US2825062A - Antenna - Google Patents

Description

' Feb. 25, 1958 LAN JEN cHu ETAL 2,825,062

ANTENNA 2 Sheets-Sheet 1 Original Filed July 9, 1945 N win INVENTORS LAN JEN CHU CHIA'SHAN PAO FIG.3

FIG.5

M QM

A HORNE) Feb. 25, 1958 LAN JEN cHu ETAL 2,825,062

ANTENNA Original Filed July 9, 1945 2 Sheets-Sheet 2 FIG-8 LAN JEN CHU /BY GHIA-ISHAN PAO ATTORNEY INVENTORS United States Patent ANTENNA Original application July 9, 1945, Serial No. 604,022. gigided and this application April 17, 1953, Serial N0. 0 4 ,450

3 Claims. (Cl. 343-780) This application is a division of our application S. N. 604,022, filed July 9,1945, now abandoned.

This invention relates to antennas for communication systems and is particularly directed to an antenna having means for shaping the primary radiation pattern of a beam of electromagnetic energy emitted by a radiating element, such as a horntype feed, without changing the geometry or dimensions of the radiating element to etfect such shaping.

Such a radiating element has wide application. It can be used to match a wave guide transmission line to free space and thus may be used for radiating waves of electromagnetic energy directly to free space. However, its principal application is in feeding energy to, and illuminating, a reflector adapted to direct the radiant energy into free space in any'desired beam pattern.

The gain, half power beam width and side lobes of the radiation pattern of an antenna having a paraboloi'clal reflector depend upon the phase and amplitude distribution of the radiation pattern of the reflector feed. I The latter' may be referred to as the primary pattern of the feed; For 1 a circular dish-like reflector the problem of feed is relatively simple. For a truncated paraboloidal reflector it is necessary to use a feed having a sharper beam in one plane than in the plane perpendicular to it.

One objective in designing a paraboloid antenna is to realize the maximum possible gain consistent with side lobe requirements. In order to get the highest gain, it is required to have a relatively uniform illumination across the aperture of the paraboloid and low spill-over at the edge. To achieve this, one can either shape the primary beam of a feed to fit a given reflector or shape the reflector such as to form a truncated paraboloid to fit a given feed, or both.

The gain of a paraboloid antenna with a given feed is a maximum if the amplitude at the edge is half the average amplitude of the illumination over the aperture of the paraboloid. For a given feed, this-provides a simple criterion for determining the size of the paraboloid. In general, the gain factor does not drop sharply with the increase of the aperture of the dish. Thus, a slightly bigger aperture can always be used to reduce the side lobes of the radiation pattern of the antenna. In most cases, however, the converse problem is encountered; namely, for a given paraboloid, how to shape the primary pattern'of a feed to achieve the best results.

The width of a primary beam of an ordinary horn feed depends primarily upon its aperture b and its flare angle 0 (Fig. 1). Thus it is seen that a horn is the simplest feed to'use for a truncated paraboloid. However,'in certain respects, this type of feed is not entirely satisfactory. First, in order to narrow the beam of a horn, it is necessary either to increase b or to decrease 0. If 0 is kept constant and b isincreased, a point is reached at which the beam width is a minimum. As the aperture dimension b increases further, some of the secondary sources in the wave front atthe aperture are out of phase with others and interfere issw y in h tow i ec ion! The ettect on the 2,825,062 Patented Feb. 25, 1958-.

pattern is to produce high side lobes or shoulders on the main lobe. To remedy this, 0 may be decreased. This type of horn, while electrically satisfactory, may be too long and heavy for some applications. Secondly, in order to broaden the beam of an E-horn (horn flared out in the E-plane) in the H-plane it is necessary to taper down its aperture in the H-plane. However, this method of broadening the beam introduces a considerable impedance mismatch. It is impossible to have this'type of feed broad band in impedance. Thirdly, if an unflared waveguide is used to feed a pillbox type feed of an aperture 4 (f is the; focal length of the pillbox) a low gain may be expected, since the beam width of an open wave guide between two parallel plates is about the same as that in free space and is too narrow for this purpose. And lastly because of spilling over theedge of the dish and severely tapered illumination, the gain of a paraboloid antenna fed by a simple flared horn is usually low. In order to design an antenna of a high gain factor, it is necessary to shape a primary pattern of some special type to provide more uniform illumination with little energy shooting outside the reflector. Such beam shaping cannot usually be achieved by merely changing the geometrical dimensions of the horn. V

The principal object of this invention is to provide var-' ious means for shaping the primary pattern of a radiating. feed element to fit specified reflectors.

