GB2105917A - Process and device for phase-shifting waves in a waveguide - Google Patents

Abstract

The invention relates to an electronic process and device for phase- shifting microwave waves propagating in a guide, wherein a row 4 of aligned wires is disposed near a wall 2 of the guide (1), the wires (5, 6...10, 11) being at a small distance (e.g. lambda ) 10 from one another and parallel to the electric field vector @ of the guided waves and bearing controllable switches such as diodes (21, 22, 23, 24), electric supply means being provided to open or close the switches in a number of adjacent wires chosen to obtain a desired phase shift. The phase shifter applies to all kinds of uses, particularly for electronic- sweep radar antennas associated with high-power radars. <IMAGE>

Description

SPECIFICATION Process and device for phase-shifting waves in a waveguide The invention mainly relates to an electronic process and device for phase-shifting microwave waves propagating in a guide.

At present there are two main kinds of phase-shifters used in electronic scanning antennas and adapted to obtain various phaseshifts in a wave guide having a rectangular cross-section.

The first kind is a ferrite wave guide producing a phase shift controlled by the action of magnetic fields. These phase shifters withstand levels of power compatible with their use in electronic scanning antennas associated with conventional radar systems. On the other hand they have two major disadvantages: 1. The switching time required for changing the state of the phase shifter is considerable (of the order of several hundred microseconds), which can be a great hindrance in certain applications and 2. The accuracy of the resulting phase values is poor. The standard deviation in the phase errors from the various phase shifters forming part of electronic scanning antenna may reach 10 . This degrades the antenna performance and more particularly gives rise to considerable secondary radiation.

The second kind of phase-shifters use PIN diodes. These diodes operate in transmission or in reflection. Conventional technologies use wave-guide transmission lines or micro-strips (= lines in the form of a strip). In all cases, the diodes must have very high avalanche voltages to enable the phase-shifter to withstand a high perak voltage as required when used on electronic scanning radar antennas.

in the known phase-shifters, a small number of very high-quality diodes are disposed at specific places, found by calculation or experiment, on the wave guides. A structure of this kind has two major disadvantages: 1. The diodes used must be very high-quality (particularly with regard to the avalanche voltage) and are therefore very expensive.

2. The accuracy of the phase-shifter is poor, since the resulting effective phase shifts depend on the chracteristics of the diodes. The standard deviation in manufacturing tolerance is directly related to the number of diodes used, and it is therefore common to find variations of 5 to 10% between two phase shifters of identical construction, i.e. using the same components.

An object of the invention is to avoid the aforementioned disadvantages of the prior art by providing a phase shifter which is very accurate, very economic in construction and can withstand a very high peak voltage.

According to one aspect of the invention there is provided a phase-shifting device com prising diodes in a wave guide having a closed, inter alia rectangular cross-section for linearly polarized waves; and comprising a row of aligned wires disposed near at least one of its walls, the wires being at a slight distance from one another and parallel to the electric field vector of the guided waves, the wires each bearing controllable switches such as diodes and electric supply means being provided to open or close the switches in accordance with the desired phase shift.

To ensure a good construction the wires are spaced from one another by a fraction of a wavelength, e.g. of the order of a tenth of the guided wavelength. In addition, a number of switches, such as conventional commercial PIN type diodes, are mounted on each wire.

According to another aspect of the inven tion there is provided an electronic method of phase-shifting, through a variable angle as required, linearly polarized waves in a wave guide having substantially closed, inter alia rectangular cross-section, wherein the appar ent width of the wave guide is electronically varied along variable lengths of the guide by disposing aligned wires inside the wave guide near at least one of its walls, the wires being at a slight distance from one another and parallel to the electric field vector of the guided waves and bearing controllable swit ches such as diodes, and the switches are opened or closed on at least some of the wires, thus bringing about apparent local vari ations in the width of the wave guide and inducing corresponding phase shifts.This method, as will be shown more clearly in the following description, give the various afore mentioned advantages and the phase shifter also has very numerous applications.

The invention will now be described by way of example only with particular reference to the accompanying drawings wherein: Figure 1 is a diagrammatic perspective view of a wave guide forming a phase shifter of the invention; Figure 2 is a larger-scale diagrammatic view showing how the PIN diodes used can be grouped near a wall of the guide; Figure 3 is a perspective view, partly cut away, of a cross-section of the wave guides; Figures 4 and 5 are electric diagrams show ing the distribution of the electric field waves guided in the guide, depending on the con trolled state of the diodes;; Figure 6 diagrammatically shows how the wires can be grouped in a rectangular wave guide in order substantially to obtain all the desired phase shifts between 0 and 360 by small increments, and Figure 7 is a perspective diagrammatic view showing the equivalent shape of the wave guide when some diodes are forward-biased and others are reverse-biased.

