US2849711A - Slotted cylinder antenna - Google Patents

Description

Aug. 26, 1958 Y e. B. M KlMMlE 2,349,711

SLOTTED CYLINDER ANTENNA Fil ed May 15, 1953 2 Sheets-Sheet 1 INVENTOR.

EEEIREE B. MHEKEMMIE ym I'M d TTORNE I 2,849,711 Patented Aug. 26, 1958 SLOTTED CYLINDER ANTENNA I George B. MacKimmie,Montreal, Quebec, Canada, as-

- Sig Ql to Radio Corporation of America, a corporation of Delaware This'invention relates to antennas and particularly to antennas especially suitable for broadcasting very-highfrequency'and ultra-high-frequency radio signals, such as television picture and sound information.

An object of the invention is to provide a simple radiating system for electromagnetic wave energy whichhas a broadfreq'uency band characteristic suitable for very-highfrequency and ultra-hig'h-frequency services.

Another object of the invention is to eliminate complicated transmission line distribution harnesses where a number of radiating elements are used.

A further object of the invention is to provide an antenna system which, by its size and configuration, eliminates theneed for a separate supporting tower, and which Within itself provides a means of conveying the energy from the transmitter to the antenna with substantially no loss of'power in the transmission line. g

These and other objects are achieved, in accordance with this invention, by an antenna system which includes an elongated conductive cylinder. The cylinder acts as the antenna tower, and as a waveguide transmission line, and has slots therein which are the radiating elements. Insidethe cylinder, a longitudinal fin extends approximately one-half the distance across the inside of the tower. When the antenna is cylindrical in cross-section, this longitudinal fin is on a radius of the cylinder. The position on the circumference of the cylinder chosen for the slots which act as the radiating elementsis such that substantially no energy is coupled from the waveguide across the slots (to be interchanged with free space by the slots) unless deliberate distortion of the internal field distribution within the cylinder is arranged. The, slots themselves arearranged end to end along an element of the cylinder parallel to the axis of the cylinder. For in-phase radiation from all slots, the slots are spaced center to center from each Iother a distance which is'a multiple of onehalf wavelength. in the'waveguide at the operating frequency. v 7

Each slot'is. excited'by a controlled distortion of the internal'field distribution within the cylinder near the center of the'flslot; In the .plane in which the slot is to be excited,' conductive elements are connected to the longi tudinal fin to shiftthe distribution of the electric field within the guide. One of theseconductive elements extends fro'rn the'center axis of the cylinder where it attaches to the free edge of the fin tov a point fonthe circumference ofthe cylinder. Another conductive element extendsin aplane at right angles. to the fin-toward the circumference of the. cylinder but is spaced therefrom.

The finned waveguide-which transfers the energy up the antenna tower ,to the slotsmay be excited by a probe fed by a coaxialtransmission line. Alternatively, a section of waveguidesuch as a rectangular waveguide may be used "to couple the energy from the radio frequency apparatus to the finriedwaveguide' part of the antenna system. I f

A more '1. detailed description follows in 1 conjunction withtheaccompanying'drawingwhercini Figure 1 is an elevation ofan antenna array in accordance with the invention;

Figure 2 is across-sectional view along the line 22 of Figure 1 showing the representative electric field dis-' tribution;

Figure 3 is across-sectional view along the line 3-3 of Figure 1 showing the representative electric field distribution;

Figure 4 is a cross-sectional view along the line 44 of Figure 1 showing the representative electric field dis tribution;

Figure 5 is a cross-sectional View. along the line 55 of Figure 1;

Figures 6A and 6B are a top sectional view and a view in perspective respectively of one arrangement for exciting energy in the tubular cylinder of Figure 1;

Figure 7' is a cross-sectional view of another method of excitingradio frequency energy in the tubular cylinder ofFigure 1; and

Figures 8, 9, 10 and 11 show other cross-sectional configurations of waveguide with which the principles of the invention may be practiced.-

Referring to Figure 1, there is shown an antenna system in accordance with the invention, having a cylindrical conductive or metallictower member 21 which serves as a waveguide means fortransferring radio frequency energy from the feed point to the-radiating elements. In the usual arrangement, the'antenna system will be erected vertically and the radio frequency energy will be coupled into the guide near the'bottom. Radiating elements in the form of vertical slots 23 (for producing horizontally polarized waves) are formed in the wall-of the waveguide near the top .of the tower 21.

