Strip transmission lines

Nov. 17, 1959

E. G. FUBIN1 ETAL

2,913,686

STRIP TRANSMISSION LINES

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Fig. 2

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Fig. 5

INVENTOR.

EOGEN£ G. FUGfN/ W/NF/Ei-D £. FROMM

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Nov. 17, 1959 e. g. fubini etal 2,913,686

STRIP TRANSMISSION LINES

Filed Sept. 17, 1953 2 Sheets-Sheet 2

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Fig. 3

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Fig. 4

INVENTOR.

EUGENE G. FUBINI WINFIELD E. FROMM

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United States Patent Office

2,913,686

Patented Nov. 17, 1959

2,913,686

STRIP TRANSMISSION LINES

Eugene G. Fiibihi, Muttontown, and Winfield E. Fromm, Hempstead, N.Y., assignors, by mesne assignments, to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware

Application September 17, 1953, Serial No. 380,674

Claims. (Cl. 333—84)

This invention relates to transmission lines, and in particular to strip transmission lines.

Ih-mariy applications of strip transmission lines in radio frequency systems, particularly in the microwave region of the frequency spectrum, dielectric losses prove to be a limitation from the standpoint of economic and efficient operation. Moreover, in Some utilizations, it is demanded that the effective electrical· length of the circuit components remain substantially uninfluenced by changes in the ambient temperature, so that performance will be of a constant and dependable nature. Also, it is important that imdesired radiation losses be kept to a minimum. In the microwave field in particular, it is im-r portant that these criteria be realized without any sacrifice Of the requirements for light weight and miniaturization.

The advantages of strip transmission line microwave components are by now well known. Among these are: light weight, ease of manufacture, cheapness, miniaturization, etc. The possibility of using printed circuit techniques ‘for the construction of microwave circuits involving strip transmission lines has been disclosed by R. M. Barrett, Electronics, June 1952, and D. D. Grieg andl-f. -F. Engelmann, Proc. I. R. E., December 1952.

In. general, these publications depict three types of transmission lines. The closed strip Or dielectric sandwich transmission line comprises a narrow ribbon conductor immersed in dielectric material, the whole being bounded by flat metallic plates. The second type, the Open strip transmission line,- comprises a ribbon conductor separated from a metallic ground plane by a dielectric slab. The third type, called the wire-above-ground transmission line, consists^: of a¹ wire or cylindrical, conductor supported above a ground plane by dielectric supports suitably positioned at spaced intervals longitudinally of the line.

It is an object of this invention to provide a strip transmission line having low dielectric -and undesired radiation losses.

It is another Object of the invention to provide a strip transmission line which has an effective electrical length which is substantially uninfluenced by Changes in. ambient temperatures.

It is another object to provide a strip transmission line which is light weight, convenient to fabricate, and Which can be arranged to occupy a relatively small amount of space.

In accordance with'the present invention, a transmission line is provided comprising a pair of metallic ground plates disposed in parallel relation to each other, and at an effective electrical distance of less than one half wave length of the highest frequency of. the energy to be propagated. A thin sheet of dielectric material is supported⁵ in spaced relation between the plates, the dielectric having conducting means^; suitably supported thereon. The conducting means cooperates With the bounding plates in guiding passage Of the radio frequency energy.

The above as well as further objects of the'invention will be more readily understood from the following description taken in conjunction with the accompanying drawings wherein:

Figure 1 is a perspective view of a section of strip 5 transmission line in accordance with one illustrative embodiment of this invention;

Figure 2 is a flux plot showing the electric field lines associated with conducting means positioned with perfect symmetry between bounding plates;

Figure 3 is a flux plot showing the distortion of the electric field lines as a result of offsetting the conducting means,

Figure 4 is a perspective view of a section of strip transmission line in accordance with another illustrative 15 embodiment of this invention, and

Figure 5 is a view, partially in longitudinal cross-section, of a section of strip transmission line bf the invention, with coaxial line fittings connected thereto.

Referring now to Figure 1, there is disclosed one illus20 trative embodiment of a strip transmission line in accordance with my invention, for use in a radio frequency system. Reference numeral 1 denotes a thin sheet of dielectric material positioned in spaced relation between a pair of metallic ground plates 2, by means of metallic 25 or dielectric support members 3. The spacing 4 between the ground plates 2 should not exceed an electrical length of λ min./2, that is, one half wavelength of the highest frequency of the energy to be propagated, arid much smaller spacings have been found advantageous and are 30 commonly used'in practice. A conducting means, shown generally at 5, for guiding passage of the radio frequency energy along the transmission line is rigidly secured to the thin dielectric sheet 1. The conducting means comprises a strip of metallic conductor material 6 which may be 35 conveniently fashioned by printed circuit techniques. As will be noted, in Figure 1 the conducting means 6 is positioned on only one side of the dielectric.

