US3796970A

US3796970A - Orthogonal resonant filter for planar transmission lines

Info

Publication number: US3796970A
Application number: US00347839A
Authority: US
Inventors: W Snell
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1973-04-04
Filing date: 1973-04-04
Publication date: 1974-03-12
Anticipated expiration: 1991-03-12
Also published as: FR2224934B1; IT1009420B; GB1410458A; AU6733274A; JPS49131357A; DE2415284A1; AU472277B2; BE813098A; FR2224934A1; SE386552B; NL7404480A; CA984473A

Abstract

The conductor pattern on a dielectric substrate includes a rectangular element of finite thickness whose two surface dimensions are chosen so that each is one-half the wavelength of a desired frequency. Accordingly, the element will support two resonant orthogonal standing waves and external coupling to each wave may be provided independently. Common coupling to both waves may also be provided so that the dual resonant structure may be used as a three-port device for frequency combining and separating and is particularly well-suited for frequency conversion applications. A two-port arrangement permits generation of specific filter characteristics.

Description

United States Patent [191 Snell, Jr. Mar. 12, 1974 [5 ORTHOGONAL RESONANT FILTER FOR 3,560,887 2/1971 Napoli et al. 333/73 s PLANAR TRANSMISSION LINES [75] Inventor: William Walter Snell, Jr., Pri ry Examinerl L. Gensler Middletown Twp., Mon uth Attorney, Agent, or Firm-D. L. Hurewitz County, NJ.

[73] Assignee: Bell Telephone Laboratories, 57 ABSTRACT Incorporated, Murray Hill, NJ. 1 The conductor pattern on a dielectric substrate in- [22] Ffled' 1973 cludes a rectangular element of finite thickness whose [211 App]. No.: 347,839 two surface dimensions are chosen so that each is onehalf the wavelength of a desired frequency. Accord- [52] U S Cl 333/6 325/445 330/4 9 ingly, the element will support two resonant orthogo- 333/82 R 533/84 nal standing waves and external coupling to each wave [51] Int Cl H01; 1/20 i 5/12 H04b 1/26 may be provided independently. Common coupling to [58] Fieid 'g' 333/73 S M 6 76 both waves may also be provided so that the dual reso- 333/83 R W 69 5 nant structure may be used as a three-port device for 330/49; frequency combining and separating and is particularly well-suited for frequency conversion applica- [56] References Cited tions. A two-port arrangement permits generation of UNITED STATES PATENTS specific filter characteristics.

3,451,015 6/1969 Heath 333/73 S 16 Claims, 9 Drawing Figures N0 COUPLING l5 3 Ila.

NO COUPLI NG PAIENTEnnm m4 3.796.970

SHEET 1 0F 3 A FIG/A K K A I9 I8 NO COUPLING TO S PAIENIEUIAR 1 2 I914 3796370 ShEEI 2 BF 3 FIG. 4

PATENIEUMAR 1 2 2974 33961970 sum 3 or 3 FIG. 6A

ORTI-IOGONAL RESONANT FILTER FOR PLANAR TRANSMISSION LINES BACKGROUND OF THE INVENTION This invention relates to planar transmission line structures, such as microstrip or stripline, and more particularly, to filter circuits utilizing transmission elements dimensioned to support resonance at selected frequencies.

Hollow waveguides have been used as transmission media formany years. However, with current interest in higher frequency signals, such as millimeter waves and above, planar transmission lines are more attractive than waveguide structures, since the smaller dimensions of planar transmission lines make fabrication easier, and reasonably sized devices provide appropriate impedance levels, such as 50 ohms, while similar impedances are difficult to obtain in waveguide structures.

Cavity resonators are commonly employed as filters with waveguide structures and the resonance phenomenon is also used to provide filtering in both microstrip and stripline types of planar transmission lines. Rectangular portions of the conductive pattern may be designed to have their major dimensions equal to one-half the wavelength of a desired frequency so that the rectangles will function as filters for that frequency. Both end-coupling and side-coupling are used vto transfer energy into and out of these filter elements, but conventional design techniques prescribe the widths of the elements. By following the accepted procedures, such as are set forth in Chapter of Microwave Filters, Impedance-Matching Networks, and Coupling Struclures, by G. L. Matthaei, L. Young and E. M. T. Jones (1964), the width is always very narrow relative to the length in order to insure that an elements conductance satisfies the design equations. This narrow dimension does not support resonance in a transverse mode at or near the frequency of operation.

