US3195075A - Variable directional coupler - Google Patents

Description

July 13, 1965 R. G. VELTROP 3,195,075

VARIABLE DIRECTIONAL COUPLER Filed Aug. 20, 1962 2 Sheets-Sheet 1 INVENTOR.

ROBERT G. VELTROP TTORNEY R. G. VELTROP 3,195,075

VARIABLE DIRECTIONAL COUPLER July 13, 1965 Filed Aug. 20, 1962 2 Sheets-Sheet 2 Fl 5 4- INVE ROBERT G. ROP

ATTORNEY United States Patent Office 3,195,075 Patented July 13, 1965 3,195,075 VARIABLE DIRECTIONAL COUPLER Robert G. Veltrop, Sunnyvale, Califi, assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Aug. 20, 1962, Ser. No. 217,978 1 Claim. (CL 333-40) This invention relates to improvements in variable directional couplers. A general object of this invention is the provision of an electromagnetic directional coupler in which the spatial relationship between two conductors in two orthogonal planes is selectively variable to permit adjustment of the center frequency of the coupling charactertisic as Well as the coupling factor.

The invention will be more fully understood from the following detailed description of the operation, reference being made to the accompanying drawings in which:

FIGURE 1 is a plan view of a coupler embodying the invention;

FIGURE 2 is a transverse section taken on lines 22 of FIGURE 1;

FIGURE 3 is an enlarged fragmentary sectional view of a modified form of the invention in which the ground planes are relocated in order to increase the bandwidth of the coupler; and

FIGURE 4 is a section similar to FIGURE 3 with the position of the parts shifted to illustrate adjustment of the coupler.

The electromagnetic directional coupler illustrated in FIGURES 1 and 2 comprises a stationary base plate 1 and connected back plate 2 together with a movable frame F consisting of side plates 3 and 3, front plate 4, support plate 5, and upper plate 6. Plates 1, 3, 3', 4, 5 and 6 are electrical ground planes and constitute an outer conducting enclosure. The opposite ends of a primary conductor 12 extend through upper plate 6 and are connected, respectively, to an input line through terminal 10 and tea direct coupled line through terminal 11. The opposite ends of a secondary conductor 9 extend through base plate 1 and are connected, respectively, to a so-called isolated line through terminal 17 and to the coupled line through terminal 18. roximate portions of conductors 9 and 12 are straight and are parallel to each other.

Longitudinal positioning of the movable frame F, FIG- URE l, adjusts the amount of longitudinal overlap of conductors 9 and 12 which overlap constitutes the coupling section having an electrical length 6, and is achieved by rotation of longitudinal drive shaft 14. This drive shaft 14 is threaded through side plates 3 and 3' and support plate 5 and is journalled in supports 24 fixed to base plate 1. Vertical movement of upper plate 6, as viewed in FIGURE 2, adjusts the spacing 1 between primary and secondary conductors 12 and 9, and is accomplished by threaded shaft journalled in plate 1 and engaging threads in plate 6. Manual adjustment of shafts 14 and 15 is provided by knobs 13 and 16, respectively.

The following explanation of the principle of the operation of a directional coupler will be helpful to an understanding of the invention. Upon connection of an external source of radio frequency energy to input terminal 10, a primary current i flows in primary conductor 12. This causes a magnetic field to be set up around the conductor, the direction of the fiux lines being clockwise as viewed along the conductor in the direction of current flow. Depending on the proximity of the second conductor to the first conductor, a number of these flux lines link secondary conductor 9 and induce a secondary voltage v,, on the conductor. This is expressed by the relationship dl v,,, M dt (1) where M is the mutual inductive coupling between the conductors, and di /alt is the rate of change of primary current with time. The negative sign in Equation 1 indicates that the voltage induced on the secondary line opposes the change in the primary current. Thus, when a current i flows in conductor 12 from input terminal 19 to output terminal 11, a voltage v is induced in conductor 9 which causes a current 1' to flow in conductor 9 from terminal 18 to terminal 17 when the primary current is increasing in value and from terminal 17 to terminal 18 when the primary current is decreasing in value.

Similarly, currents i flow in secondary conductor 9 due to the voltage v impressed on conductor 12 by the primary current i and the capacitive coupling between the conductors. The currents i flow toward the common coupling section (into conductor 9 from terminal 17 and 18) when the primary potential is increasing in value and away from the common coupling section (into terminal 17 and 18 from conductor 9) when the primary potential is decreasing in value.

To summarize, the secondary currents i and i, are degrees out of phase in terminal 17 and are in phase in terminal 18 whether the primary current i is increasing or decreasing. Therefore, if the coupler is designed such that the inductive and capacitive coupling are equal, the currents flowing in terminal 17 cancel and no output is obtained on the isolated line whereas the currents in terminal 18 add to provide maximum output on the coupled line and yield maximum directivity.

The value of the coupling characteristic, known as the coupling factor, is controlled by changing the value of mutual capacitive and inductive coupling between conductors 9 and 12. This is accomplished without changing the center frequency of the coupler by varying the spacing 1 between conductors 9 and 12 by means of drive shaft 15 with its knob 16.