Another-object of the invention is to provide a radiating V 1 feed element, for an antenna, which is adapted to produce obstacles in the feed element.

a desired primary energy beam pattern by placing Other objects, advantages and novel features of the in vention will become apparent from the following 'descrip tion with reference to the accompanying drawings, in which:

Fig. 1 is a diagrammatic top view of a horn feed flared:

out in one plane according to the present invention;

Fig. 2 is a top plan view of a horn feed similar to Fig. 1;

Fig. 3 is a perspective view of horn feed showing one embodiment of the present invention;

Fig. 4 is a top plan view of a wave guide with a shaped end and having an obstacle across its open end according Fig. 8 is a perspective view of the horn feed illustrating afurther modification according to the invention;

Fig. 9 is a diagrammatic top plan view of an open wave guide placed between two parallel plates according to a.

- a certain limit beyond which it never gets narrower,

further embodiment of the invention; 7 v

Fig. 10 is a perspective view of a pillbox type radiating element showing an obstacle placed in the aperture there- ,of according to the present invention;

Fig. 11 is a partly diagrammatic top plan view of a Ye type feed for a pillbox radiating element; and

Fig. 12 is a partly broken away perspective view of the Y-feed illustrated in Fig. 11. 7 7

Before discussing various means of shaping the pri- 'mary beam according to this invention, it is useful to examine certain results of the variation of the beam width with the aperture and the flare angle of a flared rectangular born. The dependence of the beam width upon the aperture of both an H-horn and E-horn (horns flared out in the planes parallel to the H and E vectors respectively) has been plotted. It has been observed that as 0 (Fig. 1) is kept constant and b is increased, the ten db power width W is gradually decreased to according Tables showing the results of the variation of the W with different flare angles have been prepared but it is not thought necessary for the presentdiscussion to show such tables here. W is chosen because it represents approximately the angle of interest in reflector illumination.

From the above mentioned tables it is seen that for small aperture the phase front is more or less constant; thus the W depends mainly on b, not 0. However, as b increases. the phase front becomes curved, and the beam is narrower for smaller flare angles.

p The above results merely illustrate what kind of limitations may be encountered in shaping the primary pat tern of a born by changing its geometrical dimensi"ns. It is obvious that in order to have the beam further narrowed it is necessary to have a more uniform phase front at the aperture of the horn. This is accomplished by the means of shaping the primary pattern of a radiating feed element such as a horn according to the present invention, which relies upon the fact that when metallic obstacles are placed inside a horn, currents are excited in these obstacles. determine the magnitude of the current and their positions with respect to the horn determine the phase. By superposing the radiation patterns of these obstacles on the pattern of the horn itself, a resultant pattern is obtained. Thus by changing the sizes and the positions of these obstacles it is possible to obtain the primary beam desired. This method has been applied successfully and is disclosed in the following description to the five following problems as illustrative of the inventiona (1) I narrow the beam in the H-plane.

(2) To broaden the beam in the H-plane.

(3) To narrow the beam in the E-plane.

(4) To shape the beam in the E-plane.

(5) To shape the beam of a feed in the H-plane lie-- tween two parallel plates.

A horn type feed as illustrated in the drawings is con structedand arranged in a conventional manner. In general, horn (Figs. 2 and 3) comprises a flared portion 11 attached or coupled at its smallest dimensions to a hollow pipe wave guide 12. As shown, wave guide 12 and portion 11 have a rectangular cross-section adapted to feed energy waves in the TM or E mode, the E-vector or E-plane being indicated by the arrow E (Fig. 3).

Flared portion 11 comprises upper and lower walls 13.

and 14 and side walls 15 and 16 each having one end joined to the respective walls of wave guide 11 at throat 17. The opposite ends of the walls define the aperture or mouth 18. Horn 10 may be flared in either or both the E- aud' H-planes. For example, as shown in Fig. 3, horn 10 is flared in the H-plane only, that is, the side walls 15 and 16 are disposed at an angle 0 with each other (Figs. 1 and 2). Thus the dimension b of flared portion 11 is wider in the H-plane at the mouth 18 than at throat 17. Such a horn may be called an "H-horn. Similarly an E-horn is one in which the E dimensions (distance between upper and lower walls 13 and 14) at the mouth 18 is larger than at throat 17. A horn may also be used in which both the E and H-plane dimensions are larger at mouth 18 than at throat 17.

