US 3581311 A
Description (OCR text may contain errors)
United States Patent lnventor Appl. No. Filed Patented Assignee Priority Alfred Kach Untersiggenthal, Switzerland Nov. 29, 1968 May 25, 1971 Patelhold Pa Holding AG tentverwertungs- & Elektro- Glarus, Switzerland Dec. 1, 1967 Switzerland 16921 LINEARLY POLARIZED MICROWAVE FEED ASSEMBLY FOR PARABOLIC ANTENNAS AND  Field of Search 343/770, 771,781, 818, 820, 822, 791, 840, 786
[5 6] References Cited UNITED STATES PATENTS 2,954,556 9/1960 Yang 343/770X Primary Examiner-Herman Karl Saalbach Assistant ExaminerPaul L. Gensler Att0rneyGreene & Durr ABSTRACT: A first coaxial line, short-circuited at one end and propagating a TEM wave, is surrounded by a concentric sleeve of a length approximately equal to half the operating wave length, to provide a second line with the outer conductor of the first line acting as inner conductor of the second line. The latter has one short-circuited end and an annular radiating aperture at its opposite end. The outer conductor of the first line has a pair of diametrically opposed coupling slots with the intervening segments connected, respectively, inductively and capacitatively with the inner conductor of said first line.
LINEARLY POLARIZED MICROWAVE FEED ASSEMBLY FOR PARABOLIC ANTENNAS AND THE LIKE The present invention relates to a microwave feed assembly for parabolic antennas and the like designed for energization by a coaxial feeder line and radiation of linearly polarized electromagnetic waves.
In the microwave art, dipole antennae are chiefly used as primary radiators for the illumination of parabolic reflectors, among other things on account of the low gain factor and slight directional effect of the dipole radiator.
In a well-known dipole antenna of this type energized by a coaxial line or feeder having two diametrically opposed longitudinal slots in its outer conductor, a dipole stub is fitted to each of the segments thus formed of the outer conductor. In order to couple one of the dipole stubs to the inner conductor of the coaxial line, the conductor is connected to the respective segment through a short-circuiting stub at right angles to and disposed at a point of the conductor coincident with .the bisecting plane of the slots. A splash or reflector disc arranged on the side of the slots remote from the input side of the line insures that the parabolic reflector is optimally illuminated by the dipole radiator. In the case of such an arrangement, a symmetrical coupling for the dipole stubs is provided by the effect of the half-wave (M2) resonance of the two conductor segments.
Such an antenna with pronounced dipole stubs is especially suitable for frequencies in the region of 1-4 gc. (gigacycles), corresponding to a wave length range 30-7 cm. At higher frequencies, or for still shorter wave lengths, for example 6-8 gc. or about -3 cm., respectively, the dipole stubs become so small that, regard being had to the dimension of the feeder line, it is practically no longer possible to comply with the existing geometrical and space requirements for the antenna to function satisfactorily. On the other hand, the use of dipole stubs has heretofore been one of the most simple means for the illumination of a parabolic reflector with linearly polarized electromagnetic radiation.
An important object of the present invention is, therefore, the provision of an improved dipole antenna of the referred to type which is especially, though not limitatively, suitable as a primary radiator for the illumination of a parabolic reflector with linearly polarized waves by way of a coaxial feeder line, without requiring any pronounced dipole stubs, which anten na exhibits good radiation and matching properties over a relatively wide frequency range and in spite of its relatively large dimensions compared with the operating wave length; and which is both simple in design and suitable for structural embodiment with a parabolic reflector.
The invention, both as to the foregoing and ancillary objects as well as novel aspects thereof, will be better understood from the following detailed description of a preferred practical embodiment, taken in conjunction with the accompanying drawing forming part of this specification and in which;
FIG. 1 is a longitudinal sectional view of a microwave feed assembly for parabolic antennas for the radiation of linearly polarized waves and embodying the invention;
FIG. 2 is a sectional view taken on line 2-2 of FIG. I;
FIG. 3a and 311 show substitute electrical circuits explanatory of the function and operation of the invention; and
FIGS. 4a4c further illustrate the formation of a linearly polarized radiation field in accordance with the principles of the invention.
