US2684469A - Mode selective attenuator - Google Patents

Description

July 20, 1954 s. SENSIPER MODE SELECTIVE ATTENUATOR Filed June 23, 1949 75, I CUTOFF I REG/0N INVENTOR JZ MUEL Saws/PER ATTORNEY Patented July 20, 1954 UNITED STA ES TE NT OF P ICE MODE SELECTIVE ATTENUATOR ware Application June 23, 1949, Serial No. 100,851

1 Claim.

'fication for providing an understanding of the invention. Much of this background discussion has been derived from the book Microwave Transmission Design Data by Theodore Moreno, published by McGraw-I-lill, 1948.

The present invention is primarily concerned with broadening the usable frequency band of wave guides and associated components. This desired result is accomplished generally by dis-'- crimmating against certain modes of transmis- 'sion by absorbing'them by means of devices of the present invention. Means to discriminate by mode reflectors are disclosed in my co-pending application Serial No. 95,915 filed May 28, 1949 for Reflection Filters for Wave Guides.

Accordingly, a primary object of the invention is to provide novel methods and apparatus for increasing the frequency bandwidth of wave guides.

Another object of the invention is to provide mode filter means for electromagnetic energy transmission in a Wave guide.

Another object of the invention is to provide new and improved absorption mode filters for Wave guides.

Another object of the invention is to provide means in a wave guide for absorbing certain modes of electromagnetic energy transmission.

Another object of the invention is to provide means for making Wave guides more efficient.

These and other objects will become apparent from the following specification and illustrations of which:

Figs. 1A and 1B are a pair of illustrative wave guide diagrams;

Fig. 2 is a graph illustrative of the invention;

Fig. 3 and Fig. 3A are illustrations of two embodiments of the invention;

Figs. 4A and 4B are a pair of illustrative diagrams;

Figs. 5, 6A, 6B and 7 are illustrations of other embodiments.

Hyper-Frequency Wave In the ordinary rectangular wave guide, there is an infinite number of possible modes by which energy may be carried. Each of these modes is characterized by a distinctive field configuration, and each represents a solution of Maxwells equations that will fit the boundary conditions imposed by the particular wave guide.

In any Wave guide, the possible modes of transmission may be divided into two classes, both infinite in number. In one class, the magnetic field has a component parallel to the guide axis, as well as transverse components, but the electric field is everywhere only transverse to the axis which is the axis of wave propagation. For this reason, waves of this class are referred to as transverse electric, or TE waves. Waves of this type are also called H waves, as it is only the magnetic field that has an axial component.

Waves of the second class are characterized by having components of electric field parallel to the guide axis as Well as transverse components, but a magnetic field that is everywhere only transverse to the axis. These waves are therefore known as transverse magnetic, or TM waves. They are also called E. waves.

The individual modes in rectangular wave guides are identified by giving the class of the transmission mode, followed by two numerical subscripts. These two subscripts are integers that indicate the number of half-period variations in transverse field intensity along the a and 2) dimensions of the guide, respectively. For example, the dominant mode in ordinary rectangular wave guides'is the T1310 mode, Fig. 1A, indicating that the wave is of the transverse electric type, that there is 'a single half-sinusoidal variation 9 of transverse electric field E along the a dimension, and that there is no variation of transverse field along the b dimension. The dotted lines show the magnetic field. Fig. 1B shows the 'TEzo electric field configuration, having a null in the center of the guide.

All modes of transmission in wave guides, and all modes except the principal mode in any transmission l-ine, will carry energy down the line only if the frequency is over a certain limiting or cutoff value. This value depends upon the size and configuration of the line as well as upon the particular mode of transmission. If these modes are excited in a line at frequencies below their cutoff frequencies, they will carry no real energy down the line, and the electric and magnetic fields associated with any given mode will diminish exponentially with distance from the point of excitation.

For any Wave guide the mode of transmission that has the lowest cutofi frequency is called the dominant mode. A practical wave guide is generally designed so that it is able to carry energy in only one mode of transmission. For this reason, the line is so restricted in size that only the dominant mode can transmit energy, and the line is then below cutoff for all the infinity of higher modes.

