A 700 W 12 GHz TWT FOR TV SATELLITE APPLICATIONS

Horst Seunik,* Fritz Hanf,* Sven Wallander +

Abstract

As part of a 7oo WT CW transponder for a direct TV broadcastingsatellite operating in the 11.7 to 12.5 0Hz band, a traveling-wavetube (TWT) and directly radiating multistage collector are beingdeveloped.

The state-of-the-art TWT is periodic permanent magnet (PPM) focusedand incorporates a novel type of coupled cavity slow wave structurecomprising three gain sections with a velocity taper in the finatsection for efficiency improvement.

The five-stage depressed collector for further efficiency improve-ment is made of carbon and radiates directly into space.

Introduction

Television coverage of an entire country from a direct broadcastingsatellite requlres transmitter powers between 4oo and 8oo W. This is

Fig. 1 12 GHz 700 W transponder TWT with direct radiatingcollector and power supply

* Tubes Division, Siemens AG, Munich+ Chalmers University, Gbteborg and Consultant of

Siemens Tubes Division

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an order of magnitude higher than the powers achievable withexisting transponder tubes. Weight, efficiency and thermalconsiderations of such tubes therefore determine the overallsatellite concept.

An entire transponder output stage comprising the TWT with directradiating collector and the power supply (fig. 1) are beingdeveloped at Siemens under contract to the Bundesministerium furForschung und Technologie represented by the Gesellschaft fur Welt-raumforschung. This paper describes the TWT and collector develop-ments with results so far obtained.

Optimum efficiency isachieved by two basicapproaches: A velocitytaper is introduced intothe output section of the W7V .-

slow wave structure forhigh electronic efficiency,and the kinetic energy of Fig. 2 A photograph of thethe beam is recovered byslwavstuurusing a multistage slow wave structurecollector. A carbon-metal-ceramic collector concepthas been selected not onlyto assure optimum heatdissipation, but also because of its very light weight.

The tube is focused by a periodic permanent magnet stack of samariumcobalt rings. It employs a new type of coupled cavity slow wavestructure that features unconditional stability during turn on andturn off over the entire tube life.

1. Slow wave structure

The slow wave structure consists of beam holes rings on rods asshown in fig. 2 and fig. 3. The equivalent circuit elements are theinductance L of the rod, the capacitance C between the ring and walland the capacitance C between adjacent rings.

From the equivalent circuit the dispersion equation obtained is:

Cos 1p = 1 +w 2C

2Cg

(1 )

where p is half the structur period, see fig. 3

Wa3 =- is the cold propagation constantvphph

UJ = 1

°0is the resonant frequency of the ring and rod.

The 2 i frequency is W0.57

The ratio C/C determines the cold bandwidth: The bandwidth increaseswith Cg g

gThe coupling impedance Z on the beam axis obeys. the equation

Z= -2 tan(Ap/2)1we (Pp)g

sin Qhl2O)2(0h/2)2

where a is the beam hole radius and h is the gap length.

The impedance Z thus exhibits the useful property of being zero atthe 21t point although the group velocity is zero. This elimlnatesthe danger of oscillation at the 2ff point when voltage is appliedto the slow wave structure.

Further advantages of this particular all-copper slow wave structureconfiguration are the excellent heat transfer characteristics andthe independence of the structure period from the magnetic fieldperiod because a slide-on focusing system with pole pieces externalto the tube envelope can be used. This and the actual physicalproperties of the slow wave structure lead to a relatively low slowwave structure voltage of only about 8 kV compared with 11 kV formore conventional types of.slow wave structure.

The measured coupling impedance and dispersion curves are

fig. 4.shown in

Fig. 3 The slow wavestructure and itsequivalent circuit

Fig. 4 Measured normalizedphase velocityc/v and couplingimpgance Z versusfree space wave-length

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I

I0 2(0a) (2)

2. Measurements

To investigate the basic properties of the selected slow wavestructure the first tube was built up without a velocity taper inthe output section. The results of output power and gain measure-ments on this tube are shown in fig. 5.

The small signal gain exceeded 55 dB over the entire 8oo MHz band,and for an constant output power of 400 W the minimum gain was 51 dB.The electronic efficiency varles from 12 to 16% over the band.Correlation between the measured values and large and small signalcomputer program calculations was completely satisfactory.

To increase the electronic efficiency over the 8oo ]IEz bandwidth soachieved, a second tube was built up with a, velocity taper in theouitput section. Unfortunately a somewhat excessive mismatch at theend of the output section of the slow wave structure only allowedthis tube to be operated at a reduced beam current of up to 32o mAbefore oscillatioti at 11.6 GHz started (normal beam current 450 mA).Despite this the effect of the velocity taper could be satisfactorilydemonstrated as allustrated by fig. 6. A considerable improvementin efficiency at the bottom end of the band was obtained. Presently(June 1974) the velocity taper is being optimized.

G PAS Const.d b PA x 50 W measured60-

55-

50- calculated'5-

Ps U1 a 9200 V 9et/I*%800 C

a 450mA Psat 20600

°°0 measre 0ADO caLcutGted 10

200

11,5~~ 12 . lit,5 GHz 1'3

w

800

600

400 -

200

U =8800V; JC = 50OmAmeasu-red

calculated

.g1et/%9

19/. 3-ZS7mA

J4 .

qdf%C - 300 mA

N'.

17%--of/(GHz

11,4-I 1,6 11, 12 1212---- ,

11,4 11,6 X1,8 12 12;2 12,k 12,6

Fig. 5 Small signal gainand saturated outputpower withoutvelocity taper

Fig. 6 Output power andelectronic efficiencyat saturation withvelocity taper

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- _s u r r -

5. Direct radiatingfLive stare carbon collector

The carbon-metal-ceramictechnology of the vacuumenvelope allows a directradiating collector to bebuilt with half the weightof existing designs for thesame collector power dissi-pation. Carbon also offersthe advantages of highthermal conductivity, lowsecondary emission and thebest radiation ( e = 0.9).

The carbon modules re-presenting the individualstages of the collectorare brazed together vacuum-tight by a special process.In the process involvingthe use of rhenium acopper solder wets thecarbon surface completelyand uniformly so that asubsecuent vacuum tightcopper-molybdenum seal ispossible.

Up to the present time anexperimental collector with anbeen built and has sucessfullytests .

Fig. 7 A photograph of thecarbon collector

outer diameter of 13 cm (fig. 7) hascompleted the first environmental

4. Conclusions

With the results so far obtained from the new slow wave structure,and in particular the success in brazing the carbon modules for thefive stage collector, a fully integrated state-of-the-art traveling-wave tube with directly radiating collector can be expected in thenear future. Present work is concentrated on optimizing the velocitytaper for the slow wave structure and the electronopticalcharacteristiCs of the carbon collector. The expected DC to RFefficiency of this tube will exceed 5O f.

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