US2672591A - Ultrahigh-frequency attenuator with coaxial line input and output - Google Patents

Description

March 16, 1954 R WALLAUSCHEK 2,672,591

ULTRAHIGH-FREQUENCY -AT'IEIIUA'I'OR WITH COAXIAL LINE INPUT AND OUTPUT 3 Sheets-Sheet 1 Filed Nov. 10, 1951 guvd March 16, 1954 R. WALLAUSCHEK 2,672,591

ULTRAHIGH-FREQUENCY ATTENUATOR WITH COAXIAL LINE INPUT AND OUTPUT Filed Nov. 10, 1951 3 Sheets-Sheet 2 Fig. 5

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ULTRAHIGH-FREQUENCY ATTENUATOR WITH COAXIAL LINE INPUT AND OUTPUT Filed Nov. 10, 1951 5 Sheets-Sheet 3 ZZ/ws/vrw RCHHIQ) wnmcadscne'x 49.. WW 761w Patented Mar. 16, 1954 ULTRAHIGH-FREQUENCY ATTENUATOR WITH COAXIAL LINE INPUT AND OUT- PUT Richard Wallauschek, Chromepet, Madras, India Application November 10, 1951, Serial No. 255,861

Claims priority, application France November 18, 1950 4 Claims. 1

My invention relates to a non-selective attenuator for ultra-high frequencies, matched on both sides with coaxial lines, and more specifically to an attenuator consisting of a length of a cylindrical wave guide of circular cross-section, working according to one of the type E modes, also called TM modes, i. e., transverse magnetic, at frequencies lower than the cut-off frequency of travelling wave propagation.

Wave-guide attenuators of the prior art working at frequencies lower than the said cut-off frequency are known to belong to two main types.

Those of the first type utilize guides energized in an H (or TM) mode (see for instance Technique of Microwave Measurements by Montgomery, Massachusetts Institute of Technology, 1947, page 685), more exactly in the H mode. This mode being the least attenuated for a given length of guide, the energy contained in all'the other modes rapidly becomes negligible, at least beyond a certain minimum length, compared with the energy transmitted in the H0 mode. The attenuation may then be calculated as a function of distance according to an exponential law. These attenuators are in the shape of a guide having a sufiiciently small cross-section, with which the input and output lines are coupled by suitable small loops adapted to energize especially the H0 mode. The distance between the two loops is variable and allows adjustment of the attenuation. Attenuators of this type are electrically equivalent to the assembly of a transformer, the primary and secondary windings of which are constituted by the inductances of the two loops and of resistances in series with said inductances representing the loss resistances of the loops. The coupling between these primary and secondary windings varies as a function of the distance between the loops. These attenuators are frequency-selective owing to the properties of the loops, which depend on frequency.

Attenuators of the second type utilize guides energized in an E mode. As will be set forth in detail later on, these attenuators behave like capacitive voltage dividers, but, though superior to the above-described as regards non-selectivity, they ofier certain drawbacks. The value of maximum attenuation attainable is limited by the existence of an H0 mode, liable to be excited owing to inevitable constructional asymmetries and which, though at a much lower energy level than the predominating E0 mode, benefits by a relative attenuation lower than that of the said E0 mode and finally predominates over the latter beyond a certain length of guide. The value of minimum attenuation is fairly high since the full voltage of the input wave is not applied to the capacitive voltage divider. Finally, the matching of the attenuator with the impedances of its input and output lines is generally not satisfactory, which is particularly troublesome on the output side of the attenuator.

One object of my invention is to build an attenuator of the wave-guide type, working below the cut-01f frequency of travelling wave propagation in an E mode and equivalent to a capacitive voltage divider combined with a resistance in shunt across its input and ensuring the necessary dissipation of energy and with a further resistance in series with its output, the latter determining the output impedance of the attenuator.

Another object of my invention is to obtain an attenuator in which the full voltage of the input wave is applied to the capacitive voltage divider.

Another object of my invention is to build an attenuator the output impedance of which very exactly matches the impedance of the coaxial output line.

Other objects and advantage of my attenuator will appear from the detailed description which will now be made, with reference to the appended drawings, wherein:

Figure 1 represents an attenuator of the prior art for ultra-high frequencies using a. wave-guide energized in an E mode; 7

Figure 2 shows the electrical diagram equivalent to the attenuator of Figure 1; a

Figure 3 shows the attenuation curve as a function of the length of guide, for the attenuator of Figure 1;

Figure 4 shows the electrical diagram of a highfrequency capacitive voltage divider using lumped elements.

