US3324475A - Scanning antenna array wherein feed utilizes dispersive elements to provide nonlinear scan-frequency relationship - Google Patents

Description

K. MILNE 3,324,475 SCANNING ANTENNA ARRAY WHEREIN FEED UTILIZES DISPERSIVE ELEMENTS June 6, 1967 TO PROVIDE NONLINEAR SCAN-FREQUENCY RELATIONSHIP Filed Feb. 13, 1964 3 Sheets-Sheet 1 TRMIIHIUI HIP June 6, 1967 K. NE

SCANNING ANTENNA A Y WHEREIN 1 D UTILIZES DISP TO PROVID ONLINEAR CAI -FREQUENCY REIIA Filed Feb. 15, 1964 K. MILNE 3,3

EHSIVE ELEMENT June 6, 1967 SCANNING ANTENNA ARRAY WHEREIIN FEED UTILIZES DISP 5 TO PROVIDE NONLINEAR SCAN-FREQUENCY RELATIONSHIP Filed Feb. 13, 1964 3 Sheets-Sheet 5 REF/VH7 rm'mzri mum United States Patent 3324 47s SCANNING ANTENNA ARRAY WHEREIN FEED urruzas DISPERSIVE ELEMENTS T0 PRO- This invention relates to directional aerial systems and more particularly to aerial systems giving a directional beam, the direction of which depends on the frequency of the signals fed to or received by the aerial system.

According to this invention, a directional aerial system comprises an array of radiating (or receiving elements) fed by (or feeding) a dispersive transmission line or a line including dispersive elements. A dispersive transmission line or dispersive element is a transmission line or element having group delay which is dependent on the frequency of the applied signal.

It has previously been proposed to use a serpentine waveguide with radiating slots as a directional aerial. Such a serpentine waveguide, by providing, for example, radiating slots or directional couplers feeding radiating apertures at corresponding points in a repetitive series of S-shaped bends in the line, gives a length of transmission path between successive radiators very much greater than the direct physical distance between the radiators; thus the relative phase of the radiation from the various radiators varies greatly with frequency and so the direction of the resultant beam depends on the frequency. To a first order, however, the beam angle is a linear function of frequency. In the arrangement of the present invention, however, the transmission line itself is dispersive or contains dispersive elements and thus, by suitable choice of line or of dispersive elements in the line, it is possible to obtain a non-linear relationship between frequency and beam angle.

This form of aerial system finds particular application in microwave radar apparatus where it is often required to effect repetitive scanning of a beam over a limited angular extent. The aerial system in this case would commonly be used both for radiation and reception by employing a suitable duplexer. As an example, an aerial system as described above and consisting of an array of radiating (or receiving) elements arranged in a vertical line might be used to produce a beam which is sharply directional in the vertical plane and which can be scanned in a vertical plane by changing the frequency. Such a vertical array may be combined with a reflector, e.g. a parabolic cylinder, to make the beam sharply directional in the horizontal plane and the whole aerial system may then be rotated to give a beam which is scanned rapidly up and down in elevation and is more slowly scanned in azimuth. In such radar apparatus, a short duration pulse may be employed, successive pulses being on different frequencies to efi'ect the vertical scanning, or the pulses may be of sufficiently short duration to contain all the frequency components corresponding to the desired angular coverage. Alternatively, a long duration frequency modulated pulse may be employed, for example, as in the radar system described in the specification of co-pending application Ser. No. 344,781, now Patent No. 3,266,038.

Instead of using a parabolic reflector, a number of sets of radiating elements may be used, the elements in each set being fed by a dispersive transmission line or a line including dispersive elements and the various sets being arranged side by stide. The various sets of radiating elements may be fed from a common transmission line via. controllable phase shifters; for example electri- 3,324,475 Patented June 6, 1967 cally controlled phase shifters may be employed. If the lines are vertical change of frequency produces an elevation scan whilst change of phase produces an azimuth scan.

