US3548344A - Stripline gain equalizer - Google Patents

Description

Dec. 15, 1970 J. L. PUTZ STRIPLINE GAIN EQUALIZER 2 Sheets-Sheet 1 Filed July 28, 1967 FREQUENCY FREQUENCY;

- INVENTOR. mm L. PUTZ FREQUENCY Dec. 15, 1970 J. L. PUTZ STRIPLINE GAIN EQUALIZER 2 Sheets-Sheet 2 Filed July 28, 1967 FIG. INVENTOR.

JOHN L. PUTZ I18 L9 2.0 2.'| FREQUENCY (GHZ) United States Patent U.S. Cl. 333-28 11 Claims ABSTRACT OF THE DISCLOSURE Absorption typestripline gain equalizers for providing a frequency sensitive loss characteristic are realized by utilizing one or more similar stripline resonator pairs having their major axes disposed parallel to a main transmission stripline conductor and axially spaced relative to each other such as to provide cancellation of electromagnetic wave energy reflections caused by the reactive perturbations of the individual resonators of each pair. Various types of lumped and distributed loss mechanisms are added to the resonators to control the resonator Qs and resonance characteristics. The coupling adjustment means between the main transmission stripline and the individual resonators may be provided in a simple manner and the individual resonators of a given pair may be n \/4 or n A/2 resonators where n is any odd integer and n is any integer and both 11 and 11 are preferably 1. In each case the individual resonators of a given pair are preferably axially displaced along the propagation axis of the main transmission stripline such that their respective common ends are axially spaced n)\/ 4 where n is any odd integer and preferably 1.

BRIEF DESCRIPTION OF THE INVENTION The requirements of users of high frequency electron discharge devices such as traveling wave tube amplifiers constantly become more demanding with regard to acceptable device operating characteristics as the users apply the devices to ever more complex and restrictive system design limits. The gain vs. frequency characteristic of traveling wave tubes is a prime example. The tube designer can go just so far in obtaining a flat, e.g., tapered, variable, etc., gain vs. frequency characteristic over the operating band of the tube without running into excessive cost problems if the individual desires of each user are to be satisfied by the tube design per se. Therefore the need for a cheap, compact equalizer is readily apparent. The equalizer teachings of the present invention also have separate utility in any microwave system which can benefit by means for controlling the signal amplitude vs. frequency characteristic.

The basic design philosophy of the present invention is as discussed in the abstract to couple one or more stripline resonator pairs to a main transmission stripline with the major resonator axes disposed parallel to the energy propagation axis of the main stripline. The individual resonators are provided with selective R.F. absorption means of either a lumped or distributed nature to control the absorption characteristics as desired and to provide the desired resonator Qs as well as to control the individ ual response shapes of a given resonator pair. The individual resonators of a given pair are axially displaced such that the respective common ends are spaced nA/4 apart, where n is any odd integer, although preferably 1, as determined at the self-resonant frequency of the individual resonators of a given pair. The terminology common ends is herein defined to mean electrically similar ends which in the case of the half-wavelength Patented Dec. 15, 1970 "ice open ended resonators means either end and in the case of the M4 ends means either the respective open ends I or the respective shorted ends. This provides optimum cancellation of the reflected R.F. energy introduced by the individual resonators of a pair when the individual resonators are substantially identical and thus a minimal V.S.W.R. for each inserted pair. If the self-resonant frequencies of the individual resonators of a pair differ or if the selective R.F. absorption or resistive loss levels of the individual resonators of a pair difler, the V.S.W.R. will be degraded accordingly. A plurality of individual pairs can advantageously be disposed along a single main transmission stripline to provide any desired loss vs. frequency characteristic while still retaining the low V.S.W.R. of a single pair. The individual resonators themselves may be provided with means for varying the degree of coupling to the main transmission stripline as well as with means for varying the individual resonator self-resonant frequencies. The individual resonators are n \/4 and ri k/2 types where n is any odd integer and n is any integer, and where both n and 11 are preferably 1, as determined at the desired self-resonant frequency of the individual resonators. The stripline elements are preferably simple fiat types but other variations such as rods etc., are not excluded and are included in the terminology stripline conductor and may be used to advantage especially in the tunable resonators as will be set forth in more detail hereinafter in the detailed description.

