum and that of the magnetic field at the minimum. In the case of an open end, the maximum of the electric field is at the open end of the stub and the maximum of the magnetic field will be correspondingly found at a distance of λ/4.

Because of the energy distribution explained above, the effect of the moveable section on the phase angle of the reflection coefficient S11 is determined by its position, that is, as a function of place. By the solution of the invention, a simpler and more linear method will thus be attained for tuning the summing network. A long axial adjustment movement of the moveable section enables an accurate control for instance by means of an electromechanical regulating unit by remote control or alternatively, the adjustment can be totally automatized. Furthermore, it is easy to influence the slope of the adjustment and the angle coefficient by the dimensioning of the moveable section and by material selections. The solution of the invention is also suitable to be used on a greater frequency range than previously, that is, it can also be utilized in systems operating on frequency ranges of about 1 to 10 GHz. The invention is therefore suitable to be used in DCS 1800 and DCS 1900 (Digital Cellular System) mobile telephone systems, for example, where frequency channels on ranges of about 1800 MHz and about 1900 MHz, respectively, are used.

The preferred embodiments of the summing network and the stub of the invention appear from appended dependent claims 3 and 5 to 8. In the following, the invention will be explained in more detail by way of example with reference to the accompanying drawings, wherein

Figure 1 illustrates a first preferred embodiment of a summing network of the invention,

Figure 2 illustrates a first preferred embodiment of a stub of the invention, and

Figure 3 illustrates a second preferred embodiment of a stub of the invention.

The summing network of Figure 1 can be a summing network of a base station of a cellular radio system, such as NMT (Nordisk Mobil Telefon), DCS (Digital Cellular System), or GSM system (Groupe Special Mobile).

The summing network shown in Figure 1 comprises coaxial cables 6 and T-branches 5. The coaxial cable from the uppermost T-branch is coupled to a base station antenna ANT, and a stub 1 is arranged to the connection of the lowest T-branch. The base station seen in Figure 1 comprises four radio transmitters

TX. The radio frequency signals supplied by the transmitters TX are directed through narrow-band combiner filters 4 to the summing network through the T- branches 5. In order that as much as possible of the transmitting power of the base station transmitters is supplied to the antenna ANT without being reflected back from points of mismatch, the electrical length of the cables 6 of the summing network must be a multiple of one half of the wavelength of the carrier wave of the signal to be transmitted, that is, n^*λ/2 (n=1,2,3...). Correspondingly, the length of the cables 3 leading from the outputs of the filters to the cables 6 has to be n^*λ/4 (n=1,3,5...). Thus, the summing network is completely tuned (in resonance) on one frequency only, but the mismatch usually does not at first grow very abruptly when the frequency changes away from the optimum.

The combiner filters 4 of Figure 1 are tuneable, i.e. their frequency can be adjusted in a way known per se to correspond to the frequency channels used by the transmitters TX. However, the adjustment/change of the frequency channels of the transmitters TX leads to the need for the summing network to be re-tuned to correspond to the new frequency channels. Said tuning is carried out by the stub 1 of the invention.

A control signal can be produced for the stub 1 of Figure 1 in such a manner that, for example, a base station controller or a similar control unit feeds a control signal to the stub 1 , which control signal indicates the centre frequency of the frequency channels of the base station. If the base station comprises means for measuring the power reflected back from the points of mismatch, the stub can be supplied with a control signal which is based on the power reflected back from the points of mismatch of those transmitters TX using the outermost frequency channels. Measuring means of this kind are already known in connection with automatically tuneable combiner filters, and thus they will not be discussed in greater detail herein.

Figure 2 shows a first preferred embodiment of a stub of the invention. The stub 1 shown in Figure 2 has an open structure, that is, a centre conductor 7 of the stub is not coupled to a tubular outer conductor 8. Figure 2 illustrates a partly sectional view of the stub in which case it shows a section 10 which is manufactured of a low-loss insulated material (such as Teflon) in the stub, and which holds the free end of the centre conductor 7 of the stub in place with respect to the outer conductor 8.

The stub 1 is connected to the summing network of the base station by means of a coaxial connector 9 in which case a contact will be formed between the centre conductor 7, coupled to the first pole of the coaxial connector, and the centre conductor of the coaxial cable in the summing network. Similarly, a contact will be formed between the tubular outer conductor 8 of the stub, coupled to the second pole of the coaxial connector 9, and the outer conductor of the coaxial cable in the summing network. There has been arranged to an annular air gap 12 between the centre conductor 7 and the tubular conductor 8 a moveable section 11 , which is manufactured of a low-loss dielectric material and which is moveable in the longitudinal direction of the centre conductor 7 in a space 12. A low-loss dielectric material refers to such material whose relative permittivity er>1 (and at the same time, the relative permeability μr=1), such as Teflon or ceramic, for example. Then the effect of the moveable section on the phase angle of the reflection coefficient of the stub is at its greatest at the maximum point of the electric field and at its smallest at the minimum point of the magnetic field. If, differing from the case of Figure 2, the material of the moveable piece 11 is such that er=1 and μr>1 , the effect is opposite, that is, the maximum effect is at the maximum of the magnetic field.