Like reference characters denote like parts in the different views of the drawing.
With the foregoing objects in view, the dipole antenna according to the invention is characterized generally by the connection of the inner conductor of a slotted coaxial feeder line to one of the segments of the outer conductor of said line by way of an inductive link or coupling impedance, on the one hand, and by the connection of the inner conductor of the line to the other segment by way of capacitive link or coupling impedance, respectively, the effective impedance values of these couplings being substantially equal and opposite at a mean operating frequency or wave length (A0) in the coupling plane which bisects the slots in the outer conductor of the line. Additionally, the invention involves the provision of a further hollow outer conductor or concentric sleeve surrounding the coaxial feeder line at least over the region of its longitudinal slots, whereby to provide a second line having an inner conductor formed by the outer conductor of the first line.
Due to the coupling of the second line by way of the inductive and capacitive links between the inner conductor of the first line with the segments of its outer conductor (forming the inner conductor of the second line), a TB wave is excited in the second line by a TEM wave propagated in the first line energized by a suitable source of microwave energy. This TE wave exhibiting a substantial cross-field as a result of the geometry of the concentric conductors is converted into a substantially linearly polarized field suitable for radiation by av TEM wave being directly coupled into the second line through the slots in the outer conductor of the first line, the width of the slots being such as to provide a TEM field in the second line substantially cancelling, at or near the radiation aperture of the line, the cross-field component of the TE wave being radiated, in a manner as will become further apparent from the description of the construction and function of the invention in reference to the drawing.
Referring to the latter, FIGS. 1 and 2 show a dipole antenna and parabolic reflector in longitudinal and transverse section, respectively. In the end portion 1 of a coaxial feeder line 2, energized by a suitable microwave generator (not shown), there are provided two diametrically opposite longitudinal slots 3 and 4 having a length equal to half the mean operating wave length (ho/2) and subdividing the outer conductor 5 of the line into two opposite segments 6 and 7, respectively. The inner conductor 8 of the line is linked or coupled via a shorting stub 9 to the segment 6 at a point coincident with the cross-sectional plane x-x which bisects the slots 3 and 4 and which will hereafter be referred to as the coupling plane of the line. A short-circuiting disc 10, terminating the end portion 1 of the line 2 at a distance of five-eighthsseven-eighths of the slot length (five-sixteenthsseven-sixteenths of k0) from the coupling plane X-x, constitutes a composite structural part together with the inner conductor 8 and the shorting stub 9. Additional coupling elements in the form of stubs or screws 11 and 12 are fitted in the segments 6 and 7, respectively. The screw 11 moreover serves to fasten the stub 9 to the segment 6.
An outer cylindrical conductor or sleeve 13 is secured, via a thread 14, to the outer conductor 5 of the line 2 and forms, together with said conductor a further coaxial line or feeder of annular cross section which is terminated at one end, that is, adjacent to the outer ends I5 of the slots in the example illustrated, by the base 16 of the sleeve acting as a short-circuiting disc. The opposite end of the sleeve 13 is extended by an insulating tube 17 terminating in a conical end piece 17'. Line 2 may be fitted with a suitable impedance matching means located ahead of the input-feed ends 19 of the slots 3 and 4 and consisting, in the example shown of a step or shoulder 19' on the inside of the outer conductor 5 and Ito/4 line section between said shoulder and the ends 19 of slots 3 and 4.
The function and operation of the antenna described in the foregoing will be explained with reference to FIGS. 3a, 3b and la-4c.
FIG. 3a shows an equivalent electric circuit diagram of the feeder radiator according to FIGS. 1 and 2. It is assumed that the coaxial feeder line 2 up to and including the AD/4 line sections has an internal resistance R The segments 6 and 7 of the outer conductor may then be considered as a two-wire transmission line 20 short-circuited at a distance of half a wave length from the input-feed plane and having its individual conductors connected to the inner conductor of the line at the coupling plane x-x, or at a point one-quarter wave length from the input-feed plane, via a coupling inductor L and a coupling capacitor C, respectively. The inductance L and capacitance C are so chosen that their impedance values are substantially equal and opposite at the mean operating frequency of wave length (AOL-whereby to provide a symmetrical coupling of the feeder line 2 with the antenna load or radiation resistance R,.
