Group-delay equalizer using a meander folded transmission line
United States Patent 3900806
A group-delay equalizer for microwave frequencies, comprising a meander folded transmission line, comprising means for separately changing the electric wave lengths of said folded transmission line associated with two different modes of propagation of the electromagnetic energy along said line.
Application Number:
05/457825
Publication Date:
08/19/1975
Filing Date:
04/04/1974
View Patent Images:
Export Citation:
Assignee:
Seleniz-Industrie Elettroniche Associate S.p.A.
Primary Class:
Other Classes:
333/238, 333/156
International Classes:
H01P9/00; H03H7/16; H01P9/00
Field of Search:
333/28R,31R,84M
Other References:
hewitt, A Computer Designed, 720 to 1 Microwave Compression Filter, IEEE, Trans. on MTT, December 1967, p. 687-694 TK7800I23..
Primary Examiner:
Gensler, Paul L.
Attorney, Agent or Firm:
Hill, Gross, Simpson, Van Santen, Steadman, Chiara & Simpson
Claims:
Having thus described the present invention, what is claimed is
1. A group-delay equalizer for microwave frequencies, comprising a meander folded transmission line, and adjusting means for changing separately the electric lengths of said folded transmission line associated to two different modes of propagation of the electromagnetic energy along said line, wherein said folded line is located between two ground planes, said adjusting means comprising a first block of substantially homogeneous dielectric material into which is immersed the aforesaid strip line partially folded lengthwise, a second block of dielectric material substantially similar to said first dielectric block of which it is possible to adjust the immersion of said line; means for changing the distance of one of said ground planes with respect to the outer surface of said first dielectric block, and means for changing the distance of the other of said ground planes to which is fixed said second dielectric block in order to change the immersion of said second dielectric block into said folded line.
2. A device as claimed in claim 1, characterized in that said blocks of dielectric material comprise different amounts of dielectric in the active zone.
1. A group-delay equalizer for microwave frequencies, comprising a meander folded transmission line, and adjusting means for changing separately the electric lengths of said folded transmission line associated to two different modes of propagation of the electromagnetic energy along said line, wherein said folded line is located between two ground planes, said adjusting means comprising a first block of substantially homogeneous dielectric material into which is immersed the aforesaid strip line partially folded lengthwise, a second block of dielectric material substantially similar to said first dielectric block of which it is possible to adjust the immersion of said line; means for changing the distance of one of said ground planes with respect to the outer surface of said first dielectric block, and means for changing the distance of the other of said ground planes to which is fixed said second dielectric block in order to change the immersion of said second dielectric block into said folded line.
2. A device as claimed in claim 1, characterized in that said blocks of dielectric material comprise different amounts of dielectric in the active zone.
Description:
The present invention relates to an improvement in the group-delay equalizers for micro-wave frequencies.
More particularly, the present invention relates to devices for equalizing the group-delay characteristics introduced by transmission media, such as wave guides or radio-frequency filters in circuits.
Depending upon the specific utilizations, there is a remarkable variation in the necessary operating parameters. In slopes or linear components:
1. Group-delay introduced by transmissive media in dispersive guide. Typical application: Transmission of messages with digital modulation, with desired bandwidths of (500-1000) MHz and group-delay variations up to the order of some tens of nanoseconds.
2. Group-delay, introduced by branching circuits for the oscillations which between the input port and the output port are reflected by circuits (generally passband filters) showing a reactive impedance variable with the frequency. Typical application: Branching circuits in the radio relays or repeaters with desired bandwidths in the order of 25 MHz and variations of the group-delay from about 0 to about 10 nanoseconds.
In Parabolic component:
Group-delay introduced by RF filters and RF amplifiers. The applications can be: wide band (>10%) or narrow band (<1%). In each case, the variation of group-delay is typically in the range of nanoseconds.
The typical field of utilization of the device according to this invention is therefore as follows:
A. as a component in transmission systems with millimeter waves for equalizing wide band group-delays;
B. as a component for equalizing group-delays in connection with micro-wave components in frequency bands usually not greater than (5-6) GHz.
In the prior art, problems of the related kind were solved at least partially with wave guides having progressively variable cross-sectional areas in connection with circulators or hybrids, with the possible addition of adjusting means consisting of a movable sheet of dielectric.
