US3423756A - Scanning antenna feed - Google Patents

Description

P. FOLDES SCANNING ANTENNA FEED Jan. 2l, 1969 Sheet \3 Filed sept. 1o, 1964 INVENTOR Pfff/f Fowfs BY v v Jan. l21, 1969 l Filed Sept. l0, 1964 P. FOLDES SCANNING ANTENNA FEED sheet 4 of 4 INVENTQR.

Pfff: Hawes United States Patent O 3,423,756 SCANNING ANTENNA FEED Peter Feldes, Montreal, Quebec, (Ianada, assignor to Radio Corporation of America, a corporation of Delaware Filed Sept. 10, 1964, Ser. No. 395,401

U.S. Cl. 343-775 11 Claims Int. Ci. Htllq 19/14 ABSTRACT F THE DISCLGSURE An electronically controlled conical scanning antenna feed is provided by an oversized waveguide having four tuned cavities mounted about the waveguide and coupled to it. The signal of the frequency to which these cavities are tuned is split into higher order modes thus resulting in the movement of the radiation phase center from the center of the antenna aperture. By tuning the four cavities in sequence to the frequency of this signal, it is conically scanned. Signals at other frequencies if sufficiently separated from the frequency to which the cavities are tuned continue to propagate through the waveguide without any disturbance within the waveguide.

My invention relates to antennas and particularly to an improved feed system for deflecting the antenna pattern. In a preferred embodiment of the invention the pattern is deflected so as to conically scan without the utilization of mechanical movement of any large component, or even without any mechanical movement.

The invention will be described with particular reference to an antenna designed for use as a ground station antenna for a communications satellite system. Such an antenna should transmit and receive communication signals lying within a wide frequency band, and it should also transmit a signal having a radiation pattern or patterns having a characteristic such that when directed to a target such as a satellite an error signal is provided which may be utilized to make the antenna automatically track the target. Thus, the communication signal pattern is always directed toward the satellite.

The desired antenna tracking may be obtained, for eX- ample, by employing a monopulse antenna or by employing a conical scanning antenna. The latter has certain advantages; for example, it requires only one receiver.

The present invention, as applied to a communications satellite system, is an improved conical scanning antenna. Stated more specilically, it is an improved antenna feed which, in the example to be described, feeds a Cassegrainian antenna. The conical scanning is obtained without mechanically moving either the antenna reflectors or the feed. Also, if desired, only -a narrow frequency band signal in the vicinity of a frequency fB is conically scanned; the communication signals at a different frequency fc which are transmitted or received by the antenna not being affected by the conical scan. Thus, in this case, there is provided an optimum gain characteristic for the communication signals.

An object of the invention is to provide an improved feed system for an antenna.

A further object of the invention is to provide an improved conical scanning radiating system.

A further object of the invention is to provide an improved means for deflecting an antenna radiation pattern.

A still further object of the invention is to provide an improved means for deliecting a radiation pattern without the mechanical movement of any large component.

A still further object of the invention is to provide an improved feed or radiating system that wil-l radiate energy over a wide band of frequencies within a certain radiation 3,423,756 Patented Jan. 21, 1969 pattern and that will deflect only the radiation pattern of a narrow band of frequencies within said wide band.

In practicing one embodiment of the invention, the antenna feed comprises an oversized waveguide to which the signals are fed from a waveguide dimensioned to propagate the signals in the TEN mode, for example. The signals propagate through the oversized waveguide in this same mode only so long as no asymmetry or discontinuity is introduced in the oversized waveguide.

A signal at any preselected frequency passing through the antenna feed may be made to radiate in a radiation pattern (beam) that conically scans by utilizing four bandstop filter cavities that are mounted around the oversized waveguide, each cavity mounted asymmetrically with respect to a symmetry plane of the waveguide, and each cavity coupled to the waveguide when the cavity is tuned to resonate at the said preselected frequency. Such coupling introduces an asymmetrical discontinuity in the oversized waveguide so that the mode TEN, in the instant example, is split into the additional modes 'IEZO and TEM and TMH. This results in deflection of the beam radiated or propagated from the oversized guide. By proper-ly tuning the four cavities in time succession to resonate at the selected frequency, the signal at this frequency has its radiation pattern deected so that it scans on a circular cone surface.

The present invention is particularly useful for ground stations of a communications satellite system because communication signals at widely spaced frequencies can be passed through the antenna feed without being deected, while a signal at a different frequency may at the same time be passed through the feed and its pattern made to conically scan so as to provide error signals which may be employed to make the antenna automatically track the communications satellite. The communication signals being transmitted to or received from the satellite by the antenna are not affected by the conical scanning.

