US3859600A

US3859600A - Waveguide mixer

Info

Publication number: US3859600A
Application number: US409487A
Authority: US
Inventors: George Frederick Craven; Raymond Richard Thomas; Maurice Francis Berger
Original assignee: International Standard Electric Corp
Current assignee: STC PLC
Priority date: 1973-10-25
Filing date: 1973-10-25
Publication date: 1975-01-07
Anticipated expiration: 1992-01-07

Abstract

There is disclosed an evanescent mode waveguide mixer. A first length of evanescent mode waveguide is provided having one end coupled to a signal source and the other end coupled to a local oscillator. A resonant slot, either a longitudinal or a transverse resonant slot, is provided intermediate the two ends of the first length of waveguide so that the first length of waveguide between the resonant slot and the signal source constitutes an evanescent mode filter for coupling the signal of the signal source to the resonant slot and so that the first length of waveguide between the resonant slot and the local oscillator constitutes an evanescent mode filter for coupling the signal of the local oscillator to the resonant slot. A second length of waveguide is disposed at right angles to the first length of waveguide and disposed to receive energy through the resonant slot. A mixer diode is disposed in the second length of waveguide closely adjacent the resonant slot. The second length of waveguide is dimensioned to be evanescent at the mixer generated frequency and propagating at the mixer generated sum frequency. Reactance components are provided in the second length of waveguide beyond the mixer diode to provide independent adjustment to terminate the image and sum frequencies.

Description

J/G/VAL L United States Patent 191 Craven et a1.

[ WAVEGUIDE MIXER [75] Inventors: George Frederick Craven; Raymond Richard Thomas, both of Harlow; Maurice Francis Berger, Old Harlow, all of England [73] Assignee: International Standard Electric Corporation, New York, NY.

22 Filed: Oct. 25, 1973 21 Appl. No.: 409,487

[52] US. Cl. 325/445, 321/69 W [51] Int. Cl. H03d 7/18 [58] Field of Search 325/442-445,

Primary ExaminerBenedict V. Safourek Attorney, Agent, or Firm-John T. OHalloran; Menotti J. Lombardi, Jr. Alfred C. Hill Jan.7, 1975 [57] ABSTRACT There is disclosed an evanescent mode waveguide mixer. A first length of evanescent mode waveguide is provided having one end coupled to a signal source and the other end coupled to a local oscillator. A resonant slot, either a longitudinal or a transverse resonant slot, is provided intermediate the two ends of the first length of waveguide so that the first length of waveguide between the resonant slot and the signal source constitutes an evanescent mode filter for coupling the signal of the signal source to the resonant slot and so that the first length of waveguide between the resonant slot and the local oscillator constitutes an evanescent mode filter for coupling the signal of the local oscillator to the resonant slot. A second length of waveguide is disposed at right'angles to the first length of waveguide and disposed to receive energy through the resonant slot. A mixer diode is disposed in the second length of waveguide closely adjacent the resonant slot. The second length of waveguide is dimensioned to be evanescent at the mixer generated frequency and propagating at the mixer generated sum frequency. Reactance components are provided in the second length of waveguide beyond the mixer diode to provide independent adjustment to terminate the image and sum frequencies.

12 Claims, 14 Drawing Figures A Pz r. n

S/G/VAL PATENTEDJAN H915 3.859.600

SHEET 2 BF 4 Ibo PATENTEU JAN 7 75 SHEET 3 UF 4 5.4 L n ems a f,-f (the intermediate frequencyvf a -f;vvAvEoUInE'-Mixen, iB'ACKGR OUND OF THE INV TIQ This invention relates ment.

generated frequencies include -fl,+f (the sum frequency) sorbedin the-isolator.

