US2792550A - Directional coupler - Google Patents

Description

May 14, 1957 w. c. BACKSTRAND 2,792,550

' DIRECTIONAL COUPLER Filed May 3. 1946 MAIN LINE OUTPUT FIG. 2'A

D m WR TT S EK m B C N O T S m W ATTORNEY 2,792,559 DRECTIONAL CGUPLER Winston C. Backstrand, Ithaca, N. Application May 3, 1946, Serial No. 666,877 Claims. (Cl. 333-) This invention relates to electromagnetic wave coupling apparatus and more specifically to that type of apparatus known as a directional coupler.

A directional coupler is a device which, when inserted in a transmission line on which there exist waves travelling in both directions, delivers to a pair of terminals located in an auxiliary transmission line a voltage which is largely-a function of the amplitude of the wave going in one preferred direction, and relatively independent of the wave going in the opposite direction.

The part of the directional coupler which forms a section of the transmission line in which the coupler is inserted will be referred to as the main line. it possesses a set of terminals referred to as the main line input terminals, at which the wave travelling in the preferred direction enters. The other set of main line terminals will be called the main line output terminals.

The auxiliary line contains, in addition to the auxiliary line output terminals, a resistive termination. The latter serves to absorb the wrong way energy, and to provide a good match looking into the auxiliary line output terminals. The main and auxiliary lines are connected together by a coupling mechanism which gives the coupler its directional properties.

The coupling of the directional coupler specifies that fraction of the power proceeding in the preferred direction which is delivered to the auxiliary line output terminals. More precisely, the coupling, which may be expressed either as a voltage or power ratio, is defined as the ratio of the voltage (or power) delivered to a matched detector at the auxiliary line output terminals to the voltage (or power) delivered to the couplers main line input terminals, provided that the main line output terminals are terminated by a matched load.

The directivity of a directional coupler is defined as the ratio of the voltage (or power) delivered to a matched detector at the auxiliary line output terminals under the two conditions: (1) power is fed in the main 'line input terminals, with a matched load attached to the main output terminals; and (2) the same amount of power is delivered to the main line output terminals, with a matched load attached to the main line input terminals.

The present invention concerns a slotted block coaxial line directional coupler which consists of a solid metallic lock adapted to receive and form a juncture for the main and auxiliary coaxial line sections. Two quarter wavelength sections of coaxial line having parallel axes are formed in the solid block, and coupling between these lines is provided by two slots in the solid block which are errectively waveguides operating below cut off frequency where negligible energy transmission occurs. A small amount of the mainline energy is thus coupled into the auxiliary coaxial line, and by spacing the slots apart a quarter wavelength of energy in the coaxial line, a coriventional two-coupling multiple path directional coupler is formed.

It is commonly desired to increase the coupling between the main and auxiliary coaxial lines. This may be done 17%,556 Patented May 14, 1957 by increasing the size of the coupling slot at the iuncture between the waveguide sections and the coaxial lines, but such increase is limited by interference requirements and the size of the coaxial lines themselves. The present invention sets forth simple means for increasing the coupling between the main and auxiliary coaxial lines of a slotted block directional coupler.

A principal object, therefore, of the present invention is to provide a slotted blocked coaxial line directional coupler having relatively tight coupling between main and auxiliary lines.

An additional object is to provide a directional coupier which is simple to construct with high dimensional tolerances.

Another object gether two coaxial transmission tral axes.

These and further objects will be apparent upon consideration of this specification and its accompanying drawings of which:

Figures 1 and 2 are partially cut-away perspective views of apparatus of the present invention; and Figure 2A shows an optional form of dielectric member for the apparatus of Figure 2.

Referring now to Figure 1, a solid metallic block 19 is shown with cylindrical recesses 11 and 12 disposed therein and having parallel central axes. With cylindrical conductors 19 and 20 centrally retained in recesses 11 and 12, two parallel coaxial lines are formed.

Symmetrically cut into faces 14 and 15 of block 10 are recesses 16 (only one shown). With cover plates 17 secured in place, the recesses 16 form two waveguide sections which couple together cylindrical recesses 11 and 12. By properly spacing recesses 16 a quarter wavelength is to provide a means of coupling tolines having parallel cenof energy travelling in the coaxial lines formed in block 10, a conventional multiplepath directional coupler is formed.

Substantially identical coaxial lines 21 and 22 having a common central axes are secured to the metallic block structure shown and form the main transmission line, while coaxial lines 23 and 24 are similarly secured to form the auxiliary lines. The main line output and input terminals of lines 21 and 22 respectively and the directional coupler output terminal of line 26 are indicated in Figure 1, and a matched dissipative member (not shown) terminates line 24. The center conductors of the coaxial lines shown are supported by the stub chokes 25.

The directional properties of such a coupler depend upon simple interference, constructive in the preferred forwarddirection and destructive in the backward direction. Consider energy to be travelling in both directions in the main line 21, 22. That energy which progresses in the preferred direction, from right to left, is coupled into the auxiliary line through both recesses 16. The preferred direction energy is combined constructively in auxiliary'line 23 as it travels in the ri-ght-to-left direction and proceeds to directional coupler outlet. Preferred direction energy travelling from left to right in auxiliary line 24 is absorbed in the matched dissipative load termination or cancels itself by destructive interference. The energy travelling in the wrong-way direction is also coupled through recesses 16 into the auxiliary line. That part of the coupled wrong-way energy travelling in the auxiliary line from left to right is absorbed by the line termination. That portion of the coupled wrong-way energy travelling from right to left is canceled by destructive interference, and the directivity of the entire coupler is largely terminated by the completeness of elimination of such energy.