The support members 3 are arranged at convenient locations, far enough removed from the lateral edges of 40 the conductor 6 so as to be substantially outside the region where the electric field Tines are concentrated. Roughly speaking, the distance 7 between the members 3 and the near edges of conductor 6 should be equal to, Or greater than, the distance 4 between the ground 45 plates 2.

At this point some of the advantages of the transmission line just described over prior art types will be obvious, while other advantages will necessitate further discussion.

As is known, the velocity of propagation and wavelength Of high frequency energy passing through a dielectric is dependent upon the nature of the dielectric material, and more specifically upon the dielectric constant of the dielectric medium. Further, the dielectric constant 55 itself is a function of the ambient temperature, so that with variations in temperature, the effective electrical length of the circuit elements will be changed. These variations in electrical· length are particularly undesirable and troublesome in the dielectric sandwich and open strip 60 transmission line types. Moreover, the attenuation in a:strip transmission line is increased by the presence of the dielectric material. Til the line shown in Figure 1, the dielectric material 1 is kept at a minimum consistent with the requirements for structural-rigidity, so that the effective 65 electrical lengths of the-circuit; components remain Substantially constant throughout the/range of temperature changes experienced’under practical operating conditions. In addition, dielectric losses are largely eliminated.

In Figure 2 there is shown a flux plot (TEM mode) 70 in a lateral cross-sectional-plane of the transmission: line of Fig. 1. In order to simplify the discussion, only the electric field lines have been shown, it being under2,913,686 stood, that the magnetic field lines are everywhere normal to the electric field lines. For the same reason, the thin dielectric has been removed from the figure. As will be apparent from a study of Figure 2, perfect field symmetry prevails, and the electric field lines do not cross a plane 5 exactly centered between the ground plates and parallel thereto (designated “Neutral Plane”). The outer regions 8, 9 tend to become field free because the mode existing in the central region is beyond cutoff in regions 8 and 9. Stated differently, with the ground plate spacing 4 io less than a half-wavelength, as heretofore mentioned, in the regions 8 and 9 lateral of the conducting means 5 the ground plates function as a waveguide beyond cutoff for the fundamental TEM mode of propagation illustrated in Figure 2, so that the fields in the lateral direc- 15 tions are rapidly attenuated. For this reason it is possible to effect great savings by arranging several independent circuits in close physical proximity without obtaining troublesome interference between adjacent lines (provided no mode is excited which is capable of propagating 20 between the bounding ground plates). It is usually desirable to operate in the fundamental TEM mode of propagation. In order to discourage higher frequency modes it is found advantageous to select the width W of the conductor 6 so that the sum of the width of the strip 25 plus the spacing 4 between the ground plates is less than a half-wavelength of the energy to be propagated, although theory indicates a slightly more lenient condition.

In practice there may be some departure from the exact centered parallel position of the center conductor between ³θ the ground planes, illustrated in Figure 2. In such case the electric field pattern will be distorted from that shown in Figure 2, and undesired modes of propagation may be excited. The nature and amount of such departures which are permissible depend upon the performance de- ³⁵sired of the line, and upon the particular application in which it is used.

One such type of departure is offsetting, wherein the central conductor is closer to one ground plate than to the other but remains parallel thereto. This is illustrated ⁴⁰in Figure 3.

Referring to Fig. 3, as the conducting means 5 is moved closer to one bounding plate than to the other, that is, the conducting means is offset, then the tendency is for more and more electric field lines to terminate on the nearer plate. In order to illustrate this effect, an exaggerated offsetting of the conducting means 5 is shown in Figure 4. Very few of the electric field lines terminate on the lower plate, so that the field pattern is distorted and an undesired mode can be excited. It has been ⁵⁰found from experiment that as much as 10% offset in one direction or the other can be tolerated in most applications.

In some applications the excitation of undesired modes can be tolerated, and the amount of coupling to an ⁵⁵undesired mode will depend upon the problem encountered. For example, when very high Q networks are necessitated, the coupling between different parts of the circuit plumbing and the undesired mode may be unacceptable, even if very small by ordinary standards. Roughly ⁶⁰speaking, one can state that the amount of coupling that may be tolerated when the Q is 1000, is one where the undesired mode is %ooo (60 db) of the desired mode. As a trough rule of thumb it has been found that acceptable coupling is inversely proportional to the maximum ⁶⁵Q desired.