to a Waveguide mixerarrange-f fin the second waveguide a first given distance from the first resonant slot to provide within the second wave- 'guide an evanescent mode resonator tuned to the image frequency so as to effect reactive termination of 4 I I r v M I I-thejmagefrequency; and a short circuit plate disposed As is well known,the basic mixing process generates a number offrequencies of which the following a'r'e' -im in theseciond waveguide, a second .given distance "'greate r ,than the first given distance from the first resoportant inthe present invention. Calling the signal and 5 local oscillator frequencies f, and f respectively, the T- gin general, the; properties of isolatorsmake them ,de-

i sirable in application'swhere the inputmatch m'us t' be maintained independently of minorfc-hangesfinthe charac.teristics of the mixer diode. If these advantages. aretto be"retainedwith the irnproved co'nversionloss which-is obtainedb'y reactively terminatingtheimage freque'ney,f a" 'more. complex circuit is necessary. The

necessaryaddition comprises abandstop filter, located between theisolat'orand'the mixerandltunedtothe image frequ encyl fSucna circuit is welllknown in the; waveguide art butjhjas somedisadvantages; The "rho st h serious disadvantage is that in practice it is possible to,

terminate reactivelyfth'e, image, frequency onlyl- At the sum frequency the waveguide is overmodedand atleast two or moremodes will exist i'nthe waveguidek Thus,

a satisfactory termination of the-j'sum'frequency isnot possible and in certain cases' additionalip r'oljlems,infthe group delay characteristics of the" mixerjarise from this; A further disadvantageiresults fromtheyadditional bandstopv filter requiredwhich contributestd the" com, plexity of the circuit. J

" SUMMARY OF THE INVENTIQN} A a An object of the present-invention -is ,topermitreac' tive termination of both the image and 'the sunrifre quencies in a waveguide-mixer arrangement Another object f the present inventiori is' o permi variation ofthe electrical characteristics of the wave guide mixer arrangement in a'si mple manner over wide range of impedar'ices; a

A featureof the presentinvention is the provision 'o" a waveguide mixer arrangement comprising 'a' 'firsf waveguide, one end ofwhich receivesan input vsigna having a first given frequency and thefiotherfendbf which receives alocal oscillator 'sign alh'avingja seclond given frequency different thari I-th'e first given fre- I g i i r v Zwilltbej. described =later in detail, butit' 'may be stated quency; a first resonantslot'disposed inone wall of the first waveguide intermediate theendsthereof; the first waveguide including'a first evanescentnnode waveI- guide filter tocouple the-input signal tofthe 'first'reso nant slot, and a second evanescentmodewaveguide fil- 3 slot, the second waveguide being dimensioned to be ev,-' anescent at 'the rnixer generated image frequency and propagating at the. mixer generated sum frequency; a mixer diode disposed in the second waveguide adjacent to the first resonant slot; a capacitive obstacle disposed N the mixer operation; and? v de'signed for operationat' a1 signal :fr'equen' f-i7 .570 Gl-lz (gigahertzlanda localoscillatorfrequency I 301,) of 7 5O O GHZ Theinterrrfediate frequency tfl O .O7 ,O' GHz, thes um freq u'encyKfl-lffl) is l 5.0'7( ):GHz,5 v nd'fih m se sq e y'tzfaef l 5 Ihe'fbasicmixer 'arrangenjent'coiriprises a flanged length of;rectangularlcross sectionfwaveguide ltvvhich fis so' dimensioned'as-to halveacut offfrequency abovethelf requenciesof both the signalland'thellocal oscillatOrL Exteiidin'gIinto'rthefiwaveguide"from the lower; roadwlall thereof arecapacitive screws .2. In the upperT-i firoadwallfare.two ilongitudinal narrowslots 3 fa nd 4 [BRI F DES RIPTION OF THE DRAWING I The above-mentioned and other features and objects V 2fl,f,=fi,+(f f =fl, i (fi (the image frequency).

i In a'mixer-circuit in which thesignalsource includes] j a unidirectional devicesuch as acirculatoror isolator,-

and'the isolator is coupled directly to themixer, both '1 the sumandimage frequencies will be completely ab-,

of this inventionwillfbecorhe more apparent by referwith the accompanying drawing, in which:

invention;

FIGS, 2 and3are; respectively, a sectioned plan view taken along line B-f-B of FlGQl and'an enlarged se'cti o'ned endvie'w'takenalong-line 'AI"A of FIG. 1 i I Q-FIGYSJ4 to 9- illustrate' successive stagespin presenting: i 1