The invention will be described in detail with reference to the accompanying drawing, in which:

FIG. l is a side view of an antenna feed or radiating system constructed in accordance with one embodiment of the invention;

FIG. 2 is a view taken on the -line II-II of FIG. l looking in the direction of the arrows;

FIG. 3 is a cross-sectional view of the oversized waveguide and the bandstop filter cavities shown in FIG. 1, this view being taken on the line III-III of FIG. 4 looking in the direction of the arrows;

FIG. 4 is a view taken on the line IV-IV of FIG. 3 looking in the direction of the arrows;

FIG. 5 is a view illustrating a Cassegrainian antenna that is fed by an antenna feed embodying the present invention;

FIG. 6 is a schematic diagram illustrating one way of so varying the resonant frequency of the bandstop filter cavities of FIG. l as to provide conical scanning;

FIG. 7 is a view showing a `portion of the feed system of FIG. l together with an illustration of radiation patterns that are referred to in explaining the invention;

FIG. 8 is a pair of graphs representing radiation patterns which are referred to in explaining the invention;

FIG. 9 yis a schematic diagram illustrating another way of so varying the resonant frequency of the bandstop filter cavities of FIG. l as to provide conical scanning;

FIG. l() is a cross-sectional view of the oversized waveguide with bandstop filter cavities mounted thereon in accordance with another embodiment of the invention, this view being taken on the line X-X of FIG. 11 looking in the direction of the arrows;

FIG. 11 is a top view of the waveguide and cavities shown in FIG. 10;

FIG. 12 is a block diagram of a transmitter-receiver in which the present invention is incorporated; and

FIG. 13 is a top view of the oversized waveguide with bandstop filter cavities mounted thereon in accordance with another embodiment of the invention.

In the several figures, like parts are indicated by similar reference characters.

FIGS. 1 and 2 illustrate an embodiment of the invention designed to provide conical scanning. More specically, it is designed to pass signals within a wide frequency band such as desired at the ground station of a communication satellite system, and is designed to conically scan the radiation pattern of a narrow band of energy within said wide frequency band. For example, the antenna feed of FIG. 1 will pass any communication signal from 5000 megacycles per second to 7500 megacycles per second; and within this frequency band a signal in the vicinity of 6000 megacycles per second may have its radiation pattern conically scanned.

The following description will be with reference to transmission or radiation from the feed system. However, on the basis of reciprocity theorem this is also a description of the operation when the feed system is receiving signal, as from a satellite. In the case of reception of signals, the signal that is conically scanned (the above-mentioned signal in the vicinity of 6000 mc., for example) may be the beacon signal from a satellite which is acted upon by conical scanning in the feed system to produce an error signal which may `be utilized to make the antenna track the satellite.

In the embodiment shown in FIGS. l and 2, the signal is :passed through a transmission line which preferably is either a square or a rectangular waveguide. In the present example it is a square waveguide. The largest side dimension of the waveguide 10 is greater than one-half 7\ and is less than A where x is the Wavelength of the lowest practical frequency to be passed. The signal is propagated down the line 10 in the TEm mode, and is passed in that -mode through an impedance matching or transition section 11 to a square oversized waveguide 12. The length of the transition section 11 preferably is from three to five times x so that reflection from the transition remains negligible. The waveguide 12 preferably feeds into a pyramidal square horn 13 which, in the preferred embodiment, radi- -ates the signal to the hyperboloid of a Cassegrainian antenna (see FIG. 5). The horn 13 may also be described as an oversized waveguide which is flared. If desired, the oversized waveguide 12 and the horn 13 mayl be circular. They should have two orthogonal symmetry planes. With reference to FIG. 5, it is not drawn to scale. Actual typical values are: Diameter of paraboloid=300 wavelengths; diameter of hyperboloid=30 wavelengths; aperture size of horn =3 wavelengths.

In order to provide conical scanning, four bandstop filter cavities 14a, 14b, 14C, and 14d are mounted on the four sides of the oversized waveguide. If preferred, these cavities may `be mounted at the throat cross-section of the horn or on the horn itself. These cavities and their mounting are shown in more detail in FIGS. 3 and 4 which show that there is a coupling hole for coupling each cavity to the oversized waveguide. This coupling is effective, however, only for signal at substantially the frequency at which the cavities are resonant. Thus, signals at frequencies not close to the frequency at Which the cavities are resonant are propagated down the oversized waveguide in the TEN mode and are radiated in a radiation pattern which is not deflected.

