We investigate an axially symmetrical Fabry-Perot oscillator with active devices inserted in a dielectric substrate for power combining of many more devices in the microwave and millimeter wave frequency range. Empirically in this oscillator, efficient power combining can be done when it oscillates approximately at the frequency which corresponds to the wavelength equal to twice the spacing between the devices. The wavelength in the dielectric is shorter than in free space, so we tried to insert the devices in the dielectric substrate in order to increase the number of devices. By measuring the oscillation frequency of the oscillator with sixteen devices at X-band, we confirmed that the spacing between devices was about a half wavelength in the dielectric. We achieved almost perfect power combining of sixteen device

Itoga, Toshihide

Nogi, Shigeji

Sanagi, Minoru

English

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Axially symmetrical Fabry-Perot oscillator with multiple devices inserted in dielectric substrate

Okayama University Scientific Achievement Repository

EngineeringElectrical Engineering fieldsOkayama University Year 1997Axially symmetrical Fabry-Perotoscillator with multiple devices insertedin dielectric substrateMinoru Sanagi Toshihide ItogaOkayama University Okayama UniversityShigeji NogiOkayama UniversityThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/electrical engineering/881997 Asia Pacific Microwave Conference 3PO 1-6 AXIALLY SYMMETRICAL FABRY-PEROT OSCILLATOR WITH MULTIPLE DEVICES INSERTED IN DIELECTRIC SUBSTRATE Minoru Sanagi, Toshihide Itoga, and Shigeji Nogi Department of Electrical and Electronic Engineering 0 kayama University 3-1-1 Tsushima-Naka, Okayama 700, Japan ABSTRACT We investigate an axially symmetrical Fabry-Perot oscillator with active devices inserted in a dielectric substrate for power combining of many more devices in microwave and millimeter wave frequency range. Empirically in this oscillator, efficient power combining can be done when it oscillates approximately at the frequency which corresponds to the wavelength equal to  twice the spacing between devices. The wavelength in the dielectric is shorter than in free space, so we tried to insert the devices in the dielectric substrate in order to increase the number of the devices. By measuring the oscillation frequency of the oscillator with sixteen devices a t  X-band, we confirmed that the spacing between devices was about a half wavelength in the dielectric. We achieved almost perfect power combining of sixteen devices. 1. INTRODUCTION Recently, various methods for combining output pow- ers of solid-state active devices have been developed[l] [2]. In millimeter wave frequency range, power com- bining using a Fabry-Perot resonator is considered ef- fective and several investigations using the fundamental TEMoOn-mode have been reported[3]-[5]. However, as far as the authors know, the power combining efficiency has been relatively low. One of the reasons is supposed to be insufficient device-field coupling and lack of its u- niformity. In order to achieve efficient power combining, every active device should generate the maximum out- put power cooperatively. This requires the same rfdevice current of the optimum magnitude in all the devices[6], which can be attained by uniform and sufficient device- field coupling. Inadequate coupling of the resonator to an output waveguide can be another reason of low com- bining efficiency. In order to resolve these problems, the authors pro- posed an axially symmetrical Fabry-Perot multiple- device oscillator shown in Fig.l[7]. The oscillator has a plane mirror which has a circular output window and a circular groove mounted with many solid-state devices. When it operates at  an axially symmetrical mode shown in Fig.2, uniform device-field coupling required for high efficiency combining can be realized. Installation of the output window on the plane mirror can give strong cou- pling of the resonator to the output line because of nar- Plane Mirror Concave Mirror output  Fig. 1 Axially symmetrical Fabry- Perot multiple-device oscillator[7]. row beam width a t  the plane mirror. Then the output coupling can be optimized corresponding to the negative conductance of the active devices. In experiments at  X- band, almost perfect power combining was attained for the six-, eight-, and twelve-device oscillators[7] [8]. Em- pirically in this type oscillator, efficient power combining is done when it oscillates approximately at the frequency which corresponds to the wavelength equal to twice the spacing between devices[5] [7] [8]. This yields a limitation on increasing the number of the devices. In this paper, in order to increase the number of devices mounted in the oscillator and achieve high power combining efficien- cy, we study the structure in which the active devices are inserted in a dielectric substrate on the plane mirror. 