GB2223360A

GB2223360A - Waveguide mode filter

Info

Publication number: GB2223360A
Application number: GB8822807A
Authority: GB
Inventors: R Peter John Massey
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1988-09-28
Filing date: 1988-09-28
Publication date: 1990-04-04
Also published as: GB8822807D0

Abstract

A waveguide mode filter arranged to modify the passage along a waveguide of at least two propagating waveguide modes relative to one other, comprising within the waveguide (2) a contiguous sequence (10) of respective dielectric regions (11, 12) with interfaces (13) in the form of transverse sections of the waveguide each between two regions of mutually different dielectric constant. An embodiment for extending the frequency range of an H-plane sectoral-horn direction-finding antenna 1 is described. Receiver 8 is coupled to the rectangular waveguide via a probe 4 symmetrically located to reject even-order modes, and the filter is arranged to transmit the TE(1, 0) mode but to reflect the TE(3, 0) mode. <IMAGE>

Description

DESCRIPTION WAVEGUIDE MODE FILTER This invention relates to a waveguide mode modifying filter comprising a hollow waveguide and obstructions arranged so as to modify in a selective manner the propagation along said waveguide of energy conveyed by predetermined waveguide modes, and to antenna feeders including such a filter.

Broad-band direction-finding antenna arrangements, such as that employing an H-plane sectoral horn as described in United Kingdom Patent Application No. 8,729,914, can be made which are potentially capable of:Yproviding a stable beamwidth in azimuth over a very wide frequency band well in excess of about three to one. To achieve such a performance it is necessary to ensure that only received signal energy propagating in the fundamental circumferential TM(0,1) mode of the horn, is passed via the waveguide feeder to the receiver.

Incoming signals can, however, excite higher horn modes in the sectoral horn and when received signals detected at the receiver include energy in the higher horn modes, the effective beamwidth of the antenna is found to vary with frequency to an undesirable extent. This is thought to occur because wave energy in these higher modes interfere at the horn aperture in a directionally dependent manner in the H-plane both within the higher modes and with one another including the fundamental mode, thus modifying the angular response relative to that provided by the fundamental mode alone, by altering the variation of received amplitude with direction and by causing the resultant overall directional response to vary with frequency.

In the aforementioned patent application, energy in the fundamental To(0,1) mode of the sectoral horn, whose basic transverse section conforms to a notional curved surface circular in the H-plane and concentric with the horn sector, is converted by means of a mode-converting section of waveguide so as to match the fundamental TE(1,0) mode of a rectangular waveguide type feeder. The waveguide feeder is provided at the other end with a probe which is coupled to a coaxial feeder and which is centrally placed in the wider wall of the rectangular type waveguide so that only waveguide energy conveyed by modes of odd order is detected.Because of this arrangement of the waveguide feeder, only energy in the fundamental TE(1,0) mode can be passed to the coaxial feeder throughout an operational frequency range whose upper limit is set by the critical frequency at which the first higher mode of odd order, namely the To(3,0) mode, ceases to be evanescent. This provides an operational frequency range of about three to one.

Above this critical frequency, signal energy in the higher horn mode TM(0,3) which will be readily converted to the TE(3,0) mode by the mode-converting section, will be conveyed via the feeder system to the receiver with the aforementioned adverse effect on the accuracy of the directional response of the antenna.

An attempt can be made selectively to attenuate, e.g. by scattering or dissipating, any energy propagating along the waveguide feeder in the TE(3,0) mode by means of posts or attenuation pads suitably disposed so that they act preferentially on energy distributed in the TE(3,0) mode, but such prior arrangements also tend to generate the higher mode from energy propagating in the fundamental mode and are generally unsatisfactory.

It is an object of the invention to provide an improved waveguide mode filter which can extend the range of frequencies over which an H-plane sectional-horn direction-finding antenna arrangement can be satisfactorily employed with a rectangular waveguide feeder.