Any physical structure that launches a traveling wave in a line will generally excite a large 1 number of modes at the point of excitation. Only the modes that are above cutoii, usually only one mode, are able to carry energy down the inc, and the other modes that are below cutoff are unable to carry any real energy down the line. They attenuate rapidly with distance from the point of excitation and are usually negligible at a distance roughly equal to the transverse dimensions of the line.

The higher order modes will in general also be set up by a discontinuity in the line. If there is an abrupt change in cross section, and it is not possible to meet the boundary conditions at the discontinuity by addition of components of the principle and above-cutoff modes only, be-

low-cutoff modes will be excited at the point of discontinuity.

A wave guide is a rather narrow band transmission system. if a rectangular wave guide is used in the customary manner to carry only the dominant mode of transmission, the free-space Wavelength must not exceed twice the width of the guide, or the guide will be below cutofi for the dominant mode. On the other hand, if the free-space wavelength is equal to or less than the guide width, the guide will be above cutoff for the TEzo mode of transmission. Therefore, a 2:1 frequency range is the theoretical maximum if higher modes are to be avoided. This frequency range may be extended by artificially discriminating against these higher modes, according to the teaching of the present invention, as will be discussed hereafter.

For most applications, it is desirable that the frequency range be still further restricted. The attenuation rises very sharply as cutoff is approached, and for this reason it is usually desirable to restrict the maximum operating wavelength to about 1.60 times the inside width of the wave guide. At the short-wavelength end, one should not approach too near cutoff for the TE mode, as it will then attenuate very slowly when excited by a discontinuity. The minimum operating wavelength that is recommended is about 1.05 times the inside width of the guide. If these limits are adhered to, the wave guide may be used over a frequency range only slightly greater than 1.5:1.

To avoid the TEM mode of transmission, the inside height of the guide should be less than a half wavelength at the shortest operating wavelength and should therefore not exceed half the inside width of the guide. But decreasing the height still further below this value will increase the attenuation in the guide, which in this region will be approximately inversely proportional to the height of the guide. So for minimum attenuation, the inside height of the guide should be nearly half the width.

One Of the reasons for the limitation on the bandwidth of the normal rectangular guide is possibility of TEzo excitation. As the frequency is increased the TEzo is the first higher mode excited. Also discontinuities in the guide such as elbows, and changes in dimensions cause higher order TE and TM modes and energy will propagate in the 'IEzo mode as well as the TEm mode. The TE20 power will be reflected and cause undesirable resonances to effectively appear in the desired 'IEio mode operation. In the rectangular guides used to date, the guide dimension a/b ratio is somewhat larger than 2, so that the TEzo propagates before the TEo1 mode propagates.

If TEm and TE20 propagation is present, then at the point where the T'Ezo mode is generated a hypothetical equivalent transformer arrangement exists in which a separate TEm mode line is coupled to a TE20 mode line. It may be assumed that in the equivalent circuit the lines will carry only their own mode. If then the "SE20 line is properly terminated, no reaction back on the TEm lines will occur. Of course, this may mean a slight loss of energy; but it might be well worthwhile to pay this price in order to eliminate resonances, undesirable asymmetries in the guide and in the radiation pattern, etc., if the wave guide is used for radiation or for a primary source for a reflector or lens.

One of the problems is to terminate the T1320 line without terminating the TEm line, or at least so as to allow a free choice in how the TEm line is terminated. It is desirable to do this over a broad band; it is also desirable to terminate any higher order transmission modes which are coupled, even though this may re duce the total available power in the TEm mode. The present invention does this. Once higher order modes which can be transmitted are generated, it is desirable to absorb them rather than to attempt to convert them back into 'IEm mode energy. Although means for the reconversion are possible to devise, all likely ones would appear to be quite narrow band devices and would possibly do more damage than good to the propagation of the 'IEm mode. One approach is to separate the mode energies and to feed power back into the T1311) line in the proper phase, but the improbability of doing this properly over even a narrow band is so high as to make the absorption of the higher modes preferable.

In my copending application Serial No. 95,915 entitled Reflection Filters for Wave Guides filed May 28, 1949, I have disclosed mode filter means operating mainly on the principle of reflecting a desired mode of propagation, whereas the apparatus of the present invention is directed mainly towards absorbing or removing undesired modes, and passing through the desired modes without reflection.