337 Fig.2 Axially symmetrical TEMoln mode derived by sum of two linear-polarized modes. at concave mirror \+ 0 20 40 60 80 r ["I Fig.3 Magnetic field distribution of TEM016 mode at  mirrors for the case of resonator length 100". 2. INCREASING OF THE NUMBER We consider the Fabry-Perot oscillator which consists of a plane mirror and a concave mirror. Active devices are located at  equal spacing on a circle of radius rd on the plane mirror. Figure 1 shows the Fabry-Perot oscil- lator which we proposed in our previous study[7]. The axially symmetrical TEMoln mode in Fig.2 used as a combining mode is obtained by combination of two lin- early polarized TEMoln mode, one in %-direction and the other in y-direction. Thus electric field of this mode has only radial component of azimuthally uniform mag- nitude. Active devices are mounted at equal spacing in the circular groove on the plane mirror and are coupled to the axially symmetrical field in the groove. Figure 3 shows the magnetic field distribution of TEM016 mode at  the plane mirror and the concave mir- ror with neither a groove nor an output window when the radius of curvature of the concave mirror is 150m- m and the resonator length L is 100". The resonant frequency is equal to 9.91GHz for this case. When the radius rd is equal to 22.5" and the num- ber of the devices is eight, the spacing between devices is 17.7" corresponding to half wavelength a t  10GHz and almost perfect power combining was achieved[7]. For twelve devices case, when the devices were located on a larger circle of radius rd =34[mm] where the spacing be- tween devices is 17.8mm, high combining efficiency was OF DEVICES 33 8 obtained with short resonator length. However, when the resonator length increases, efficiency reduces because the field at r = rd becomes weak compared with the max- mum value on the plane mirror and the diffraction loss increases[8]. It is considered that the combining efficien- cy decreases if the larger radius rd is used in order to combine powers of many more devices. The wavelength in a dielectric is inversely proportion- al to the square root of the dielectric constant and shorter than in free space, so the real spacing between devices corresponding to half wavelength in the dielectric at  the oscillation frequency can be reduced. Therefore, many more devices can be mounted with Fabry-Perot oscilla- tor without degrading combining efficiency on the case of the radius rd a t  which the field is almost maximum on the mirror. 3. EXPERIMENT An experiment was carried out at  X-band for sixteen Gunn diodes oscillator. Figure 4 shows the structure of the oscillator. Both the radius of the plane mirror and that of the concave mirror are equal to 120". A radius of curvature of the concave mirror is equal to 150". The diodes are inserted in a dielectric ring on the circle of radius rd=29[mm]. The dielectric ring with an inner radius rgi of 22.9" and an outer radius r, of 36.5" is composed of Rogers Duroid 5870 substrate with a thick- ness h of 3.17" and dielectric constant of 2.33. The electromagnetic wave generated from diodes propagates radially to +r and -r directions. The wave propagating to -r excites Fabry-Perot resonator through a coaxial groove with an outer radius rgo of 26.5" and a depth dp  of 4". In order to alleviate the discontinuity in impedance a t  the groove opening, a tapered transition was provided. The wave propagating to +r is converted from TEM mode of a radial line filled with dielectric to TEM mode of a coaxial line with width 'w, at rs =43[mm] through a radial line filled with air. Figure 5 shows an equiv- alent circuit of converting part from the radial line to the coaxial line. Z z  is a characteristic impedance of the coaxial line and is given by ZOa(r) and ZOd(r) denote the characteristic impedance of the radial line filled with the air and the dielectric, respectively and are given by Plane Mirror Concave Mirror Short Plunger + 1. ....... ~ ......... .......... i-0 LX ........ - L ) c  h ,ric Short P1 Devices Radius of Curvature 150" (a) structure of part mounted with devices (b) Entire structure Fig.4 Oscillator structure for experiment in which sixteen diodes are inserted in dielectric substrate. where k is propagation constant in free space, E, is di- electric constant , and H i 2 ) ( r )  and H12)(?)  