The action of the bandstop filter cavities will now be described with particular reference to FIGS. 3 and 4. The oversized waveguide 12 is dimensioned so as to support the four waveguide modes TEN, TEZU, and TEM and TMm. These modes can be propagated for either vertical polarization or horizontal polarization or both. The inside width a (and also height since the waveguide is square) is greater than )t and less than 1.5K where A is the longest operational wavelength. By longest operational wave length is meant the wavelength of the signal of the lowest practical frequency to be propagated by the feed system. In the example being described, this frequency is 5000 megacycles per second. The longest practical wavelength is usually limited by losses in the waveguide. By the use of a ridge-loaded waveguide this wavelength can be made greater.

The four bandstop filter cavities and their coupling to the oversized waveguide are alike. Referring to the cavity 14a as shown in FIGS. 3 and 4, it is on the top of the waveguide 12 and is coupled to the waveguide 12 through a circular hole 16a. The cavity 14a and its coupling hole are mounted off-center with respect to the center axis of the top of the waveguide 12 (with respect to the vertical symmetry plane) so that the cavity 14a is coupled asymmetrically to the wave guide 12.

If the bandstop filter cavity 14a is tuned to a frequency in the vicinity of a frequency B which is not close to the operational frequency fc, the cavity has practically no effect on the propagation of frequency fc signal in the waveguide 12. As an example, if the cavity is tuned to a -frequency of 6005 mc. and the operational frequency fc is a 6010 mc. carrier wave carrying voice channels, the 6010 mc. carrier `wave is substantially `unaffected by the cavity. In this case the incoming 6010 mc. signal being propagated in the TEN mode travels through the waveguide 10 (FIG. l), the matching section 11, the waveguide 12 and the horn 13 without any disturbance, and the horn 13 radiates a symmetrical pencil beam 17 (radiation pattern) as illustrated in FIGS. 7 and 8 in solid line. Thus, any conical scanning of the radiation pattern of a signal at a frequency is the vicinity of fB (f3 being 6000 mc. in this example) that is produced by the action of the cavities 14a, 141), 14o and 14d does not deflect or otherwise affect the signal at frequency fc (6010 mc. in this example). The minimum separation between fB and fC which provides unaffected operation for fc depends upon the geometry of the bandstop cavities.

Now consider the case where conical scan is desired so that an error signal is obtained which may be employed to make the antenna automatically track a target such as a satellite. The four cavities 14a, etc., are tunable through a frequency range from fm to BZ where the center frequency is fB, which in this example is 6000 mc. In this example it will be assumed that fB1i=6005 rnc. and that fB2=5995 mc. A signal of frequency fm, for example, is propagated down the antenna feed (FIG. l) in the TEM, mode to the oversized waveguide 12. Now the signal at frequency fm as it passes through the Waveguide 12 is affected lby one or more of the cavities 14a, etc., since they are tuned successively to the frequency fm and are, therefore, successively coupled to waveguide 12 at this frequency. Since the coupling is asymmetrical with respect to waveguide 12, the incoming TEN mode splits into the above-mentioned four modes, namely, TEN, TE20, and TEM and TMm. These four modes are represented by graphs in FIG. 3. The T1310 and T1220 graphs represent the voltage read from a probe that is moved horizontally through the waveguide. The TE11+TM11 graph is for the sum of these two modes and represents the voltage read from a probe that is moved vertically through the waveguide. The presence of these modes results in an asymmetrical field distribution in the antenna feed from vthe cavity coupling to the output aperture of the horn 13.

Referring for the moment only to the effect of the one cavity 14a when it is resonant at frequency fm and the signal frequency is fm, the asymmetry can be characterized by an amplitude and phase asymmetry, and furthermore by a movement of the radiation phase center from the center of the aperture. This shift in the position of the phase center is illustrated in FIG. 7 where the radiation pattern 17a in broken lines represents the pattern of the fm frequency signal that is being deflected by the cavity 14a. Also, it will be noted that the radiation pattern 17a is tilted otf axis.

The frequency JBl at which the cavity is resonant, and at which the radiation pattern of a signal is tilted, can be adjusted by selecting the geometrical dimensions b, c and h of the cavity (see FIGS. 3 and 4), and by selecting the amount t that a tuning pin 18a extends into the cavity.

The amount of angular tilt 0 of the radiation pattern can be controlled by the diameter d of the coupling hole 16a andby the distance x that the center of the coupling hole is away from the center line of the top of the oversized waveguide 12.