The closed strip transmission line has a characteristic impedance (Z_o) which, to a first approximation, is independent of the exact centering of the inner strip con- ₇₀ductor between the ground plates. This is not the situation in the open strip and the wire-above-ground transmission lines where the characteristic impedance (Z_o) is a function of the distance between the conductor and the ground plate. However, in the structure of Figure 1, it 75 may at times be desirable to off-center the conducting means slightly in order to compensate for the presence of the conducting means 5 on only one side of the dielectric.

The open strip transmission line and the wire-aboveground line of the prior art are of course limiting cases of complete off-centering, since one of the ground plates is at infinity with respect to the other. The desired and the undesired modes then coincide, and the electrical coupling between adjacent circuit elements is such as to prevent their utilization in many applications, such as for example, high Q circuits and feeds for antenna systems. This is of course a serious limitation, particularly in application to strip transmission line plumbing system. The transmission line of Figure 1 on the other band possesses no such design limitations.

Another illustrative embodiment of the invention is disclosed in Figure 4. The structure shown is similar to that shown in Figure 1, except that the conducting means 5 is formed by two conductive strips 6, 6' fixedly secured to opposite sides of the thin dielectric sheet in registered relationship. Thus the strips 6, 6' normally operate electrically as a single conductor. This embodiment has the additional advantage of being inherently symmetrical, since the conductors 6, 6' are equidistant respectively Ί from the ground plates 2, with only air-dielectric between respective conductors and ground plates. Another advantage derives from the fact that the thin dielectric 1 » lies substantially in a neutral plane in so far as the electric field between the bounding plates 2 is concerned, and in a substantially zero electric field between conductive strips 6, 6', so that dielectric losses are reduced to such an extent as to be almost entirely eliminated.

The above discussion concerning departures from an exactly centered parallel position of the central conducting means 5 applies also to the embodiment of Figure 4.

In the usual operation of the line shown in Figure 4, the conductors 6, 6' are fed in parallel. In such case spurious resonances may be introduced. In order to avoid this result, the opposed conductors 6, 6' are shown electrically connected by means of metallic connectors 11 spaced longitudinally along the transmission line. Advantageously these connectors 11 may be rivets, metalized holes or the like for establishing the requisite electrical contact between the conductors. The purpose in establishing electrical contact between opposed conductors 6, 6 is to maintain them at the same potential at each cross-section, so that spurious resonances cannot occur. To assure freedom from such spurious resonances, the distance 12 between adajcent connectors 11 may be made less than one-half wavelength at the highest operating frequency.

With the transmission line shown in Figure 4, circuit elements have been designed with Q’s equal to or higher than those obtained with conventional air-filled coaxial lines.

Figure 5 shows a conventional manner of coupling a strip transmission line to ordinary coaxial line. Here each end of a strip transmission line section 13 is provided with coaxial line fittings 14, 14' of conventional construction. As shown, the strip transmission line section 13 is constructed as in Figure 4, with the central conducting means comprising a pair of flat conductive strips 6, 6' affixed to opposite sides of the dielectric sheet &gt; 1 in registered relationship. The dielectric sheet is sup- ’ ported by members 3 in parallel relationship with the ground plates 2 and midway therebetween. J|

The coaxial fitting 14 has its outer conductor 15 con- nected to the ground plates 2 in any suitable manner, here shown as by flanges 16, 16' which are bolted together. The conductor 17 of the coaxial fitting is centrally supported in outer conductor 15 by any suitable means, here shown as an insulating ring 18. This central conductor 17 is conductively attached to the two conductive strips 6, 6'. The coaxial fitting 14' is similarly

2,913,686 connected to the other;end of the strip transmission line 13. Thus the conductive strips 6, 6' are fed in parallel as above described. As is common practice, the coaxial fittings are threaded to permit ready attachment of coaxial lines thereto. 5

The parallel feeding of strips 6, 6' causes corresponding ends thereof to be at the same relative potential at all times, and thus, in the absence of spurious resonances, causes opposed areas of the strips throughout the length thereof to be at substantially the same potential, io However,: if spurious resonances are troublesome, the strips may be electrically connected at spaced points along the length thereof by rivets, etc., as described in connection With Figure 4.

As used in this specification, and in the claims to fol- 15 low, the term transmission line is used in the broadest sense in accordance with current usage in the radio frequency art. Thus in contemplation of the instant invention, the transmission line may be used to transmit energy as a resonant circuit, as a circuit element such as a wave filter, metallic insulators or the like, as an aid in impedance matching etc.

It should be understood that the embodiments of the invention are illustrative only, and other modifications within the scope of the appended claims will occur to those skilled in the art from a consideration of the structures shown together, with the teachings hereof.