SFIGF IO shows the performancecharacteristic a mixer diodemount;. I

I los l 1 and ;1 2 relate to th eory "'FIo'. 3 indicatesan alternative slot the mixerarrangementof thepresentiinvlention I g D SCRIPTIO of THE Ii'IiEjF'EI'iRE D LEMBOD I M ENT "{The mixer arrangement shown. in Fl G v l i si z" nil} i (IQXO ach centered, :on'jthe. respective transverse center line of twocapa'citive"screws};Amixer diodes is mounted cloisely ladjacen't fthe-planel'of slot, 3 Extending, from "jea chfof'slotsfi and 4am waveguide lengths 6 arr-(1,7,; eachoffrectangular-cross sectiOn;each containing my 7, 7 adjustable

capacitive screw

8 and 9, respectively, and, 1 each' te rminatedbyalconducting,plate' l0 and ll,"re"-" fspectively TheQnature and fiinctio'nof"these twoarms here. that these two armsjc'on'stituite, respectively,' image (and sum frequency reactive terminating'circuitsj and a Y rejection filter for theiocal oscillator frequency. a

the left hand end'ofnw'aveg uide 1; fan dithe "local oscillator. signal having a frequency fliS, 'fedfintogthe right hand end of waveguide 1.-, En'ergy?transferthrough the wave-E; guide at the respective frequencies-isachieved by adjustin'g the depth of penetration of the ,c'apacitivef screws 2 sothat the wave'guide'sejctions oheitherside' of the mixer diode 5 functionaseyanescent dominant, mode bandpass filters centered res pec tively-, onjtheg'l The input signal havin'g'a frequency flvis fe'd'into the principleof operation ofth etmixer arrangement of; f the presentinvention; 1

arrangement for? I input signal and local oscillator signal frequencies. The basic functioning of such filters is more fully described in British Pat. No. 1,129,185.

With regard to the filtr for the input signal, this has a two-fold purpose. It is convenient to have a short length of waveguide between the mixer and the input flange, where connection to the signal isolator is made. This section must be resonated in order to .effect power transfer. I-Iowever,,the filter satisfies a more basic purpose in providing a broad band harmonic rejection filter, which provides protection from undesired frequencies (say above GHz in the present example) which would otherwise pass through the transparent (at these frequencies) signal filter and mix with harmonics of the local oscillator to produce an undesired I.F. signal. A broad passband of signal and image frequencies ensures that the source impedance, as seen by the mixer, at signal and image frequencies, is essentially that of the isolator connected at the input flange.

With mixer diode 5 suitably matched to the two frequencies, the input signal frequency and the local oscillator signal frequency, the basic technique of matching a mixer diode mount in evancescent mode waveguide filters being more fully described in British Pat. No. 1,187,283, the resulting intermediate frequency is derived from mixer diode 5 via a low pass filter 12 in a coaxial output connector 13.

Before describing the image terminating technique, an understanding of the principles on which the mixer mount functions is necessary. This can be obtained with the aid of FIG. 4 which shows diode 5 mounted across the narrow longitudinal slot 3 in the broad wall of waveguide I. The effect of the slot in the guide can be better understood by considering evanescent mode resonance from a slightly different standpoint from that of the above first-mentioned British patent.

If one considers a thin slice of cut-off waveguide along its transverse axis, FIG. 5, it may be considered as a transmission line less than half a wavelength long (because it is below cut-off). By adding a capacitance C, FIG. 6, the electrical length can be increased until it is a half wavelength long, i.e., 6 4) M2, where 0 the electrical length of the waveguide and 4) equivalent electrical length contributed by C, resonance will be modified somewhat by the environment on either side. This is the fundamental way in which resonance occurs. Although this is simply a way of describing a phenomenon which may be described more accurately by other means, it is a viewpoint that leads to new configurations. For example, a transmission line which is shorter than a half wavelength may be brought to the resonant half-wave condition by adding series inductance, FIG. 7, where 2(6/2 (1),) )\/2. This series inductance required to realize the transverse resonance in cut-off waveguide may be obtained by a short (less than M2) longitudinal slot, FIG. 8.

It will be evident that the narrow slot in FIG. 8 will provide an ideal interconnection point between small semiconductor devices, such as mixer diodes, etc., and the waveguide circuit. Diodes can be directly integrated into the slot, FIG. 9, if desired, thus, reducing the external lead inductance of the device to zero. Wide band mounts may be realized in this way.