If the input signal to waveguide 12 is dual or circularly polarized then one arrangement that is possible to obtain coupling to both the TEM, and TEM modes is one that uses four cavities with the one cavity on each side being rotated as shown in FIGURE 1. Still referring only to the elfect of a single cavity, the tilt of the radiation pattern occurs in a plane indicated in FIG. 3 as P0. The angle that the plane P0 makes with a horizontal plane passing through the center of the waveguide 12 is identified as the angle This angle of the plane in which the tilt occurs can be controlled by the angle a shown in FIG. 4. It will be seen that the angle et is the angle at which the wide side of the cavity is tilted with respect to a plane drawn perpendicular to the longitudinal axis of the oversized waveguide 12.

In the example illustrated, the angle a is adjusted so that the angle of the p-lane in which the tilt occurs is 45 degrees. In this example the angle a is 12.5 degrees. Therefore the radiation pattern or beam is tilted in the same plane regardless of whether the exciting wave in the waveguide 12 is the TEN mode or the TEO, mode. In this case the antenna feed may =be used for either linear or circular polarizations. If linear polarization is used, the desired conical scanning at the frequency fm may be obtained regardless of the particular value of the angle or and, therefore, of the angle [3.

In order that conical scanning may be provided, rather than deflection in one plane, four cavities are mounted around the oversized waveguide 12, one cavity on each side of the cavity as shown in FIGS. 1 to 4. The four cavities indicated at 14a, 14h, 14C and 14d are mounted symmetrically around the waveguide 12, but with each cavity mounted asymmetrically with respect to the symmetry plane. Each cavity is a duplicate of the other, and each cavity is mounted on and coupled to the waveguide the same as described in connection With the cavity 14a.

Assume a signal at frequency fm is propagated down the antenna feed and is to be conically scanned. As shown in FIGS. 3 and 9, in order to provide conical scanning, movable tuning pins 18a, 18b, 18C and 18d are provided for the cavities 14a, 141i, 14e and 14d, respectively. Each pin is mounted so that it may be slid in and out of its cavity. The pins are slid in and out sinusoidally with a 90 degree or one quarter time period delay in the motion of the pin in consecutive cavities. There are FA complete periods per second, FA preferably being an audio frequency. One suitable drive to provide such motion for the tuning pins is illustrated in FIG. 9. As shown in FIG. 9, each tuning pin is driven by a crank, and the cranks are phased so that when pin 18a is fully in the cavity (and the cavity resonant at frequency fBl), the pin 18h is one-half way in, the pin 18e is completely out (180 degrees out of phase with pin 18a), and the pin 18d is one-half way in. At this position of the tuning pins the cavity 14a is producing the entire deflection of the signal fBI radiation pattern, the cavity 14a` is producing no deection since it is not resonant to the signal at frequency fBl, and the cavities 14b and 14d are each producing less than maximum deection but in equal amounts and in opposite directions so that their deflections cancel.

In the example being described, each cavity is resonant at frequency fBZ (5995 mc. in this example) when its tuning pin is completely out. Therefore a signal at frequency im propagated down the antenna feed will be yconically scanned as previously described by driving the tuning pins as shown in FIG. 9.

A signal at the center frequency fB, the frequency lat which a cavity is resonant with its tuning pin half way in, will not be deflected by driving the tuning pins. However, a signal at any frequency between fB and fm or between fB and fm may be conically scanned by driving the tuning pins, although the cone will not be a circular one. In practice this deviation of the cone from the circular has no importance because it only makes the horizontal and vertical error signals slightly different, and it causes a slight reduction in the error voltage because the audio filter in the detector circuit for this voltage removes the higher harmonics 0f the error signal which result from a noncircular cone scanning. At frequencies fm and fBg the cone of scanning is practically circular.

The communication frequency band of this antenna system when used for communication ideally is cSBz-i 0f cfBi-i-fi where f1 is the 3 db bandwidth of a cavity and is With fc so located, a signal at either fBl or fm, or at a frequency between fBl and fBZ, may be conically scanned without deecting (conically scanning) the communication signal.

By tolerating a small amount of conical scanning loss, the communication frequency band fc can be in the frequency band of scanning; for example, fc might be somewhere between fB1 and fm. In general, this is not a preferred arrangement, but it is a useable one for communication.

FIG. 6 illustrates an arrangement for obtaining conical scanning without even a minor mechanical motion. Instead of tuning pins, each cavity is provided with a ferrite rod and a magnetizing or control coil for changing the permeability of the ferrite rod. In this case the cavities should be of a non-magnetic material such as brass. Each ferrite rod may be held in place by a sheet of insulating material, such as Teon, frictionally supported in the cavity.