Other known systems provide for the use of circulators or hybrids in connection with reactive circuits. One known solution is that proposed by the U.S. Pat. No. 3,277,403 of S. B. Cohn, entitled "Microwave dual mode resonator apparatus for equalizing non linear phase angle of other components" of October 4, 1966.
A further solution of the problem has been proposed in the literature by Jones and Bolljahn (E. M. T. Jones, J. T. Bolljahn: "Coupled Strip Transmission Filters . . . ", I.R.E. Microwave, April 1956). In this publication, reference is made to an all pass structure with a line folded between two ground planes.
The present invention aims to solve the problem of equalizing the characteristics of group-delay in terms of the frequency with a device of the all pass structure with a folded line capable of operating with wide band (>10%), with reduced overall dimensions, and adjustable for permitting the necessary flexibility of use of the device and for compensating the manufacture tolerances which can become severe at the opening frequencies. The solution according to this invention allows also to obtain with reduced overall dimensions said equalization of group-delay with narrow band (<1%) at relatively low frequencies (<4GHz) where the use of dual mode circularly polarized cavity reactance elements would imply excessively large mechanical overall dimensions (see the above mentioned U.S. Pat. No. 3,227,403).
The present invention will be now described with reference to one embodiment thereof described by way of example, and with reference to the attached drawings, wherein:
FIG. 1 shows the base unit of a meander folded line for introducing the symbols used in the specification;
FIG. 2 diagrammatically shows the static capacitances correlated to two propagation modes (odd and even respectively) for lines coupled to each other and located between two ground planes;
FIGS. 3 and 4 are diagrams showing the trend of the group-delay versus the electric lengths of the transmission line;
FIG. 5 shows diagrammatically a sectional view of a folded line between two ground planes;
FIGS. 6 and 7 show further views of folded lines;
FIG. 8 shows a first sectional view taken along the plane A-B of FIG. 9 of an embodiment of the equalizer according to this invention of the uniform line type;
FIG. 9 shows a second sectional view according to the plane C-D of FIG. 8;
FIG. 10 shows a perspective view of the module of dielectric material used in the structure shown in FIGS. 8 and 9;
FIGS. 11a, b, c show examples of embodiment of the detail A shown in FIG. 10;
FIG. 12 shows a first sectional view taken along the line C-D of FIG. 13 of an embodiment of the equalizer according to this invention with a lumped loading line;
FIG. 13 shows a second sectional view taken along the plane A-B of FIG. 12;
FIGS. 14, 15 and 16 show respectively partial sectional views taken along the planes E-F, G-H and I-L as shown in FIG. 12.
With reference now to the drawings, and particularly to FIG. 1, a folded line can be considered as a quadripole of two-port structure of the all pass type with image impedance corresponding to Z o at all frequencies. It is capable of introducing a group-delay variable with the frequency since the phase of the oscillation between the input port and the output port is a non-linear function of the electric length of the line.
The symbols as follows will be now introduced, useful for the subsequent specification of this invention.
With reference to FIG. 1, let us assume:
f c the frequency for which is θ = λ/4
θ the electric length of the line;
Z oo , Z oe the characteristic impedances associated with the two modes of propagation along the line, "odd" and "even" respectively;
θ 00 , θ oe the electric lengths of the line associated to the two odd and even modes of propagation ##EQU1## characteristic parameter or coupling coefficient;
A = (1 + K) / (1 - K).
It can be demonstrated that when θ oo = θ oe = θ we shall have ##EQU2## where τ is the group delay. In FIG. 3 there has been shown the trend of the group delay τ versus θ expressed by the relation (1) for a given f c .
As the features of the structure of FIG. 1 arise from the "directional" properties of two coupled lines, this structure can be used twice to sum the contributions represented by the relation (1) relative to one element with a single folding. The checking of the values which appear in the relation (1) leads to the result that by a suitable selection of said values it will be possible to synthesize various characteristics of the group-delay versus the frequency.
The example of the above appears in the FIG. 3 wherein it can be noted that it is possible to change the slope of the characteristic of group-delay frequency introduced into the circuitry by changing the value taken by the θ at the operative frequency, in other words by changing the value of the characteristic frequency f c .
Also the non linear component or almost parabolic component can be adjusted by operating a part (for instance one-half) of the structure with determined values of the electric length θ. This possibility is shown in the diagram of FIG. 4.
