Wave transmission lines and networks – Plural channel systems – Having branched circuits
Patent
1996-02-13
1997-11-18
Gensler, Paul
Wave transmission lines and networks
Plural channel systems
Having branched circuits
333134, 333136, 333263, 455103, 455120, H01P 512, H04B 102
Patent
active
056892195
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/FI95/00372, filed Jun. 27, 1995.
TECHNICAL FIELD
The present invention relates to a summing network for combining and feeding radio frequency signals supplied by radio transmitters to common antenna means, which summing network comprises conductors, connectors and a stub.
BACKGROUND OF THE INVENTION
The invention especially relates to a summing network of combiner filters of a base station in a cellular radio network. A combiner filter is a narrow-band filter which resonates exactly on the carrier frequency of a transmitter coupled to it. In the base station of a cellular radio system, for example, the signals obtained from the outputs of the combiners are combined 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), if the electrical length of its cables corresponds to a multifold of half the wavelength 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 increase very fast when the frequency changes away from the optimum. In practice, the summing network is usually optimized to approximately 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.
In practice, however, the usable frequency band of a summing network is too narrow for the frequency channels of the base station transmitters to be changed very much without having to deal with the tuning of the summing network. So, need has arisen for a fast and simple adjustment of the tuning of the summing network.
A prior art solution is known 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 coaxial cable and connectors, i.e. the frequency to which the summing network is tuned.
The most serious weakness of the aforementioned, prior art, stub is the difficulty in adjusting it. The stub has to be adjusted manually by shifting the position of the short-circuit screw. The measure in question requires a visit by a service man to the site, which in turn takes a lot of time and increased costs.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the aforementioned problem, and to provide a solution for making the tuning of a summing network easier. This object is achieved by a summing network of the invention characterized in that the stub comprises tuning means for changing the electrical length of the summing network as a response to a control signal fed to the stub.
The invention is based on the idea that the tuning of the summing network for a new frequency range is made considerably easier and faster as the stub is provided with tuning means for changing the electrical length of the summing network by means of a control signal fed to said stub. Thus, for example, the tuning of the summing network can be carried out by remote control without service personnel having
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Gensler Paul
Nokia Telecommunications Oy
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