Fig. 1A illustrates the well-known configuration of the T5110 mode in a rectangular waveguide; Fig. 1B illustrates the TE20 mode. It will be seen that the T2310 mode has a point of maximum intensity midway along the a dimension of the guide whereas the 'I'Eao mode has a null at this midpoint and has maximums at the onequarter and three-quarters points along the a dimension. Advantage of this electromagnetic field configuration may be taken for the purpose of selecting or discriminating against one or more modes of propagation.

In a rectangular wave guide, for any particular mode of transmission, the cutofl wavelength M is given in terms of the guide dimensions a and b by We) (t) In this formula, m and n are the subscripts denoting the particular mode under consideration (a g., TE'm and is is the limiting or cutoif wavelength. The equation holds for either TE or TM modes of transmission.

The size of guide necessary for transmission of some of the lower modes may be determined from the above equation. For example, the TE mode can carry energy in all sizes of wave guide in which d/A is greater than unity. To allow only the dominant mode to carry energy, one dimension of the guide should'not exceed A and should be less than M2 and the other should not exceed M2. The TEio mode alone will carry energy when a/i is between .5 and 1.0.

Assume a=2b and consider Fig. 2, .in which I all possible modes are plotted against cutoff Ac. At a A of more than 2a at the right hand edge of Fig. 4 all modes are cut off. As is decreased, 1. e., frequency increased, to where l=2a then the dominant TE io mode carries energy. When A becomes equal to a then the 'IEzo carries energy, etc.

It may be seen from Fig. 2 that as the frequency is increased the TEzo mode may be troublesome as it is the first of the vertically polarized higher modes to 'be excited. The present invention by absorbing the 'IEzo mode and passing the TEiu mode extends the useful frequency range of a given size wave guide.

As previously discussed in connection with Fig. 2, it is desirable in order to extend the useful bandwidth of a given size guide to discriminate against the 'IEzc mode. Fig. 3 shows means to ,do this, assuming that there is an undesirable TEzu :mode present.

Fig. 3 comprises a rectangular guide I having coaxial line input and output connections is and Hi, and a second guide 2 connected to the first guide in a T joint. The side arm extension of the main guide, that is, guide 2, has a septum or fin 3 of conductive material connected perpendicular to and midway along the longer dimension of the guide, thus, effectively shorting the two longer sides of the guide together at their middle point. the fin 3 will tend to reflect the TEm mode and prevent it from propagating up the guide '2. However, there is nothing to prevent the TE20 mode from propagating along the guide 2 and in this way a large proportion of the TEzo and J.

TEOl mode of the main guide will be diverted in the side guide 2.

It is desirable to terminate guide 2 for instance by filling in the spaces between fin 3 and the outer walls of the side guide with absorbing material i in order to absorb the energy of the TE2o and TEM modes and prevent its reflection back into the main guide. The absorbing material a may be polyiron, resistance card, cloth which is impregnated with carbon, or other material having high electrical resistive properties. Sand mixed with carbon has been used in some cases as an absorber. Any equivalent microwave attenuating material may be used. The guide 2 may be also terminated by some apparatus such as a receiver arranged to utilize the TEzo mode.

Fig. 3A shows another construction for achieving the same purpose. In this embodiment the side guide 2 contains two fins 5 and 6 of absorbing material placed at the one-quarter and threequarter points along the longer dimension, that is, the points of maximum intensity of the TEzo mode. Therefore, these absorbing structures at the points of maximum intensity of the TEzo t will be seen therefore that effect the Tiho mode current.

mode will tend to absorb that mode and they will tend to pass the TEm mode between them. This embodiment is preferable where it is desired to transmit some TEm energy in the side guide 2 for some purposes while at the same time achieving the desired result of attenuations or absorbing TEzo energy. The fins or struts in the embodiments of Fig. 3 do not have to be inserted flush with the surface of the main guide I as illustrated but may be withdrawn further up into the guide 2, if desirable. The arrangement of Fig. 3A may be used in a coupling device as well as a mode filter, as the TEm mode will tend to pass through it.

Fig. 4A shows current flow in the walls of the guide due to the TEm mode and Fig. 4B shows the current flow due to the TEzo mode. The slots 1 and 8 substantially interrupt the TEm current whereby the TElo mode will tend to be radiated through the slots.

However, the slots 1 and 8 do not appreciably Therefore, the 'IEzo mode will not be radiated through the walls but will be transmitted along the guide.