Figure 5 shows an axial section of my attenuator;

Figures 6 and 7 respectively show, in perspective, the energizing portion and the receivingporw tion of my attenuator; Figure 8 shows the electrical diagram equivalent to the attenuator of Figure 5.

Referring to Figure 1 which represents an. attenuator of the prior art, excited in an E mode, I represents a guide with a circular cross-section, and 2 and 3 the inner conductors of the input and output coaxial lines, of which the wall I of the guide constitutes the outer conductor. At the ends of each one of the conductors 2 and 3 are respectively mounted series resistances 4 and 5 secured to said conductors'by means of resilient clamps 6 and l,

Each one of these resistances carries a metal disc (8 and 9 respectively), mounted perpendicularly to the axis of the tube l and exactly centered within the latter, with no direct contact and forming therewith a sumciently large shunt capacity.

The equivalent diagram of the attenuator of Figure l is represented on Figure 2, wherein 2i and 22 are the input terminals and 23 and 2 1 the output terminals. The capacities C; and C'o, existing respectively between the-discs 8 and 9 and the tube i are assumed to be sufficiently large so that their reactances may be neglected when compared with the values R1 and R: of the resistances 4 and 5.

The input impedance of the attenuator is thus practically determined by R1 and it may be matched with the characteristic impedance of the coaxial input line. Similarly, the output impedance of the attenuator is practically determinedby R 2 and it may be matched with the characteristic impedance of the coaxial output line. Th'e capacity C is the capacity of the surfaces facing "the discs 8 and 9 audit varies as a function of the distance between said surfaces.

The attenuation is practically independent of frequency, at least for frequencies sufliciently remote from the cut-ofi frequency of the guide and "for not too small distances between the two discs. The attenuator is electrically symmetrical and'fnay be used, in principle, in the two directions, that is 23-24 may be used as input terminals and 2l22 as output terminals.

Figure 3 shows, as a function of distance d between the discs 8 and 9 the relative attenuatiohs of the E mode (which predominates over the higher modes E1 and E2 also excited by the disc 8), and of 'the Ho mode excited at a much lower level by the inevitable constructional asymmetries.

Between the distance do, beyond which the higher modes E1 and E2 become negligible and the distance d1 beyond which the H0 mode, which has a lower attenuation than the E0 mode becomes predominant with respect to the latter, the attenuation can be calculated according to an exponential law as a function of distance. In general, such attenuators operate correctly between "30 and 150 db, approximately.

'A 'drawback of these attenuators consists, as already stated, on-the one hand in the existence of an upper limit of the possible attenuation, on the other hand in the relatively high value of their'minimum attenuation (30 db), and finally in the fact that the matching with the impedances of the input and output lines is not sumciently ensured by the series resistances R1 and R'z, which is particularly troublesome at the output of the attenuator.

Itwill'be further noted that, due to the presence of the series resistance R'i, the whole input voltage is not applied to the terminals of the voltage divider consisting of the capacities C and C's; hence an additional cause for the relatively high value of the minimum attenuation.

Figure 4 shows the diagram of an attenuator consisting of a capacitive voltage divider of a conventional type, for attenuating high frequencyvoltag'es. On Figure 4, 4i and 42 are the input "terminals and =13 and M the output termirials. -Cois afixed capacity and C an adjustable capacity, "the assembly'of whichforms the voltage divider. R1 isa resistance dissipating the energy delivered by the high frequency energy source and R2 'is "a suitably selected resistance 4 connected in series at the output of the attenuator and determining its output impedance. The attenuator, being a four-terminal non-symmetrical network can Work correctly only in one direction, which is that indicated by the arrow 45. The chief advantage of this type of attenuator resides in the fact that its attenua'tion'does not depend on frequency to the extent to which the assumption of its equivalence to lumped elements (capacities and resistances) remains valid. If W1 is the power available at the output from a generator connected with the terminals 6| and 42, and matched with R1, the voltage at the input to the attenuator is:

V1=\/W1R1 The voltage at the terminals of C0, assuming C to be smaller than C0 and wC much smaller than 1/31 is:

V 2 W1-R2W1-Z R. B]

which shows that does not depend on frequency.

The main object of the invention is to build an attenuator for ultra-high frequencies whose operation is more closely represented by adiagra'm consisting of lumped-elements than is the case for the attenuator of Figure l and the diagram of Figure 2.

Referring to Figure 5, it will be seen that the attenuator comprises an energization portion designated as a whole by the reference numeral l9 and "mounted inside a conducting tube comprised of two juxtaposed parts IMP-I01 and which constitute the wallof the guide and a receiving portion designated as-a whole by the reference numeral 28 and mounted inside a conducting tube H32 sliding inside the tube IOI.