It is, therefore, an object of the present invention to provide an absorption type stripline equalizer with an improved V.S.W.R. characteristic within the design band of the equalizer.

A feature of the present invention is the. provision of an absorption type equalizer incorporating a main transmission stripline with at least one pair of stripline resonators coupled thereto with the individual stripline resonators provided with R.F. absorbing means and having their main axes disposed substantially parallel to the propagation axis of the main transmission stripline with the ends of the individual resonators of a pair axially displaced to cancel the R.F. reflections introduced by the individual resonators of a pair.

These and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a fragmentary and partially sectioned high frequency electron discharge device of the traveling Wave amplifier type incorporating an equalizer at the R.F. input port.

FIG. 2 is an enlarged perspective view of an equalizer incorporating the teachings of the present invention.

FIG. 3 is a fragmentary perspective view of a stripline resonator having means for varying the self-resonant frequency of the resonator.

FIG. 4 is an illustrative graphical protrayal of a nonsaturated gain vs. frequency characteristic for a traveling wave tube.

FIG. 5 is an illustrative graphical portrayal of loss vs. frequency characteristics realizable over the design band for a typical equalizer of the present invention.

FIG. 6 is an illustrative graphical portrayal of the V.S.W.R. vs. frequency characteristics for a single resonator and a properly spaced pair of matched resonators.

FIG. 7 is a perspective view of an embodiment of a \/2 stripline resonator incorporating lumped R.F. loss means and means for varying the self-resonant frequency of the resonator.

FIG. 8 is a perspective view of a variation of the embodiment depicted in FIG. 7.

FIG. 9 is a perspective view of a M 4 stripline resonator incorporating a lumped resistor.

FIG. 10 is a perspective view of a \/2 resonator having series R.F. loss means.

FIG. 11 is a plan view of another equalizer embodiment incorporating the teachings of the present invention.

FIG. 12 is another plan view of a stripline equalizer incorporating the teachings of the present invention.

FIG. 13 is a loss vs. frequency plot for the equalizer depicted in FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION Turning to FIG. 1 there is depicted a high frequency electron discharge device of the traveling wave type 10 including any conventional beam forming and projecting means 11 disposed at the upstream end portion thereof and beam collector means 12 disposed at the downstream end portion thereof. Disposed within a tube envelope 13 between the upstream and downstream end portions of the device is a slow wave interaction circuit 14, e.g. a ring-and-bar circuit supported on dielectric rods 15 in a conventional manner. A PPM focusing means 16 or any other suitable focusing means may be employed to control the beam along the device axis. Coaxial input and output port or terminal means 17 and 18 are coupled to the respective ends of the slow wave circuit 14 for introducing R.F. energy for amplification and for extracting the amplified energy.

An equalizer 20 is coupled to the R.F. input port 17 as shown. The equalizer could be coupled to the output port if desired. However, the power handling capabilities would have to be increased accordingly. The function of the equalizer is to introduce variable attenuation vs. frequency within the operating band of the tube. The precise variable attenuation vs. frequency characteristic introduced by the equalizer will depend on the tube buyer and user specifications or it could simply be tailored by the tube builder to provide an e.g. fiat gain vs. frequency characteristic over the operating band of tube as a standard practice.