However, the fact that we must have θ oo 32 θ oe , as it appears from the literature, and the fact that it is practically impossible to obtain a mechanical adjustment of the length of the line, the adjustment of the device becomes a problem (besides the actuation for fixed frequencies with acceptable tolerances), and therefore will limit the practical embodiments to nonadjustable structures.
Notwithstanding the above, according to the present invention, the adjustment is still possible if a method is discovered for embodying adjusting means capable of varying separately the two propagation velocities relative to the even and odd modes, such as by a mechanical adjustment of the dielectric structure connected to the folded line, and/or by the adjustment of lumped discontinuities embodied by metallic or dielectric elements, that is operating with non-uniform and non-homogeneous lines.
In the following disclosure, by way of clearness but not for limiting purposes, reference will be made to a non-homogeneous uniform line to a homogeneous and non-uniform line.
First case: uniform and non-homogeneous line
Let us consider the structure of FIG. 2 wherein have been diagrammatically shown two ground planes, electrically conductive and denoted by the numeral references 10,11 spaced apart from one another through a distance b . Between these ground planes 10,11 is located in a substantially centered location the folded line, consisting of a strip, shown in sectional view and referenced as 12,13,14,15 for an example of an indefinite repetitive structure.
In the conductor sections 13, 14, the static capacitances have been respectively shown relative to the even and odd modes of propagation along the line.
With reference to this Figure, it is therefore possible to observe that the partial or total presence of a dielectric in the zone 1, as shown by a shadow and/or in the symmetrical zone, affects primarily the characteristic impedance Z oo and the velocity v o relative to the odd mode, and the value of C oo = 4C'f o + 2 C p , will be altered by the effect on the value of C'f o .
On the contrary, the total or partial presence of dielectric in the region 2 near the ground planes (shown in FIG. 2 by the same criteria as the region 1) affects essentially the characteristic impedance Z oe and the velocity V e of the even mode, and the value of C oe = 4C'f e + 2C p will be altered. Also, the value of the capacitance C p will easily take account of this fact when the unit is designed or adjusted. In fact, however, in case of w/b<<1, corresponding to the cases of practical interest, and for values of coupling between the lines like those necessary in the practical cases, the contribution of C p will be negligible. Consequently, a variation of the amount of dielectric present in the above indicated regions (1 and 2) allows the values of the electric lengths of the lines associated to the two even and odd propagation modes to be separately changed.
In the most general case, a practical embodiment of means for changing the capacitive loading would render necessary the study of the electric field between the two lines in the partial presence of dielectric. For the cases of practical interest, it is possible to proceed to a strict real dimensioning of a structure partially filled with a dielectric, by using the methods for the determination of the capacitances of C'f e and C'f o as appeared in the literature.
In connection therewith, let us consider the FIG. 5. In this Figure the structure comprising the line folded between two ground planes is substantially entirely filled with a dielectric. This structure comprises the ground conductor planes A,B; the dielectric blocks D1,D2 and the conductor of the folded line denoted by L1,L2,L3 . . .
The two blocks of dielectric material D1, D2 can be spaced apart from one another along the plane a--a. A variation of the distance b2, being b = b 1 + b 2 , obtained by spacing away the conductor plane A and leaving fixed the position of the upper face of the dielectric block D2, obtained by introducing an air gap Δb 2 between the conductor plane A and the upper surface of the dielectric block D 2 , produces substantially a decrease of the capacitance value C'f e . Correspondingly, a variation of the distance b 1 , obtained by spacing away the dielectric block D 1 in the interface of a--a and introducing an air gap Δ a between the two dielectric blocks by spacing away the conductor plane B together with the dielectric block D 1 , produces substantially a decrease of the value of the capacitance C'f o .
Then, it is possible according to this invention to have two separate adjustments, practically embodied for C'f e and for C'f o permitting the embodiment of a folded line transmission structure wherein it will be possible to adjust the values θ oo and θ oe satisfying the condition θ oo = θ oe .
As a particular case, considering the dielectric blocks D 1 , D 2 as homogeneous, and with a relative dielectric constant ε r ≅ 2.3, we shall have with reference to the symbols shown in FIG. 5 and for the case where w/b<<1, and
0.6 < t/b < 0.8
0.15 < s/b < 0.25
Δ b 1 ≉ Δ b 2 = Δ b, we shall have: ##EQU3##
The above mentioned adjustments will influence also the coupling coefficient K and the image impedance Z o as follows: ##EQU4##
The contribution of (3) is substantially a parasitic effect of the adjustment which however can be corrected by the adjusting means. The effect of (4) is that of changing the impedance Z o of the structure, limited, however, to a practically allowable amount which can be compensated by acting in a way substantially analogous to that as previously described, or with lumped constants on the line tracts connecting to the load.