The magnetizing coils have a sine wave current owing through them which is supplied from a source 21. The current in the magnetizing coils is displaced degrees in consecutive coils by means of the phase Shifters 22, 23 and 24. Each cavity is also provided with a bias coil carrying a steady direct current supplied from a D-C source such as a battery. The magnetic iield of the bias coil causes the ferrite rod to have a permeability intermediate its maximum and minimum permeabilities produced by the sine wave current owing through the magnetizing coil. The conical scanning action is the same as that described in connection with FIG. 9. Specifically, taking the instant when maximum positive polarity iS flowing through the magnetizing coil of cavity 14a, the ferrite rod of 14a has maximum permeability (the fields of the magnetizing and bias coils adding) and cavity 14a is tuned to resonance at frequency B1. At the same time, maximum current of negative polarity is owing through the control coil of 14e and the ferrite rod of cavity 14e has minimum permeability (since the sine wave eld cancels the -bias field) so that cavity 14C is not resonant at frequency fBl. No current is flowing through the magnetizing or control coils of cavities 14b and 14d at this instant so that the ferrite rods in these cavities have the same permeability, Which is a value intermediate the maximum and minimum permeabilities. Thus, the deection effects of cavities 14h and 14d cancel out at this instant. All deflection of the radiation pattern at this instant is caused by the cavity 14a.

In the foregoing examples, the coupling hole or aperture between each bandstop cavity and the oversized waveguide has been described and illustrated as a circular hole. However, it may have other shapes. For maximum coupling, the coupling hole should be a slot having a length about equal to a half free space wavelength of the signal to be conically scanned, and the length to width ratio of the slot should be about ten. Also, the largest coupling is obtained if the coupling hole is about halfway ybetween the symmetry plane and the side wall of the cavity. The coupling is most frequency sensitive when adjusted for maximum coupling as described above. Considerable frequency sensitivity may be desirable so that the existence of the TE20 and TEM and TMm modes can be restricted to a narrower frequency band.

If the input wave from waveguide is dual or circularly polarized, the arrangement using four cavities at an angle a described in the foregoing example may be used where the angle a is adjusted so that the cavity has an equal amount of coupling to both the TEM, and the TEM exciting modes, and the corresponding beam tilt occurs in the diagonal plane of the radiating horn 13.

A second arrangement using eight cavities as shown in FIGS. l0 and 11 may be used where the input wave is dual or circularly polarized. Each cavity is provided with a tuning pin as previously described. In this arrangement, for a signal to be deflected, the cavities 100, 101, 102 and 103 are coupled to the signal being propagated in the" TEU, mode. The cavities 100 and 102 at the top and bottom of the waveguide 12, respectively, have their wide dimension perpendicular to the direction of propagation of the signal. The cavities 101 and 103 at the sides of the waveguide 12 have their wide dimensions parallel t0 the direction of propagation of the signal.

In the example now being described, the signal is also being propagated in the TEO1 mode. The cavities 100A, 101A, 102A and 103A are coupled to the signal in this mode. As illustrated, the cavities 100A and 102A on the top and bottom of the waveguide 12, respectively, have their wide dimension parallel to the direction of propagation of the signal. The cavities 101A and 103A on the sides of the waveguide 12 have their wide dimension perpendicular to the direction of propagation of the signal.

In the case of either dual polarization (illustrated in FIG. l0) or circular polarization, the actuation of the tuning pins for producing conical scanning is the same as previously described. It may be noted that when the tuning pin of cavity 100 is extended into its cavity a maximum amount, the tuning pin of its adjacent cavity 100A is also extended into its cavity a maximum amount. The same is true with respect to the operation of the tunings pins of the other adjacent cavities.

FIG. l2 illustrates in block diagram a communication or radar system which incorporates an embodiment of the present invention. If employed as a radar system, it may be of the pulse-echo type for tracking a target and measuring its range.

The system comprises a radio transmitter 51 which feeds signal through a diplexer 52 to the waveguide 10, matching section 11, oversized waveguide 12 and multimode horn 13 previously described. The received signal passes through the horn 13, waveguide 12, matching section 11, and waveguide 10, and through the diplexer 52 to a preamplifier 53. The amplified signal is passed to a mixer 54 where it beats with signal from a local oscillator 56. The resulting IF signal is amplified in amplifier 57 and detected by detector 58 to supply the demodulated received signal.

The error signal of frequency ,A resulting from the conical scanning is detected by an amplitude modulation detector 59 and supplied to a bandpass filter 61 that passes the error signal and applies it to phase sensitive detectors 62 and `63. Each of the detectors 62 and `63 is sensitive to both the relative phase and the relative amplitudes of the signals applied to it. Each of these detectors preferably is phase locked in accordance with well-known practice.