An absorption type stripline equalizer 20 could be of the type shown in FIG. 2 which includes a main transmission stripline 21 with a pair of the stripline resonators 22, 23 coupled thereto with an approximately M4 axial overlap as determined at the self-resonant frequency of the individual resonators. This one-quarter electrical wavelength overlap as discussed previously between the individual resonators along the propagation axis of the main stripline has been found to produce a very low V.S.W.R. in each direction for the pair. In other words, energy reflections due to reactive discontinuities produce mutual cancellation with the results shown in FIG. 6. Curve C is an illustrative plot of V.S.W.R. vs. frequency for an equalizer as depicted in FIG. 2 with only a single resonator coupled to the main line, whereas Curve D illustrates the reduction in V.S.W.R. for a pair of resonators having the same self-resonant frequency and a \/4 overlap where A is determined at the common self-resonant frequency. The reduction in R.F. energy reflections due to reactive discontinuities achieved with a 50-ohm stripline design using a pair of resonators of substantially \/2 length with a M4 overlap was substantial. The equalizer of the above design was terminated in a 50-ohm load, in both directions, and V.S.W.R. l.2:l over a 15% design band, in both directions, was measured which provides a graphic illustration of the effectiveness of the M4 overlap design in producing an essentially completely absorptive equalizer with substantially no reactive mismatch.

Turning again to FIG. 2 the stripline equalizer 20 is seen to be of a compact, simple, and rugged design enclosed in a housing 24 which includes a conductive wall 25 which serves as the ground plane for both the main and coupled resonator striplines. Any conventional conductive materials such as e.g. copper, silver-plated brass, may be used for all conductive portions of the equalizer if desired. The dielectric substrates 26, 27, 28 between the main stripline conductor 29 and the resonator stripline conductors 30, 31, respectively, may be of any suitable dielectric materials such as e.g. polystyrene, Teflon, ceramic, and any suitable bonding cement such as e.g. polystyrene cement, epoxy, may be used to bond the dielectric and conductive elements of the equalizer together to form a rigid design. The remaining wall portions of the housing 24 can be joined by any conventional means. Of course, the entire housing could be made out of an integral cup design with a single cover member if desired. A pair of coaxial couplers 33, 34 are fastened to opposite end walls 35, 36 of the housing as shown and their center conductors 37, 38 are bonded to the respective ends of the conductor 29 of the main stripline 21 by any conventional metal joining technique e.g. brazing. Dielectric insulation beads 40, 41 provide a rigid hermetic seal between the inner and outer conductors of the coaxial input and output couplers. Since the equalizer has essentially identical response characteristics regardless of which way RF. energy is directed through it, either port may be used as the input or output.

The degree of RF. attenuation provided within the design band of the equalizer is controlled via the introduction of R.F. attenuation or loss in the individual resonators 22, 23. For example, the resonator conductors 30, 31 may be made of lossy conductive materials such as e.g. stainless steel, thin alloy films such as platinum, and/ or the substrates 27, 28 may be made of lossy dielectrics such as e.g. fiberglass, carbon loaded epoxy, and/or lossy-conductive films 42 may be deposited directly on the substrates as shown, Suitable types of lossy-conductive films are e.g. Aquadag which can be deposited by painting or by spraying, and metallic alloy films such as platinum or nichrome which can be deposited by evaporating techniques. The lossy films can be deposited near the ends of the respective resonators as shown to obtain maximum effect, since the electromagnetic E-fields are strongest at the ends of the resonators. The lossy film conductors can extend completely around the substrates between the ground plane and above-ground conductors or other approaches such as set forth hereinafter may be used to advantage.

If it is desired to make the individual resonators adjustable either from the standpoint of providing a fine tuning mechanism for adjusting the individual resonances by varying the lengths of the conductors 30, 31 such that the frequency at which they are M2 or self-resonant is varied or for varying the frequency at which 11 M4 displacement occurs, the design in FIG. 3 may be used to advantage.

In brief, the tunable stripline resonator depicted in FIG. 3 includes a metal rod 44 with threaded bores at each end for receiving tuning screws 45, 46 as shown. The rod 44 is affixed to the dielectric substrate 47 and an elongated slot 48 is provided in the substrate for receiving a preferably dielectric clamp screw 49 which is screwed in a threaded bore in the ground plane 25 (not shown). By simply turning the screws 45, 46 the self-resonant frequency of the stripline resonator is varied as desired. By loosening the screw 49 the spacing between the strip conductor (rod 44) and the main stripline can be varied as desired to adjust the individual resonator to main transmission line coupling as desired.

