US2986735A - Radiation suppressor - Google Patents

Description

May- 30, 1961 M. w. SCHELDORF 2,986,735

RADIATION SUPPRESSOR Filed May a, 1959 0 4o 8 0 I20 I60 200 240 250 57.0 560 nited States tent RADIATION SUPPRESSOR Marvel W. Scheldorf, Palos Heights, 11]., assignor to Andrew Corporation, Chicago, 11]., a corporation of Illinois Filed May 6, 1959, Ser. No. 811,502

4 Claims. (Cl. 343-828) proper phasing being accomplished by the feed transmission line. In one adaptation of this structure, employing a single feed-point, a single long conductor is fed at one end and is provided with radially folded portions between adjacent half-wave radiating segments to provide the desired phasing, this type being sometimes known as the Franklin antenna. A pair of such structures may be employed as a center-fed antenna. Such antennas, however, are poorly suited for comercial manufacture and widespread use.

It has heretofore been suggested that a collinear array may in effect he formed from a long conductor with the portions from which radiation is not desired having such radiation suppressed by shielding. For example, in British Patent 573,436, there is suggested a construction wherein the antenna is formed from a linear radiator of an integral number of half-wavelengths, with alternate half-wave segments covered by tubular conducting sleeves. As recognized by this patent, however, such a construction, without modification, is impractical because of the fact that the radiation currents induced in the sleeve suppressors will in turn cause radiation from the suppressors themselves, so that the suppressors will not accomplish their desired function of producing radiation only from the alternate exposed segments of the structure. Accordingly, there are proposed in that patent a number of supressor structures which are stated to be capable of suppressing the undesired radiation from the surronded conductor while themselves not being excited to produce radiation. In 11.8. Patent 2,852,774 of the present inventor, there is described a novel construction of a half-wave suppressor which produces more highly eflective radiation suppression in such usage. However half-wave suppressors, so constructed that radiation from the suppressor itself is inhibited either by the constructions of the suppressors shown in the mentioned British patent or in the mentioned US. patent of the present inventor, are relatively diflicult to manufacture as compared with a simple cylindrical sleeve.

It is the principal object of the present invention to provide an effective radiation suppressor which is capable of suppressing radiation from portions of a conductor without itself producing radiation, but which is capable of construction in such a simple manner as to make the cost of such suppressors, and of antennas employing such suppressors, relatively small as compared with the suppressors and suppressed radiation antennas heretofore known. 4

In the British patent mentioned, it' is suggested that for certain purposes assertedly connected with the'elimination of undesired lobes in the radiation pattern, it may be desirable to shorten the physical length of the suppressors below a half-wavelength (it will be understood that the term wavelength as herein used as a unit of physical length, refers, unless otherwise qualified, to free, space wavelength), and to load the annular spaces between the conductor and the sleeve (no matter how constructed) with suitable dielectric to maintain the elec trical length (phase change) constant. There are therein suggested for this purpose the use of dielectrics such as polythene and polystyrene.

The present invention stems from the discovery that if the suppressor sleeves, such as those described in the British patent, are loaded with dielectrics of substantially higher dielectric constant than those suggested in that patent, and the sleeves further shortened correspondingly, fully satisfactory elimination of radiation from the suppressors themselves may be accomplished without the necessity of employment on the sleeves of flanges or other complex constructions, such as those shown in the British patent or in the mentioned patent of the present inventor. If the physical length of the suppressor, which may be a simple and inexpensive cylindrical conducting tube; is shortened to from approximately 20 percent to' approximately 30 percent of a wavelength, the radiation from the suppressor sleeve is eliminated, thus producing the desired suppressed radiation efi'ect in which the power is radiated solely from the uncovered portions of the long conductor. In order to achieve this eflect, it is necessary to employ dielectrics having a dielectric constant of at least 3.0. It is found experimentally that best results are achieved, when employing such simple tubular sleeves, with sleeve lengths of from 22 /2 percent to 27 /2 percent of a wavelength, with the annular space between the central conductor and the sleeve wholly or partially filled with dielectric in an amount suflicient to reduce the velocity of propagation within each suppressor to a velocity bearing the same ratio to free-space velocity as the physical length of each suppressor bears to a half-wavelength. i