In considering problem 1 above, viz. narrowing the beam in the H-plane, reference is made to Figs. 2 and 3 which illustrate an H-horn 10 of the kind described herein. beam pattern ordinarily produced by horn 10 is made narrower in the H-plane by placing metallic obstacles in the path of the energy, for example, within the flaredportion 1 1 or across aperture or mouth 18. Such obstacles may be in the form of rods, bars. or pins of metal or other suitable electrically conductive material having a relatively small crosssectional dimension .compared to that of aperture 18. The obstaclesare of s'u'tficient length that their ends abutror connect opposite walls of flared According to one embodiment of the invention'the' portion 11. According to this embodiment, the obstacles comprise one, two or three pins 20, 21 and 22 of circular or rectangular cross-section. arranged with their longitudinal axes in the E-plane and perpendicular to upper and lower walls 13 and 14.

For the single pin 20, this is placed in the plane substantially bisecting the angle 0 at a distance a from the mouth or aperture 18. From data showing the primary pattern as a function of the dimension d it has been de tcrmined that when pin was located adjacent mouth 18, the beam is comparatively broad and consists of three peaks, a main one at the center flanked by two smaller ones at about 3140" from the main peak. As d, is in- The sizes and shapes of these obstacles creased the main peak increaseswhile the two side peaks decrease; consequently the beam width became narrower. With further increase of d the W passes through a distinct minimum beyond which it becomes broader again.

Instead of a single pin 20, two pins 21 and 22 may be placed symmetrically across the mouth 18 in the E-plane. Pins 21 and 22 are separated by a distance d which may be varied for obtaining the desired beam pattern. With d small, the beam is very broad and as d is increased W passes through successive minima and maxima. It has been determined that the best pattern is obtained when.d \=0.884 where A is the wavelength of the radiant energy, and the flare angle of 6 is At this position, the W was 52 with side lobes 20 db down. Phase measurements of a horn having two obstacles or pins 21 and 22 are satisfactory, and within W the phase front is practically constant. Also the square of the standing wave ratio, (SWR) with pins 21 and 22 is less than. half of that without pins.

In order to narrow the beam still further three metallic pins 20, 21 and 22 may be used together and located as previously described. A satisfactory narrow beam pattern can thus be obtained although the side lobes are somewhat higher. Additional pins or obstacles would have the effect of narrowing the beam pattern further but most of the energy would be contained in the side lobes rather than the main beam. For this reason it is considered inadvisable to use more than three pins for this type of horn feed although this depends somewhat on the dimensions of the horn.

An. important characteristic is that of the. impedance match of the horn. For a horn with a single pin 20 as described above, the impedance match is somewhat unsatisfactory for use with a broad band. .However in a horn with two pins 21 and 22 symmetrically placed across its aperture, the impedance characteristics are satisfactory.

For ease of adjustment of pins 21 and 22, it is preferable tomount them on the weatherizing flange of the horn. Sucha flange is of conventional design comprising a flange element 23 mounted around the aperture 18 of the horn and a sheet of suitable dielectric material (not shown) is placed across the mouth 18 to enclose the horn. Dielectric material is secured in place on the flange by any suitable means such as by screws.

The impedance of a horn feed depends in part on the following factors.

(1) Shape of the horn itself: Usually it is not desired to use a matched horn to begin with, because of the additional effect of the obstacles.

(2) Sizes of the pins: Both the impedance and the primary pattern depend upon the size of the pin. For one type of horn, the size of pins 20, 21 and 22 for the best pattern is about in diameter or width.

(3) Positions of these pins: Within a certain region the positions of these pins are not very critical to the pattern, while they are sensitive to impedance matching. Thus, the positions of these pins at which both the pattern and the matching are satisfactory can always be chosen.

(4) Screws holding the flange, the weatherizing material and the horn together: It is noted that the positions of these screws are fairly critical'to the impedance match ing. The explanation is that when some screws short theflange to the horn at some particular positions, current of higher mode are excited inside these obstacles, and consequently change the matching. 1

(5 Thickness of the weatherizing material: The change of the thickness of the weatherizing material changes both the reflection due to the weatherizing material and the phase of the reflection due to the pins.

Without using any matching device the type of horn as described hereinabove can usually be matched to (SWR) less than 1.5 in free space. For a horn with threev pins, it can also be fairly well matched by changing the above variables. Thus it can be seen that a'horn of the type described can bemade extremely broad banded according to this invention.

