US2648839A - Direction finding antenna system - Google Patents

Description

1953 .1. R. FORD ET AL DIRECTION FINDING ANTENNA .SYSTEM 2 Shets-Sheet .1

Filed Oct. 2, 1950 22 EEC.

EEC.

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N E 6 z m 6 INVENTO JokrzzRFord fmorzfiollzrzgerg' z I ATTORNEY Wit 560M542 Aug. 11,.19'53 Filed Oct. -2, 1950 J. R. FORD ET AL DIRECTION FINDING ANTENNA SYSTEM 2 Sheets-Sheet 2 aldmz! BY g f ATTORNEY Patented Aug. 11, 1953 UNITE DERECTION FINDING ANTENNA SYSTEM ware Application October 2, 1950, Serial No. 187,992

3 Claims.

This invention relates to radio echo detection systems and more particularly to such systems employing a wave guide feed.

A type of antenna which is now well known is that utilizing a waveguide to which is coupled a plurality of antenna elements. These antenna elements may comprise spaced slots in a hollow pipe waveguide wall through which energy within the waveguide is radiated. Again, a plurality of dipoles or the like may be suitably coupled at spaced intervals along a waveguide. The antenna slots may overlap longitudinally, thus to form a continuous staggered slot. In the past, such systems have been excited so that all of the elements radiate simultaneously during at least a major portion of the transmission or reception.

It is an object of the present invention to provide a new and novel method of operation of such an antenna.

It is another object of the invention to provide a novel combination of an antenna structure employing a plurality of such antenna elements.

A further object of the invention is to provide a novel radio echo detection system in which such an antenna is employed.

In accordance with the invention, the operation of the antenna is entirely different from the operation which has been employed heretofore. The novel operation is secured by associating with the antenna a pulsed energy generator, the pulses of energy in their travel through the waveguide being short in length with respect to the total waveguide length. Thus, an entirely novel system of operation is secured in which only a fractional portion or" the antenna elements are radiating at any moment of time and at no tim are all of the elements thereof simultaneously excited.

The foregoing objects, advantages and novel features of the invention will be more apparent from the following description in which like reference numerals refer to like parts and in which:

Fig. 1 is a perspective view of a slotted waveguide antenna;

Fig. 2 is another perspective view of a slotted waveguide antenna with an associated circuit schematically indicated;

Fig. 3 is a cross-sectional view of a somewhat different type of waveguide antenna which may be utilized in practicing the invention;

Fig. 4 is a schematic view of another system embodying the invention;

Fig. 5 is a sketch illustrative of the geometry of the system illustrated in Fig. l;

Fig. 6 is illustrative in block diagram form of a circuit which may be used in connection with the system of Fig. 4.

Fig. '7 is a perspective view of an assembly which may be used in a novel radio echo detection system embodying the invention; and

Fig. 8 is illustrative of a variation in the system of Fig. 4.

Referring now more particularly to Fig. 1, a long rectangular waveguide Iii has transverse slots I2 cut in one of the broad walls M. These slots l2 are spaced longitudinally a half guide wavelength, the slots being successively staggered on each side of the median line of the broad wall id, as shown, thus adjacent slots have radiation reversed in phase due to the spacing of half guide wavelengths, but also reversed due to the staggering current on opposite sides of the broad wall center line being in opposite directions. Thus adjacent slots radiate in-phase. Wave-guide ID has an absorptive matched termination it at one end thereof. The other end is supplied with electromagnetic energy from a transmitter 18 having a very high frequency of operation and being modulated in very short pulses. The energy is supplied to wave-guide ii] from the transmitter I8 to input end l9 through a T-R arrangement 20 of any of the various .well-known kinds whereby during transmission energy is supplied to waveguide ill but upon the cessation of transmission the energy returning from waveguide H1 is switched to a receiver 22. Let the period of one of the pulses from transmitter l8 be represented by T. The energy will travel through waveguide It at a velocity which may be assumed to equal substantially the group velocity 1). Let the length of the waveguide H! from the input end to the termination l6 be L. Then the energy advances from the input end toward the termination l6 and the pulses are so short that the length of the pulse, in the waveguide IB is substantially qual 11 t. and is less than L, and preferably only a small fraction, say about 6 or less of L. Thus the length L is substantially longer than the distance traversed in the waveguide It by the energy in the tim of a duration single pulse. Other antenna elements than the slots shown might be used. Accordingly, only a portion of the antenna elements spaced along the guide lengths are radiating energy at a time. As an example, suppose that there are a hundred apertures l2. The length of the pulse may be only ten times the spacing of the aperture, and consequently at th most ten adjacent apertures l2 are emitting energy at any instant of time. After the first ten nearest the input end !9 of the waveguide are radiating energy, a few moments in time later the first apertureis not radiating energy because the pulse a has passed it and the next ten are radiating energy. This continues until the last ten are radiating energy and thereafter as the energy strikes the matched absorbing termination H5 it is absorbed so that successively the last nine apertures, the lasteightapertures, etc..,.near. the end 96, are radiating. The: effect is best seen by refering to Fig. 2. The radiation pattern from Waveguide it may assume a shape such as that illustrated by the lobe 24 which at a later time may be in the position illustrated by the; dotted line lobe 25. The waveguide. l orv apertures. or