In certain applications narrow band mounts may be more convenient; provided the mount has a high unloaded Q-factor it may serve as the final resonator of an evanescent mode filter associated with the mount.

The loaded Q-factor may be raised by simply shortening the slot, which will then provide insufficient loading inductance to obtain transverse resonance. It Wlll then be necessary to add a tuning screw in order to realize resonance. Clearly, a wide range of loaded Qs is ob tainable by adjusting the ratio of these two parameters: slot length and tuning screw penetration. The practical range of loaded Q-factors obtainable in this way is from about 3 to 300, the upper limit depending on the unloaded factor of the resonator and the loss which may be tolerated. FIG. 10 illustrates the performance of a mount of medium bandwidth with slot and tuning screw.

To return now to the mixer arrangement of FIGS. 1, 2 and 3, the basic mount resonator may be coupled into on either side by, respectively, power from the signal and local oscillator sources. A third arm 6 on the outer broad face of waveguide 1 is necessary to prevent radiation from the slot. Image and sum termination circuits are placed in this arm,

The theory and practice of image termination has been discussed by several authors, Barber, M. R. Noise Figure and Conversion Loss of the Schottky Barrier Mixer Diode," Trans. M.TIT. (I.E.E.E.) MTT-IS, No. 11, Nov. 1967, and Johnson, K. M. X Band Integrated Circuit Mixer with Reactively Terminated Image." Trans. M.T.T. (I.E.E.E.) MTT-l6, No. 7, July 1968. Johnson discusses various types of termination but shows that short-circuit termination has the incidental benefit of lowering the IF. impedance.

Combined sum and short circuit image terminations produce substantial improvements in conversion loss, with corresponding improvements in noise figure.

In the mixer arrangement shown, the length of waveguide 6 is dimensioned to be below cut-off for the image frequency, i.e., it is evanescent at this frequency, and short circuit image termination is achieved quite simply with the capacitive screw 8. This evanescent mode resonator is tuned to resonate at image frequency, thus placing a short circuit across the terminals of the mixer diode 5. The length 1 between tuning screw 8 and the slot 3 determines the bandwidth over which the short circuit is maintained. The value is not critical, but if it is too short the match at local oscillator frequency will be affected. If! is too long the bandwidth of the mount will be inadequate.

Short circuit termination of the sum frequency 18 achieved in a different way. The size of the waveguide 6, which conveniently but not necessarily is the same size as that of waveguide 1, is chosen so that while the image frequency is below cut-off, the sum frequency is above cut-off. Thus, the latter frequency propagates in the usual way and a short circuit at the plane of diode 5 is achieved by locating plate 10 an appropriate distance from the diode (electrically half a wavelength bearing in mind that the presence of the capacitive screw will modify the physical distance necessary).

FIG. 11 shows a plunger 14 which for test purposes was used to replace plate 10 of FIGS. 1 and 3, and FIG. 12 shows the effect of varying the distance L of plunger 14 from diode 5, for a sum frequency of 15 243 GHz and g/2 1.25 cms (centimeters). It will be observed that the distance between maxima corresponds closely to the guide wavelength of the sum frequency in this guide. This is not a definitive test, however, because the local oscillator second harmonic is closely adjacent frequency. In order to confirm the theory it was therefore Conversion'Loss f dB Noise figure (calculated) dB Noise figuretmeasured) dB LF. Admittance (normalized 2+j0.l3

at the local oscillator signal frequency, and the capacitive screw 9"contained therein 'is adjusted to provide an evanescent mode rejection filter at that frequency. This rejection filter is notes sential and will only be required if the local'os'cillator'pojwe r is lirnited, toprevent power loss in the signal source;

The following performance'figures of ,amixer-local oscillator unit of the type shownin'FlGS. 1, 2 and 3 are typical of the improvement which-is obtainedby' the "above-described image termination arrangement. v The first column figures are foran unterminated iir'r-t age, i.e. image absorbedinsource, and .th'e s econd .cole. umn figures are for short circuit terminatedsumfand image frequencies.

pedance the diode. The improvement, compared with resistive image termination, is shownin 1 16.13.