The achievement of the desired radiation suppression without the'necessity of adding any complexity of structure to the simple cylindrical sleeve is believed to be attributable to the fact that a sleeve of a length reduced to the vicinity of a quarter-wavelength will not radiate appreciably irrespective. of its'coupling to a radiating source. The optimum physical length (with loading se-,

lected in accordance with the principle mentioned above) is found experimentally by measurements of gain'to be very close to aquarter-wavelength. It is believed that the optimum performance in this region. is due to' two, opposing factors. As the physical length of the suppressors is decreased beyond the length suggested in the British patent for differentpurposes, the radiation from the simple sleeves diminishes. At the same time, however, there is some loss of gain due to the reduction of end-to-end spacing between the exposed radiating portions of the central conductor; it is known, in connection with conventional collinear arrays, that the optimum spacing between the adjacent ends of the collinear radiators is one-half-wavelength, the directivity or gain being gradually and slowly degraded as this end-to-end spacing is decreased. However, this degradation, it is believed,

is minor compared to the large increase ingain which is achieved by the diminution of the radiation fromlthe suppressor sleeves as their length is decreased in theregi where they still remain of a physical lengthinje s5, f

a quarter wave. As the length of the suppressors isde Patented May '30, "1961 creased beyond a quarter wave, however, there is little if any further decrease in radiation from the sleeve, which is already substantially eliminated at a quarter wave, so that large further reductions of length tend only to reduce the gain in the same manner as with. separately fed collinear arrays. The fall-off in gain so found may also be. due, in part to increase of direct coupling between the radiating segments.

The desired velocity of propagation within the suppressors may be obtained either by using a single body of solid dielectric extending throughout the length of each suppressor, with the dielectric selected to have the proper dielectric constant (4.0 in the case of quarter wave physical length) or may be formed from spaced discs of a dielectric having a dielectric constant which is higher, thus producing the desired average velocity. A convenient form of construction employs discs of ceramic having a dielectric constant in the neighborhood of 6, thus permitting, by the use of simply fabricated spacers of desired lengths, the construction of suppressors for any desired frequency of operation without the necessity of having dielectric bodies of a variety of lengths.

It will of course be seen that the simple suppressor of the invention may be used in a variety of antenna types. In the embodiment of the present invention illustrated in the drawing, the array is of the center-fed type with a balanced feed.

In the drawing:

Figure 1 is a longitudinal cross-section of a radiation suppressor constructed in accordance with the present invention;

Figure 2 is a schematic representation of an antenna to which the suppressor shown in Figure 1 has been applied; and

Figure 3 is an E-plane radiation pattern obtained from the antenna shown in Figure 2.

Referring to Figure 1 there is shown a radiation suppressor mounted on a conductor 11. Surrounding the conductor 11 is a coaxial cylinder 12 which has a physical length of approximately one-quarter wavelength. Interposed between the outer cylinder sleeve or member 12 and the inner conductor 11 are a plurality of groups of insulator discs at spaced intervals. Preferably such groups are arranged more or less in symmetry with respect to the middle of the cylinder 12. Thus, as shown in the drawing, these insulator discs may be in groups 13, 14, and 16. The groups 14 and 15 adjacent the center are separated by a sleeve 17 which surrounds the inner conductor 11. The groups 13 and 14 are likewise separated by another sleeve 18, and in a similar manner a sleeve 19 separates the groups 15 and 16. The end groups 13 and 16 are retained in position by suitable snap rings 21 and 22 surrounding the inner conductor 11 adjacent the ends of the outer cylindrical sleeve or member 12. The dielectric insulator members arranged in the groups 13, 14 and 15 are so selected and arranged as to reduce the velocity of high frequency current propagation within the cylinder 12 to substantially fifty percent of the velocity if air were the dielectric medium.

In Figure 2 there is shown a simple center-fed collinear antenna 23, each half of which is equal to one and onequarter wavelengths in physical dimensions. Two radiation suppressors 24 and 25 are located at half wavelength intervals from the center of the dipole and extend a distance which is a physical dimension substantially equal to one-quarter wavelength. Each of the conductive elements of the dipole 23 therefore extends a half wavelength beyond each of the suppressors 24 and 25.

In Figure 3 there is shown a typical E-plane radiation characteristic plotted from measurements made from an antenna of the type diagrammatically shown in Figure 2. Anyone skilled in the art will readily appreciate from comparison of this radiation pattern with patterns characteristic of ordinary dipole antennas that the array efiectively formed by use of the present suppressor provides 4 great advantage in gain, with a minimum of added cost and complexity.