FIG. 4 merely depicts an example of a response curve of gain vs. frequency for a hypothetical system, device, etc., which may be tailored as desired through use of an equalizer incorporating the teachings of the present invention. For example, if the equalizer user wants to control the gain to the limits between A and B over the frequency range between f and f so that the shadowed region above A is lowered, he merely tailors the loss vs. frequency characteristic of the equalizer to obtain e.g.,

curve 1 depicted in FIG. 5. If the equalizer user wants a more or less equal overall loss introduced across a given band, curve 2 in FIG. may be of interest. If the equalizer user wants an asymmetric loss vs. frequency characteris tic, then the equalizer can be designed to provide a loss vs. frequency response such as e.g., curve 3 in FIG 5.

Curve 1 in FIG. 5 is illustrative of equal coupling between main stripline 21 and each individual resonator with equal R.F. loss for each resonator. Curve 2 in FIG. 5 is respresentative of stagger-tuned plural pairs of resonators with equal coupling and loss for each individual resonator, and curve 3 is representative of staggered tuning and different coupling and loss between plural pairs of resonators. Staggered tuning simply means adjusting or designing the individual stripline resonators of different pairs to slightly different resonant frequencies by varying the self-resonant frequency as discussed above. To preserve the low V.S.W.R. response characteristics of the equalizer, the individual self-resonant frequencies, loss, and coupling of the resonators in a given pair should be made equal and variations in the above parameters should be made between pairs. The width of a given response curve for a pair is a function of the amount of loss used in each resonator pair. The greater the loss the less sharp the response curve will be. The degree of attenuation or loss at midband is a function of the coupling between the individual resonators and the main stripline. The coupling is primarily a function of the spacing between the individual resonators and the main stripline and the width of the respective above-ground conductors and can be made adjustable. Obviously, then the equalizer techniques taught herein are capable of providing many different variable attenuation vs. frequency response characteristics.

Turning now to FIG. 7 there is depicted a variation of an individual stripline resonator which may be used to advantage. The resonator 50 includes a dielectric slab 47 and adjustable rod conductor line 44 with tuning screws 45, 46 as in the FIG. 3 embodiment, disposed on ground plane 25 together with slot 48 to provide adjustable couling if desired. The R.F. loss means used in the FIG. 7 embodiment is a lumped resistor 52 which is soldered or the like via terminal conductors 53, 54 which are axially displaced along the major axis of the rod 44 as shown. The resonator as in the previous cases is a self-resonant /2 A resonator. The Q of the resonator can be controlled by the resistance values, with some accompanying change in the resonant frequency. The use of 500 to 10,000-ohm resistors connected as shown in FIGS. 7 and 8 produces an increase in resonator Q with increasing resistance as well as a small reduction in resonator frequency. Values of resistance lower than a critical minimum value which can be determined by experimental empirical approaches will produce an opposite effect, namely an increase in Q for a decrease in resistance. Increasing the axial spacing between terminals 53, 54 increases the resistor effectiveness as it will increase the E-field differential between the terminals.

In FIG. 8 another resonator 57 is depicted which is a modified version of the FIG. 7 embodiment. The modification involves the tuning means. In FIG. 8 the selfresonant conductor 58 is a simple strip conductor e.g. copper which is bonded to a dielectric substrate 47 e.g. 6.13. T exolite copper-clad laminate, grade 11711, manufactured by General Electric Corporation. The selfresonant frequency of the /2 wavelength resonator can be varied by curling up the ends 60, 61 as shown to vary the resonator length.