A longitudinal fin or partial septum 25 extends the entire length of the conductive tower 21 and .is positioned on a radius of the cylinder 21. The longitudinal fin 25 is electrically and mechanically connected to the cylinder 21 and extends substantially one-half the distance across the inside of the tower; that is, its free edge lies approximately along the central longitudinal axis of the cylinder. The position of the finor partial septum 25 may be visualized best by an inspection of Figures 2, 3, 4, and 5.

Figure 2 is a cross-sectional view along the line 22 of Figure 1 and shows the configuration of the finned waveguide 21, 25. The electric field distribution within the waveguide is also indicated in Figure 2. This mode of propagation in a finned cylindrical waveguide is de-' noted the TEg The maximum concentration. of the electric field on the periphery of the cylinder 211's opposite the fin 25. No peripheral currents exist along this line and a longitudinal slot cut in the cylinder 21 opposite the fin 25 will-not couple to the TEM mode.

Referring now toFigures 3 and 4, there are shown cross-sectional views along the lines 3-3 and 4-'4of Figure 1 of the 'two layers of slots 23 spaced apart by an odd multiple of a waveguide half wavelength. Con sidering Figure 3 first, in the horizontal plane containing the center of the slot 23'two conductive elements connect ing to the fin 25 are usedto alter the distribution of the electric field inside the cylinder and thus cause the radio frequency energy propagated in-the finnedwaveguideto couple to the slot; The first of these conductive elements in a strip of conductive material 27' which extends from the free edge of the fin 25 to a point on the circumference of the cylinder 21. This strip of conductive mate rial ormetal 27 rotates the distribution of the electric field at the point of connection of the strip 27 by an amount which is determined byjthe angle/included between the strip ZfT-and the longitudinal fin 25.

The second of these conductive elements utilized to alter I the field distribution is a plate 29 which extends toward the wall of the tower 21 opposite to that to which the strip 3 27 is connected. This plate 29 may conveniently be arranged at right angles to the longitudinal fin 25, although of course other angular positions may be utilized as deter-- mined by the degree of coupling desired and the impedance matching considerations'of the entire antenna assembly. The dimensions of the strip 27 and plate 29 may be made identical for all slots in a particular antenna array, but their orientation to produce in-phase radiation from all slots will depend upon the spacing between adjacent slots. If adjacent slots 23 in the antenna array are spaced an even multiple of a half Wavelength in the finned waveguide, all of the strips 27 and plates 29 will be oriented in the same direction; for example, all will be identical to that shown in Figure 3. If the center to center spacing is an odd multiple of a half wavelength in the waveguide, the orientation of strips 27 and plates 29 will be reversed at adjacent slots so as to produce equal and in-phase radiation from all the slots in the array.

There may be some applications where a controlled amplitude and phase distribution (or either one) among the different slots is required. Such operation can readily be obtained by proper dimensioning of the individual stripplate elements 27, 29 behind each slot, and by the spacing between adjacent slots 23 and strip- plate elements 27, 29.

Referring to Figure 4 as well as Figure 3, Figure 4 shows an example of the reverse orientation to that shown in Figure 3 which is used to obtain in-phase radiation from slots spaced an odd multiple of a half Wavelength.

Referring to Figure 5, which is a cross-sectional view along the line of 5 of Figure 1, there is shown one 9 method of feeding radio frequency energy to the finned waveguide tower 21, 25. A coaxial line having an inner conductor 31 and an outer conductor 33 terminates in a probe 35 which may be positioned on the same diameter occupied by the partial septum or fin 25. The inner conductor 31 of the coaxial line is conductively connected to the probe 35, and the outer conductor 33 is conductively connected to the conductive cylinder 21. The shape and length of the probe 35 and its orientation around the periphery of the cylinder 21 are used to aid in matching the impedance of the coaxial line 31, 33 to the finned waveguide 21, 25.

In Figures 6A and 613 there is shown a top sectional view and a view in perspective respectively on one arrangement for exciting the TE mode in the finned waveguide 21, 25 using a rectangular waveguide 37. The TE mode is propagated in the rectangular waveguide 37 with the broad walls of the rectangular waveguide 37 oriented parallel to the longitudinal axis of the cylindrical waveguide 21. The energy is introduced through a rectangular opening in the finned waveguide 21, 25 near but to one side of the partial septum or fin 25.