In accordance with the invention there is provided a waveguide mode filter comprising a hollow waveguide and obstructions arranged so as to selectively interact with energy propagating in predetermined waveguide modes, characterised in that the obstructions comprise a sequence of respective individual dielectric regions distributed along the waveguide with interfaces therebetween each substantially in the form of a transverse section of the waveguide and separating regions of mutually different dielectric constant, the spacing of the interfaces and the respective values of the dielectric constant in the individual dielectric regions being selected so that the passage along the waveguide of at least two predetermined propagating waveguide modes is selectively modified by the interference of energy propagating in the predetermined modes and reflected from said interfaces.

Preferably each of the respective individual dielectric regions forming the sequence is formed from a corresponding rigid dielectric medium so that the correct interboundary distances can be achieved by a simple process of assembly. If a relative dielectric constant similar to that of air is required for any section, a rigid foam dielectric of suitably low density can be employed.

When a waveguide mode filter in accordance with the invention is employed to extend the range of frequencies over which signal energy is conveyed to a receiver through a rectangular waveguide solely in the fundamental TE(1,0) mode, the receiver is coupled to the rectangular waveguide via a probe symmetrically located so as to reject signal energy conveyed via waveguide modes of even order, and the sequence of respective individual dielectric regions, is so selected and dimensioned that energy propagating in the TE(1,0) mode is transmitted substantially without reflection, and that over a predetermined frequency range extending upwardly from the critical frequency of the TE(3,0) mode, energy propagating in the TE(3,0) mode is substantially reflected by the sequence of respective individual dielectric regions.

In an embodiment, a waveguide mode filter in accordance with the invention is included in a rectangular type feeder waveguide forming part of a sectoral horn antenna arrangement comprising an H-plane sectoral horn antenna whose throat is coupled to the rectangular type feeder waveguide by means of a mode conversion waveguide section arranged to convert signal energy propagating in the fundamental horn mode at the throat of the sectoral horn into signal energy propagating in the fundamental TE(1,0) mode of the rectangular type feeder waveguide.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, of which: Figure la illustrates in perspective and partial section a sectoral H-plane horn antenna arrangement including a waveguide mode filter in accordance with the invention, Figure Ib is a cross section of part of Figure la, and Figure 2 is a diagram illustrating the structure of the waveguide mode filter.

Figure la illustrates in perspective and partially in section an H-plane sectoral horn arrangement as described in United Kingdom Patent Application No. 8,729,914 the contents of which are to be assumed to be incorporated herein, and including a waveguide mode modifying filter in accordance with the present invention. An H-plane sectoral horn antenna 1 having an H-plane flare angle of 180 degrees, feeds a feeder waveguide assembly comprising a main waveguide section 2 of uniform cross section coupled to the throat of the sectoral horn 1 via a mode-converting waveguide section 3. The main waveguide section 2, illustrated in cross section in Figure Ib, comprises a basically rectangular cross-section the width of which is considerably greater than the height and which is formed into a circular arc whose angular extent is equal to the H-plane flare angle of the sectoral horn 1. A centrally located probe 4 feeding a coaxial cable 5, is arranged to be excited by or alternatively to excite the fundamental TE(1,0) mode of the rectangular type waveguide 2. A tapered ridge 6 is provided adjacent the probe 4, in conventional manner, in order to provide a wide bandwidth feed. The fundamental TE(1,0) waveguide mode of the feeder waveguide 2 is transformed by the mode conversion waveguide section 3 into the fundamental TM(0,1) mode of the H-plane sectoral horn 1 at the throat 7 of the sectoral horn.

Because the system is reciprocal, a corresponding inverse mode conversion will be similarly effected in the reverse direction.

The coaxial feed probe 4 is located symmetrically at the centre of the broad face of the feeder waveguide 2 so that it can only receive or generate energy in rectangular waveguide modes of odd order. This means that the feeder system 2,3 can carry signal energy from the sectoral horn antenna 1 to a receiver 8 connected to the coaxial feed 5, uniquely in the fundamental rectangular waveguide mode TE(1,0) over a frequency range of about three to one since the probe 4 will be substantially insensitive to the first mode TE(2,0) of even order whose cut off frequency lies within this frequency range. At frequencies above this range the higher order modes of odd order will cease to be evanescent and will propagate contributing to the transfer of signal energy from antenna to receiver commencing with the TE(3,0) mode.