It is an object of the present invention to provide a planar transmission line structure capable of supporting multiple resonances. In particular, it is an object to provide such a structure capable of supporting two orthogonal resonances.

SUMMARY OF THE INVENTION In accordance with the present invention the conductor pattern of a planar transmission line, such as a microstrip or stripline, includes a rectangular conductive element physically isolated from, but electrically coupled to, the rest of the pattern by conductive leads. A first standing wave can be excited to resonate between two opposite edges of the element by coupling a signal having a wavelength equal to twice the distance between these opposing edges. Simultaneously, another standing wave can be excited to resonate normal to the first wave by coupling a signal having a wavelength equal to twice the distance between the two edges orthogonal to the first. Coupling to the resonator is reciprocal so that energy may be coupled into or out of the resonator by either capacitive or inductive coupling between the resonator element and the conductive leads.

The resonating element has a finite thickness and is deposited upon a dielectric substrate by conventional techniques identical to those used to create the rest of the conductor pattern. The dimensions of the resonator are chosen so that two signals of selected frequencies each resonate in orthogonal modes with total isolation within the resonator. A coupling conductor located at an edge but off its midpoint couples to both modes while a coupling conductor located at an edge and symmetrically positioned with respect to the edges midpoint, couples only to the resonance transverse to that edge.

Thus, a three-port device can be used as both a dual signal filter and combiner by locating a pair of conductors so that each exclusively excites one resonant mode and a common conductor coupling energy from both. This configuration, which can be used reciprocally as a separator, finds application in frequency converters.

Supporting dual resonant signals in accordance with the principles of the invention also permits specific filter resposes to be generated. For example, a two-port device can be arranged to couple to a nearly square rectangular resonator element so that a single input excites the two orthogonal resonances at nearly identical frequencies and a single output lead couples energy from both resonating modes out of the resonator. With specific coupling combinations the relative phasing between the energy associated with the two modes can be adjusted to generate either notch or doubly-tuned characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a section of a microstrip type of planar transmission line illustrating a resonator circuit in accordance with the present invention;

FIG. IA is a section view taken along line 1A of FIG. 1, showing the shielding structure used in conjunction with a planar transmission line;

FIG. 2 is a representation illustrating the instantaneous standing wave patterns of the orthogonal resonances in the resonator of FIG. 1; I

FIG. 3 illustrates a modified version of the structure of FIG. 1;

FIG. 4 illustrates a frequency converter circuit utilizing the resonator of FIG. 1;

FIG. 5 illustrates a specific filter arrangement in accordance with the principles of the invention;

FIG. 5A is a graphical presentation of the filter characteristic associated with the structure of FIG. 5;

FIG. 6 illustrates another specific filter arrangement in accordance with the principles of the invention; and

DETAILED DESCRIPTION FIGS. 1 and 1A illustrate a section of planar transmission line 10, whose conductor pattern includes resonator element 1 1 capable of supporting two orthogonal resonant modes. These figures depict a microstrip type of planar transmission line having dielectric substrate 12 mounted upon ground plane 13 and conductor pattern 14 deposited upon the substrate parallel to ground plane 13. The conductor pattern, which has a finite thickness, at least as great as the electrical penetration at the transmission frequency (on the order of a few microns for GI-lz frequencies), is normally deposited by conventional evaporation techniques. In the section of transmission line shown, this conductor pattern 14 consists of resonator 1 1 and conductor leads 15, 16 and 17 for coupling to the resonator.

The microstrip line is enclosed by a conductive shield conveniently formed by two separable portions. The first portion forms a channel 18, whose internal surface constitutes ground plane 13 and whose side walls define the width of the channel. The other portion, which forms a cover 19, mates with channel 18 by conventional means to complete the enclosure and form a rectangular cross section. The shield is not absolutely necessary for operation of the invention, but microstrip lines having substrates of low dielectric constant are conventionally enclosed by such a shield to prevent loss due to radiation of energy from resonating elements. Accordingly, channel 18 and cover 19 are included in the interest of completeness. Of course, the planar transmission line may also be of the stripline variety where two ground planes are provided and the conductor pattern is suspended between the ground planes. The stripline embodiment may also be shielded in which case the two surfaces of the shielding structure parallel to the conductor pattern would constitute the ground planes.