Theenergizing portion of the attenuator comprises the parts 10?, W8, W8 and'llfi. I

it! is a cylindrical metal rod connected by one of its ends to the innerconductor Hi3 of the input 'coaxial line, by a spring clamp I04, and which goes through the center of a circular insul'atingdis'c iUB'arr'anged in the plane of a crosssection perpendicular to the axis of the .guide. The other 'or far end of the rod N31 has, as a terminal plane, the farther "face of the disc I08. The outer conductorof the input coaxial lineis constituted by 08.

The disc W8 has about the same diameter as.

contact with [00 and Iill, while, in the plane common to the end of the rod I01 and to the farther face of the disc I08 and of the ring I09, there has been applied, by metallization, a homogeneous electrically resistant layer H the thickness of which is small with respect to the depth of penetration of high frequency currents due to the skin effect. The far end of I0! is in electrical contact with H0.

The receiving portion for the attenuated wave comprises the parts III, H2, H3, H4 and H5.

' Part H3 is a metal body having circular symmetry and in the shape of a pot, having an outer bottom H6, an inner bottom III and a cylindrical wall I I0 the edge of which is bevelled, I I9.

Inside part H3 is secured a cylindro-conical insulating support I I2, the cylindrical portion of which enters the pot H3 and the conical portion of which extends the bevelled surface H9. This part H2 comprises a central hole entered by and securing a cylindrical metal rod III con nected by a spring clamp I06 to the inner conductor I of the output coaxial line. The rod III, therefore, is not in direct metallic contact with the metal pot H3.

On the conical surface of the insulating support H2, extended by the bevelled surface of the pot I I3 there is applied, by metallization, a homogeneous electrically resistant layer H4, having a thickness of the same order as that of layer H0, and which also covers a very short length of the rod III at the place where said rod issues from the insulating support.

The diameter of the pot I I3 is slightly less than the inner diameter of the tube I02 and the receiving assembly is centered and secured inside a metallic tube I02 by means of a thin annular insulating layer H5 intermediate between I02 and H3. Thus the metal parts I02 and H3 are not in direct contact but offer a certain electrical capacity between them. I02 constitutes the outer conductor of the output coaxial line.

The energizing assembly I07, I08, I09, H0 may, for example, be obtained by sealing a glass disc between a central metal rod and a concentrical metal ring, by lapping the common plane surface, opposite to the output side of the rod, and by applying a resistant layer by cathodic evaporation. One could also use a ceramic disc, bored at its center with an axial hole, secure in this hole, by means of putty, a metal rod whose end would be flush with one face of the disc, metallize a thick layer over the cylindrical portion of the disc to form a deposit playing the part of the ring and metallize a thin layer on the face of the disc which is flush with the end of the metal rod.

Similar embodiments may be used for the receiving assembly. The insulating layer H5 may be obtained by introducing between I02 and H3 a cement capable of drying in air or a plastic blue capable of being polymerized by heat.

The attenuator behaves electrically at its working frequencies like a guide length working below the cut-off frequency of travelling waves. The corresponding guide length then consists of the portion of the tube IOI comprised between the energizing and receiving portions. Due to the configuration of the electromagnetic field in the vicinity of the layer H0 the guide is almost exclusively energized in the E modes.

To adjust attenuation between the input and output of the attenuator, the tube I02 may be moved inside the tube IOI by means of a knurled knob I23, a pinion I24, and a rack I25 (or by any equivalent control system).

:Thereds no drawback to having .the tube I02' slide within the tube IOI with a fairly large clearance, as it is well known that in a guide energized at a frequency lower than the cut-off frequency of E modes, there is practically no current in the guides metal wall. The quality of the contact between the tubes IN and I02 practically plays no part in the value of the attenuation obtained.

The equivalent diagram of the attenuator of the invention is represented on Figure 8, wherein:

8I82 are the input terminals of the attenuator and 83-84 its output terminals.

The layer N0 of the energizing device may be represented by a number n of elementary resistances Z1 to Z1 in series;the sum of which must be taken equal to Z1, the characteristic impedance of the coaxial input line, each elementary resistance Z1 to Z1 representing the resistance of an annular zone of the layer I I0.

The capacity between the energizing'surface of layer I I0 and the receiving surface may be represented' by a number n of elementary capacities C1 respectively corresponding to the various resistances Z1 to Z1, 2' being any integer number from 1 to n, the said elementary capacities each having one terminal connected to the junction point of two successive resistances,. the other terminals being connected to one another. The capacity C1, for instance, has one of its terminals connected with the junction point of resistances Z1 and Z1 The diagram of Figure 8 represents perfectly the distributed nature of the coupling between the energizing system and the receiving surface. The system of capacities O1 to C11 forms, with the capacity Co between I02 and I03, a voltage capacities C1 to C11 very small relatively to nZ1 the voltage between point Q and point P1 of the set-up is:-

P1 being the junction point between resistance Z1 and. Q a point at a constant potential to which the last resistance Z1 is connected where U1 is the input voltage, which, in the case of a generatorwith a power W1, matched with R1 is equal to \/R1W1.

