GB2266155A

GB2266155A - Measuring standing wave ratios

Info

Publication number: GB2266155A
Application number: GB9208236A
Authority: GB
Inventors: Christopher Ian Mobbs
Original assignee: Filtronic Components Ltd
Current assignee: Teledyne Defence Ltd
Priority date: 1992-04-14
Filing date: 1992-04-14
Publication date: 1993-10-20
Also published as: GB9208236D0; WO1993021667A1

Abstract

A directional coupler comprises a main signal conductor (10), probes (13, 14) capacitively coupled to the conductor (10) at points spaced a quarter wavelength apart along the conductor (10), one probe (13) providing a first terminal (4), and a quarter wavelength line and a half wavelength line connected respectively from the probes (13, 14) to a second terminal (3). Signals on the first and second terminals (4, 3) represent the power of the forward and reverse signals, respectively, on the conductor (10), so that the ratio between these signals represents VSWR (voltage standing wave ratio). The two terminals may be connected to a logarithmic circuit giving an output proportional to the difference between the logarithms of the forward and reverse powers (therefore representing VSWR).

Description

VSWR Measurement This invention relates to the measurement of VSWR (voltage standing wave ratio) in the feed to an antenna, particularly in high power communications systems.

There is a requirement for the measurement of the VSWR in the feed to an antenna in high power communications systems.

The newly introduced digital modulation systems make these measurements difficult owing to the pulsed nature and wide dynamic range of the signals. The application also places stringent limits on the intermodulation distortion which will be tolerated (-150dB), so that any device which follows the transmit output filter is a very critical component.

We have now devised a directional coupler with characteristics suited to this application.

In accordance with this invention, there is provided a directional coupler which comprises a main signal conductor, first and second probes capacitively coupled to said main signal conductor at points spaced a quarter wavelength apart along said main signal conductor, the first probe providing a first terminal, and a quarter wavelength line and a half wavelength line connected respectively from the first and second probes to a second terminal.

In this arrangement, signals on the first and second terminals represent the power of the forward and reverse signals, respectively, so that the ratio between these signals represents the VSWR.

The first and second terminals may be connected to a logarithmic circuit which gives an output proportional to the difference between the logarithms of the forward and reverse powers, and therefore represents the ratio of these powers (i.e. the VSWR). This circuit may comprise a successive detection log amplifier.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic longitudinal section through a directional coupler in accordance with this invention FIGURE 2 is an equivalent circuit diagram of the directional coupler of Figure 1; and FIGURE 3 is a schematic block diagram of a circuit for determining VSWR from the directional coupler of Figure 1.

Referring to Figure 1, there is shown a directional coupler which comprises a conductive metal rod 10 disposed axially within a cylindrical metal tube 12. One end 1 of the rod 10 receives the signal to be transmitted and the opposite end 2 is connected to the transmit antenna. Two adjustable probes 13,14 project through the tube 12 into the air-space around the rod 10, the probes 13,14 being spaced apart along the length of the rod 10 by a quarter wavelength. The probes 13,14 couple capacitively with the rod 10. Probe 13 is connected to a signal output 4, and probes 13 and 14 are connected respectively by quarter wavelength and half wavelength lines (e.g. implemented by strip lines) to a signal output 3.

Figure 2 is the equivalent circuit diagram of the directional coupler of Figure 1. The equivalent circuit corresponds to the "rat race" hybrid with some of the quarter wavelength lines thereof replaced by lumped capacitor impedance inverters. The negative shunt capacitors are however negligible. The directional coupler of Figure 1 is aligned in simple manner, by adjusting the two probes 13,14 for the correct forward coupling and reverse isolation response.

The output signal from terminal 4 represents the power of the forward signal and the output signal from terminal 3 represents the power of the reflected signal: the outputs from terminals 4 and 3 are however very small, typically - 40dB relative to the signals whose power they represent. A comparison of the output signals from terminals 4 and 3 therefore enables the VSWR to be determined.

Figure 3 shows a circuit for determining the VSWR from the signals on terminals 3 and 4. A successive detection log amplifier LA samples the forward and reverse power signals alternately, under the control of a driver D acting on a switch S, typically at a rate of 2OKHz. The output of the log amplifier may be AC coupled and comprises a square wave having an amplitude proportional to the difference between the logarithms of the forward and reverse powers, therefore represents the ratio of these powers, i.e. the VSWR.

Preferably the arrangement is such that the forward power is attenuated relative to the reverse signal, typically by lldB.

The square wave from the log amplifier is detected using a synchronous detector SD controlled by the driver oscillator D.

The output voltage varies above or below zero level depending whether the return loss is greater or less than lldB. Usually the critical levels of return loss are between 6dB and 16dB: thus the measurement range may be confined to 11 + 5dB, allowing accurate measurement without a severe linearity requirement of the log amplifier.

Claims

1) A directional coupler which comprises a main signal conductor, first and second probes capacitively coupled to said main signal conductor at points spaced a quarter wavelength apart along said main signal conductor, the first probe providing a first terminal and a quarter wavelength line and a half wavelength line connected respectively from the first and second probes to a second terminal.

2) A measuring circuit for measuring voltage standing wave ratio in a signal line, comprising a directional coupler as claimed in claim 1 and a logarithmic circuit to which said first and second terminals are connected, said logarithmic circuit being arranged to give an output proportional to the different between forward and reverse powers in said signal line.

3) A measuring circuit as claimed in claim 2, in which said logarithmic circuit comprises a successive detection log amplifier.

4) A directional coupler substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

5) A measuring circuit substantially as herein described with reference to Figure 3 of the accompanying drawings.