Conductor lead couples one signal to resonator 11 by the capacitive effect between the narrow end oflead 15 and edge 22 of resonator 11. Similarly, a second signal'is capacitively coupled to resonator 11 by lead 16 which couples to edge 21. Lead 16 is shown passing through the side wall of channel 18 where it may couple to the second signal propagating, for example, in a waveguide (not shown) adjacent the shielded transmission line 10.

Resonator 11 is rectangular in shape and

adjacent edges

21 and 22 are therefore perpendicular. The electrical length l, of edge 21 is chosen to be one-half a wavelength of the frequency of the first signal coupled from lead 15 and the electrical length 1 of edge 22 is chosen to be one-half the wavelength of the frequency of the second signal coupled from lead 16. The physical lengths of

edges

21 and 22 are minutely shorter than the electrical lengths l, and 1 since the effects of fringing and loading extend the electrical length slightly beyond the boundaries of resonator 11.

The dimensions of resonator l1 permit the two signals to simultaneously resonate in transverse modes without interacting with each other. Furthermore, the intrinsic Q of each resonance is many times higher than in a conventional narrow width, single resonant filter for the same frequency since the dimension transverse to the direction of resonance is much wider than in the conventional filter.

The operation of the orthogonal mode resonator may be further explained with reference to FIG. 2 where it is assumed that signal S, at frequency m is applied via conductor lead 15 and signal S at frequency m is applied via conductor lead 16. S couples to edge 22 and establishes a sinusoidal voltage which varies with the distance along edge 21 as shown instantaneously as IE at the bottom of FIG. 2. Since the overall electrical length 1 is one-half M, where A; is the wavelength corresponding to an, a standing wave is established and a null results at the electrical midpoint of length 1,. If the other signal, S is coupled transversely to the length 1 at this midpoint, it will resonate in a transverse mode without coupling to 8;. Thus, if conductor lead 16 is positioned symmetrically with respect to centerline 25, which is the perpendicular bisector of dimension 1,, it will see a null of the E voltage. If the end of lead 16 extends to both sides of line 25, it is noted that coupling to S will occur to the right of line 25, but simultaneously an equal and opposite coupling will occur on the left of line 25 and the net result will be no coupling to the resonating S signal if lead 16 is centered on line 25.

Similarly, signal S will generate the standing wave pattern shown instantaneously as E at the right of FIG. 2 and a null will result at the midpoint of length 1 along its perpendicular bisector, centerline 26. Accordingly, if lead 15 is centered along this line, resonating E energy will not couple to the resonating E energy.

It is noted that the degree of coupling to the resonator is dependent upon the spacing between the ends of the conductors and the edges of the resonator. The loaded Q of the resonance can, of course, be adjusted by adjusting this coupling. As can be seen in FIGS. 1

i and 2, tighter coupling is provided between lead 16 and resonating energy from resonator 11 to external circuitry. For convenience, however, conductive lead 17 in FIG. 1 will be referred to as the output port. This lead is centered on neither line 25 nor line 26 and will accordingly couple to both orthogonal resonating modes. Lead 17 is located at corner 23 so that it couples substantially equally to both standing waves, but if it were located along one edge between the centerline and a corner, it would also couple to both waves, to a greater or lesser degree, depending upon its location. Any lead locatedoff the centerline will therefore commonly couple to the two orthogonal modes.

The conductor leads are shown in FIG. 1 as being capacitively coupled but they may also be inductively coupled. As shown in FIG. 3, a modified version of FIG. 1,'common lead 37, which couples to both orthogonal resonances, can be inductively coupled by placing its side, or longer edge, parallel to an edge, such as 31, of resonator 11. The side-coupling of lead 37 also pro vides a capacitive effect sothat the total coupling is a result of the combination of inductive and capacitive coupling.