It should be understood that the energy in a signal of a given frequency received by the sectoral horn 1, will tend to be distributed among the various horn modes which are capable of propagating in the horn at that frequency. When the frequency falls below the cut-off frequency of a mode at any point, e.g. in the taper or at the output aperture, that mode will become evanescent and will neither propagate nor transfer energy, in fact signal energy carried by a mode which becomes evanescent at some point along the horn taper will be reflected. In some cases a mode may propagate in the horn and become evanescent, or ineffective as in the case of modes of even order, in the feeder waveguide assembly 2,3,4, in which case energy in that mode will not reach the receiver 8 connected thereto via the cable 5.

However, when the signal frequency is above the cut off frequency of a higher order rectangular waveguide mode, e.g. the TE(3,0) mode, signal energy present in the corresponding sectoral horn mode will be converted by the conversion section 3 into and excite that higher order waveguide mode, and in the case of a mode of odd order, will pass to and be combined with the fundamental mode signal at the receiver 8.As a consequence of the adverse relationship between beamwidth and frequency which characterise the higher order horn modes together with the variation with frequency of the phase of the higher order mode signal relative to that of the fundamental mode, the resultant H-plane beamwidth of the sectoral horn as measured from the signal at the receiver 8, will no longer be constant with frequency in this range rendering the horn antenna unsuitable for direction finding above the cut off frequency of the TE(3,0) mode.

In order to extend the operational frequency range of the antenna system, there is inserted in the waveguide feeder 2 a mode filter 10 in accordance with the invention. The filter 10 comprises a sequence of respective individual dielectric regions 11,12, in which adjacent regions have mutually different values of dielectric constant. The interfaces 13 between each region and an adjacent region are each substantially in the form of a transverse section of the waveguide.The spacing of the interfaces 13, and the respective values of the dielectric constant in the individual dielectric regions 11,12, between successive interfaces 13, are selected so that the passage along the waveguide of at least two predetermined propagating waveguide modes, in the present example the fundamental rectangular TE(1,0) mode and the higher TE(3,0) rectangular mode, is selectively modified relative to one another by interference of energy propagating in the respective predetermined modes and reflected from the interfaces 13. In the present example the aim is to maximise reflection of signal energy propagating in the higher TE(3,0) mode while at the same time maximising transmission of signals propagating in the fundamental TE(1,0) mode.

The mode filter assembly 10 of individual dielectric regions 11,12, is preferably made up entirely of rigid blocks of suitable dielectrics arranged in contact with one another, thus providing a simple and accurate method of providing the required spacing of the respective interfaces 13. It is quite possible to alternate blocks of dielectric with dielectric regions containing a suitable gaseous medium such as air, however, some form of spacer would be required to locate the block interfaces on either side of the gas filled region and such spacers would form discontinuities within the waveguide and can therefore cause the unwanted generation of higher waveguide modes. If it should be found desirable to employ regions having a dielectric constant similar to air, blocks of rigid low density foam dielectric can be employed, formed for example from expanded polystyrene.

The mode filter 10 will now be further described and illustrated with reference to Figure 2 which is a schematic diagram of a longitudinal axial section of the mode filter. In the present example, the mode filter is assumed to comprise, basically, a rectangular type waveguide whose longitudinal axis lies along the z-axis of a cartesian coordinate system, and whose cross-section is parallel to both the x- and y-axes, and within which a succession of dielectric regions are disposed. The relative dielectric constant of the ith section is indicated by epsilon-suffix-i.