Fig. 5 illustrates an embodiment as explained in connection with Fig. 4 in which a pair of tapering slots 1 and t are in the top of the guide I, at the points of maximum intensity of the T1220 mode, leaving a continuous narrow portion in the center or the a dimension. A similar pair of slots may also be made in the bottom of the guide. This device will operate in the manner described in connection with Fig. 4, that is, the slot will radiate the TEim mode whereas it will not tend to radiate the 'IEzo mode.

Since it is desirable to separate the TEm and TEzo modes, a continuation guide it may be inserted over the cuts or slots, which are made sufficiently long to insure that only the TEio mode exists in the continuation guide l0 and only the TEzo mode in the main guide. The main guide is terminated with resistance material 4, absorbing the TE20 mode energy. Continuation guide ill or it becomes the main guide, depending on whether slots are cut in top only or top and bottom, Embodiments of this are shown in Fig.6 A and Fig. 613.

Fig. 7 illustrates another embodiment of the invention in which the TEzo mode is absorbed by a pair of resistance fins I! and I2 which are placed at the one-quarter and three-quarter distances across and perpendicular to the longer side of the guide. These resistance fins being placed at the points of maximum intensity of the TEzo mode absorb that mode but transmit the T1310 mode through, although with some attenuation.

The different embodiments may be used in cooperation, for instance the embodiments of Fig. '7 may be used in cooperation with that of Fig. 3. Thus any TEzo mode which was not absorbed in guide 2 of Fig. 3 would be further absorbed and attenuated by the resistance fins l2 of Fig. 7.

The teaching of the present invention has many advantages. By utilizing the mode sensitive filters of the present invention it is possible to utilize a given size wave guide for a greater band of frequency. In other words, the filters of the present invention tend to broaden the frequency band of conventional wave guides.

This technique is particularly useful for test equipment where flexibility and versatility are important factors. The present invention is not limited to air filled wave guides but may be used with other type guides.

Another application in which the present device may be utilized is the using of a single wave guide for several simultaneous communications, employing frequency division to multiplex them. The higher frequencies may be excited in the guide so as to propagate at the higher modes and the mode filters of the present invention may then be used to separate the diiierent frequencies.

To conclude, it has been shown that the present invention offers means and methods for utilizing rectangular guide over a wider than normal band without higher mode diiiiculties by virtue of absorbing the undesired higher modes.

Many immaterial variations may be made without departing from the scope of the present invention, for instance, in the types of absorbing material and in various combinations of the different constructions shown. Also specific designs may be evolved for particular applications involving particular modes of propagation within the scope of the teaching of the present invention.

What is claimed is:

A broad frequency band wave guide transmission system comprising a main rectangular wave guide transmission line, means for coupling a high frequency signal into said line at one end thereof, means for coupling said signal out of said line at the other end thereof, the broad cross-sectional dimension of the rectangular wave guide transmission line being greater than the free space wavelength at the frequency of said signal whereby the wave guide transmission line is capable of supporting higher order modes including the TEzo mode of propagation, and means positioned along said line between the signal input and output means for filtering ofiE energy propagated in the TEzo mode, said last-named means including a branch rectangular wave guide section joined at one end to the broad wall of said wave guide transmission line to form a series T junction therewith, a conductive fin inserted in said branch section parallel to and midway between the two narrow walls thereof, one edge of the fin lying in the plane of the broad wall of said transmission line at said junction whereby said branch section provides a very lcw impedance in series with said line for energy in the dominant TElO mode of propagation, and high frequency energy absorbing material positioned in the space between said fin and the adjacent walls of said branch section whereby said branch section provides a matched impedance in series with said line for absorbing energy in the TEzo mode of propagation.

References Qited in the file of this patent UNITED STATES PATENTS Number Name Date 2,088,749 King Aug. 3, 1937 2,425,345 Ring Aug. 12, 1947 2,472,038 Yando May 31, 1949 2,491,662 I-loughton Dec. 20, 1949 2,527,619 Brehm Oct. 31, 1950 2,589,843 Montgomery Mar. 18, 1952 OTHER REFERENCES Publication, i'iicrowave Transmission Circuits, vol. 9, Radiation Laborary Series pub lished May 21, 1948, by McGraw-I-Iill Publishing Co. of New York, pp. 425-427. (Copy in Div. 69.)

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