FIG. 3 also shows an additional lead 37' which may be used as a second output'port. This auxiliary lead is positioned at corner 24 and therefore couples in common to both orthogonal modes. The absolute values of energy coupled into leads 37 and 37' are identical. However, a degree phase difference exists between

corners

23 and 24 for the energy resonating as a result of excitation at lead 16. This can be seen by the instantaneous plot of the standing wave shown at the right side of FIG. 2. No phase difference exists between

corners

23 and 24 for energy resonating'as a result of excitation from lead 15. Due to the phase reversal of only one resonance the outputs at leads 37 and 37' may be usefully combined in applications such as balanced downconverters.

FIG. 4 illustrates rectangular resonator element 41 in a frequency conversion circuit. Lead 47 forms a series connection with mixer element 42, filter 43 and port 44. If leads 45 and 46 provide the pump and RF signal inputs, respectively, resonator 41 will provide filtering of each signal while the output on lead 47 will contain a combination of both signals. This combination is then heterodyned in mixer 42 which may be Schottky Barrier Diode if the circuit is used for downconversion. The product of the mixer is applied to a conventional lowpass filter, such as lumped element filter 43, which passes the IF output frequency to port 44. Due to the frequency selectivity of resonator 41 with regard to the RF signal, only one sideband is applied to mixer 42 so that the circuit has the characteristics of a singlesideband downconverter.

Of course, the resonator operates reciprocally and single-sideband upconversion may be provided by utilizing as the mixer instead of a Schottky diode another appropriate mixing device. For instance, a nonlinear resistive diode or a nonlinear capacitor, such as a varactor diode, may be used. ln this case, the pump signal from lead 45 would be coupled from resonator 41 to lead 47. It would then be heterodyned (or parametrically amplified ifa varactor diode were used) in 42 with an IF input signal from port 44 to produce a doublesideband product at lead 47. Only one sideband couples from lead 47 to resonator 41 where it is removed via lead 46.

Another application of the orthogonal resonator is shown in FIG. 5. This is a two-port device with each port connected to a lead 52 or 53. These leads are placed respectively at

opposite corners

54 and 55 of resonator 51, and thus located each is provided with essentially equal coupling to both orthogonal modes in resonator 51. The electrical dimensions d and d are selected so that both resonant frequencies add constructively in parallel to produce a doubly-tuned bandpass characteristic illustrated in FIG. 5A. This is accomplished by making a and d nearly identical, differing only enough so that their respective resonant frequenciesf andf are separated by Af, which is one-half the frequency of the overall desired doubly-tuned filter. It is noted that the coupling between the leads and the resonator can be adjusted to realize loaded Qs for both orthogonal resonances which are twice the loaded Q of the overall filter response as required to produce the desired doubly-tuned characteristic.

FIG. 6 illustrates another two-port orthogonally resonant filter designed to produce a notch filter characteristic. Resonantor element 61 is, like resonator element 51, a nearly square rectangle. The input is coupled via lead 62 to corner 64 and the output is coupled at adjacent corner 65 to lead 63. The notch filter operates similarly to the doubly-tuned filter of FIG. 5, except that the location of lead 63 at corner 65 causes the energy of one resonating wave to be coupled I SQ degrees out-of-phase from that coupled to output lead 53 in the doubly-tuned filter. This results in destructive interfer' ence, rather than constructive interference, with the resulting filter characteristic shown in FIG. 6A. The notch frequency f,., may be located as desired within the band by appropriate selection of the dimensions h and hg of resonator 61, and by adjustment of the coupling at leads 62 and 63.

In all cases it is to be understood that the abovedescribed arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a resonant filter operative in a selected frequency band, said filter being included in said conductive pattern and comprising:

a rectangular element of conductive material having a first pair of opposite edges being dimensioned equal to one'half the wavelength of a first selected frequency and the other pair of opposite edges adjacent the first edge being dimensioned equal to one-half the wavelength of a second selected frequency, both said first and second frequencies being within said frequency band,

said element beingcapable of supporting two orthogonal resonant standing waves, the first wave producing a first resonant mode between the two edges of the other pair of edges, the second wave producing a second resonant mode orthogonal to the first and between the two edges of the first pair of edges,

at least three ports,

means for exclusively coupling energy between the first port and the first resonant mode, I means for exclusively coupling energy between the second port and the second resonant mode, and means for simultaneously coupling energy between the thirdport and the orthogonal first and second resonant modes.