In one embodiment of the illustrated structure, employing radiating conductors of 0.625 inch diameter and low-loss ceramic insulating discs of approximately 0.9 inch radial thickness appropriately spaced to produce an average dielectric constant of 4.0 with simple conducting sleeves of 7 inch length and exposed radiating portions of 14 inch length, there was obtained a directivity gain (nor mal to the antenna) of 4.4 db as compared with the pattern of a reference dipole.

Although the employment of the one-quarter wave suppressor with internal dielectric loading sufiicient to produce an internal velocity of propagation of approximately 50 percent of free-space velocity, as described above and illustrated in the drawing, produces the most desirable results, i.e., highest gain, the length and loading may be varied over a substantial range while still retaining the basic advantage of the invention in providing high gain in a simple and inexpensive manner, so long as the proper relation between suppressor length and loading (ratio of internal velocity to free-space velocity equal to ratio of suppressor length to one-half of free-space wavelength) is maintained. The practical upper limit of suppressor length is approximately 30 percent of free-space wavelength at the frequency employed with dielectric loading producing an internal velocity of 60 percent of free-space velocity; with such construction, a gain of well in excess of 3 db is readily achieved at a substantial saving of space, cost and complexity as compared with other manners of achieving the same gain and type of radiation pattern. However for substantially lower values of gain, corresponding to substantially greater suppressor lengths, the present invention is not economically applicable, since even the relatively low cost of the present suppressors is not warranted by the small gain thus achieved.

The lower limit of desirable suppressor length is approximately 20 percent of free-space wavelength, for the reasons previously set forth.

The application of the invention to types of antennas other than the center-fed array illustrated, suchv as endfed single vertical long conductors, rectangularly folded conductors in which radiation is to be suppressed from one pair of opposite sides, and many others, will be obvious. Likewise it will be obvious that the suppressor illustrated as a solid cylinder may be of any mesh or cage construction in which conductors are separated by a distance which is small compared to the wavelength at the frequency of operation.

Thus, while for the purpose of illustrating and describing the present invention a preferred embodiment has been shown in the drawing, it is to be understood that the principles employed are susceptible of other embodiments and that variations may be made without departing from the spirit and scope of the invention as set forth in the accompanying claims.

What is claimed is:

1. An antenna comprising an elongated radiator having a plurality of exposed longitudinal segments each onehalf wavelength long and a segment between said halfwave segments of a length of substantially onequarter wavelength, said latter segment being effectively surrounded by a conducting enclosure of a length of one-quarter wavelength, the enclosure consisting of a conducting tube of substantially uniform inner and outer diameter throughout its length and containing dielectric material of a dielectric constant of at least 4.0 in an amount to reduce the internal propagation velocity by one-half, the exposed segments being substantially free of such dielectric material.

2. An antenna comprising an elongated radiator having a plurality of exposed longitudinal segments each onehalf wavelength long and a segment between said halfwave segments of a length of from 20 percent to 30 percent of a wavelength, said latter segment being effectively surrounded by a conducting enclosure, the enclosure consisting of a conducting tube of substantially uniform inner and outer diameter throughout its length and containing dielectric material of a dielectric constant of at least 3.0 in an amount to reduce the internal propagation velocity by the rato of the length of the enclosure to a half-wave length, the exposed segments being substantially free of such dielectric material.

3. An antenna comprising an elongated radiating element having a longitudinal portion thereof elfectively surrounded on all sides by a coaxial conducting enclosure of a length of substantially one-quarter wavelength, the enclosure consisting of a conducting tube of substantially uniform inner and outer diameter throughout its length and containing dielectric material of a dielectric constant of at least 4.0 in an amount to reduce the internal propagation velocity by one-half.

4. An antenna comprising an elongated radiating element having a longitudinal portion thereof effectively surrounded on all sides by a coaxial conducting enclosure of a length of from percent to percent of a wavelength, the enclosure consisting of a conducting tube of substantially uniform inner and outer diameter throughout its length and containing dielectric material of a dielectric constant of at least 3.0 in an amount to reduce the internal propagation velocity by the ratio of the length of the enclosure to a half-wavelength.

References Cited in the file of this patent UNITED STATES PATENTS 2,311,535 Goldmann Feb. 16, 1943 FOREIGN PATENTS 573,436 Great Britain Nov. 21, 1945

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