Broadening of the beam in the H-plane may be is complished by tapering inwardly and toward .its end the broad sides of the wave guide 30 (Fig. 4), thus decreas ing its aperture, but this introduces a considerable mismatch in impedance. It has been found that when the corners of the wave guide 30 are cut off at an angle so that more diffraction is produced, the primary beam in the H-plane is broadened. a a

According to the present invention, it has been found possible to obtain a W as high as 320. This is ac-- complished by placing an obstacle such as a metallic pin 32 of the character hereinbefore described across the: center of the aperture 33 of the cut corner-wave guide 30. The beam width is a function of both 21 and (shown in Fig. 4). It has been found that-with a constant angle qt, the beam is broader for larger 41; and for constant a, the beam is broader for an increasing qt. A plot of the relationship between W of a cut corner wave guide 30, as shown in Fig. 4 and the angle of cut shows that W increases as is increased with a general maximum at 45:45". For a larger 45, the beam becomes more and more flat with a practically constant W For 60, the beam has two peaks at an approximate angle of :55 with a slight dip in the middle.

From studies of the secondary patterns inside a truncated paraboloidal reflector fed by the cut-corner wave guide as 30 there is shown a substantial advantage in using this type of feed over an open wave guide.

A modified application of this type of feed is shown in Fig. 5. A horn 35 similar to that described with reference to Figs. 1, 2 and 3 is shown with flaring in the E- plane. In an ordinary horn of this type, without tapering in the H-plane a low gain would be expected due to poor illumination in the H-plane. The beam shaping of this E-horn 35 is accomplished by attaching two substantially semi-circular or semi-polygonal plates or sheets 36 to the flared walls of the horn. The plates 36 are disposed substantially parallel to each other or with their free ends inclined slightly toward each other. An obstacle in the form of a rectangular or circular bar or rod 37 is disposed centrally across the mouth of the horn 35, in the H-plarie as shown, and is attached in any suitable manner to the plates 36 preferably near the periphery thereof as at 38. As an example, with an E-horn having a 45 flare angle and a circular metallic rod with a diameter of .25", the Wp/lO Obtained was 246 in the H-plane. It was found that over 150 on either side of the peak, the phase was constant and the center of the feed was 1.5 inward from the front of the horn. Although this type of horn has a higher side lobe in the secondary pattern of the H-plane, the gain is about 8% greater than that of a similar ordinary horn.

Referring now to Figs. 6 and 7, there is shown a means for narrowing the beam in the E-plane. Horn 40 is constructed and arranged in the manner described with reference to Figs. 1, 2 and 3. An obstacle in the form of a rectangular strip or sheet member 41 is so placed inside the flared portion 42 of horn 40 that its width w is parallcl to the electrical lines of force and at a distance x from the mouth 43 of the horn. 1When so placed,- current is induced on this plate. The primary pattern of horn40 may be studied as a function of x and w. It may be observed that for a constant 1:, the W decreases markedly as the width w of the strip is increased. Variation of dimension x for a constant width w causes the W to pass through a distinct minimum. However, the side lobes decrease constantly as x increases. Thus, by properly positioning strip 41 an improved pattern having a narrow beam in the E-plane may be obtained.

In order to get a high gain of a paraboloid antenna it is necessary to have a uniform illumination in the E-plane. It was found possible to shape the beam of a horn feed in this plane by'changing the width of a weatherizingflange as hereinbefore described in front of the horn. With a certain width, the currents excited in the flange contribute out ofphase components to the field at the center of the primary pattern, and in phase components in some; other directions. The radiation pattern thus obtained has double peaks at an angle from the center and a slight dip. at the center.

Such results may be obtained with a born 50 (Fig. 8) constructed as hereinbefore described withreference to, Figs. 1, 2 and 3. with two pins 51 and 52 placed symmetri cally across the mouth 53 in the E-plane. vA Weatherizing flange 54 having a 'width m is secured in any desirable manner around and extends beyond the dimensions 'of' mouth 53. Pins 51 and 52 narrow the beam in the H plane as described hereinbefore with reference to Figs. 1, 2 and 3 while the extended flange 54 is effective to flatten.

Impedance measurements show that horn 50 as described is relatively broadbanded.

Figs. 9 and 10 illustrate the adaptation of the means of;

shaping a beam, as described with reference to Figs. 1, 2 and 3, to a pillbox type feed element. Pillbox 60 cornprises parallel plate members 61 and 62 closing the ends of a parabolic cylindrical reflecting strip 63 and aiford-' ing a rectangular aperture or mouth 64. Pillbox 60 is fed by a wave guide 65 in a conventional manner.

The W of the open wave guide 65 in the H-plane is about Thus when an open wave guide is used to f e s lt c 60mi s a p tu w dth n 1 (r in the focal length of pillbox 60) the gain of the antenna embodying the pillbox is low. It is, of course, possible to shape the pillbox according to the best contour of illumination of the wave guide but this usually occupies more space.