2. A resonant filter as claimed in claim 1 wherein each means for coupling energy between a port and a resonant mode includes a conductor lead, an edge of the lead being positioned parallel to an edge of the element and spaced apart therefrom so that capacitive coupling exists between the lead and the element.

3. A resonant filter as claimed in claim 2 wherein the spacing between the edge of the lead and the edge of the element is selected to establish the Q of the resonance coupled via said lead.

4. A resonant filter as claimed in claim 2 wherein at least one of the leads has its side positioned parallel to and spaced apart from the edge of the element so that inductive coupling exists between the lead and the element.

5. A resonant filter as claimed in claim 1 wherein said means for exclusively coupling energy between the first port and the first mode includes a first conductor lead being positioned symmetrically with respect to the centerline perpendicularly bisecting the electrical length of one edge of the other pair of edges.

6. A resonant filter as claimed in claim 5 wherein said means for exclusively coupling energy between the second port and the second resonant mode includes a second conductor lead being positioned symmetrically with respect to the centerline perpendicularly bisecting the electrical length of one edge of the first pair of edges.

7. A resonant filter as claimed in claim 1 wherein said means for simultaneously coupling energy between the third port and the orthogonal first and second resonant modes includes a common conductor lead being positioned at a first corner of the element to couple substantially equally to both orthogonal resonating modes.

mayhem shiftedin phase by 180 degrees from tlie energy of that mode coupled at the first corner.

9. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a frequency converter circuit included in said conductive pattern and comprising:

a rectangular element of conductive material being dimensioned to support two orthogonal resonant modes at two selected frequencies,

at least three ports,

means for exclusively coupling a pump signal at the first port to resonate in a first mode within theelement,

means for exclusively coupling energy between the second port and a second resonant mode orthogonal to the first mode within the element,

means for simultaneously coupling energy between a common point and both orthogonal modes, and

mixing means connected to the common point and to the third port for interacting the pump signal and another signal applied thereto to produce a third signal.

10. A frequency converter circuit as claimed in claim 9 wherein an RF signal is applied via the second port to the element and both the pump signal and the input RF signal are coupled from the element at the common point, and wherein said mixing means is a Schottky Barrier Diode which heterodynes the pump signal and the input RF signal to produce an IF output at the third port.

1 l. A frequency converter circuit as claimed in claim 9 wherein an input IF signal is applied to the mixing means via the third port, and the pump signal is coupled from the element at the common point, and wherein said mixing means is a varactor diode which parametrically amplifies the input IF signal to produce an RF output signal at the common point.

12. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a resonant filter included in said conductive pattern, and comprising:

a rectangular element of conductive material being dimensioned to support two orthogonal resonant modes at a first and second frequency, respectively,

means for coupling energy from external the element to excite the two orthogonal resonant modes, and

common means for simultaneously coupling energy from both the excited orthogonal modes to external the element.

13. Aresonant filter as claimed in claim 12, wherein said filter has a first and second. port, and wherein said means for coupling energy from external the element is a single input conductor lead located at one corner of the element and wherein said common means for simultaneously coupling energy from both the excited orthogonal modes is an output conductor lead located at another corner of the element.

14. A resonant filter as claimed in claim 13 wherein the. element is nearly square, the locations of the input and output leads being selected so that energy of each of said resonant modes couples to the output conductor lead with constructive addition to produce a doublytuned frequency response with the center frequency of the response located between the first and second frequencies. Y

15. A resonant filter as claimed in claim 13 wherein the element is nearly square, the location of said leads being selected so that the energy of each of the resonant modes couples to the output conductor lead with destructive interference to produce a notch frequency response.

16. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a resonant filter operative in a selected frequency band, said filter being included in said conductive pattern and comprising:

a rectangular element of conductive material having a first pair of opposite edges being dimensioned equal to one-half the wavelength of a first selected frequency, and means for coupling energy into the element to excite a resonant standing wave at the first frequency,

characterized in that the other pair of opposite edges of the element adjacent the first edges are dimensioned equal to one-half the wavelength of a second selected frequency, both said first and second frequencies being in said selected frequency band,

said element is capable of supporting two orthogonal resonant standing waves at the first and second frequencies,

said means for coupling energy includes at least one conductor lead and is capable of exciting the two orthogonal resonating waves, and

said filter further includes means for simultaneously coupling energy from both the excited resonant orthogonal waves to external the element.

"UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,796 97O Dated March 12, l9Z 4 Inv nt William w. Snell Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column5, line 3 3, "opposite" should read -adjacent--.

In column 5, line 5 "adjacent" should read --opposite--.

In claim 1, at column 6, line 16, "edge" should read --edges-.

Signedjand sealed this 16th day of July 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 603764 69 =ORM PO-105O (10-69) u.s sovimmun mu'rms ornc: nu o-aee-au.

Claims

1. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a resonant filter operative in a selected frequency band, said filter being included in said conductive pattern and comprising: a rectangular element of conductive material having a first pair of opposite edges being dimensioned equal to one-half the wavelength of a first selected frequency and the other pair of opposite edges adjacent the first edge being dimensioned equal to one-half the wavelength of a second selected frequency, both said first and second frequencies being within said frequency band, said element being capable of supporting two orthogonal resonant standing waves, the first wave producing a first resonant mode between the two edges of the other pair of edges, the second wave producing a second resonant mode orthogonal to the first and between the two edges of the first pair of edges, at least three ports, means for exclusively coupling energy between the first port and the first resonant mode, means for exclusively coupling energy between the second port and the second resonant mode, and means for simultaneously couplinG energy between the third port and the orthogonal first and second resonant modes.

8. A resonant filter as claimed in claim 7 wherein said means for simultaneously coupling energy between the third port and the orthogonal first and second resonating modes further includes an auxiliary conductor lead being positioned at a second corner of the element adjacent the first corner to couple substantially equally to both orthogonal modes, the energy of one of said modes being shifted in phase by 180 degrees from the energy of that mode coupled at the first corner.

9. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a frequency converter circuit included in said conductive pattern and comprising: a rectangular element of conductive material being dimensioned to support two orthogonal resonant modes at two selected frequencies, at least three ports, means for exclusively coupling a pump signal at the first port to resonate in a first mode within the element, means for exclusively coupling energy between the second port and a second resonant mode orthogonal to the first mode within the element, means for simultaneously coupling energy between a common point and both orthogonal modes, and mixing means connected to the common point and to the third port for interacting the pump signal and another signal applied thereto to produce a third signal.

11. A frequency converter circuit as claimed in claim 9 wherein an input IF signal is applied to the mixing means via the third port, and the pump signal is coupled from the element at the common point, and wherein said mixing means is a varactor diode which parametrically amplifies the input IF signal to produce an RF output signal at the common point.

12. In a planar transmission circuit having a ground plane, a substrate, and A conductive pattern on the substrate parallel to the ground plane, a resonant filter included in said conductive pattern, and comprising: a rectangular element of conductive material being dimensioned to support two orthogonal resonant modes at a first and second frequency, respectively, means for coupling energy from external the element to excite the two orthogonal resonant modes, and common means for simultaneously coupling energy from both the excited orthogonal modes to external the element.

13. A resonant filter as claimed in claim 12, wherein said filter has a first and second port, and wherein said means for coupling energy from external the element is a single input conductor lead located at one corner of the element and wherein said common means for simultaneously coupling energy from both the excited orthogonal modes is an output conductor lead located at another corner of the element.

14. A resonant filter as claimed in claim 13 wherein the element is nearly square, the locations of the input and output leads being selected so that energy of each of said resonant modes couples to the output conductor lead with constructive addition to produce a doubly-tuned frequency response with the center frequency of the response located between the first and second frequencies.

16. In a planar transmission circuit having a ground plane, a substrate, and a conductive pattern on the substrate parallel to the ground plane, a resonant filter operative in a selected frequency band, said filter being included in said conductive pattern and comprising: a rectangular element of conductive material having a first pair of opposite edges being dimensioned equal to one-half the wavelength of a first selected frequency, and means for coupling energy into the element to excite a resonant standing wave at the first frequency, characterized in that the other pair of opposite edges of the element adjacent the first edges are dimensioned equal to one-half the wavelength of a second selected frequency, both said first and second frequencies being in said selected frequency band, said element is capable of supporting two orthogonal resonant standing waves at the first and second frequencies, said means for coupling energy includes at least one conductor lead and is capable of exciting the two orthogonal resonating waves, and said filter further includes means for simultaneously coupling energy from both the excited resonant orthogonal waves to external the element.