The previously described arrangement for conical scanning employing ferrite rods in the bandstop filter cavities is shown in block diagram form. Signal at frequency fA from the oscillator 21 is applied without phase shift over a lead 60 to the phase detector 62 where it functions as a reference signal for the elevation error signal. The output of detector `62 is a signal that controls, by a servo system (not shown), the elevation tracking of the antenna being fed `by the multimode horn. Also, signal at frequency fA is applied with degree phase shift over a lead 64 to the phase detector 63 where it functions as a reference signal for the azimuth error signal. The output of detector 63 is a signal that controls the azimuth tracking of the antenna by means of a servo system (not shown).

If the conical scanning is produced Iby means of tuning pins driven as shown in FIG. 9, the reference signals foi the detectors 62 and 63 may be obtained from signal supplied from a generator coupled to the driving motor as illustrated.

As illustrated in FIG. 13, the conical scanning can be restricted to a narrower frequency band by locating more than one cavity along the direction of propagation of the signal. In the example illustrated, two cavities are located along this direction, these being 14a and 14a1 on the top of waveguide 12, 14b and 14b1 on one side, and 14d and 14d1 on the other side. The two cavities on the bottom of waveguide 12 are not shown. The tuning pins for the cavities 14a and 14a1 are indicated at 18a and 18a1, respectively. The two cavities in line are tuned to the same frequency. When a signal is being conically scanned, the tuning pins of the two cavities in line are moved together, the tuning pin 18a1 being completely out when the tuning pin 18a is completely out.

If, for some reason, the target tracking is to be achieved in a wide frequency band, then the cavities which are in line along the direction of signal propagation may be tuned to different and equally spaced frequencies with their frequency spectrums overlapping. There may be three or more such cavities rather than only the two in line cavities shown in FIG. 13.

It may be desirable to design the antenna feed so that the antenna is capable of tracking any one of several satellites or targets. Such satellites may each be radiating a beacon signal at a frequency different from the beacon frequencies of the other satellites. For such a design the cavities which are in line may be designed to be resonant at widely spaced frequencies located in the overall communication frequency band of the antenna.

The narrowness of the frequency band to which the conical scanning is restricted depends in large part upon how sharply resonant the cavities are. A cavity is resonant at a signal frequency if its electrical length is equal to one-half the signal wavelength. However, the cavity may be made more sharply resonant to this frequency if the cavity is given a length that is a multiple of this one-half wavelength.

The bandstop filter cavities have been described as being mounted on the oversized waveguide 12. Instead of being mounted on waveguide 12, they may be mounted at the throat of the horn 13 or on the multimode horn 13 itself. The conical scanning operation is the same whether the bandstop filter cavities are mounted on the waveguide 12 or on the horn 13. As previously mentioned, the horn 13 is a flared oversized waveguide. The phrase oversized waveguide in the claims refers to the horn 13 as well as to the waveguide 12.

It may be noted that the conical scanning system can be optimized for the tracking signal. For example, assume the antenna feed of FIG. l is operated with the communication signal having a frequency far enough removed from the frequency of the signal being conically scanned so that the communication signal is unaffected by the conical scanning. Then the antenna patterns angle of tilt 01, shown in FIG. 8, preferably is made equal to 0.5 00 where, as shown in FIG. 8, 00 is the half angle between the 3 db points of the pattern. On the other hand,

if the communication signal itself is being conically scanned, then the angle of tilt 01 preferably is given a value between 0.3 and 0.4 00.

There will now be described a way in which -a system, such as that illustrated in FIG. 12, may be made less sensitive to error caused by amplitude modulation of the received signal due, for example, to fading, target scintillation or target rotation. Instead of rotating the antenna beam at one rate such as 30 cycles per second, it may be rotated alternately at two different rates such as 30 c.p.s. and 45 c.p.s. This may be done as illustrated in FIG. 12 by providing a second oscillator 21a and a second ybandpass lter 61a. As an example, the oscillator 21 has a frequency of 30 c.p.s. and the oscillator 21a has a frequency of 45 c.p.s. Then the filters 6-1 and l61a are designed to pass 30 c.p.s. and 45 c.p.s., respectively. A motor M drives switches as illustrated to connect alternately the oscillator 21 and filter y61 to the circuit and the oscillator 21a and filter 61a to the circuit. This switching may be done, for example, at the rate of once per minute. Thus, an undesired amplitude modulation of a frequency of approximately 30 c.p.s. due to target scintillation, for example, will be less likely to prevent the antenna from tracking the target.