Fig. 1, firstly, shows a wave guide 1 mainly comprising a metal pipe of rectangular cross section having a long side a and a short side or height h. A wave train linearly polarized propagates in the wave guide and the direction of its magnetic field t as shown in the drawing is parallel to the short side or height h of the guide. L denotes the length of the guide.

In the illustrated embodiment, most of one of the small side surfaces of height h of the guide is made up of an inset metal plate 2 which, when the guide is in the operating position, exactly blocks the corresponding aperture 3 formed in the surface.

In the invention, a row 4 of aligned wires are disposed near wall 2 and inside the wave guide when wall 2 is fitted in place (see inter alia Fig. 3). The wires are at a small distance from one another and extend parallel to the electric field vector E and each wire bears diodes, as shown more clearly in Fig. 2, to which reference will be made for details. In practice, the parallel aligned wires 5, 6, 7, 10, 11, 12, 13, . . are disposed on a support plate 20 advantageously made by the printed circuit technique. Wires 5, 6, 7, 10, 11, 12, 13 .... are also grouped in families containing different numbers of wires and simultaneously energized and brought into a conductive or non-conductive state.

In Fig. 2, each wire bears four diodes such as 21, 22, 23, 24 which are grouped in pairs in opposition and supplied with one polarity by a wire 25 in the substantially central plane of plate 2 and with the other polarity by wires 26, 27 near the edges of surface 2. Advantageously wires 26, 27 are connected to earth, i.e. to the metal wall 2 of the guide, whereas wire 25 supplies the current for changing the diodes from the conductive to the non-conductive state.

In the example illustrated in Fig. 2, wire 25 can supply a family of five parallel adjacent diode-bearing wires, whereas a wire 28 similarly supplies a family of six diode-bearing wires regularly spaced by a distance e. In practice, the groups will be different and as illustrated e.g. in Fig. 6, which will be described hereinafter. The object of Fig. 2 is to show on a legible scale how sets of families of aligned wires can be mounted in practice as required for the operation of the phase-shifter according to the invention.

In Fig. 4, it has been assumed that some of the diode-bearing wires in row 4 have been forward-biased by an electric current travelling from the positive pole connected to earth (and therefore to the supply wires 26. 27 in Fig. 2) to collector 25 connected by wire 29 to the control logic unit 30. In practice, wire 29 can extend through the short side 2 (see Fig. 3) of the wave guide via a capacitive bushing in the middle of the aforementioned side. When the diodes are biased in the aforementioned manner, the situation is the same as if wall 2 were moved towards the interior of the guide by the distance d between the plane 4 and the wall, the electric field t being distributed in the guide as shown by the curve.In such cases the length of the wave guided by the wave guide is given by the formula:

If on the contrary the polarities are reversed, as shown in Fig. 5, the diodes are blocked. The latter situation is the same as if the wires in plane 4 in front of wall 2 did not exist; the field is distributed as indicated in the drawing. In that case, the length of the guided wave is given by the formula:

As the preceding explanation clearly shows, the width of the wave guide varies depending on whether the rows of wires are interrupted (blocked diodes) or continuous (conductive diodes), i.e. there is a variation in the propogation speed of the waves, i.e. a certain phase shift. When the diodes are conductive, the wires are sufficiently close together to act as another continuous wall.In practice, very good results are obtained if the spacing pitch e between two adjacent wires is made a fraction of the guided wavelength Ag. i.e. a tenth of the wave length.

Referring now to Fig. 6, there is described an embodiment of the invention and the method of operating it.

The wave guide 1 has a length L of about 1 metre, a width a of 72 millimetres and a height h of 34 millimetres.

70 lines one behind the other are mounted at a distance d = 10 millimetres from the wave guide wall 2 by the printed circuit technique described previously with reference to Fig. 2, and form a substantially continuous plane 4. The space e between pairs of adjacent wires is about 14 millimetres. The lines are grouped in families marked 32 to 37 and respectively comprising 1, 2, 4, 9, 18 and 36 wires disposed one behind the other and supplied in each family in parallel and simultaneously by separate power supplies marked 38-42, by the method explained hereinbefore with reference to Figs. 3, 4 and 5.

Each wire bears four diodes mounted in groups of two in opposition, as explained with reference to the preceding Figures, the diodes being of the conventional commercial HEWL ETT-PACKARD 5082-3379 type, having an avalanche voltage of about 300 V and a reverse capacitance of the order of 0.2 pF.