are second kind of Hankel functions. If Z, which is impedance look- ing forward to the coaxial line at  r = r, is equal to Z0d(r0), reflection do not take place on the converting part and the wave is reflected by only a short plunger in the coaxial line. Therefore, output power maximum can be obtained by adjusting the position of the short plunger. Z, is expressed as follows: where Figure 6 shows the variation of Z, versus T,  for the case of frequency lOGHz, E, = 2.33, h = 3.17[mm], tu, = 5[mm], and r,  = 36.5[mm]. From this result, when r,=43[mm], Z, is nearly equal to Zod(ro). Figure 7 shows a typical measured variation of the oscillation frequency and the output power with the res- onator length. In Fig.7(a), the theoretical resonant fre- quency is also plotted. Compared with the theoretical frequency, it is considered that the oscillator operates coaxial line ZL I h1 radial line filled with air Z, radial line filled with dielectric Fig.5 Equivalent circuit of converting part from radial line to coaxial line at  TEMoln mode. The oscillation frequency was about lOGHz, so the spacing between devices was nearly e- qual to half of the wavelength in the dielectric substrate. 339 I I 40 45 50 55 r.9 [“I Fig.6 Variation of 2, versus r, for the case of frequency 10GHz, E, = 2.33, h = 3.17[mm], w, = 5[mm], and r, = 36.5[mm]. ”6 0 80 100 120 140 Lesonator Length [mm] (a) Oscillation frequency I I 80 1 0 0  120 140 (b) Output power Lesonator Length [mm] Fig.7 Experimental result Thus the effect of inserting the active devices in t,he di- electric was confirmed. Defining the combining efficiency as the ratio of the output power of the multiple-device oscillator to the sum of the maximum output powers of each device when they are measured in a usual waveguide cavity, the maximum value obtained was 107% when the resonator length was equal to 101”. 4. CONCLUSION We investigated the axially symmetrical Fabry-Perot oscillator with active devices inserted in the dielectric substrate on the plane mirror. The wavelength in the 340 dielectric is inversely proportional to  the square root of the dielectric constant, so the real spacing between de- vices corresponding to a half of the wavelength in the dielectric a t  the oscillation frequency can be reduced. Therefore, the higher dielectric constant of the substrate is, the more the number of the devices can be without degrading the combining efficiency. By experiments a t  X-band, the usefulness of the dielectric was confirmed and almost perfect power combining of sixteen devices was achieved. The future subject is power combining of more many devices and in higher frequency range. REFERENCES [l] K. J. Russell : “Microwave Power Combining Tech- niques,” IEEE Trans. Microwave Theory & Tech., MTT-27, pp.472-478 (May 1979). [2] K. Chang and C. Sun : “Millimeter-Wave Power- Combining Techniques,” IEEE Trans. Microwave Theory & Tech., MTT-31, pp.91-107 (February 1983). [3] J .  W. Mink : “Quasi-Optical Power Combining of Solid-state Millimeter-Wave Sources,” IEEE Trans. Microwave Theory & Tech., MTT-34, pp.273-279 (February 1986). [4] L. Wandinger and V. Nalbanian : “Millimeter-Wave Power Combiner Using Quasi-Optical Techniques,” IEEE Trans. Microwave Theory & Tech., MTT-31, pp.189-193 (February 1983). [5] H. Kondo, M. Hieda, M. Nakayama, T. Tanaka, IC. Osakabe, and K. Mizuno : “Millimeter and Submil- limeter Wave Quasi-Optical Oscillator with Multi- Elements,” IEEE Trans. Microwave Theory & Tech., MTT-40, pp.857-863 (May 1992). [6] S. Tanaka, S. Nogi, K. Fukui, and Y.-A. In : “Crucial Factors in Power Combining by Oversized Cylindrical Cavity Multiple-Device Structure,” IEEE Trans. Mi- crowave Theory & Tech., MTT-37, pp.1755-1760, (November 1989). [7] M. Sanagi, S. Nogi, K. Fukui, and K. Watanabe : “Fabry-Perot Multiple-Device Oscillator Using an Axially Symmetric Mode,” IEICE Trans. Electron., E78-C, pp.949-956 (August 1995). [8] M. Sanagi, E. Yamamoto, and S. Nogi :’ “Axially Symmetric Fabry-Perot Power Combiner with Active Devices Mounted on Both the Mirrors,” IEEE MTT- S Int. Microwave Symp. Dig., 3, pp.1259-1262 (June 1996). 

Axially Symmetric Fabry-Perot Power Combiner with Active Devices

Fabry-Perot Multiple-Device Oscillator Using an Axially Symmetric Mode,”

Millimeter and Submillimeter Wave Quasi-Optical Oscillator with MultiElements,”

Millimeter-Wave Power Combiner Using Quasi-Optical Techniques,”

Millimeter-Wave PowerCombining Techniques,”

Mink : “Quasi-Optical Power Combining of Solid-state Millimeter-Wave Sources,”

Russell : “Microwave Power Combining Techniques,”

Axially symmetrical Fabry-Perot oscillator with multiple devices inserted in dielectric substrate

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