In a simple form, the aerial system of the present invention may comprise a sraight or serpentine form waveguide with dispersive elements between successive radiating slots spaced along the Waveguide. It is often convenient to describe the form of a complex microwave impedance in a waveguide by reference to the analogous lumped circuit elements in a two-wire transmission line and each dispersive element in the waveguide may be considered as the microwave analogue of a lattice filter for a two-wire line having an inductance and capacitance in shunt in each of the two lines and having an inductance and capacitance in series between the input end of each wire and the output end of the other wire. Such a lattice filter network gives a uniform amplitude of response over a wide range of frequencies but the phase varies nonlinearly with frequency and hence the time delay varies with frequency. The network remains substantiauy matched however as the frequency varies.

The dispersive elements may be adjustable so that the relationship between frequency and beam angle may be varied; for example the elements may be electrically eon. trollable.

In the following description reference will be made to the accompanying drawings in which:

FIGURE 1 is a diagram illustrating a dispersive aerial system;

FIGURE 2 is an equivalent lumped circuit diagram for explaining the operation of a dispersive element;

FIGURE 3 is a perspective view, partly cut away, showing part of an aerial system;

FIGURE 4 is a perspective view illustrating part of another form of aerial system;

FIGURE 5 is a diagram illustrating a dispersive linear array arranged for scanning in a vertical plane in accord ance with the frequency of an input signal and mechanically scanned in a horizontal plane; and

FIGURE 6 is a diagram used for explanatory purposes.

FIGURE 1 illustrates diagrammatically a number of radiating elements 10 which typically are arranged in a Vertical plane one above another and which are fed from a transmitter 11 via an input waveguide 12 of generally serpentine form. Commonly this type of aerial would be used for both transmitting and receiving, a suitable duplexer 13 being provided so that received signals from the waveguide 12 are fed from the duplexer 13 to a receiver 14. For convenience in terminology, however, in the following description reference will be made more particularly to the transmitting conditions in which signals are fed from the transmitter 11 through the waveguide 12 to the radiating elements 10. The various radiating elements are coupled to the waveguide 12 by directional couplers 15 and short lengths of waveguide 16. The directional couplers 15 have coupling values chosen to suit the desired aperture illumination, enabling the proportion of the transmitted energy fed to each radiating element to be determined in accordance with the required conditions. As shown in FIGURE 1, each radiating element 10 is at one end of the short waveguide 16 and the other end of this waveguide is terminated by a matched load 17, this waveguide 16 being coupled by the directional coupler 15 to the main waveguide 12. The end of the main waveguide 12 remote from the transmitter 11 is terminated in a matched load 18. The main waveguide 12 is of serpentine form and has a dispersive element 19 between each of the couplers 15. The mechanical construction of these dispersive elements will be described later with reference to FIGURE 3 and for the present it may be stated that such a dispersive element in its simplest form may comprise a 3 db directional coupler with two of its de-coupled arms constituting signal input and output arms and with the other two arms terminated in identical resonant cavities (each shown as a filter 21 and short circuit 22).

FIGURE 2 shows the equivalent lumped circuit elements in a two-wire transmission line; each dispersive element 19 may be considered as the microwave analogue of a lattice filter for a two-wire line having an inductance and a capacitance 31 in shunt in each of the two lines 32 and 33 and having an inductance 34 and capacitance 35 in series between the input end of each line and the output end of the other line. This filter contains resonant and anti-resonant circuits. Assuming that the magnitudes of each of inductances 30 is L /2, each of the capaeitances 31 is 2C each of the inductances 34 is 2L and each of the capacitances 35 is Cg/Z, then in the simplest case the tuned circuits are made to have the same resonant frequency f Thus I 1 f0 l l Z W 2 2 The sharpness of resonance factor m is given by if C2 The phase shift B at a frequency f is given by Such a lattice filter network gives a uniform amplitude of response over a wide range of frequencies but the phase varies non-linearly with frequency and hence the timedelay varies with frequency. The form of the relationship between the differential delay and frequency can be varied by choice of the two parameters available in this filter. The network remains substantially matched however as the frequency varies. In a microwave analogue using a 3 db directional coupler with two arms terminated in resonant cavities, the phase characteristic is determined by the cavity and it will be appreciated that, by the use of complex forms of cavities, it is possible to control the dispersive characteristics. Alternatively or additionally the transmission line may be reactively loaded to provide a dispersive characteristic.