The voltages appearing respectively on the coupled line (terminal 18) and on the direct line (terminal 11) are:

1/1-70 cos 6+j sin 9 where k is the voltage coupling coefiicient, E is the coupled voltage, E is the direct or output voltage, E is the input voltage, and 6' is the electrical length of the common coupling section between conductors 9 and 12. The coupled signal is maximum for 6: (2n-1)1r/ 2 radians, n=1, 2, 3, or when sin 0:1. The coupling coeificient, determined by the physical relationship between the conductors and the ground plane, determines the bandwidth of the coupler.

With a known common coupling section of electrical length 6 which is M4 long at some frequency it is possible to determine the center frequency of the coupling characteristic and the directional coupler from the relationship where f is the center frequency, A is the wavelength, and

c is the velocity of light. Adjustment of the electrical length 0 of the common coupling therefore makes it possible to vary the center frequency of the coup-ling characteristic.

In the embodiment of FIGURES l and 2, the electrical length 0. of the common coupling section is varied by rotating knob 13 and shaft 14 to adjust the relative longitudinal displacement of conductors 9 and 12. For coupling values of less than db, the spacing of the conductors from each other and from ground planes is suflicient that vertical movement of the upper plate 6 does not affect the characteristic impedance of the conductors and the coupling characteristic remains substantially constant as the center frequency is varied.

In the modified form of the invention shown in FIG- URES 3 and 4, the ground planes of the coupler are relocated to provide broadband operation and to provide a constant coupling characteristic for tight couplings. Input and output lines 22 and 23 are connected to primary conductor 21. Ground plane 24 is located between lines 22 and 23. The line designated by reference character 28 is the edge of ground plane 24 adjacent to primary conductor 21. Ground planes 25 and 26 are located adjacent to lines 22 and 23, respectively. Lines 22 and 23' are connected to secondary conductor 21'. Ground plane 24' is located between lines 22' and 23. Ground planes 25' and 26' are located adjacent to lines 22' and 23', respectively. The junctions of ground planes 25, 25' and 26, 26' are designated by reference characters 27' and 27, respectively. The phantom lines 21a, 27a and 28a of FIGURE 4 represent another position to which conductor 2'1, ground plane junction 27 and edge 28 of ground plane 24, respectively, may be moved in order to change the coupling factor; i.e., to change the tightness of the coupling.

The relative positions of the input and output lines, the conductors and the ground planes determine the characteristic impedance and match of the coupler, and these parts are adjusted relative to the conductors and with respect to each other to obtain the proper coupling value. When the coupled conductors 21 and 21' are longitudinally offset (in a right-left direction as viewed in PI"- URE 4) to change the center frequency, the coupling factor and the characteristic impedance and match of the lines remain constant because the effective spacing (in the vertical direction, as viewed in FIGURE 4) of lines, conductors and ground planes are constant. The spacing d between conductor 21 and ground plane 25 is initially adjusted to provide the desired characteristic impedance and is effective for this purpose when the conductors 21 and 21 are longitudinally offset as shown in FIGURE 4. The same is true of the spacing between conductor 21 and ground plane 26'. f

When the conductors and ground planes are moved to the positions shown by the phantom .lines in FIGURE 4, junctions 27 and 2'7 are offset by a distance X. The spacing d between conductor 21 (coincident with phantom line 21a) and ground plane junction 27a( coincident with phantom lines 27a) is maintained constant at its initial value in order to maintain proper match and characteristhe center frequency of the coupling characteristic. Thus, the physical relationship between the ground plane and the conductors is maintained constant and adjustment of the coupling characteristic and the center frequency of the coupling characteristic are independent of each other.

As many modifications of this invention can be made without departing from its true spirit, the scope of this invention is to be determined from the appended claim and not from the specific embodiment described herein.

What is claimed is:

A variable directional coupler for coupling electromagnetic energy at microwavevfrequencies over a frequency band, said coupler comprising an electrically conducting base member,

a first conductor supported on and insulated from said base member, said first conductor having a straight portion 0 predetermined length spaced from and extending parallel to one side of said base member, an electrically conducting frame supported on said base member for parallel movement relative thereto and forming an enclosure therewith,

said frame comprising side plates projecting from said one side of the base member and an upper plate having an inner side spaced from said base plate, I a second conductor supported on andinsulated from said upper plate,

said second conductor having a straight portion of predetermined length spaced from and extending parallel to said one side of the upper plate and being parallel to and overlying said straight portion of said first conductor in a common plane whereby the straight portions of said first and second conductors are totally enclosed by said enclosure, means for moving said frame parallel to said base memher in a direction parallel to said straight portions of said conductors for changing the length that said straight portions overlie each other while maintaining said straight portions parallel and in the common plane whereby to shift the midband frequency of the handover which the energy is coupled, and means for moving said second conductor relative to said base plate and said first conductor in a' direction normal to the said parallel movement to equally change the spacing between the said first and said second conductors at all points along the said conductors whereby the amount of energy coupled between the conductors is changed.

3/24 Birch-Field 336l2l 1/65 Bock et al. 333-l0 JOHN F. BURNS, Primary Examiner.

LARAMIE E. ASKIN, Examiner.

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