According to a first modification of the invention, the coupling inductance L and coupling capacitance C take the form, respectively, of the shorting stub and a trimmer screw 12 protruding into the coaxial feeder and adjusted to provide equal and opposite inductive and capacitive coupling .reactances between the inner conductor 8 and segments 6 and 7 of the line 2. Such a solution, while suitable for certain uses and applications is less advantageous than the modification,
. utilizing a fixed coupling screw 11 in conjunction with a shortcircuited end section of the conductor having a length equal to five-sixteenthsseven-sixteenths A from the coupling plane Xx, in the manner as will become further apparent from the following description in reference to the modified circuit diagram of FIG. 3b.
Referring to the latter, the end portion 1 of the line 2, being terminated by the short-circuiting disc at a distance of fiveeights-seven-eigh ths of the slot length, or five-sixteenthsseven-sixteenths of the operating wave length, from the coupling plane x-x, is represented by an equivalent two-wire line 21 being short-circuited at its end in the manner shown. The stub 9 which connects the inner conductor 8 to the segment 6 is represented in the equivalent circuit by the inductor L. As may easily be seen, the end portion 1 of the feeder line 2, being terminated at the stated distance from the coupling plane x-x, provides a capacitive coupling for the segment 7 corresponding to the capacity C according to FIG. 30, on the one hand, while additionally acting to transform the inductance L of the stub 9 up to approximately twice of the inductance L according to FIG. 3a, on the other hand.
Thus with, the same stub 9, the arrangement as shown in FIG. 1 provides a doubling of the effective coupling inductance compared with the previously mentioned variant of a capacitive coupling by means of a trimmer stub or screw 12, thus providing a four times impedance transformation for matching the feeder line to the load resistance R of the antenna. As a consequence, the cross section of the stub 9 may be made large enough, even at very high operating frequencies, to impart good mechanical stability to the arrangement when using impedance-transformation ratios as required in practice.
The symmetrical coupling of the segments 6 and 7 results in the excitation of an electric TE wave in the annular space or outer line enclosed by the sleeve 13 and conductor 5, the field of said wave in the coupling plane x-x being illustrated in FIG. 40. Due to the circular cylindrical surfaces or geometry of the electrodes, this field has a relatively strong transverse or crosscomponent, greatly distorting thereby the ideal linearly polarized field. Besides the TE wave, a standing TEM wave, whereof the field in the coupling plane x-x is shown in FIG. 4b, is excited via and in the region of the slots 3 and 4, in the outer wave guide or line between conductors 5 and 13.
Inasmuch as the cross-components of the TE field and of the TEM field are in the same direction in the coupling plane x-x, superimposition of the fields at this point further increases the distortion of the linearly polarized wave set up in the outer line. However, since the cross-sectional dimensions of the outer line are so chosen that, at the operating frequencies involved, only the TE wave is excited in the outer line, while the field of the standing TEM wave penetrates into said line only in heavily attenuated form, optimal compensation of the interfering cross-components of the TE field may be achieved by rotation of the phase of the TE wave by 180. It is for this purpose that the length of the sleeve 13 is so chosen that the distance of its open end or radiating aperture from the coupling plane x-x is at least substantially equal to half the operating wave length of the TE, wave. Optimal compensation of the cross-components of the TE field may thus be achieved at or near the radiating aperture or open end of the outer line or wave guide. The strength of the field of the TEM wave can moreover be so adapted, such as by varying the width of the coupling slots 3 and 4, that the crossfield components of the TE wave are substantially completely compensated at the open end or radiating aperture of the antenna.