Figure 7 is an alternative way of exciting the "PE mode in the finned waveguide 21, 25 by means of a rectangular waveguide 37'. In the arrangement of Figure 7, the rectangular waveguide 37 is joined to an opening in the finned cylindrical waveguide 21, 25, the center of the opening shown, but not necessarily being diametrally opposed to the point of connection aof the fin 25. In this arrangement, the broad walls of the rectangular waveguide 37 are in planes transverse to the longitudinal axis of the cylinder 21 and the narrow walls are parallel to that axis. Energy is introduced into the rectangular waveguide 37 in the TE mode, the same as in Figure 6, but the rectangular guide 37' is rotated through an angle of 90 In all three feed methods shown in Figures 5, 6, and 7,

diflierent orientations of the feeder transmission line around the periphery of the cylinder 21 may be utilized to aid in obtaining broadband matched transitions. Additionally another set of strip-plate elements may be included between the lowest slot 23 and the feedpoint of the guide 21 for the purpose of broadbanding the input admittance.

'4 A suitable diameter of a waveguide cylinder like that shown in Figures 1 through 7 is given by the equation where is the frequency in megacycles at the center of the desired band and D is the diameter in feet. This choice of size gives a ratio of operating frequency to cut-01f frequency of approximately 1.30 and a ratio of wavelength in air to the wavelength in the guide of about 0.63. These parameters allow a slot spacing of approximately 0.8 free space wavelengths for electrical half wave spacing of the slots in the guide. With the diameter of cylinder described, only the dominant mode, that is, the TE can propagate. The waveguide is below cut-off frequency for all other modes which might be spuriously generated.

The length of each slot 23 of Figures 1, 3 and 4 is chosen to be self resonant at the center of the operating frequency band. The slots themselves are preferably wide; good characteristics have been observed for slots having a ratio of length to width of approximately six to one and with an overall length of about 0.44 of a free space wavelength.

When coupled to the finned waveguide 21, 25 by means of a strip 27 and plate 29, the slot 23 accepts and radiates some of the energy passing by in the waveguide. The effect of each slot is to shunt an admittance across the waveguide at the center of the slot. This admittance exhibits properties typical of a series resonant circuit, and the magnitude of the admittance is readily controlled by altering the mechanical configuration of the strip 27 and plate 29. The slot admittance is reduced to zero (as long as the slot is diametraily positioned with respect to the point of connection of the fin 25) if there is no strip or plate to alter the field in the vicinity of the slot. The slot in such case does not aifect propagation in the waveguide, and no radiation takes place.

Besides controlling the coupling between the slot 23 and the waveguide 21, 25, the strip 27 and plate 29 each present a susceptance load to the waveguide energy. The strip 27 behaves as a shunt inductance, and the plate 29 as a shunt capacitance. The two together are adjusted so as to be approximately anti-resonant at the design frequency in the absence of a slot. Thus when a self resonant slot is placed at the level of the strip-plate elements, a pure conductance is shunted across the waveguide. With the arrangement described, a very broad band termination is obtained, since the susceptance slope of the admittance due to the slot is opposite to the susceptance slope of the strip-plate combination, and therefore a partial cancellation takes place.

The closed portion of the Waveguide 21, 25 above the top slot 23 and below the feed point (the probe in Figure 5 or the rectangular waveguide of Figure 6) is made an odd multiple of a quarter wavelength, measured from the center of the slot or the center of the feed aperture, so as to present an open circuit admittance. In the case of the waveguide coupling arrangement of Figure 7, the closed end of the waveguide is spaced an even number of quarter wavelengths from the center of the feed aperture.

Figure 8 shows another cross-sectional configuration of waveguide with which the principles of the invention may be practiced. The circular waveguide 21 is divided by a septum 39 extending entirely across a diameter of the cylindrical guide 21. Two partial septa 25' extend from the center of the full septum 39 to form a structure consisting of two finned semi-circular Waveguides with their flat sides adjacent. Slots 23 are cut in the conductive cylinder 21 opposite to the attachment of the partial septa 25' to the common Wall 39 of the two waveguides. These slots 23 are excited by conductive strips 27 and plates 29 in the manner explained above with reference to Figure 3 for the embodiment with a circular cross-section.

Figure 9 illustrates how the principles of the invention Figure 10 shows another modification of slotted waveguide which utilizes the slot feeding principles of the invention. A circular tubular guide 21 is divided by a total septum 39 into two waveguides of semi-circular cross-section. The slots cut in the periphery of the tubular guide 21 are located at points of maximum electric field potential and where the peripheral currents are normally zero. Strips 27 extend from near the center of the total septum 39 to a point on the periphery of the circular tube 21. Plate elements 29' extend from the septum 39 toward, but still spaced from, the wall'21 on the opposite side of the slot 23 from which the conductive strips 27 are positioned. Strip and plate elements of Figure 10 act-to alter the field distribution inside the semi-circular guides in a manner like that explained above in connection with Figures 1 and 3 so that the slots 23 in the outer wall of the waveguide are coupled to the energy propagated therethrough.