Instead of switching as described above, the oscillators 21 and 21a may be connected to feed simultaneously into the system so the antenna beam will scan at different rates, i.e., the beam will speed up and slow 'down in the course of making several rotations. With this arrangement the Ibandpass filters 61 and `61a are connected in parallel so that their outputs are connected at all times to the two phase sensitive detectors.

What is claimed is:

1. An antenna -feed for providing angular deliection of signals within one frequency band while simultaneously passing undeected signals within other frequency bands, said feed comprising:

a waveguide having two orth'ogonal symmetry planes,

a transmission line connected to said waveguide for propagating to said waveguide said signals within said one frequency band and within said other frequency bands in a dominant mode,

said waveguide being dimensioned so as to support the propagation of said signals in additional modes,

means for only splitting said dominant mode of said signals within said one frequency band into said additional modes causing deflection of said signals within said one frequency band and for simultaneously passing undeflected said signals within said other frequency bands.

2. An antenna feed for providing angular deliection of signals within one frequency band while simultaneously passing undeflected signals within other frequency bands comprising:

an oversized waveguide having tw-o orthogonal symmetry planes,

a transmission line coupled to said waveguide, said line being dimensioned to propagate signals within said one frequency band and within said other frequency bands in at least one of two dominant TE modes,

said waveguide being dimensioned so that it can propagate said signals in the additional modes TE20, T1311l and TMm,

and means for introducing asymmetry into said waveguide only for said signals within said one frequency band and for simultaneously passing undeliected said signals within said other frequency bands whereby said dominant mode of said signals within said one frequency band is split into said additional TEzo, TEM and TMm modes and said signals within said one frequency band are deflected.

3. An yantenna feed comprising an 'oversized waveguide having two orthogonal symmetry planes,

a transmission line coupled to said waveguide, said line being dimensioned to propagate signals in a certain mode,

said waveguide being dimensioned so that it can propagate signals in the additional modes TEZO, TEM and TMll:

means for introducing asymmetry into said waveguide whereby said certain mode is split into said additional TEm, TEM and TMm modes, said last named means includes a resonant cavity mounted on each side of said waveguide, each of said cavities being coupled asymmetrically to said waveguide for a propagated signal of a frequency to which the cavity is resonant, and

means for tuning said cavities to resonance successively in the same order in which they are positioned around the waveguide whereby said propagated signal is conically scanned when radiated.

4. In an antenna feed for providing angular deflection of signals within one frequency band while simultaneously passing undeected signals within other frequency bands, the combination comprising:

an oversized waveguide having two symmetry planes,

a transmission line coupled to said waveguide, said line being dimensioned to propagate said signals in a dominant mode such as the TEM mode or the TEM mode,

said waveguide being dimensioned so that it can propagate said signals in the additional modes TVE-20, TEM and TMn,

a tunable cavity mounted on one side of said waveguide,

means for only coupling said cavity to said waveguide when a signal having a frequency within said one frequency band propagated through said waveguide and for passing uncoupled and undeected said signals within said `other frequency bands,

said coupling means bein-g located asymmetrically with respect to the longitudinal center axis of said one side of the waveguide whereby said dominant mode of only said signals within said one frequency band is split int-o said additional T1520, TEM and TMH modes and only said signals within said one frequency band are deflected.

5. An antenna feed for providing conical scanning, said feed comprising an oversized waveguide having two symmetry planes,

a transmission line connected to said waveguide for propagating to it signal in a dominant mode,

said waveguide being dimensioned so as to support the propagation of said signal in higher-order modes,

a resonant cavity mounted on each side of said waveguide, each cavity having a coupling aperture extending into the waveguide,

each coupling aperture being located asymmetrically with respect to the longitudinal center axis of the waveguide side in which it is located, each cavity being coupled t0 said waveguide when the cavity is resonant to a signal passing through said waveguide whereby said signal is passed through said waveguide with said dominant mode of propagation split into said higher-order modes, and

means for tuning the successive cavities positioned around said waveguide successively to resonance in a sine wave cycle so that each cavity is resonated degrees out of phase with the preceding cavity.

6. In an antenna feed for providing angular deflection of signals within one frequency band While simultaneously passing undeflected signals within other frequency bands, the combination comprising:

an oversized waveguide,

a transmission line coupled to said waveguide, said line being dimensioned to propagate said signals within said one frequency band and said other frequency bands in the TElo mode,

said waveguide being dimensioned so that it can propl l agate said signals in the additional modes TEZO, TEn and TMm,

a tunable cavity mounted on one side of said waveguide,

means for only coupling said cavity to said waveguide when a signal having a frequency within said one frequency band is propagated through said waveguide and for passing uncoupled and undeflected said signals within said other frequency bands,

said coupling means being located asymmetrically with respect to the longitudinal center axis of said one side of the waveguide whereby said TEU, mode of only said signals within said one frequency band is split into said additional T1320, TEU and TMll modes and only said signals within said one frequency `band are deflected.