Referring now to FIGURE 3 the various radiating elements are illustrated as radiating apertures 40. In FIGURE 3 five such apertures are shown but it will be appreciated that typically a very much greater number might be employed, for example, 100. Each of the apertures 40 is coupled by a transition section 41 to one end of short rectangular waveguide 42 the other end of which is terminated in a matched load 43. The waveguides 42 are coupled by coupling slots 44 to spaced points along the length of a main feed guide 45 of serpentine form. This waveguide 45 corresponds to the waveguide 12 of FIGURE 1 and the coupling slots 42 for the various radiating apertures 40 coupled into portions of the waveguide 45 separated by the necessary dispersive elements. The input end of the waveguide 45 is illustrated at 46. The other end 47 is terminated in a matched load. The waveguide 45 is arranged like the waveguide 12 in FIGURE 1 with 3 db couplers 48 and filter elements, consisting of posts 49, with short circuits in the portions of the waveguides beyond the posts that is to say at the left-hand of FIGURE 3.

The arrangement of FIGURE 3 employs a rectangular waveguide and is arranged for radiating (or receiving) linearly polarised signals which are transmitted through the waveguide in a TE mode. It is often a requirement however in radar systems to be able to radiate signals polarised in two orthogonal planes; depending on the relative phase of the two signals, the resultant radiation may be circularly or elliptically or linearly polarised. FIGURE 4 illustrates a construction of a dispersive aerial for radiating signals in two orthogonal planes of polarisation simultaneously. Referring to FIGURE 4 there are shown two serpentine feed guide systems 50, 51 each generally similar to the serpentine feed guide 45 of FIGURE 3 and incorporating dispersive elements such as are shown in FIGURE 3. These two serpentine feed guides 50, 51 are arranged side by side and each radiating aperture is fed from both these feed guides. FIGURE 4 illustrates one such radiating aperture 52 in the form of a fin-loaded square-section horn fed via a polarisation coupler 53 from two square-section waveguides 54 and 55. The square waveguide 54 takes its input from the serpentine feed guide 50 by means of a combined coupler and differential phase equaliser 56 and having an output waveguide of rectangular section (corresponding to the waveguide 42 of FIGURE 3) which leads via a twist 57 to a rectangular-to-square waveguide transition 58 and an E-plane single mitre bend 59 to the aforementioned square-section waveguide 54. The square-section waveguide 53 is fed from the serpentine feed guide 51 by a coupler 60 feeding the signals into a rectangular waveguide which leads via a rectangular-to-square transition 61 into the squaresection waveguide 55. The relative amplitude and phase of the signals into the two feeds 50 and 51 is adjusted to provide the desired polarisation at the radiating apertures; this may be done using known techniques for adjustable polarisation radar systems.

These aerial systems of FIGURES 3 and 4 consisting of a stack of radiating elements with the dispersive feeds can give a directional beam which is sharply beamed in one plane but the direction of which, as will be further explained below, is dependent on the frequency of the signal. Assuming that the radiating elements are stacked in a vertical line, the beam will be sharply directional in a vertical plane. Commonly it is required that a radiated beam should be directional in two planes and FIGURE 5 illustrates diagrammatically a typical way in which the stacked radiating elements of FIGURES 3 or 4 might be used in a radar system. Referring to FIGURE 5 the radar system is illustrated diagrammatically as comprising a transmitter 70 and a receiver 71 coupled via a duplexer 72 to a serpentine feed 73 which may be of the form shown in FIGURES 3 or 4 and which has a series of radiating elements 74. The radiation from these elements is directed into a cylindrical parabolic type reflector 75 for forming the horizontal beam and the complete aerial system is rotated about a vertical axis as indicated diagrammatically at 76. It will be seen that the beam is directional in the horizontal plane in a manner determined by the reilector 75 and is rotated mechanically for scanning in this plane. In the vertical plane the beam shape is determined by the feed system 73, 74, but the elvation angle will depend on the frequency. Such an aerial might be used with radar apparatus in which successive pulses are radiated on different frequencies to effect the vertical scanning or the pulses may be of sufliciently short duration to contain all the frequency components corresponding to the desired angular coverage. Alternatively long duration frequency-modulated pulses may be employed as described, for example in the specification of co-pending application Ser. No. 344,781, now Patent No. 3,266,038.