It can be shown (see for example Robert E. Collin "Field Theory of Guided Waves" pub. McGraw Hill (1960) chapter 5 that the electric and magnetic field components, E and H, respectively, of a given waveguide mode are described by equations (1) and (2) where the wavey barred field quantities represent vectors dependent only on the transverse x and y variables. GAMMA is the propagation constant for a mode and is given by equation (3). In this equation k-suffix-o is the wave-number for free space modified by the relative dielectric constant epsilon of the medium as given by equation (4) wherein f is the frequency, c is the speed of light in free space, and k-suffix-c is the critical wave-number of the mode which is determined from the mode and from the transverse dimensions of the waveguide. In the case of a TE(m,n) or a TM(m,n) mode in a rectangular waveguide, k-suffix-c is given by equation (5). It will be apparent from equation (3) that GAMMA will be real when k-suffix-c is greater than k-suffix-o and from equations (1) and (2) that the mode will then not propagate along the guide, i.e.

it will be evanescent. However, when k-suffix-c is less than k-suffix-o the propagation constant GAMMA is imaginary and can be represented by equation (6) in which beta is the imaginary component of the propagation constant of the mode, and the mode propagates along the waveguide.

Considering only one mode propagating in the waveguide structure illustrated by Figure 2, within each individual dielectric-filled section the mode can propagate in either the positive z-direction, i.e. the desired direction taken in Figure 2 to be from left to right, or in the negative z-direction. Let the electric fields associated with this mode in the ith dielectric section be represented by equation (7) in which GAMMA-suffix-i is calculated for the given mode for each section from equations (3) and (4) using epsilon-suffix-i which is the dielectric constant of the medium in the ith dielectric section in the waveguide. The first term on the right hand side of equation (7) represents the contribution of a forward travelling wave, i.e. travelling from left to right and conventionally represented by the negative sign in the exponent, and having an amplitude T-suffix-i.The second term represents the contribution of a backward travelling wave having an amplitude R-suffix-i generated, for example, by subsequent reflection of part of the forward travelling wave.

At the boundary between the ith and the (i+1)th sections the following boundary conditions apply namely the total electric field on either side of the boundary must be the same, as expressed by equation 8. Similarly the first differential with respect to z of either side of equation 8 must also be the same and this as expressed by equation (9). Thus in a cascade of N dielectric sections, each boundary will lead to two equations e.g. (8) and (9), relating four variables, namely T-suffix-i, R-suffix-i, T-suffix-(i+1) and R-suffix-(i+1). There will be N+1 boundaries which will result in 2N+2 equations. To this must be added the waveguide at the input to the filter and the waveguide at the output of the filter as further dielectric regions (the dielectric being a gaseous medium such as air) and this will increase the total to N+2 sections given 2N+4 variables.The equations can therefore be used to eliminate all but two of the variables. The equations for the terminal boundaries will include the electric field of the forward and reflected waves propagating in the input waveguide and having amplitudes T-suffix-o and R-suffix-o respectively, and the corresponding electric field amplitudes in the output waveguide which will be represented by T-suffix-(N+1) and R-suffix-(N+1).

When considering the operation of the mode filter it will be assumed that the output waveguide will contain no reflected wave, e.g. from other parts of the system, and the term R-suffix-(N+1) will therefore be set to zero. This means that there are 2N+2 equations to be solved for 2N+3 variables and that an expression for T-suffix-(N+1), i.e. the forward propagating output wave, can be found in terms of the forward propagating input wave T-suffix-o. This expression is the transmission response of the mode filter for the given mode (which determines k-suffix-c) at a particular frequency f. It should be noted that conservation of energy is represented by the identity given as equation (10).

These basic mathematical relationships (1) to (10) are employed in known manner as a basis for generating a computer program, for example in FORTRAN, which enables the transmission and the reflection response of any given sequence of dielectric regions to be calculated for any selected mode and frequency.