7. An antenna feed for providing conical scanning, said feed comprising an oversized waveguide,

a transmission line connected to said waveguide for propagating to it signal in a dominant mode,

said waveguide being dimensioned so as to support the propagation of said signal in higher-order modes,

a resonant cavity mounted on each side of said waveguide, each cavity being coupled asymmetrically to said waveguide when the cavity is resonant to a signal passing through said waveguide whereby said signal is passed through said waveguide with said dominant mode of propagation split into said higherorder modes, and

means for tuning the successive cavities positioned around said waveguide successively to resonance in a sine wave cycle so that each cavity is resonated 90 degrees out of phase with the preceding cavity.

8. An antenna feed comprising an oversized waveguide having two orthogonal symmetry planes,

a matching waveguide section,

a transmission line coupled to said oversized waveguide through said matching section, said line being dimensioned to propagate signals in either the TEN mode or the TEM mode or in both of said modes,

said waveguide being dimensioned so that it can propagate signals in the additional modes TE20, TEU and TMll,

means for introducing asymmetry into said waveguide whereby said first-mentioned modes are split into said additional TE20, TEM and TMll modes,

said last named means including a resonant cavity mounted on each side of said waveguide, each of said cavities being coupled asymmetrically to said waveguide for a signal propagated in at least one of said first-mentioned modes and of a frequency to which the cavity is resonant, and

means for tuning said cavities to resonance successively in the same order in which they are positioned around the waveguide whereby said propagated signal is conically scanned when radiated.

9. An antenna feed comprising an oversized waveguide having two orthogonal symmetry planes,

a transmission line coupled to said waveguide, said line bein-g dimensioned to propagate signals in either the TEN, mode or the TEM mode or in both of said modes,

said waveguide being dimensioned so that it can propagate signals in the additional modes TE20, TEM and TMH,

means for introducing asymmetry into said waveguide whereby said first-mentioned modes are split into said additional TE20, TEU and TMH modes,

said last means including a resonant cavity mounted on each side of said waveguide, each of said cavities being coupled asymmetrically to said waveguide for a propagated signal of a frequency to which the cavity is resonant,

12 each of said cavities having a wide side and being positioned with its wide side tilted at a certain angle with respect to a line drawn perpendicular to the longitudinal axis of said waveguide, the value of said certain angle being such that each cavity has an equal amount of coupling to both the TEM, and the TEM metry planes,

a transmission line coupled to said waveguide, said line being dimensioned to propagate signals in a certain mode,

said waveguide being dimensioned so that it can propagate signals in the additional modes T1520, TEn and TMll, ,v

means for introducing asymmetry` into said waveguide whereby said certain mode is split into said additional TE20, TEU and TMm modes, said last named means including a resonant cavity mounted on each side of said waveguide,

each of said cavities being coupled asymmetrically to said waveguide for a propagated signal kof a frequency to which the cavity is resonant, and

means for tuning said cavities to resonance successivelyv in the same order in which they are positioned around the waveguide whereby said propagated signal is conically scanned when radiated, said means for tuning said cavities including means for varying the rate of said successive tuning so that the conical scanning of said radiated signal speeds upand slows down in accordance with the variation of said rate.

11. An antenna feed for providing conical scanning,

said feed comprising an oversized waveguide, a transmission line connected to said waveguide for A propagating to it signal in a dominant mode,

said waveguide being dimensioned so as to support the propagation of said signal in modes which are multiples or harmonics of said dominant mode,

a resonant cavity mounted on each side of said waveguide,

each cavity being coupled asymmetrically to said waveguide when the cavity is resonant to a signal passing through said waveguide whereby said signal is passed through said waveguide with said dominant Inode of propagation split into additional modes of propagation,

means for tuning the successive cavities positioned around said waveguide successively to resonance in a sine wave cycle so that each cavity is resonated 90 degrees out of phase with the preceding cavity, and

means for making said sine wave cycle occur at successively different frequencies. i

References Cited UNITED STATES PATENTS 2,433,368 12/1947 Johnson et al 343-786 X 2,905,940 9/1959 Spencer et al. 343-768 X 3,021,524 2/1962 Kompfner 343-787 X HERMAN KARL SAALBACH, Primary Examinar.

WM. H. PUNTER, Assistant Examiner.

U.S. Cl. X.R.

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