Assuming b = 10 mm, the percent variation of the characteristic frequency f c which can be expected according to (2) is: ##EQU5##
Hereinafter a possible practical embodiment of the device according to this invention will be described by way of example.
Second Case: non-uniform line either immersed or not in homogeneous dielectric
Still considering the diagrammatical representation of FIG. 2 let us now have reference to FIGS. 6 and 7 showing the diagrammatical representation of a line with a single folding useful for understanding the second embodiment of this invention concerning non-uniform lines, immersed or not immersed in a homogeneous dielectric.
The line comprises a U-shaped conductor 20 located between two conductor ground planes A,B. A susceptance b e located in the regions R 1 and R 2 allows the adjustment of the electric length θ oe relative to the even mode and a susceptance b o also located in the regions R 1 and R 2 allows the adjustment of the electric length θ oo relative to the odd mode. The practical embodiment can be obtained by placing the conductor 20 forming the line in a dielectric, either air or solid, and the susceptances b e , and b o can be formed by metal screws or dielectric screws.
The possibility of adjusting independently the two susceptances allows the condition of equal electric length to be obtained for the modes of propagation. If b e = b o = b, the distance θ between the regions R 1 , R 2 will be ##EQU6## to which corresponds an electrical length between the regions R 1 , R 2 equalling θ Rs .sbsb.2 = π
Therefore the percent variation of the electrical length which is to be obtained being η, we have ##EQU7## from which we obtain: ##EQU8##
Now two practical embodiments of the group-delay equalizer for microwave frequencies will be described.
EXAMPLE 1 -- Equalizer with uniform line
Reference is made to FIGS. 8 to 11. The device comprises a first plate 30 with at least the inwardly turned surface conductive, and a second plate 31 similar to the first cited plate. Between the plates 30, 31 a spacer 32 is interposed. To the plate 31 is fixed a module of dielectric material shaped as shown in FIG. 10 fixed to the plate 31 by the screws 36. A similar module 34 is fixed by screws 35 to the spacer 32.
The transmission line, meander folded, shown as a unit by 37, is inserted and fixed in the grooves 38 provided in the dielectric module 33. Two tracts of line 39, 40 serve for the electrical connection to the connecting terminals of the device to the balance of the device.
In FIGS. lla, 11b and 11c there is shown the relationship between strip transmission line 37 and the fingers of the dielectric module 33, in the folding zone. In FIG. 11a the strip 37 follows a segmented fold and a portion of dielectric 33 is removed in the corners as shown. In FIG. 11b in the fold zone the strip 37 follows a curvilinear path and the dielectric 33 is correspondingly shaped. In FIG. 11c, the strip 37 is bent as in FIG. 11a and the dielectric 33 has a corresponding shape but without empty spaces.
The adjustment of the characteristics of the equalizer is obtained by varying the distance of the lower surface of the plate 30 with respect to the upper surface of the dielectric module 34 and changing the immersion of the dielectric block or module 33 in the line by changing the distance of the plate 31 with respect to the spacer 32 (see FIG. 5 for reference to the previous disclosure).
The adjustment of the aforesaid distances can be obtained by the bushings 41, 41', 41" and 41'". The structure of these bushings and the associated mechanisms will be described with reference to the bushing 41 as both the other shown bushings and others corresponding by symmetry and not shown in the Figure, are identical. The bushing 41 in the shape of an overturned cup, bears against the outer surface of the plate 30 and is provided with an internal threading 42 which engages a threaded pin 43 forming the prolongation of a post 44 along which can slide, due to the holes 46, the plate 30. A contrast spring 46 tends to space away the plate 30 from the spacer 32. The lower module 33 can have reduced dimensions with respect to the dimensions of the module 34 and if a single adjustment of the constants will be sufficient, it can also be entirely dispensed with.
Of course, it is possible to provide for the connection in series and/or parallel of more equalizers according to this invention, as well as to modify the mechanical structure of the unit in order to obtain independent adjustments of the same tract of folded line, with a longitudinal division of the dielectric modules. Said alternatives are obviously capable of being designed by a person skilled in the art and will not be described in detail.