The voltage transmitted to the terminals of Go through the capacity C1 is:

Therefore, the sum of all the voltages superposed at the terminals of Co is and the power W2, available at the output terminals 83 and 84 of the attenuator is, assuming access I the admittance of Co to be very llarge compared with 1 2 l'U 1121 1 1 1 121+ C1] Hence the value-of the ratio Kall This expression is very similar to Formula 1 valid for the diagram of 'Figure 1. It shows the non-selectivity of the attenuator of the invention.

The restriction imposed for (2) to be valid, namely that the distance between the energizing and receiving surfaces be larger than a certain value, means, physically, that for too small distances, the contents in higher modes of the field excited in the guide depends on frequency. This fact does not constitute a drawback of the attentuator according to the present invention with respect "tothat of Figure 5 since in the latter, the existence of higher E modes already introduces a certain dependence between attenuation and frequency below a certain distance between the energizing and receiving surfaces, and since'the difference in the contents of the fieldin higher modes practically introducesn'o additional drawback.

Experiments have shown that the novel attenuator makes it possible to obtain an attenuation which is practically independent of frequency above a certain minimum attenuation value, with a good matching of the impedances of the attenuators output'and'of the output ccaxial line and an input impedance sufiiciently matched with that or" the input coaxial line to ensure a satisfactory operation of the generator vious attenuators since in the latter only a small fraction of the ultra-high frequency input voltage is applied to the voltage divider C'Cn (due to the presence of theseries resistance R'1) while, in the novel attenuator, the'full ultra-high frequency voltage is applied to the voltage divider. In practice, due to this fact, the minimum attenuation may ice-lowered by to db with re spect to that obtainable in prior systems.

Iclairn:

1. An attenuation device for ultra-high irequencies including a length of cylindrical wave guideoi circular cross-section, workingat a frequency below the cut-off frequency-of propagation of travelling waves and adapted to be respectively connected .atits input and atits output to input and to output coaxial lines and energized on its input side so as .to .develop an electromagnetic field of the type having a magnetic component transverse with respect to the axis of the said guide, comprising on its input side an energizing conducting surface set up inside and 8 perpendicular to the axis of the guide and consisting :of a homogeneous electrically resistant layer deposited on an insulating disc and connecting the central conductor of the said input coaxial line to the inner wall of the said guide which at the same time forms the outer :conductor of the input coaxial line, the thickness and resistivity of said layer being such that its effective resistance measured between said inner and outer conductors be substantially equal to the characteristic impedance of said input coaxial line, *and'a receiving metal plate perpendicular to the axis of and set up inside a metal tube mounted inside the :said guide and concentric therewith and electrically insulated from the said tube so as to form an electric capacity with the inner wall :of the said tube the said plate being secured to a flared transition :conductor consisting of .a homogeneous electrically resistant layer deposited on an insulating support andconnected'at its larger 'end to the said plate and at its smaller end to the .inner conductor of the said output coaxial line, the outer conductor of the said output line being formed by the said metal tube, and the efiective resistance of said transition conductor measured betweensaid receiving plate and said innerconductor of said output coaxial line being substantially equal to the characteristic impedance of output coaxial line.

2. A device according to claim 1, wherein receiving plate, the conical or flared transition conductor and the central conductor of the-c0axial output line are mounted inside the said metal tube wi ichis slidingly movable inside the said guide, thereby to enable variations of the distance between theenergizing and receiving-surfaces and adjust the attenuation.

3. A device according to claim 1, wherein the energizing surface of the guide is made in the form of an insulating disc bored with an axial hole, through which passes a metalrod one .end of which is flush with one .face of the disc, and the other end of which is electrically connected to the central conductor of the input coaxial line, the said disc beingsurrounded with a metal ring fitting into a groove provided in the inner wall of the guide and coated, on the side flush with the rod,with a homogeneous electrically resistant layer in contact with the ring and with the said rod.

4. A device according to claim 1, wherein the receiving device is made of an insulating body comprising a flared or conical portion, mechanically secured-at its larger end, to a metal plate insulated from and forming an electric capacity References Cited in the file or" this patent UNITED STATES .PATENTS :Name Date Hansen July .11, 1950 Number

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