As an alternativeitofthe above describ'e'dlongitudinal in common withthos'e of "the longitudinal slot, except slot, a transverse slot 15; FIG. 14, :may"be use'd.'Thef .1 principles involved -with' this transverse slot have much that the transverse slot must be u'se'd above its natural resonant frequency, i.e., a..length t/2. This is-becausej simple unloaded slot such as'previo'usly'- described the-slot must present a. series, capacitive reactanc'e, g

. X=2X in order'to achieve evanescent-mode resonance.

i.e., include the sidewalls, and)thereforeaj'dumb-bell" slot is preferable. Although very simil'arftothepreviqus g i arrangement there is less flexibility becausetheloaded",'.

Q-factorcannqt be varied in the" same-wayaswiththe' longitudinal slotJ'Thus the transverse sljot 'rnountis I likely to} be of value in broad band applic'ationsg} Y r In theabove described mixer arrangement, ihe use of" waveguide both below "cut-off and above 'cu t-offlpec mits the independent adjustmentfof the terrnination at. t i sum and image frequency. Prescribed terminations'fc'an'j-* be obtained at frequencies. appfoximately an octavey apart, whereas usingconventional w holly propagatin g'f} Y waveguide techniques the sum'frequency wouldl'be v propagating in two modesj'andwould, therefore, be unv manageable. 1 g 3 v While we have described abovethe principles offour invention in-connection with specific apparatus, it is to be cle'arlyunderstood that thisdescription is made only by way of exampleand not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims. 1

, 6 We claim: 1. A waveguide mixer arrangement comprising:

a first waveguide, one end of which receives an input signal havinga first given frequency and the other end of which receives a local oscillator signal having a second given frequency different than said first given frequency;

a first resonant slot disposed in one wall of said first waveguide intermediate the ends thereof;

. s'aid firstwav'eg uide including a first evanescent-mode waveguide filter to couple said input signalto saidfirst resonant slot, and

a second evanescent mode waveguide filter to couple said local, oscillator signal to said first resonant slot;- 1

a second waveguide coupled at right angles to said first waveguide encompassing said first "resonant ,slot, said second waveguide being dimensioned to x be evanescent at the mixer generated image frequency and prop'agating at-the mixer generated {sum frequency; i

' j an: to said first resonant slot;-

' u-a capaciiive obstacle disposed in said second-waver fguide'a first given distance from said first resonant 1 fi-s'lot to provide within said second waveguidean ev v anesce'nt mode resonator tuned to said image-fie} f quency so as to effect reactive termination of said image frequency; and

ashort circuit plate disposed said secondwave reactive terminatio nfof said.sumfrequencyy 2. An arrangement according to claim 1 further in -Q s a coaxial connector coupled to.,said second wave- Y guide and-saidnrixerdiode and saidgsecon'dgiven-frequencytf said firstjresonant slot fihp'ludes ,1 q

I saidlfirst resonant slot includes transverseydirnension of sai'djfirst waveguide;

said secondgiven frequencyr a'ndi- 1. a capacitive obstacle disposed.infsaidthifd wave g guide a third given distance different than said first I and second given distances from said resonant slot" I 1 ejectionfilter for i to provide an'evanescent mode "said second giverrfrequency. j

6. An arrangement according to claim 5, wherein .said first-and second reso'nant slots each inclu de I guid'e-a'secondgiven distance greater than said first; 3 given distance from saidfirst resonant slot to effect Ariarrangement'accordingto claim wherein i an-unloaded narrow'slot disposed parallel- "to the longitudinalidimensionfof' said first waveguide. 4. An arrangement. according to claim 2 w-he'rein a dumb-bell shaped," slot disposed parallel-to the i I V i SJAharrangement,aecordjn toclairn antithesi as ecjond resonantslor di p s in d onewalljbfx v. i Said. -iW gui e 'said first resonant a'thirdwaveguide coup ed at right ariglesttosaid first vzvvaveguide parallellto said second"waveguideem tjcompassing said second resonantslot j;said third wave guide being dimensioned'tozb'e evanescent at;