FIGURE 6 is an explanatory diagram used in the following analysis of the relationship between beam angle and group delay and of the radiation pattern of a dispersive aerial system such as has been previously described. Re-

ferring to FIGURE 6 there is shown diagrammatically a linear array comprising a serpentine feed 80 with radiating elements 81. The direction of the main beam is indicated diagrammatically by the dash lines 82 which are at an angle to the normal line of the array. The beam angle 6 is determined from the equation d0 t n a a]? sec w r a where -1 dtbtf) (f)-y T is the group delay of the feeder at frequency J. For operation close to broadside (0:0), Equation 2 reduces to:

d0 X Zimand since A/d is the approximate value of beamwidth we obtain the important relation: Beamwidths per c./ s. frequency change' Group Delay (5) For normal waveguide feeds, the group delay is m if E x (a where s=total feeder length c=fk=velocity of electro magnetic waves in free space A :guide wavelength.

Waveguide group delay thus reduces as the frequency increaes, but the dispersion is very small except near the cut-off frequency.

The radiation pattern received at a point at a considerable distance R from the centre of the array shown in FIGURE 6 is proportional to where a is the excitation of the mth radiator, and the phase is measured with respect to the input feeding point.

For symmetrical amplitude distributions (i.e. a =a it is convenient to write (7) in the form:

where n e f Ea... Pj{ Sm m=o n A 2 is a real function which defines the shape of the radiation pattern.

If i and A are the frequency and corresponding wavelength at which the beam is normal to the array (i.e. fif is an even multiple of Zmr), the pattern at becomes where g(sin 0, f0) =22 m EXP The pattern at any other frequency f can then be written in terms of the pattern at I as:

E(sin 0,

where The beamwidth at frequency f is thus f /f times the beamwidth at I and the beam maximum occurs at slnaf d which is the same result as that given in Equation 2. Differentiation of the phase term in Equation 12 shows that the total group delay at the target is simply R 1 FW 15) Equation 12 gives the variation of field strength with angle at a fixed frequency, or the spectrum obtained at a fixed angle when the array is fed with a uniform spectrum. If the array is uniformly illuminated (spatially) for example, Equation 10 reduces to and the pattern at frequency f is where (10) is given by Equation 13. The angular width of a major lobe between zeroes at a fixed frequency is 20 where 9 lm i i E n+1dfn+1d 18) whilst the width of the major lobe in the spectrum at a fixed angle is 2B c.p.s. where B is given by the equation a .lh-B f n+1 (19) in which f is the frequency giving maximum response at the angle of interest. If D(j) does not change too rapidly over the interval 2B, Equation 19 yields &7! n 1 D( f (20) The half-power beamwidth is thus approximately Vd and the half-power bandwidth is approximately equal to the reciprocal of the aerial group delay.

I claim:

1. A directional aerial system comprising an array of radiating elements fed by a transmission line including dispersive elements so that because of the dispersion the sweep of the radiation beam of said array is non-linearly related to the frequency.

2. A directional aerial system comprising a waveguide with a series of radiating elements coupled to the guide at spaced points along its length and having dispersive elements spaced along the waveguide between the successive couplings to the radiating elements so that because of the dispersion the sweep of the radiation beam of said system is non-linearly related to the frequency.

3. A directional aerial system as claimed in claim 2 wherein the waveguide is of serpentine form.

4. A directional aerial system as claimed in claim 2 wherein each dispersive element comprises a 3 db directional coupler with a pair of de-coupled arms connected to resonant cavities and the other two arms constituting signal input and output arms so that the signal transmission path through said waveguide passes into one of said other arms and out through the other.