both may be graduated in sizeinorder to assure;

that each of th apertures lzradiateisubstane tially the same amount of poyer as every other aperture. Waveguide I0 may be of a length to extend for example, from one en'd 'of an' air-- plane runway to the other. The energy isitransmitted from transmitter I 8 and the lapse in time before it is-returned to: receiver 22 will give information regarding the time requiredforthe pulse to leave the transmitter; pass through the waveguide into the p'arti'cular'position substantially directly underneath anairplane, thence-to reach. the airplane, and for the echo to return vertically to waveguide l0 and thenceto the TR arrangement 20' to receiver 22;

An alternative waveguide arrangement is'shown in Fig. 3 in which a waveguide 30' semi-circular in cross-section feeds dipoles 32spaced" equally along the length of the. Waveguide 30. These are arranged to radiate in phase. The waveguide 30 may be substituted for the waveguide Hi bearing in mind again that it is to be long with respect to the length of the pulse being transmitted through. it, appropriate spacing, of the antenna elements 32, and appropriate orientation of the guide to direct. the energy from the dipoles 32 in the vertical plane (or. such other plane a may be desired) Referring now more particularlyto Fig. 4', three waveguides 40, 42., and Mlarespacedequally and laid out in parallel lines. parallel to therunway of an-airplane landing field. These. three Wave.- guides each may be similar. to waveguide It having radiated elementswhichin. the plane longitudinal thereto atthefrequency, of .operation are quite narrow and are restricted to an. area directly above theslots momentarily ratiating. In the transverse plane, the. waveguidesfl, 42, and M may have respectively radiation patternssuch as it, 48, and 553. The transmitter l3, whichmay be the same as thetransmitter ofFig. l, feeds one end'of the waveguideAlthrougha T.-R.arrange.- ment 52.

Areceiver 56 is: also. connectedtothe T-A ar.-. rangement to receivetheecho-signal as from an airplane 61 from waveguideA-Z and. for. simplicity receivers '3and 62' are shown connectedto waveguides ii? and it respectively to receive. the signals therefrom. However,,waveguides.40 and 44 could also be connectedito the input. of receiver 68 instead of beingcorr-ected assshown.v Receivers 53, Eli, and 52- may be connectedto anoscililoscope 6d (the face only of. which isshown in the drawing) as a common element todisplay the signals received therein. It will be understood that the oscilloscope sweep circuitifi5-may be syn:- chronized with the transmitter:by-connection 66 so that the sweep startsfrom thedeft with transmission of a pulse of energyby' transmitter 18 and moves uniformly with: time to the right. A pip it will appear at'the initiation of the sweep-representing the mainbang, and three; other pips,

one 12 and two pips 'lzappear-at somezdistancei to the right (assuming the sweep to progress from left to right) representing the return of signals from waveguide 42 and waveguides 16, and 4 respectively. The velocity of the signal in the waveguides, each of which may be presumed to be constructed similarly to the others, may be known or measured. It will be assumed that an approaching plane "M is at an altitude small compared to the slant range from the receiver ends, that is the ends of waveguides it, 42, and M to which the receivers 53', (iii, and 52 are connected. In order to. center the plane ti on the runway down. which the waveguides are laid, it is now only necessary to. direct the pilot to bring his plane toward one or the other side of the field until two of the pips F2 (from guides 48 and M) merge into one. The distance bteween the pips it 311(11171'18 pip 12' on the oscilloscope screen 5 is proportional to the ground range of the aircraft plus its altitude.

