METHOD AND DEVICE FOR SUPERVISING THE CONDITION OF A RECEIVING ANTENNA

Abstract: The invention relates to a method and a device for supervising the condition of a receiving antenna by transmitting a radio-frequency measuring signal through an antenna line (2) to the antenna and by measuring (8) the magnitude of a measuring signal component reflected back from the antenna. To improve the measuring accuracy, the invention is characterized in that the same measuring signal is measured also when transmitted directly in the direction of the receiver and the condition of the antenna (1) is determined on the basis of the relative difference between the two measured signal magnitudes.

Description: Method and device for supervising the condition of a receiving antenna The invention relates to a method of super- vising the condition of a receiving antenna by transmitting a radio-frequency measuring signal through an antenna line to the antenna and by measuring the magnitude of a measuring signal component reflected back from the antenna over a pre- determined measuring path. Receiving and transmitting antennas form an essential part of radio systems, such as cellular radio telephone systems and their base stations, and their condition affects the quality of transmissions. The condition of the antennas can be supervised e.g. by measuring their standing-wave ratio, that is, the electric matching of the antennas to the rest of the receiving and transmitting system. With methods in use today, the measurement is carried out by supplying power to the receiving antenna through the antenna line and by measuring the power reflected from the antenna through the antenna line by means of a wideband powermeter. Due to the wide bandwidth of the powermeter, high power has to be used in the measurement in order that the measure¬ ment would not be sensitive to interferences, that is, signals received by the antenna would not inter¬ fere with the measurement. The use of high power causes intermodulation distortion in the receiver. Such problems can be avoided by using a duplex filter which separates the measuring signal and the signals to be received from each other. The duplex filter, however, has to precede the receiver parts, which deteriorates the sensitivity of the receiver as the filter causes losses in the antenna signal. In one prior art arrangement, both of the above-mentioned problems are avoided by using a measuring frequency outside the receiving band and a narrow-band power measurement matched to this frequency. EP Patent Application 0 261 828 discloses a microwave range analyser in which both a signal supplied directly from the measuring source and a signal reflected from a microwave network under analysis is measured for a vector-based relative power measurement. In the measurement of the reflected power, a sample signal extracted from the output power of the measuring signal generator is applied to a detector over a different path from that of a sample signal extracted from the power reflected from the microwave network under analysis. Thus, in the EP Patent Application, the relative measurement is rather insensitive to variation in the output level of the measuring signal generator, but as the signals to be compared with each other propagate over different paths to the detector, the measurement does not automatically take into account imperfections and variations occurring in the properties of signal path components. For the measurement of reflected power the analyser has to be calibrated by first measuring a known microwave standard by it and using the calibration values so obtained in subsequent measure¬ ments. In the measurement of the condition of the antenna, this would imply that the antenna supply should be connected to a microwave standard in place of the antenna for calibration. The object of the invention is to improve the accuracy, reliability and long-term stability of prior methods intended for supervising the condition of the antenna. This is achieved by means of a method of the type disclosed in the introduction, which according to the invention is characterized in that the same measuring signal is measured over the same measuring path also when transmitted directly in the direction of the receiver, and the condition of the antenna is determined on the basis of the relative difference between the two measured signal magnitudes. The invention is also concerned with a device for supervising the condition of a receiving antenna, comprising a transmitting means for generating a radio-frequency measuring signal; a first directional coupling means for applying the measuring signal to the antenna line in the direction of the antenna; and a measuring means for measuring the magnitude of a measuring signal component reflected from the antenna and propagated over a predetermined measuring path. The device is characterized in that it further com¬ prises a second directional coupling means for apply- ing the measuring signal directly in the direction of the receiver to said measuring path; and a switch means for switching the measuring signal from said transmitting means first to the second directional coupling means for measuring the magnitude of the measuring signal and then to the first directional coupling means for measuring the magnitude of a reflected measuring signal component, the condition of the antenna being determined by the measuring means on the basis of the relative difference between the two measured signal magnitudes. The basic idea of the invention is to measure not only the signal reflected from the antenna but also separately the magnitude of the measuring signal transmitted directly in the direction of the receiver. This other measurement provides a reference value for the magnitude of the measuring signal, with which value the magnitude of the measuring signal component reflected from the antenna is compared. This reference value takes into account the transmission power of the measuring signal generator and the properties of signal path components, such as amplifiers and branching devices, at the measuring moment. With such a relative measurement the measuring accuracy is not affected by deviation occurring between the properties of different individual components forming part of the measuring arrangement and the measuring signal path or variation in time in the properties of one and the same component. By means of the invention, it is also possible to substantially reduce and simplify manual check-ups and calibrations during installation and use or even completely avoid them. In the following the invention will be described in greater detail by means of exemplifying embodiments with reference to the attached figure, which shows a block diagram of a device realizing the method according to the invention. The invention will be described herein only to such an extent as is necessary for the disclosure and understanding of the invention. A receiving antenna 1 is connected by means of an antenna cable 2 through a bandpass filter 7 and a dividing amplifier 5 to a branching device 6 which divides a signal coming from the direction of the antenna into two or more branches. The branching device 6 is used especially in radio systems comprising several frequency channels when each frequency channel has a separ -e receiver after the branching device. A signal generator 9 generates a radio- frequency measuring signal for use in the measurement of the condition of the antenna. The frequency of the measuring signal is preferably outside the normal receiving band of the receiver so as to minimize interference occurring in normal reception. An adjustable attenuator or amplifier 9 may be connected to the output of the signal generator 10 for adjust¬ ing the magnitude of the measuring signal to be transmitted. The measuring signal is applied from the attenuator 9 to a change-over switch SI, which passes the measuring signal to a directional coupling means 3 in one position and to a directional coupling means 4 in the other position. The directional coupling means 3 and 4 are connected to the antenna line 2 in such a way that the directional coupling means 3 directs the measuring signal to be transmitted through the antenna line in the direction of the antenna, and the directional coupling means 4 directs the measuring signal to be transmitted through the antenna line in the direction of the receiver. One output of the branching device 6 is con¬ nected to a measuring unit 8, which measures the magnitude of the measuring signal coming from the branching device. In the preferred embodiment of the invention, the measuring unit 8 also controls the change-over switch SI. The measuring unit may be e.g. a powermeter with an associated logic circuitry for the required control and calculation operations. The measuring can also be carried out by means of a field magnitude meter possibly already existing in the device. The measuring device used is preferably arranged to measure within a narrow bandwidth exactly at the measuring signal frequency. When the condition of the antenna 1 is measured, the switch SI passes the measuring signal from the signal generator 10 to the directional coupling means 4, which directs the measuring signal in the direction of the receiver, whereby it proceeds through the amplifier 5 and the branching device 6 to the measuring unit 8, which measures the magnitude or power of the signal and stores the measured value for subsequent use as a reference value. Thereafter the state of the switch SI is changed, and so the measuring signal is passed to the directional coupling means 3, which directs the measuring signal through the antenna line 2 to the antenna 1. A measuring signal component reflected from the antenna 1 back to the antenna line 2 passes through both directional coupling means 3 and 4, the amplifier 5 and the branching device 6 to the measuring unit 8, which again measures the value of the magnitude of the reflected signal. Thereafter the measuring unit 8 calculates the relative difference between the reference value and the reflected signal magnitude. The relative value so obtained is compared with a preset alarm limit; if the alarm limit is exceeded, the measuring unit gives an alarm in a suitable manner. The figure and the description related to it are only intended to illustrate the present invention. In their details, the method and the device of the present invention may vary within the scope of the attached claims.