an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide. 7. An arrangement according to claim 5, wherein said first and second resonant slots each include a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide. 8. An arrangement according to claim 1, wherein said first resonant slot includes an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide. 9. An arrangement according to claim 1, wherein said first resonant slot includes a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide. 10. An arrangement according to claim 1, wherein a second resonant slot disposedin said one wall of said first waveguide adjacent said first resonant slot; a third waveguide coupled at right angles to said first waveguide parallel to said second waveguide encompassing said second resonant slot, said third waveguide being dimensioned to be evanescent at said second given frequency; and a capacitive obstacle disposed in said third waveguide a third given distance different than said first and second given distances from said second resonant slot to provide an evanescent mode rejection filter for said second given frequency. 11. An arrangement according to claim 10. wherein said first and second resonant slots each include an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide. 12. An arrangement according to claim 10. wherein said first and second resonant slots each include a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 I 859 I 600 Dated J y 1 1975 Inventor) George Frederick Craven; Raymond Richard Thomas and Maurice Francis Berger It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the Front Page, between Data Element Identifier [21] and Data Element Identifier K52] insert "E301 Foreign Application Priority Data Nov. 14, 1972 Great Britain a 52433/72."

signed and sealed this 151:1". day of April 1 75.

/ n I: ..'"X....)

At test I m C Z:..P.SEZ;ALLL- I E-Tar. PETE: C Ii -.361 Cormissi oner of Patents ,rttestin-g Oi" Cicer anal 'i'racietflarl-zs FORM PO-105O (IO-69)

Claims

1. A waveguide mixer arrangement comprising: a first waveguide, one end of which receives an input signal having a first given frequency and the other end of which receives a local oscillator signal having a second given frequency different than said first given frequency; a first resonant slot disposed in one wall of said first waveguide intermediate the ends thereof; said first waveguide including a first evanescent mode waveguide filter to couple sAid input signal to said first resonant slot, and a second evanescent mode waveguide filter to couple said local oscillator signal to said first resonant slot; a second waveguide coupled at right angles to said first waveguide encompassing said first resonant slot, said second waveguide being dimensioned to be evanescent at the mixer generated image frequency and propagating at the mixer generated sum frequency; a mixer diode disposed in said second waveguide adjacent to said first resonant slot; a capacitive obstacle disposed in said second waveguide a first given distance from said first resonant slot to provide within said second waveguide an evanescent mode resonator tuned to said image frequency so as to effect reactive termination of said image frequency; and a short circuit plate disposed in said second waveguide a second given distance greater than said first given distance from said first resonant slot to effect reactive termination of said sum frequency.

2. An arrangement according to claim 1, further including a coaxial connector coupled to said second waveguide and said mixer diode; and a low pass filter contained in said connector to extract the desired intermediate frequency signal which is equal to said first given frequency minus said second given frequency.

3. An arrangement according to claim 2, wherein said first resonant slot includes an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide.

4. An arrangement according to claim 2, wherein said first resonant slot includes a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide.

5. An arrangement according to claim 2, further including a second resonant slot disposed in said one wall of said first waveguide adjacent said first resonant slot; a third waveguide coupled at right angles to said first waveguide parallel to said second waveguide encompassing said second resonant slot, said third waveguide being dimensioned to be evanescent at said second given frequency; and a capacitive obstacle disposed in said third waveguide a third given distance different than said first and second given distances from said resonant slot to provide an evanescent mode rejection filter for said second given frequency.

6. An arrangement according to claim 5, wherein said first and second resonant slots each include an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide.

7. An arrangement according to claim 5, wherein said first and second resonant slots each include a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide.

8. An arrangement according to claim 1, wherein said first resonant slot includes an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide.

9. An arrangement according to claim 1, wherein said first resonant slot includes a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide.

10. An arrangement according to claim 1, wherein a second resonant slot disposed in said one wall of said first waveguide adjacent said first resonant slot; a third waveguide coupled at right angles to said first waveguide parallel to said second waveguide encompassing said second resonant slot, said third waveguide being dimensioned to be evanescent at said second given frequency; and a capacitive obstacle disposed in said third waveguide a third given distance different than said first and second given distances from said second resonant slot to provide an evanescent mode rejection filter for said second given frequency.

11. An arrangement according to claim 10, wherein said first and second resonant slots each include an unloaded narrow slot disposed parallel to the longitudinal dimension of said first waveguide.

12. An arrangement according to Claim 10, wherein said first and second resonant slots each include a dumb-bell shaped slot disposed parallel to the transverse dimension of said first waveguide.