5. A directional aerial system comprising a main waveguide, a series of auxiliary guides coupled by directional couplers to said main guide at spaced points along its length, dispersive elements spaced along the waveguide between the successive couplings to the auxiliary guides so that because of the dispersion the sweep of the radiation beam of said system is non-linearly related to the frequency, said auxiliary guides each having a radiating aperture.

6. A directional aerial system as claimed in claim 5 wherein the waveguide is of serpentine form.

7. A directional aerial system comprising a number of radiating elements coupled to spaced points on a signal transmission path for feeding all said elements, which signal transmission path between each of said spaced points includes a waveguide from one of the points leading to a first arm of a 3 db directional coupler and a waveguide leading from a second arm of the coupler tie-coupled from the first arm, the other two arms of the coupler each containing a filter and being terminated in a short circuit, some of said elements including a dispersion element so that the sweep of the radiation beam of said system is non-linearly related to the frequency.

8. A directional aerial system comprising a number of radiating elements, coupling means for feeding to each radiating element from separate input signals with different planes of polarisation, a pair of transmission lines each coupled at spaced points along its length to one of the inputs of the successive radiating elements, dispersive elements on said lines situated between each adjacent pair of coupling points so that because of the dispersion the sweep of the radiation beam of said system is non-linearly related to the frequency and means for feeding signals of the same frequency in a predetermined amplitude and phase relationship to the two transmission lines.

9. A directional aerial system comprising a signal transmission path, a number of radiating elements arranged in line and coupled to spaced points on said signal transmission path for feeding said elements, which signal transmission path between each of said spaced points includes signal delay means having a non-linear relationship between delay time and frequency so that because of the said non-linear relationship the sweep of the radiation beam of the said radiating elements is nonlinearly related to frequency.

10. A directional aerial system as claimed in claim 9 wherein said signal delay means comprise a 3 db directional coupler with two de-coupled arms of the coupler each containing a filter and being terminated in a short circuit.

11. A directional aerial system as claimed in claim 9 wherein said radiating elements are arranged in an upright line to give beaming in a vertical plane and feed a reflector shaped to give beam in a horizontal plane.

12. A directional aerial system as claimed in claim 11 and mounted for rotation to scan the beam about a vertical axis.

13. A directional aerial comprising a number of sets of radiating elements, the elements in each set being fed by a dispersive transmission line and the various sets being arranged side by side, and because of the dispersion the sweep of the radiation beam of said aerial is nonlinearly related to the frequency.

14. A directional aerial as claimed in claim 13 wherein the various sets of radiating elements are fed from a common transmission line via controllable phase shifters.

15. A directional aerial system comprising a plurality of radiating elements evenly spaced apart and fed from evenly spaced points on a serpentine feed system to give a directional beam, which feed system, between suocessive spaced points, has dispersive elements whereby there is a non-linear relationship between the direction of the radiated beam and the frequency of signals applied to said feed system.

16. A directional aerial comprising a number of sets of radiating elements, the elements in each set being fed by a transmission line including dispersive elements and the various sets being arranged side by side and because of the dispersion the sweep of the radiation beam of the said aerial is non-linearly related to frequency.

17. A directional aerial as claimed in claim 16 wherein the various sets of radiating elements are fed from a common transmission line via controllable phase shifters.

References Cited UNITED STATES PATENTS 2,530,580 11/1950 Lindenblad 343777 X 2,605,413 7/1952 Alvarez 343-854 X 2,878,472 3/1959 Stems 343-853 3,020,549 2/1962- Kales et al. 343-77l 3,041,605 6/1962 Goodwin et al. 343 3,105,968 10/1963 Bodmer 343-77l 3,142,028 7/1964- Wanselow 33383 X HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Examiner.

R. F. HUNT, Assistant Examiner.

Claims (1)

1. A DIRECTIONAL AERIAL SYSTEM COMPRISING AN ARRAY OF RADIATING ELEMENTS FED BY A TRANSMISSION LINE INCLUDING DISPERSIVE ELEMENTS SO THAT BECAUSE OF THE DISPERSION THE SWEEP OF THE RADIATION BEAM OF SAID ARRAY IS NON-LINEARLY RELATED TO THE FREQUENCY.

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