The resultant field of the TE wave with its distortion removed in this manner at the open end or radiating aperture of the outer line is shown in FIG. 4c, illustrating the attainment of substantially symmetrical linearly polarized waves radiated from the open end of the outer line and in turn by the antenna 22. The insulating tube 17 which extends the outer line from its open end serves to partially delay the radiated waves, whereby as the electromagnetic field emerges from the open end of the insulating tube, the electric field lines extend at least in part tangentially to the conical surface of the end piece 17. Experiments have shown that it is advantageous for the length of the cylindrical part of the insulating tube 17 to be approximately half the length of the radiated TE wave, with the tube extending over about half its length into the hollow conductor or sleeve 13.
When the antenna is used as a rear feed radiator of a parabolic reflector, as shown at 22, FIG. 1, the insulating tube 17 imparts increased divergence to the radiated waves, resulting thereby in a more uniform illumination of the parabolic reflector. The optimum cone angle depends on the irradiation angle which it is required to attain. Good results have been achieved with a cone angle of 30 for most practical purposes. The insulating tube 17 furthermore acts at least partly as a transformation means between the wave-resistance of free space and the radiation resistance (R of the line 5, l3, and also protects the antenna from the effects of atmospheric infl uence.
The screws 11 and 12 furthermore serve as coupling stubs for the purpose of matching the line 2 to the symmetrizing transformer formed by the means for the symmetrical coupling of the segments 6 and 7 to the inner conductor 8 of line 2.
In the foregoing, the invention has been described in reference to an exemplary illustrative device. It will be evident that variations and modifications, as well as the substitution of equivaLent parts or devices for those shown for illustration, may be made in accordance with the broader scope and preview of the invention.
l. A linearly polarized microwave feed assembly for parabolic antennas and the like comprising in combination:
I. a first coaxial feeder line having an inner conductor and an outer conductor,
2. said outer conductor being provided with a pair of opposed longitudinal slots located adjacent to the output end of said feeder line and having a length approximately equal to one-half the operating wave length, to provide a pair of intervening segments of said outer conductor between said slots,
3. means to short circuit the output end of said line,
4. a concentric metallic sleeve surrounding said feeder line in overlying relation to said slots, to provide a second coaxial feeder line with the outer conductor of said first feeder line forming the inner conductor of said second feeder line,
5. means to short circuit one end of said second feeder line, to provide a radiating aperture at the opposite end of said second feeder line, and
6. a pair of coupling means to inductively couple the inner conductor of said first feeder line with one of said segments and to capacitively couple said inner conductor of said first feeder line with the other of said segments of said outer conductor of said first feeder line,
7. said inductive and capacitive coupling means offering substantially equal and opposite impedances, to excite a symmetrical TE wave in said second feeder line by a TEM wave propagated through said first feeder line, and
8. the length of said sleeve from the bisecting plane of said slots to said aperture being about one half of the operating wave length and said slots having a width sufficient to apply an additional standing TEM wave to said second feeder line having a cross field component substantially cancelling the cross field component of said TE, wave at said aperture.
2. In a microwave feed assembly as claimed in claim 1, wherein said inductive coupling means consists of a short-circuiting stub coincident with said bisecting plane and connecting said inner conductor to one of said segments, and wherein said capacitive coupling means consists of an adjustable coupling stub mounted in the other of said segments coincident with said plane.
3. in a microwave feed assembly as claimed in claim 1, wherein said inductive coupling means consists of a short-circuiting stub disposed within said bisecting plane and connecting said inner conductor to one of said segments, and wherein said capacitive coupling means consists in the short-circuited end of said first line having a spacing distance from said plane equal to five-sixteenthsseven-sixtecnths of the operating wave length. i
4. In a microwave feed assembly as claimed in claim 1, including an insulating tube coaxial with and extending from the aperture ofsaid sleeve.
5. In a microwave feed assembly as claimed in claim 1, wherein the short-circuiting means of said second line is disposed on the side adjoining the short-circuiting means of said first line, and a parabolic reflector traversed by said first line and disposed in spaced relation to said aperture.
6. In a microwave feed assembly as claimed in claim 1, wherein the short-circuiting means of said second line is disposed on the side adjoining the short-circuiting means of said first line, an insulating tube coaxial with and extending from the aperture of said sleeve, said tube terminating in a conical-shaped and piece, and a parabolic reflector traversed by said first line and disposed in spaced relation to said end piece.