The system of Fig. 4 is operative only when the plane is over the landing field, that is above the assembly of waveguides 691,52 and 3%. For ground control approach (GCA) systems, the range indicated by the distance between pips if! and i2 approximates the true range of the aircraft, and approaches this true range more closely as the aircraft approaches touchdown over the waveguideAZ. Moreover, in mose cases, the error involved issmall. However, if desired, the altitude of the plane may be measured by employing a slight modification. of the system, whereby the error may be (in effect) subtracted from the range to indicate the true range.

Referring to Fi 5, the applicable geometry is illustrated. When the pips i2 merge, the distance. between the pip l2 and the merged pips l2is indicative of a time of travel which in turn indicatesa distanceq.

Now

q=b-c (l) and Therefore:

/2 (a /q-q) (3) Therefore, the altitude C can be found by standard, computer techniques. For example, a special potentiometer may be wound leaving the requisite variations with q, at least over a limitedrange,.to. give C, whcnmotion of a pick-up arm is linear withq. 6n the oscilloscope St, Fig. 4, for example, may be impressed a voltage proportionalto C derived from such a computer. Fig. 6 i1- lustrates how the voltage proportional to q may be derived. From receiver till a signal is supplied via a connection at to a flip flop circuit 32 which is flipped in one-direction by the signal indicated byv pip 12;" and inthe other direction by merged pips-l2; fed by connection 853 also. The rectangular wave out-put of nip-flop circuit 82 is fed to a triangular shaper S4 (or integrator). The average output voltageof the triangular shaper 84 is proportional'to the distance (1.

The system of Fig. 4 functions preferably when the plane 61 is somewhere over the runway, as pointed out hereinbef-ore. Standard known methods may be used to bring the plane in thus far, or for example, the spacing of slots 52 only at the far end of the runway, may be altered to direct the energy radiated from the end remote from the receiver to be directed longitudinally away from the waveguide. A continuous slot near the end of eachof waveguides 4t, 42, and 44, such as illustrated in Fig. 8, may alsobe. usedfor this;

purpose. Assuming, waveguides 40, 42, and 44 to be so constructed, causing a plane to move so that pips 72 coalesce lines up the course of the plane with the runway.

Referring now more particularly to Fig. 7, there is illustrated a hollow pipe waveguide I which is preferably substantially rectangular in crosssection to avoid Inoding difficulties. A series of antenna elements I02 spaced around the waveguide I00 of Fig. may be excited by energy in the waveguide. Waveguide I60 is preferably bent into a substantially circular shape and the antenna elements i022 are placed on the side of the circular guide facing away from the interior of the circle. The waveguide I00 may be laid substantially the ground plane and with the elements I02 thus facing outwardly the energy travels radially from the circle in which the waveguide I00 is shaped. A waveguide with a crosssection such as that of Fig. 3 may here be employed to advantage with antenna elements at a slight angle to the ground plane. A transmitter I8 is connected to a T-Rarrangement I04 which in turn is connected to the end I38 of waveguide 00. The receiver H0 is connected through the T-R arrangement N14 to the end I08 of waveguide I00. The other end H2 of waveguide N50 is connected also to a receiver III). An indicator, which may be cathode ray oscilloscope I I4 shown in face view in the drawing, has the signals from the receiver displayed thereon. The sweep is from left to right, linear with time and initiated to start with each pulse transmission from transmitter it. A connection I I6 is employed to synchronize the sweep. The receiver pulses are applied to the vertical plates.