Claims: Claims: 1. Method of supervising the condition of a receiving antenna by transmitting a radio-frequency measuring signal through an antenna line to the antenna and by measuring the magnitude of a measuring signal component reflected back from the antenna over a predetermined measuring path, c h a r a c t e r ¬ i z e d in that the same measuring signal is measured over the same measuring path also when transmitted directly in the direction of the receiver, and the condition of the antenna is determined on the basis of the relative difference between the two measured signal magnitudes. 2. Method according to claim 1, c h a r a c ¬ t e r i z e d in that the measurements are carried out by means of a field magnitude meter of the receiver. 3. Device for supervising the condition of a receiving antenna, comprising a transmitting means (9, 10) for generating a radio-frequency measuring signal; a first directional coupling means (3) for applying the measuring signal to the antenna line (2) in the direction of the antenna (1); and a measuring means (8) for measuring the magnitude of a measuring signal component reflected from the antenna (1) and propagated over a pre¬ determined measuring path, c h a r a c t e r i z e d in that the device further comprises a second directional coupling means (4) for applying the measuring signal directly in the direction of the receiver to said measuring path; and a switch means (SI) for switching the measuring signal from said transmitting means first to the second directional coupling means (4) for measuring the magnitude of the measuring signal. and then to the first directional coupling means (3) for measuring the magnitude of a reflected measuring signal com- ponent, the condition of the antenna being determined by the measuring means (8) on the basis of the relative difference between the two measured signal magnitudes. 4. Device according to claim 3, c h a r a c - t e r i z e d in that a branching device means (6) is coupled to the antenna line (2) before the receiver, the branching device means passing part of the signal coming from the direction of the antenna (1) into a branch comprising said measuring means for measuring the magnitude of the measuring signal. 5. Device according to claim 4, c h a r a c t e r i z e d in that the frequency of the measuring signal is outside the frequency band of the receiver.

Inventors: JAENTTI ARTO, SAVUSALO JARI, JAENTTI, ARTO, SAVUSALO, JARI
Applicants: TELENOKIA OY
Application year: 1991
Main classification: G01R27
IPC codes: G01R29