In FIG. 9 a variation of a stripline resonator is depicted which is of the A/ 4 type as opposed to the A/ 2 types discussed previously. The resonator 65 includes dielectric substrate 66 and strip conductors 67, 25 with a lumped resistor 68 shorting the conductors together at the one end as shown via lead terminal 69 with the other lead terminal 70 soldered or the like to the conductor 67 as shown.

In FIG. 10, a N2 strip resonator 71 using a series resistor for providing a controlled amount of loss is depicted. The line 72 is segmented at the center and a carbon paint or lossy carbon loaded ceramic, etc. resistor 73 disposed therebetween as shown. This resistor serves as a part of the half-wave resonator and by varying the length or resistivity of same the effectiveness of same as a loss mechanism may be increased.

FIG. 11 is another embodiment of an equalizer incorporating the teaching of the present invention. The equalizer 80 includes ground plane 81 forming a part of a housing 82 similar to the housing shown in FIG. 2. A main transmission stripline conductor 83 is coupled between the end terminals e.g. as shown in the FIG. 2 em- .bodiment. Two pairs of resonators 84, 85 of the types such as discussed above are included in the equalizer. The one pair of resonators 84 includes a pair of 2 strip conductors forming resonators 86, 87 with 4 end spacing for reasons discussed previously, and the other pair of resonators includes a pair of M4 strip conductors 88, 89 again using nA/4 axial spacing between similar ends, where n is an odd integer. Since the individual resonator pairs are effectively decoupled their individual loss vs. frequency responses may be simply superimposed to obtain the overall resultant equalizer loss vs. frequency response characteristic. The M4 resonators are symmetrically oriented such as to have similar ends facing in opposite directions. This orientation will provide a bi-directional R.F. match whereas if the orientation is such that similar ends face in the same direction, the lack of symmetry will in general result in a good R.F. match in one direction only. For the non-symmetrical orientation, the optimum spacing along the main line may depart somewhat from the ideal n7\/ 4 value previously given.

The M4 resonators of a given pair can be symmetrically disposed with total overlap on opposite sides of the main transmission line 83 and with the common ends still M 4 spaced to provide a reasonable V.S.W.R., although in general the match is inferior to that obtainable with 3M4 spacing between outside ends. A possible advantage of this configuration is the attainment of a broader absorption characteristic than is possible with a more widely disposed pair, due to the mutual coupling between members of the pair.

In FIG. 12 an equalizer 90 incorporating 7 individual resonators using 3 similar pairs and a single resonator is depicted. The half-wave length pair 91 was tuned for selfresonance at 1750 me. and the loss resistors were each 1000 ohms. The half-wave length pair 92 was tuned for self-resonance at 1820 mc. and the loss resistors were each 10,000 ohms. The )r/ 4 pair 93 was tuned for self-resonance at 1790 mc. and the loss resistors were each 5000 ohms. The overlap for the M2 pairs was )\/4 at their respective self-resonant frequencies and the M4 pair were spaced approximately A/ 4. A single M4 resonator 94 was made self-resonant at 2120 mc. and a SOOO-ohm resistor used.

The loss in db vs. frequency characteristic for the absorption equalizer depicted in FIG. 12 is shown in FIG. 13. The plot is self-explanatory and indicates the high degree of flexiblity in design which can be achieved in designing stripline equalizers as taught herein. Since the equalizer package for a commercial design includes a cover, the Q, coupling, and tuning adjustments are helpful for compensating for the variations in the self-resonant frequencies, Q and coupling produced by the addition of the cover. If perfect symmetry with regard to V.S.W.R. is desired for a given equalizer, only matched pairs should be used and mechanical accuracy as well as control of the electrical parameters will have to be watched carefully.