A slot (not shown in the figure) in the longitudinal direction of the tube has been formed in the tubular outer conductor 8, a projection 13 attached to the moveable piece 11 protruding from the slot, by means of which projection the moveable section 11 can be moved in the tubular conductor 3. By the structure shown in Figure 2, a simple and sufficiently linear phase angle adjustment of the reflecting wave can be attained, the axial control movement of the adjustment being easy to combine to a frequency control movement of the filter. Furthermore, the slope of the adjustment and the angle coefficient of the adjustment can be easily influenced by the dimensioning of the moveable section and by material selection.

In Figure 2, the propagating wave a/ ° is indicated with arrow a and the reflecting wave b/β° with arrow b. The reflection coefficient T is then: T = ^/ α⁰ - β° b

Moving the moveable section 9 from one place to another has an effect on the reflection coefficient of the switching element 1 as shown in the lower part of Figure 2. That is, depending on the position of the moveable piece (described by the horizontal axis) in the stub 1 , the phase angle of the reflection coefficient will change (the vertical axis).

Figure 3 illustrates a second preferred embodiment of a stub of the invention. A stub V shown in Figure 3 corresponds to the stub 1 shown in Figure 2 in other respects but in the case of Figure 3, the structure of the stub is closed, that is, its centre conductor 7 is coupled to the outer conductor 8 at a short-circuit point 14. Furthermore, the electrical length of the structure of the stub in Figure 3 is 3L/4, whereas in the case of Figure 2, it is L/2. It is to be understood that the foregoing explanation and the figures related thereto are only intended to illustrate the present invention. The variations and modifications of the invention will be evident to those skilled in the art without deviating from the scope and spirit of the invention disclosed in the appended claims.

Claims

1. A method for tuning a summing network of a base station, which summing network includes a stub (1 , 1') comprising an elongated centre conductor (7) and a substantially tubular conductor (8) encompassing the centre conductor, the tubular conductor being arranged concentrically with the centre conductor (7), and in which method the summing network is tuned by means of the stub (1 , 1'), c h a r a c t e r i z e d by arranging to the stub (1 , 1') a moveable section (11) manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor (7), and adjusting the phase angle of a wave reflecting from the stub (1 , 1 ') of the summing network for accomplishing a phase transfer and for tuning the summing network by moving said moveable section (11) in the longitudinal direction of the centre conductor (7).

2. A summing network for combing radio frequency signals supplied by radio transmitters (TX) and for supplying the signals to a common antenna means (ANT), which summing network includes conductors (3, 6), connector pieces (5) and a stub (1 , 1') comprising a connector (9) for connecting the stub to the summing network, an elongated centre conductor (7) whose first end is connected to a first pole of the connector (9), and a substantially tubular conductor (8) encompassing the centre conductor (7), the tubular conductor being arranged concentrically with the centre conductor, and whose first end is connected to a second pole of the connector (9), c h a r a c t e r i z e d in that the stub (1, 1') comprises a moveable section (11) which is manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor (7), and which is moveable in the longitudinal direction of the centre conductor (7) for adjusting the phase angle of a wave reflecting from the stub (1 , 1') of the summing network for tuning the summing network of the base station.

3. A summing network according to claim 2, c h a r a c t e r i z e d in that the output of each radio transmitter (TX) is connected to the input of a corresponding filter (4), that the input of each filter (4) is connected by means of a branching member (5) to an antenna cable of the summing network, and that the stub (1 , 1 ') is connected to said antenna cable.

4. A stub for tuning a summing network of a base station, which stub (1 , 1') comprises: a connector (9) for connecting the stub to the summing network, an elongated centre conductor (7) whose first end is connected to a first pole of the connector (9), and a substantially tubular conductor (8) encompassing the centre conductor (7), the tubular conductor being arranged concentrically with the centre conductor, and whose first end is connected to a second pole of the connector (9), characterized in that the stub (1, 1') comprises a moveable section (11) which is manufactured of a low-loss dielectric or ferrimagnetic material encompassing at least the centre conductor (7), and which is moveable in the longitudinal direction of the centre conductor (7) for adjusting the phase angle of a wave reflecting from the stub (1, 1') of the summing network.

5. A stub according to claim 4, characterized in that the end of the centre conductor (7) situated further from the connector (9) is connected to the tubular conductor (8) to achieve a short-circuited stub (1').

6. A stub according to claim 4 or 5, c ha racter ized in that said moveable section (11) is a cylindrical solid section manufactured of ceramic or Teflon where an opening extends through the middle section.

7. A stub according to any one of claims 4 to 6, characterized in that the moveable section (11) encompasses the centre conductor (7), and that tubular conductor (8) encompasses the moveable section (11), in which case the moveable section (1 ) can be moved along the centre conductor (7) in an airspace (12) confined by the centre conductor and the essentially tubular section (8).

8. A stub according to claim 7, characterized in that the sheath of the tubular conductor (8) comprises a slot extending in the longitudinal direction of the tubular conductor, and that the moveable section (11) comprises a projection (13) protruding through said slot, in which case the moveable section (11) of the stub (1, 1') can be moved by means of the projection (13) protruding from the slot of the tubular section.