The dimensions of the recesses 16 are such that they act as waveguides beyond cut-oh" and thus direct only a small sample portion of the main line energy to the auxiliary line, thus making the apparatus shown highly adaptable to directional power monitoring problems. Because of the extremely high attentuation of a cut-off waveguide, it is often desired to increase the coupling between the main and auxiliary lines. As stated above, this may be done by increasing the size of the slots joining the recesses 16 and the coaxial lines formed in the block 10, but such techniques have serious limitations.

The present invention sets forth directional coupler apparatus wherein the coupling between the main and auxiliary lines is increased by proper design of the recesses 16. Heretofore, these cut-off waveguide sections (16) were made with an electrical slot Width w not in excess of the diameter of the cylindrical cavities 11 and 12. In the apparatus shown, tighter coupling is achieved by increasing the physical dimension w so as to permit the greater flow of energy from main to auxiliary lines.

Another form of the present invention is shown in the partial view of Figure 2. Metallic block 10, cylindrical recesses 11 and 12, center conductors 19 and 20 and cover plate 17 are identical to the corresponding elements of Figure 1. In Figmre 2, however, the waveguide coupling recesses cut into face 14 of block 10 (only one recess shown) has a physical width w equal to the diameter of cylindrical cavities 11 and 12. Disposed in these recesses are members 30 (one shown) made of a dielectric material such as, for example, polystyrene. These dielectric members 30 cause the electrical width of the waveguide coupling recesses to be elfectively made greater than the actual physical width w, and, as stated above, such increase in width provides the desired increase in electrical coupling between the main and auxiliary transmission lines.

In Figure 2A, a second possible form of the dielectric member is shown which is interchangeable with the members 30 of Figure 2. This member 35 fills the entire waveguide coupling recess and, in addition, extends into coaxial line cavities 11 and 12. Any interruption of energy flow caused by these members does not cause appreciable back reflection because of their quarter wavelength spacing. Openings 36 in member 35 accommodate coaxial line center conductors 19 and 20.

It will be obvious to those skilled in the art that the coup 'ng structure described in this specification is adaptable to a great variety of electromagnetic wave apparatus in addition to the directional coupler. The scope of this invention shall not be limited to the specific details set forth above.

What is claimed is:

1. A directional coupler for the transfer of a portion of the microwave energy propagated in a main transmission line to an auxiliary transmission line comprising, a metallic block having formed therein two quarter wave length sections of transmission lines positioned to have parallel axes, said block being recessed at each end of said transmission line sections to provide coupling apertures therebetween, means for coupling said main transmission line and said auxiliary transmission line to said quarter wave length sections of line, and solid dielectric tween said main and auxiliary transmission lines.

2. A directional coupler for the transfer of a portion of the microwave energy propagated in a main transmission line to an auxiliary transmission line comprising, a metallic block having formed therein two quarter wave length sections of transmission lines positioned to have parallel axes, said block being recessed at each end of said sections to provide coupling apertures therebetween, the width of said recesses being equal to the cross-section dimension of said transmission line sections, means for coupling said main transmission line and said auxiliary transmission line to said transmission line sections, and solid dielectric inserts positioned in said recesses for increasing the electn'cal width of said recesses to increase the coupling between said transmission lines.

3. A directional coupler for the transfer of a portion of the microwave energy propagated in a main transmission line to an auxiliary transmission line comprising, a metallic block having formed therein two quarter wave length sections of transmission lines positioned to have parallel axes, said block being recessed at each end of said sections to provide coupling apertures therebetween, the width of said recesses being equal to the cross-section dimension of said transmission line sections, the depth of said recesses being such as to form a wave guide section operating below cut-off frequency, means for coupling said main transmission line and said auxiliary trans mission line to said transmission line sections, and solid dielectric inserts positioned in said recesses for increasing the electrical width of the wave guide so formed to increase the coupling between said auxiliary transmission line and said main transmission line.

4. A directional coupler for the transfer of a portion of the microwave energy propagated in a main transmission line to an auxiliary transmission line comprising, a metal block having two parallel faces separated by a quarter wavelength of the propagated energy, said block having a pair of cylindrical transmission line sections with parallel axes bored between said faces, a cover plate for each of said faces, each of said faces having a recess cut therein forming with said cover plate a wave guide section to couple said transmission line sections, means or coupling said main transmission line and said auxiliary transmission line to said transmission line sections and a solid dielectric insert having a dielectric constant greater than that of air positioned in each of said recesses for increasing the electrical width thereof to increase the coetficient coupling between said transmission line sections.

5. A directional coupler for the transfer of a portion of the microwave energy propagated in a main transmission line to an auxiliary transmission line comprising, a metal block having two parallel faces separated by a quarter wave length of the propagated energy, said block having a pair of cylindrical transmission line sections with parallel axes bored between said faces, a cover plate for each of said faces, each of said faces having a recess cut therein forming with said cover plate a wave guide ing the electrical width thereof to increase the coefficient of coupling between said transmission line sections.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 16, 1940 2,423,390 Korman July 1, 1947 2,531,777 Marshall Nov. 28, 1950 2,562,281 Mumford July 31, 1951

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