Since many changes could be made in the above construction and many apparently widely different embodiments could be constructed without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An absorption equalizer for microwave energy for providing a predetermined variable loss vs. frequency characteristic within the design band of the equalizer including, an unbroken main stripline transmission conductor means disposed between a pair of coupling ports for introducing and extracting electromagnetic wave energy into and out of said equalizer, at least one pair of stripline resonators electrically insulated from and coupled to said main stripline transmission conductor means, the individual resonators of said pair of stripline resonators having their major axes disposed substantially parallel to the major axis of said main transmission stripline conductor means, said individual resonators of said pair of stripline resonators each being self-resonant at approximately the same frequency within the design band of the equalizer, said individual resonators of said pair of stripline resonators each being provided with lossy R.F. attenuation means for absorbing R.F. energy coupled therein from said main transmission stripline conductor means, said individual resonators of said pair of stripline resonators having their common ends axially displaced approximately n \/4 electrical wavelengths apart along the major axis of the main transmission stripline conductor means where n is any odd integer and preferably 1 and where A is determined at substantially the self-resonant frequency of the individual resonators.

2. The absorption equalizer as defined in claim 1 wherein each of said individual stripline resonators include a stripline conductor which is 11 M4 in length where 11 is any odd integer and preferably 1 and is determined at the self-resonant frequency of the resonators, each of said resonators having the one end thereof electrically shorted between the two conductors forming said stripline resonator.

3. The absorption equalizer as defined in claim 1 wherein each of said individual stripline resonators includes a stripline conductor which is 11 M2 in length where 11 is any integer and preferably 1 as determined at the self-resonant frequency of said resonators.

4. The absorption equalizer as defined in claim 1 wherein said lossy R.F. attenuation means includes axially spaced lossy conductive coatings deposited on the resonator surface portions.

5. The absorption equalizer as defined in claim 1 wherein said lossy R.F. attenuation means includes at least one lumped resistor shunted across a single stripline conductor of each of said resonators.

6. The absorption equalizer as defined in claim 1 wherein said lossy R.F. attenuation means includes at least one lumped resistor shunted between a pair of conductors forming each of said stripline resonators.

7. The absorption equalizer as defined in claim 1 wherein said resonators are provided with coupling adjustment means for varying the coupling between the main stripline transmission conductor means and the individual resonators.

8. The absorption equalizer as defined in claim 1 wherein said resonators are provided with means for varying the individual self-resonant frequencies of said resonators.

9. The absorption equalizer as defined in claim 1 wherein said absorption equalizer includes a conductive ground plane forming a common conductor and support base for said main transmission stripline conductor means and said individual stripline resonators, said common ground plane forming a wall portion of a housing structure for said absorption equalizer, said individual stripline resonators and said main transmission stripline conductor means including dielectric substrates disposed on said common ground plane with the frequency determining conductor means being disposed on said dielectric substrates, said frequency determining conductor means being provided with means for adjusting the self-resonant frequencies of said individual resonators.

10. The absorption equalizer as defined in claim 1 wherein a plurality of pairs of stripline resonators are coupled to said main transmission stripline conductor means with the individual resonators of each of said pairs of resonators having approximately the same self-resonant frequency and wherein the common ends of the frequency determining conductors of the individual resonators of a given pair of each of said plurality of pairs of resonators are axially displaced approximately Il)\/4 along the major axis of said main transmission stripline conductor means where n is any odd integer and where is determined for each pair of resonators at the self-resonant frequency of the individual resonators of that pair of resonators.

11. The absorption equalizer defined in claim 1 wherein said equalizer includes resonator pairs of n )\/4 and n M2 coupled to said main transmission stripline conductor means at axially displaced sections along the major axis of said main transmission stripline conductor means, where 11 is any odd integer and preferably 1 and n is any integer and preferably 1 and where A is determined for a given pair at a common self-resonant frequency of the resonators of a given pair of resonators.

References Cited UNITED STATES PATENTS 2,984,802 5/1961 Dyer et al. 2,820,206 1/1958 Arditi et al. 2,859,417 11/1958 Arditi. 2,937,347 5/1960 Matthei et al. 3,104,362 9/1963 Matthei 333-73 3,215,958 11/1965 Isaacson. 2,961,621 11/1960 Tannenbaum 33--81 HERMAN KARL SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner US. Cl. X.R.

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