EXAMPLE 2 -- Equalizer with line with sole lumped adjustments
Reference is to be made to FIGS. 12 to 16. The device comprises a box type structure generally denoted by 47, which is conductive at least internally, including an upper plate 48, a lower plate 49 and a spacer 50. Inside the box type structure 47, the folded line 51 is supported by little blocks of low loss insulating material 52,53,54. The line tracts 55,56 serve for connecting the equalizer to the electrical terminals for the connection to the remainder of the circuit.
FIG. 12 shows in 57,58, adjusting susceptances in a possible embodiment thereof. In 59,60,61 said susceptances have been shown in a different possible embodiment.
As it will be better noted in FIG. 13 the susceptances 57 include a first screw element 62 which is threaded in the lower plate 49 and which presents toward the line a flat end, circular with a diameter greater than the maximum lateral dimension of the part of line on which it operates, at the center of which is screw threaded a screw 63 which can extend inside the space between the lines. The elements 62 and 63 act therefore also on the electric lengths of the folded line. FIGS. 14,15 16 show partially interrupted sectional views of FIG. 12 showing the geometrical relation between the adjusting elements 59,60 operating on the electrical lengths.
FIGS. 12 to 14 show the line of the equalizer device immersed in air. Said line could also be immersed into a homogeneous dielectric.
Furthermore, the lumped susceptances can also be practically made of conductor material or if dielectric material.
More particularly, the present invention relates to devices for equalizing the group-delay characteristics introduced by transmission media, such as wave guides or radio-frequency filters in circuits.
Depending upon the specific utilizations, there is a remarkable variation in the necessary operating parameters. In slopes or linear components:
1. Group-delay introduced by transmissive media in dispersive guide. Typical application: Transmission of messages with digital modulation, with desired bandwidths of (500-1000) MHz and group-delay variations up to the order of some tens of nanoseconds.
2. Group-delay, introduced by branching circuits for the oscillations which between the input port and the output port are reflected by circuits (generally passband filters) showing a reactive impedance variable with the frequency. Typical application: Branching circuits in the radio relays or repeaters with desired bandwidths in the order of 25 MHz and variations of the group-delay from about 0 to about 10 nanoseconds.
In Parabolic component:
Group-delay introduced by RF filters and RF amplifiers. The applications can be: wide band (>10%) or narrow band (<1%). In each case, the variation of group-delay is typically in the range of nanoseconds.
The typical field of utilization of the device according to this invention is therefore as follows:
A. as a component in transmission systems with millimeter waves for equalizing wide band group-delays;
B. as a component for equalizing group-delays in connection with micro-wave components in frequency bands usually not greater than (5-6) GHz.
In the prior art, problems of the related kind were solved at least partially with wave guides having progressively variable cross-sectional areas in connection with circulators or hybrids, with the possible addition of adjusting means consisting of a movable sheet of dielectric.
Other known systems provide for the use of circulators or hybrids in connection with reactive circuits. One known solution is that proposed by the U.S. Pat. No. 3,277,403 of S. B. Cohn, entitled "Microwave dual mode resonator apparatus for equalizing non linear phase angle of other components" of October 4, 1966.
A further solution of the problem has been proposed in the literature by Jones and Bolljahn (E. M. T. Jones, J. T. Bolljahn: "Coupled Strip Transmission Filters . . . ", I.R.E. Microwave, April 1956). In this publication, reference is made to an all pass structure with a line folded between two ground planes.
The present invention aims to solve the problem of equalizing the characteristics of group-delay in terms of the frequency with a device of the all pass structure with a folded line capable of operating with wide band (>10%), with reduced overall dimensions, and adjustable for permitting the necessary flexibility of use of the device and for compensating the manufacture tolerances which can become severe at the opening frequencies. The solution according to this invention allows also to obtain with reduced overall dimensions said equalization of group-delay with narrow band (<1%) at relatively low frequencies (<4GHz) where the use of dual mode circularly polarized cavity reactance elements would imply excessively large mechanical overall dimensions (see the above mentioned U.S. Pat. No. 3,227,403).