Figure 11 shows a triangular waveguide 21' having a slot 23 cut in one corner thereof. A conductive strip 27 and a plate element 29 are positioned on the wall of the waveguide opposite the slot 23 to alter the'field distribution and operate in the same manner as the similarly numbered elements of Figure 10.

As an example of an antenna in accordance with the invention constructed for very-high-frequency television service, the following dimensions are representative: the cylinder 21 for operation on television channel 4 (66 to 72 megacycles) has anoutside diameter of 6 9 /2 and is constructed of A" thick mild steel plate, giving an inside diameter of 6' 9". The total length from the base of the antenna to the top of the tower is 75. The slots 23 are 6' 4" long by l 1" wide and are spaced ll apart, center to center. The center of the top slot is spaced 6" from the'closed upper end of the cylinder 21. The septum or fin 25 is 7 mild steel 40 /2 wide and continuously butt-welded to the inside of the cylindrical tower 21. The strip and plate are also of 7 mild steel and arelO" high, positioned at the center of each slot 23. The strip 27 makes an angle of 71 with the septum 25 while the plate 29 makes an angle of 90, extending a distance 1 3 toward the opposite side of the cylinder. The coaxial line 31, 33 is 1 /6 outside diameter 51 /2 ohm line and the probe 35 has an overall length of'32 from the inner wall of the cylinder 21 extending toward the septum 25.

The antenna thus constructed in accordance with the invention has certain important practical advantages. Since it is its own structural support it does not require a separate tower. If made very high, it may be guyed at any point below the lowest layer of slot radiators. Further, it is unlikely that electrical troubles could develop in the antenna portion after installation, since there are no friction contacts, no insulating materials, no dissimilar metals to create differential expansion and corrosion difficulties, and no de-icing equipment is necessary. The transmission line eificiency is higher than that which is obtainable by even the largest commercial coaxial or two-wire transmission line. The transmission line itself, as well as the slot radiators, have a power handling ability many times that which it is contemplated will be authorized by government agencies having jurisdiction over frequency and power allocations for broadcast serv- 1ces.

What is claimed is:

1. A tubular waveguide adapted to interchange energy between the interior thereof and surrounding space, said waveguide comprising a tubular conductive wall and a longitudinal conductive septum attached to the interior of said tubular wall, said tubular wall being provided with a longitudinal slot located in the portion of said wall farthest removed from the attachment of said septum to said wall, a conductive strip connected between a point on said wall and a point on said septum, said conductive strip being located near the center of said slot in the lengthwise dimension of said waveguide.

2. A tubular waveguide adapted to interchange energy between the interior thereof and surrounding space, said waveguide comprising a tubular conductive Wall and a I longitudinal conductive septum attached to the interior of said tubular wall, said tubular wall being provided with a longitudinal slot located in the portion of said wall farthest removed from the attachment of said septum to said wall, a conductive strip connected between a point on said wall and a point on said septum, a further conductive plate member attached to said septum and extending toward but having a free end spaced from said tubular wall in a portion on the other side of said slot from that to which said strip is connected, said conductive strip and said further conductive plate member being located near the center of said slot in the lengthwise dimension of said waveguide.

3. A tubular waveguide antenna comprising a tubular metallic wall and a longitudinal metallic septum extending the entire length of said tubular wall, said septum being attached at one of its edges to said tubular wall and having its other edge located near the center axis of said tubular wall waveguide, said tubular wall having a plurality of longitudinal slots diametrally disposed with respect to the point of attachment of said septum, a metal lic strip connected between a point on said metallic wall spaced from the point of attachment of said septum and said other edge of said septum near the center of the length of each of said plurality of slots, and a further conductive plate member extending from the connection between said septum and said strip toward but having a free end spaced from a portion of said tubular wall on the other side of said slot from that to which said strip is connected.

4. A tubular waveguide adapted to interchange energy between the interior thereof and surrounding space, said waveguide comprising a tubular metallic wall and a longitudinal metallic septum attached at one of its edges to said tubular wall and having its other edge located near the central axis of said waveguide, said tubular wall having a longitudinal slot therein diametrally disposed with respect to the point of attachment of said septum, a metallic strip connected between a point on said metallic wall and said other edge of said septum and making an angle with said septum in a plane transverse to said central axis of said guide at the center of said slot.