In operation, the transmitter I8 feeds a pulse into the end I08 of waveguide I00. This pulse again is so short in time duration that at any moment of time only a fractional portion of the elements I02 are radiating. Accordingly the pulse travels out in a circular sweep starting at the end I03, travelling circumferentially around waveguide I00. The system is intended to give warning indications of approaching planes at a considerable distance from the waveguide I00. Accordingly, the time of travel to/2 of the signal path through waveguide I00 may be considered short and negligible compared with the time of travel through space to the object to be indicated and the return of the echo therefrom. Accordingly the distance from the main bang I22 of the echo pips I24 and I26 on screen of oscilloscope H0 is representative of the distance from the waveguide I00 to the plane. The angular location of the object to be indicated may be determined from the differential between the pips I24 and I26. More accurately, let tr be the time of travel of energy radiated from a point I I8 on waveguide I 00 nearest some reflecting object such as an airplane. Let is be twice the time of travel of energy in the waveguide I00 from the point IIB to the nearest end H2. Then the elapsed time between initiation of transmission of a pulse (main bang) and the first echo is:

Similarly, the elapsed time between the main bang and receipt of the second echo is:

1'2: /gta-f-tr-I-Vz (tin-to) =tT+ /zto where to is twice time of travel of the pulse from one end I08 to the other end II2 of the energy in the waveguide I00. Thus ta (proportional to the azimuth angle) is proportional'to the differ ence in sweep distance between the two echo pips plus a constant, and tr (range) is proportional to the difierence in sweep distance between the main bang and the second pip minus a constant. Computer circuits are well known which give a solution of these equations. Again, as a simple means, a specially wound potentiometer may be employed. To derive sensing, it may be desirable to shape the output pulses of the different receivers differently to resolve the ambiguity of Whether the first pulse is received from the left or right semi-circle of the waveguide I00 as viewed in the drawing.

Referring now more particularly to Fig. 8, there is illustrated a modification of the construction of the end of waveguide I0 01 Fig. 1 remote from connection to the receiver. For a short distance from this end, say a, length equal to the number of simultaneously radiating elements, the waveguide is opened at the top by a continuous slot I20. From slot I20 the energy leaves substantially without reflection. However, the slot acts in this instance as an end fire array, causing the energy to be projected forwardly, instead of vertically, at an angle inclined from the horizontal to a degree determined largely by the phase velocity in this portion, that is, the portion having slot I20. Thus in a system as that of Fig. 4, there may be detected a warning signal by returned reflections of planes approaching the runway from a position well in advance of the waveguide I0. If this variation is used, one should preferably have some means for distinguishing high altitudes from a distance in advance of the end of the waveguide I 0 as perhaps by another search radar. More simply, only one of the waveguides, say 42 of Fig. 4 is so made. Then the absence of an echo from the other waveguides is an indication that signal is being received from the slot portion of the one waveguide 42, and that the reflecting object or plane is an advance of the runway.

It will be apparent that there is disclosed a new and novel method of operation of a waveguide antenna, as well as new and novel combinations utilizing such a waveguide.

What we claim as our invention is:

1. In a system for radiating or receiving energy and having a long leaky waveguide, the method of operation comprising the step of exciting said waveguide with a succession of pulses of high frequency energy the time duration of each of which is substantially less than the time required for said pulse energy to traverse the length of said waveguide.

2. In a system for radiating energy, said system having a long waveguide and a substantial plurality of antenna elements equally spaced along the length of said waveguide and coupled thereto, the method of operation comprising the steps of exciting said waveguide with successive pulses of high frequency energy the time duration of each of which is substantially less than the time required for said pulse energy to traverse the length of said waveguide, and exciting each of said elements successively along the waveguide length with said pulses of energy but substantially less than all of said elements at any one time, thereby to establish an antenna pattern for said radiation which travels in space in the direction of motion of said energy through the waveguides.

3. In a system for receiving energy, said system having a long waveguide and a substantial plurality of antenna... elements equally; spaced; along thelength; of: saidi waveguide. and; coupled thereto; the: method of. receivingjpulse energy: comprising the steps of energizing; successively andprogressively a. few ofthe adjacent. antenna elementsspaxced along said waveguide with.each' of aasuccession. of pulses of. high. frequency energy the; duration. ofeach of- Which i substantiallyless than. the :time required for'said. pulse energy to traverse the length-ofsaid.Waveguide, and energizing said waveguide with said successive pulses of=- energyfrom. the: energized. an-- tennas thereby to couple: energy inthewaveguide moving in. the direction in which the elements.

are-successively energized.

JOHN. R. FORD. WALDON PEARSON BOLLINGER.

References Cited in the file of this patent UNITED STATES PATENTS

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