The present invention will be now described with reference to one embodiment thereof described by way of example, and with reference to the attached drawings, wherein:
FIG. 1 shows the base unit of a meander folded line for introducing the symbols used in the specification;
FIG. 2 diagrammatically shows the static capacitances correlated to two propagation modes (odd and even respectively) for lines coupled to each other and located between two ground planes;
FIGS. 3 and 4 are diagrams showing the trend of the group-delay versus the electric lengths of the transmission line;
FIG. 5 shows diagrammatically a sectional view of a folded line between two ground planes;
FIGS. 6 and 7 show further views of folded lines;
FIG. 8 shows a first sectional view taken along the plane A-B of FIG. 9 of an embodiment of the equalizer according to this invention of the uniform line type;
FIG. 9 shows a second sectional view according to the plane C-D of FIG. 8;
FIG. 10 shows a perspective view of the module of dielectric material used in the structure shown in FIGS. 8 and 9;
FIGS. 11a, b, c show examples of embodiment of the detail A shown in FIG. 10;
FIG. 12 shows a first sectional view taken along the line C-D of FIG. 13 of an embodiment of the equalizer according to this invention with a lumped loading line;
FIG. 13 shows a second sectional view taken along the plane A-B of FIG. 12;
FIGS. 14, 15 and 16 show respectively partial sectional views taken along the planes E-F, G-H and I-L as shown in FIG. 12.
With reference now to the drawings, and particularly to FIG. 1, a folded line can be considered as a quadripole of two-port structure of the all pass type with image impedance corresponding to Z o at all frequencies. It is capable of introducing a group-delay variable with the frequency since the phase of the oscillation between the input port and the output port is a non-linear function of the electric length of the line.
The symbols as follows will be now introduced, useful for the subsequent specification of this invention.
With reference to FIG. 1, let us assume:
f c the frequency for which is θ = λ/4
θ the electric length of the line;
Z oo , Z oe the characteristic impedances associated with the two modes of propagation along the line, "odd" and "even" respectively;
θ 00 , θ oe the electric lengths of the line associated to the two odd and even modes of propagation ##EQU1## characteristic parameter or coupling coefficient;
A = (1 + K) / (1 - K).
It can be demonstrated that when θ oo = θ oe = θ we shall have ##EQU2## where τ is the group delay. In FIG. 3 there has been shown the trend of the group delay τ versus θ expressed by the relation (1) for a given f c .
As the features of the structure of FIG. 1 arise from the "directional" properties of two coupled lines, this structure can be used twice to sum the contributions represented by the relation (1) relative to one element with a single folding. The checking of the values which appear in the relation (1) leads to the result that by a suitable selection of said values it will be possible to synthesize various characteristics of the group-delay versus the frequency.
The example of the above appears in the FIG. 3 wherein it can be noted that it is possible to change the slope of the characteristic of group-delay frequency introduced into the circuitry by changing the value taken by the θ at the operative frequency, in other words by changing the value of the characteristic frequency f c .
Also the non linear component or almost parabolic component can be adjusted by operating a part (for instance one-half) of the structure with determined values of the electric length θ. This possibility is shown in the diagram of FIG. 4.
However, the fact that we must have θ oo 32 θ oe , as it appears from the literature, and the fact that it is practically impossible to obtain a mechanical adjustment of the length of the line, the adjustment of the device becomes a problem (besides the actuation for fixed frequencies with acceptable tolerances), and therefore will limit the practical embodiments to nonadjustable structures.
Notwithstanding the above, according to the present invention, the adjustment is still possible if a method is discovered for embodying adjusting means capable of varying separately the two propagation velocities relative to the even and odd modes, such as by a mechanical adjustment of the dielectric structure connected to the folded line, and/or by the adjustment of lumped discontinuities embodied by metallic or dielectric elements, that is operating with non-uniform and non-homogeneous lines.
In the following disclosure, by way of clearness but not for limiting purposes, reference will be made to a non-homogeneous uniform line to a homogeneous and non-uniform line.
First case: uniform and non-homogeneous line
Let us consider the structure of FIG. 2 wherein have been diagrammatically shown two ground planes, electrically conductive and denoted by the numeral references 10,11 spaced apart from one another through a distance b . Between these ground planes 10,11 is located in a substantially centered location the folded line, consisting of a strip, shown in sectional view and referenced as 12,13,14,15 for an example of an indefinite repetitive structure.
In the conductor sections 13, 14, the static capacitances have been respectively shown relative to the even and odd modes of propagation along the line.