In designing a mode filter 10 in accordance with the invention, the first stage is to select an initial dielectric sequence i.e. of interboundary distances, of relative dielectric constants and total number of dielectric sections, which may be arbitrary, and to apply the computer program to determine the response of the sequence for each of the modes whose passage along the waveguide is to be selectively modified relative to the others, namely in the present example for both the TE(1,0) and the TE(3,0) modes, over the proposed operational frequency range of the mode filter, in the present example a range of frequencies upward from the critical frequency of the TE(3,0) mode.This set of calculations is then repeated for progressively different sets of values for, e.g. increments or decrements in, the interboundary distances and relative dielectric constants in order to identify for that dielectric sequence, the corresponding dependent trends in the values of overall transmission T-suffix-(N+1) and overall reflection R-suffix-o relative to a given input T-suffix-o for the respective modes over the desired frequency range. In general, several different initial dielectric sequences will be examined by this method and the trends observed will be noted and employed, if suitable, in selecting, if necessary, further more favourable initial dielectric sequences.The aim is to identify a dielectric sequence which indicates at least a trend towards the desired mode filter response for the selected modes, in the present example towards a low or zero overall transmission value and a high overall reflection value for the TE(3,0) mode for a range of frequencies above the critical for the To(3,0) mode, and a high overall transmission value for the fundamental To(1,0) mode for frequencies from the critical frequency of the TE(1,0) mode.

When a satisfactory dielectric sequence has been assembled in this manner, the values associated with the sequence are subjected in the computer to an optimising routine in order to maximise the match to the required response. This would-mean in the present example to increase the rejection (i.e. reflection) of the TE(3,0) mode as far as possible and to extend the frequency in the upward direction at which it is effective while at the same time maintaining transmission of the TE(1,0) mode.

It will be understood with reference to Figures 1 and 2 that when the arrangement shown in Figure 1 is employed to feed a receiver 8 in a direction finding system, the input end of the mode filter 10 in the feeder waveguide 2 will be located nearest to the mode converter 3 and the antenna 1. In this way signal energy propagating along the feeder 2 in the unwanted TE(3,0) mode will be largely reflected by the mode filter 10 back to the antenna 1 and can be effectively prevented from reaching the receiver 8 and therefore degrading the directional characteristics of the sectional horn antenna 1 over the range of frequencies for which reflection of this mode can be adequately maintained by the mode filter 10.

It is not intended to limit a mode filter in accordance with the invention to use in the specific embodiment herein described with reference to Figure 1, and it can be employed whenever there is a need to selectively modify the passage of two or more predetermined waveguide modes relative to one another along a waveguide. The waveguide need not be rectangular but can, for example, be of circular cross-section. The mode filter can alternately be arranged and computed to provide selective responses which differ from those described herein and to provide mutually different responses for more than two different waveguide modes.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of waveguide mode filter means, sectoral horn antennas, feeders and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

CLAIM(S)

1. A waveguide mode filter comprising a hollow waveguide and obstructions arranged so as to selectively interact with energy propagating in predetermined waveguide modes, characterised in that the obstructions comprise a sequence of respective individual dielectric regions distributed along the waveguide with interfaces therebetween each substantially in the form of a transverse section of the waveguide and separating regions of mutually different dielectric constant, the spacing of the interfaces and the respective values of the dielectric constant in the individual dielectric regions being selected so that the passage along the waveguide of at least two predetermined propagating waveguide modes is selectively modified by the interference of energy propagating in the predetermined modes and reflected from said interfaces.

2. A waveguide filter as claimed in Claim 1, characterised in that each of the respective individual dielectric regions forming said sequence, is formed from a corresponding rigid dielectric medium.

3. A waveguide filter as claimed in Claim 1 or Claim 2, characterised in that said hollow waveguide is effectively a rectangular waveguide for feeding signal energy to a receiver via a probe symmetrically located so as to reject signal energy conveyed via waveguide modes of even order, and the sequence of respective individual dielectric regions is such that energy propagating in the TE(1,0) mode is transmitted substantially without reflection and that over a predetermined frequency range extending upwardly from the critical frequency of the TE(3,0) mode, energy propagating in the TE(3,0) mode is substantially reflected by said sequence.

4. A waveguide mode modifying filter substantially as herein described with reference to the accompanying drawings.

5. A sectoral horn antenna arrangement comprising an H-plane sectoral horn antenna whose throat is coupled to a rectangular type feeder waveguide by means of a mode conversion waveguide section arranged to convert signal energy propagating in the fundamental horn mode at the throat of the horn into signal energy propagating in the fundamental TE(1,0) mode of the rectangular type feeder waveguide, and including in said rectangular type feeder waveguide a waveguide mode filter as claimed in any one of Claims 1 to 4.