5. A tubular waveguide adapted to interchange energy between itself and surrounding space, said waveguide comprising a tubular metallic wall and a longitudinal metallic septum attached at one of its edges to said tubular wall and having its other edge located near the central axis of said waveguide, said tubular wall having a longitudinal slot therein diametrally disposed with respect to the point of attachment of said septum, a metallic strip connected between a point on said metallic wall and said other edge of said septum and making an angle with said septum in a plane transverse to said central axis of said guide at the center of said slot, and a further conductive plate member in said plane having a free end spaced from said wall but extending toward a portion of said wall on the other side of said slot from that to which said strip is connected.

6. A tubular waveguide antenna comprising a tubular metallic wall and a longitudinal metallic septum extending the entire length of said tubular wall, said septum being attached at one of its edges to said tubular wall and having its other edge located near the center axis of said tubular Wall waveguide, said tubular wall having a plurality of longitudinal slots oppositely disposed with respect to the point of attachment of said septum, a metallic strip connected between a point on said metallic wall spaced from the point of attachment of said septum and said other edge of said septum near the center of the length of each of said plurality of slots, a further conductive plate member at theconnection between said septum and each of said strips extending toward but having a free end spaced from said tubular wall on the other side of said slot from that to which each of said strips is connected, and means remote from said slots for coupling transverse electric waves between said waveguide and a further transmission line.

7. A tubular waveguide adapted to interchange energy between itself and surrounding space, said waveguide comprising a tubular metallic Wall and a longitudinal metallic septum attached at one of its edges to said tubular wall and having its other edge located near the central axis of said waveguide, said tubular wall having a plurality of longitudinal slots therein diametrally disposed with respect to the point of attachment of said septum, said slots being spaced apart along the length of said waveguide, a metallic strip connected between a point on said metallic wall and said other edge of said septum in a plane transverse to said central axis of said guide at the center of each of said slots, a further conductive plate member extending from the connection of each of said strips and said septum toward but having 'a free end spaced from. a portion of said tubular Wall on the other side of said slot from that to which each of said strips is connected, and a probe entering said waveguide remote from said slots for coupling transverse electric Wave energy, said probe being positioned on the opposite side of said waveguide from and spaced from said septum.

8. A tubular waveguide adapted to interchange energy between itself and surrounding space, said waveguide comprising a tubular conductive wall and a longitudinal conductive septum attached to said tubular wall, said tubular wall having a longitudinal slot in the portion of said wall farthest removed from the attachment of said septum to said wall, a conductive strip connected between a point on said wall, and a point on said septum, a further conductive plate member attached to said septum and extending toward but having a free end spaced from said tubular wall in a portion of said wall on the other side of said slot from that to which said strip is connected, said conductive strip and said further conductive plate member being located near the center of said slot in the lengthwise dimension of said waveguide, and

8 means remote from said slot for coupling transverse electric wave energy having a maximum electric field between said septum and the opposite wall portion into said waveguide.

9. A tubular waveguide adapted to interchange energy between the interior thereof and surrounding space, said waveguide comprising a tubular conductive wall and a longitudinal conductive plate attached to the interior of said tubular wall, said tubular wall being provided with a longitudinal slot located in said wall, a conductive strip connected between a point on said wall and a point on said plate, said points between which said conductive strip is connected being positioned in a plane that is substantially transverse to the longitudinal axis of said waveguide and that passes through said slot.

10. A tubular waveguideantenna comprising a tubular metallic wall, a rectangular metallic partition plate positioned within said tubular wall in a plane in which the central longitudinal axis of said tubular wall lies, said plate extending the entirelength of said wall and being attached at one of its longer edges to said tubular wall, the other longer edge of said partition plate being physically spaced from said tubular wall, said tubular wall having a plurality of longitudinal slots therein positioned with their centers substantially in said plane at points in said tubular wall remote from said one longer edge of said partition plate, a metallic strip connected between a point on said tubular wall and a point on said other edge of said partition plate in the region of each of said slots, said points between which each of said metallic strips is connected being positioned in a plane that is substantially transverse to said longitudinal axis and that passes substantially through the center of its respective slot, and a metallic extension member fastened to said partition plate adjacent each of said metallic strips, each of said extension members extending away from said points of attachment of said strips to said tubular wall, said extension plates being physically spaced from said tubular wall.

References Cited in the file of this patent UNITED STATES PATENTS 2,477,510 Chu July 26, 1949 2,573,461 Lindenblad Oct. 30, 1951 2,591,695 Hansen Apr. 8, 1952 2,635,188 Riblet Apr. 14,1953 2,660,674 Brown Nov. 24, 1953

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