With reference to this Figure, it is therefore possible to observe that the partial or total presence of a dielectric in the zone 1, as shown by a shadow and/or in the symmetrical zone, affects primarily the characteristic impedance Z oo and the velocity v o relative to the odd mode, and the value of C oo = 4C'f o + 2 C p , will be altered by the effect on the value of C'f o .
On the contrary, the total or partial presence of dielectric in the region 2 near the ground planes (shown in FIG. 2 by the same criteria as the region 1) affects essentially the characteristic impedance Z oe and the velocity V e of the even mode, and the value of C oe = 4C'f e + 2C p will be altered. Also, the value of the capacitance C p will easily take account of this fact when the unit is designed or adjusted. In fact, however, in case of w/b<<1, corresponding to the cases of practical interest, and for values of coupling between the lines like those necessary in the practical cases, the contribution of C p will be negligible. Consequently, a variation of the amount of dielectric present in the above indicated regions (1 and 2) allows the values of the electric lengths of the lines associated to the two even and odd propagation modes to be separately changed.
In the most general case, a practical embodiment of means for changing the capacitive loading would render necessary the study of the electric field between the two lines in the partial presence of dielectric. For the cases of practical interest, it is possible to proceed to a strict real dimensioning of a structure partially filled with a dielectric, by using the methods for the determination of the capacitances of C'f e and C'f o as appeared in the literature.
In connection therewith, let us consider the FIG. 5. In this Figure the structure comprising the line folded between two ground planes is substantially entirely filled with a dielectric. This structure comprises the ground conductor planes A,B; the dielectric blocks D1,D2 and the conductor of the folded line denoted by L1,L2,L3 . . .
The two blocks of dielectric material D1, D2 can be spaced apart from one another along the plane a--a. A variation of the distance b2, being b = b 1 + b 2 , obtained by spacing away the conductor plane A and leaving fixed the position of the upper face of the dielectric block D2, obtained by introducing an air gap Δb 2 between the conductor plane A and the upper surface of the dielectric block D 2 , produces substantially a decrease of the capacitance value C'f e . Correspondingly, a variation of the distance b 1 , obtained by spacing away the dielectric block D 1 in the interface of a--a and introducing an air gap Δ a between the two dielectric blocks by spacing away the conductor plane B together with the dielectric block D 1 , produces substantially a decrease of the value of the capacitance C'f o .
Then, it is possible according to this invention to have two separate adjustments, practically embodied for C'f e and for C'f o permitting the embodiment of a folded line transmission structure wherein it will be possible to adjust the values θ oo and θ oe satisfying the condition θ oo = θ oe .
As a particular case, considering the dielectric blocks D 1 , D 2 as homogeneous, and with a relative dielectric constant ε r ≅ 2.3, we shall have with reference to the symbols shown in FIG. 5 and for the case where w/b<<1, and
0.6 < t/b < 0.8
0.15 < s/b < 0.25
Δ b 1 ≉ Δ b 2 = Δ b, we shall have: ##EQU3##
The above mentioned adjustments will influence also the coupling coefficient K and the image impedance Z o as follows: ##EQU4##
The contribution of (3) is substantially a parasitic effect of the adjustment which however can be corrected by the adjusting means. The effect of (4) is that of changing the impedance Z o of the structure, limited, however, to a practically allowable amount which can be compensated by acting in a way substantially analogous to that as previously described, or with lumped constants on the line tracts connecting to the load.
Assuming b = 10 mm, the percent variation of the characteristic frequency f c which can be expected according to (2) is: ##EQU5##
Hereinafter a possible practical embodiment of the device according to this invention will be described by way of example.
Second Case: non-uniform line either immersed or not in homogeneous dielectric
Still considering the diagrammatical representation of FIG. 2 let us now have reference to FIGS. 6 and 7 showing the diagrammatical representation of a line with a single folding useful for understanding the second embodiment of this invention concerning non-uniform lines, immersed or not immersed in a homogeneous dielectric.
The line comprises a U-shaped conductor 20 located between two conductor ground planes A,B. A susceptance b e located in the regions R 1 and R 2 allows the adjustment of the electric length θ oe relative to the even mode and a susceptance b o also located in the regions R 1 and R 2 allows the adjustment of the electric length θ oo relative to the odd mode. The practical embodiment can be obtained by placing the conductor 20 forming the line in a dielectric, either air or solid, and the susceptances b e , and b o can be formed by metal screws or dielectric screws.