In the case of an operating band between 2 300 and 3 500 MHz, the resulting phase shift is directly proportional to the number of lines of forward-biased diodes, each line producing a shift of 5 . Under these conditions, the six successive families produce respective phase shifts of 5, 10, 20, 45, 90 and 180'.

Consequently, by supplying one or more families with forward biasing, practically all phase shifts between 0 and 360 can be obtained by 5" increments. If required, another line can be added at the beginning for producing an additional phase shift of 5% if it is desired to cover all effective phase shifts between 0 and 360 by increments of 5 . Measurements show that at 3 000 MHz, the guided wavelength in the wave guide is about 1 40 mm when the diodes are blocked, compared with 1 64 mm when diodes are forward biased.

In a frequency band from 2 500 to 3 200 MHz, i.e. 25%, the operation of the phase shifter is satisfactory. Irrespective of the phase states, losses remain below 1.5 dB and the proportion of standing waves ( = TOS) is less than 1.5.

The measured resistance to peak power was 50 KW with diodes having an avalanche voltage of only 300 V.

The main properties of this kind of phaseshifters, therefore, are the considerable width of the frequency band (25%) which they can use and the very good resistance to power.

They are also simple to construct and inexpensive to manufacture.

Since a number of diodes (four mounted in series in the illustrated example) are used on each wire, and as a result of the capacitance of the diodes used, there is practically no perturbation in the propagation mode of the microwave wave in the guide when the diodes are reverse-biased (since the apparent capacitance is divided by four by connecting the four diodes in series). The phase shift produced by the reverse-biased diodes, relative to an empty wave guide, is practically indetectable. In addition the four diodes are mounted in series on each wire and consequently the peak voltage capable of destroying the diodes is increased.

Fig. 7 diagrammatically illustrates the operation of wave guide 1 when used when some of its rear diode wires (AR) are reverse-biased whereas the wires in front (AV) are forwardbiased. This is equivalent to electronically moving the side wall 2 of the wave guide at the front, forwad-biased part, the wall being moved to the plane 4 of the thus-polarized wires. The invention thus provides a wave guide having a side wall which can be electronically moved as required along all or part of its length.

Of course, numerous variants can be made to the embodiments illustrated and described.

For example, the principle of the invention can be applied to wave guides having nonrectangular sections, so as to "contract" or "adapt" the dimensions of the guides.

The diode wires could be disposed not against one wall but e.g. against both facing walls of the rectangular guide in order to obtain similar effects with a shorter guide.

In addition, the guide can be filled completely or partly with a dielectric material for reducing its length.

Claims (8)

1. An electronic method of phase-shifting, through a variable angle as required, linearly polarized waves in a wave guide having a substantially closed, inter alia rectangular cross-section, wherein the width of the wave guide is electronically varied along variable lengths of the guide by disposing aligned wires inside the wave guide near at least one of its walls, the wires being at a slight distance from one another and parallel to the electric field vector of the guided waves and bearing controllable switches such as diodes, and the switches being opened or closed on at least some of the wires, thus bringing about apparent local variations in the width of the wave guide and inducing corresponding phase shifts.

2. A phase-shifting device comprising diodes in a wave guide having a closed, inter alia rectangular cross-section for linearly polarized waves, and comprising a row of aligned wires disposed near at least one of its walls, the wires being at a slight distance from one another and parallel to the electric field vector of the guided waves, the wires each bearing controllable switches such as diodes and electric supply means being provided to open or close the switches in accordance with the desired phase shift.

3. A device as claimed in claim 2, wherein the wires are spaced from one another by a fraction of a wavelength, e.g. of the order of a tenth of the guided wave length.

4. A device as claimed in claim 2 or 3, wherein a number of switches in series are disposed on each wire.

5. A device as claimed in claim 4, wherein groups containing equal numbers of diodes are mounted in series on each wire and supplied in opposition, the supply being made at one polarity substentially in the central plane of the row of wires and at the other polarity towards the edges of the adjacent surface of the guide.

6. A device as claimed in any of claims 2 to 5, wherein the wires are grouped in families containing different numbers of wires, supplied and simultaneously made conductive or non-conductive.

7. A device as claimed in any of claims 2 to 6, wherein the switches are conventional commercial PIN type diodes having an average avalanche voltage e.g. of the order of 300 V.

8. A device for phase shifting waves in a waveguide and substantially as hereinbefore described and as shown in the accompanying drawings.