According to this embodiment of the invention, the feed may be modified by placing an obstacle such as a metallic pin 66 of circular or rectangular cross-section along the pillbox axis at a distance d (Fig. 9) in front of wave guide 65. Pin 66 is disposed between and connects plates 61 and 62 with the longitudinal axis of pin 66 substantially perpendicular to plates 61 and 62. As d is increased, the W increases from 140 to 210 resulting in a beam which is very flat and has a satisfactory cut-off at the edge.

Another means of shaping the beam of a feed between the parallel plates of a pillbox type element is illustrated in Figs. 11 and 12. The pillbox 70 (Fig. 12) is constructed as described hereinbefore with reference to Fig. 10. Instead of being fed with a simple open wave guide the feed comprises a wave guide somewhat similar to that described with reference to Fig. 4. Thus, an open wave guide 71 has its corners cut off at an angle 6 and two sections of open wave guide 72 and 73 are connected such as by solder to the openings formed by the cut-oif corners to form a Y-shaped feed element. Thus the junction or point of meeting 74 of members 72 and 73 (Fig. 12) forms an obstacle in the path of energy fed to pillbox 70. By changing the geometrical dimensions 0,

1 and 1 it is possible to obtain a satisfactory W For example with 6:60" it is possible to obtain -a'W as broad as 2110 with three peaks, one at the center and two at :60" from the center and having a satisfactory (SWR'P in :free space and between the plates of the pillbox.

It will be evident that the above description discloses novel, easy and economical means of shaping the primary beam pattern of ,a feed element without substantially changing the shape or dimensions of the feed element itself. The invention comprises the broad concept of placing obstacles .in the path of the feed energy whereby the radiation pattern of the feed element such as a horn is modified by the superposition thereon of the radiation pattern produced by the obstacles.

The above description and the figures of the accompanying drawings are indicative of certain embodiments of the invention. However, it will be obvious to those skilled in the art that various other charges, modifications and improvements may be made without departing from the spirit of the invention.

What is claimed is:

1. An antenna having a reflector and a wave guide means for feeding electromagnetic waves of high frequency energy in a primary beam pattern toward said reflector, said reflector comprising a beam-shaping reflecting element having spaced parallel plate members between which energy having its electric vector oriented perpendicular to said spaced plate members is fed from said wave guide, a parabolic cylindrical reflecting strip enclosing one portion of said element whereby said energy is reflected from said element along a path between said parallel plates through an unenclosed portion of said element to produce said primary beam, and a metallic pin conductively connected to said parallel plates and located in the path of energy fed by said wave guide into said beam-reflecting element, said pin being spaced a predetermined distance from said wave guide to modify the shape of said primary beam.

2. A high frequency electromagnetic device for directing energy into free space comprising a first wave guide including two substantially parallel plates and a curved conductive surface joining said metallic plates, a second wave guide for feeding energy to said first wave guide with the electric field thereof perpendicular to one pair of opposite walls of said second wave guide, said second aszaoea wave guide being so positioned with respect to said first wave guide that the longitudinal axis of said second wave guide when extended is substantially parallel to said two parallel plates and is substantiallyperpendicular to the tangent to saidcurved conductive surface at the point of intersection of said axis and said tangent, and a metallic object iocated along the extended axis of said second wave guide for -re-r-adiating energy impinging thereon to said first wave guide, said object having a dimensionpan allel to the axis of said second wave guide that is small compared to the wave length of the directed energy, said object being mechanically and electrically connected between said one pair of Opposite walls of saidsecond wave guide.

3. An antenna comprising a parabolic shaped reflect ing element having ,perpendicularspaced parallel plate members between which a metallic bar is oriented, said bar being conductively and mechanically connected to at least one of said plate members, and radiating means for feeding electromagnetic waves of high frequency energy in a primary beam pattern toward said reflector, said radiating means comprising a hollow pipe wave guide having an end portion for directing said energy in a broad beam toward said reflector, said metallic bar located in the path of said directed energy between said wave guide and said reflector, said metallic bar being oriented with respect to the electric field of said energy to produce a radiation pattern which is effective to vary the said primary beam pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,273,447 0111 M. Feb. 17, 1942 2,283,935 King May 26, 1942 2,298,272 Barrow Oct. 13 1942 FOREIGN PATENTS 222,399 Switzerland July 15, 1942 894,803 France Mar. 20, 1944 OTHER REFERENCES Rectangular Hollow-Pipe Radiators, by Barrow and Greene; Proceedings of the I. R. B, vol. 26, No. 12,

December 1938.

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