The possibility of adjusting independently the two susceptances allows the condition of equal electric length to be obtained for the modes of propagation. If b e = b o = b, the distance θ between the regions R 1 , R 2 will be ##EQU6## to which corresponds an electrical length between the regions R 1 , R 2 equalling θ R
Therefore the percent variation of the electrical length which is to be obtained being η, we have ##EQU7## from which we obtain: ##EQU8##
Now two practical embodiments of the group-delay equalizer for microwave frequencies will be described.
EXAMPLE 1 -- Equalizer with uniform line
Reference is made to FIGS. 8 to 11. The device comprises a first plate 30 with at least the inwardly turned surface conductive, and a second plate 31 similar to the first cited plate. Between the plates 30, 31 a spacer 32 is interposed. To the plate 31 is fixed a module of dielectric material shaped as shown in FIG. 10 fixed to the plate 31 by the screws 36. A similar module 34 is fixed by screws 35 to the spacer 32.
The transmission line, meander folded, shown as a unit by 37, is inserted and fixed in the grooves 38 provided in the dielectric module 33. Two tracts of line 39, 40 serve for the electrical connection to the connecting terminals of the device to the balance of the device.
In FIGS. lla, 11b and 11c there is shown the relationship between strip transmission line 37 and the fingers of the dielectric module 33, in the folding zone. In FIG. 11a the strip 37 follows a segmented fold and a portion of dielectric 33 is removed in the corners as shown. In FIG. 11b in the fold zone the strip 37 follows a curvilinear path and the dielectric 33 is correspondingly shaped. In FIG. 11c, the strip 37 is bent as in FIG. 11a and the dielectric 33 has a corresponding shape but without empty spaces.
The adjustment of the characteristics of the equalizer is obtained by varying the distance of the lower surface of the plate 30 with respect to the upper surface of the dielectric module 34 and changing the immersion of the dielectric block or module 33 in the line by changing the distance of the plate 31 with respect to the spacer 32 (see FIG. 5 for reference to the previous disclosure).
The adjustment of the aforesaid distances can be obtained by the bushings 41, 41', 41" and 41'". The structure of these bushings and the associated mechanisms will be described with reference to the bushing 41 as both the other shown bushings and others corresponding by symmetry and not shown in the Figure, are identical. The bushing 41 in the shape of an overturned cup, bears against the outer surface of the plate 30 and is provided with an internal threading 42 which engages a threaded pin 43 forming the prolongation of a post 44 along which can slide, due to the holes 46, the plate 30. A contrast spring 46 tends to space away the plate 30 from the spacer 32. The lower module 33 can have reduced dimensions with respect to the dimensions of the module 34 and if a single adjustment of the constants will be sufficient, it can also be entirely dispensed with.
Of course, it is possible to provide for the connection in series and/or parallel of more equalizers according to this invention, as well as to modify the mechanical structure of the unit in order to obtain independent adjustments of the same tract of folded line, with a longitudinal division of the dielectric modules. Said alternatives are obviously capable of being designed by a person skilled in the art and will not be described in detail.
EXAMPLE 2 -- Equalizer with line with sole lumped adjustments
Reference is to be made to FIGS. 12 to 16. The device comprises a box type structure generally denoted by 47, which is conductive at least internally, including an upper plate 48, a lower plate 49 and a spacer 50. Inside the box type structure 47, the folded line 51 is supported by little blocks of low loss insulating material 52,53,54. The line tracts 55,56 serve for connecting the equalizer to the electrical terminals for the connection to the remainder of the circuit.
FIG. 12 shows in 57,58, adjusting susceptances in a possible embodiment thereof. In 59,60,61 said susceptances have been shown in a different possible embodiment.
As it will be better noted in FIG. 13 the susceptances 57 include a first screw element 62 which is threaded in the lower plate 49 and which presents toward the line a flat end, circular with a diameter greater than the maximum lateral dimension of the part of line on which it operates, at the center of which is screw threaded a screw 63 which can extend inside the space between the lines. The elements 62 and 63 act therefore also on the electric lengths of the folded line. FIGS. 14,15 16 show partially interrupted sectional views of FIG. 12 showing the geometrical relation between the adjusting elements 59,60 operating on the electrical lengths.
FIGS. 12 to 14 show the line of the equalizer device immersed in air. Said line could also be immersed into a homogeneous dielectric.
Furthermore, the lumped susceptances can also be practically made of conductor material or if dielectric material.