The tube with Onde S progressive ( dolly-wave tubes ) is a Vacuum tube used in ultra high frequencies to carry out Amplificateur S of weak, average or strong power. It makes it possible to produce amplifiers with wide strip and very weak Bruit basic. It is appropriate particularly well for the amplifiers of the communications satellite .

Origins of the travelling wave tubes

The travelling wave tube was invented by Rudolf Kompfner in 1942 in England. The first used at the end of the Second world war provided some Milli Watt S of power and functioned at the frequency of 2 GHz.

They are the logical development of the tubes ultra high frequency appeared in the Thirties, which made it possible to be freed from the limitations of the tubes to traditional grids.

They belong to the family of the tubes to linear beam.

Principle of operation of a SIGNAL Collecting bi- S

The travelling wave tube (SIGNAL, Travelling Wave Tubes ) is composed of four principal parts:

• the electron gun (Filament, Cathode, Wehnelt and Anode);
• the propeller;
• collecting ;
• the vacuum chamber.

The electron gun

The Cathode is the source of the electron S which constitute the electronic Beam in all the tubes ultra high frequency. It is made of a complex mixture of metals (Tungstène porous, Baryum) and is brought up to a temperature of 1050°C, approximately, by a tungsten filament coated with a moulding in Alumine which supports heat exchange between the filament and cathode.

When cathode reached its operating temperature, one can apply an electric field between Anode and cathode. Cathode then emits a beam of electron S very dense which is accelerated by the potential positive of the anode.

The particular form of cathode and the wehnelt has a convergent effect on the beam, which is thus focused in a cylinder a few millimetres in diameter in the area of the anode.

The propeller

The propeller is a spiral in Cuivre or tungsten, inserted in the metal envelope of the tube, of which it is insulated by means of ceramics bars, chosen for their good thermal Conductibilité.

The Focalization of the electron beam is ensured by alternate Aimant S permanent.

The propeller is connected to the envelope in a point indicated “not propeller” of the food.

The signal ultra high frequency of entry is applied to the propeller at its end side gun.

To obtain a good interaction between the beam and the wave ultra high frequency which moves along the propeller, it is necessary that they have an axial speed near. As the propagation of a wave along a wire in the vacuum is carried out at a speed close to that of the light, it is necessary to increase the length of its way to synchronize it with the beam, this explains the shape of the propeller. During their displacement in the tube, the electrons are slightly slowed down by the wave, and of this fact communicate to this one part of their energy. The amplitude of the wave will be thus larger at the end of the propeller than at its beginning: the signal was amplified.

The precise adjustment the speed of the beam is obtained by the adjustment of the tension propeller-cathode.

Collectors

Their role is to collect the electrons after their passage through the propeller. They receive all the energy which the beam did not provide to the wave ultra high frequency.

• the new tubes with two collectors, resulting from space technology, allow an appreciable profit of Rendement and a reduction even more important in the power dissipated by the SIGNAL in small signal.

Role of collector 1

The tension collecteur1 - cathode must be sufficiently important so that the electrons more slowed down, when the tube functions with Saturation, are not likely to turn back to come to fall on the last whorls from the propeller.

• This collector is thus polarized with a tension close to that of the single collector of the SIGNAL conventional.

Role of collector 2

It has the role of collecting all the electrons having preserved a sufficient speed to cross the first collector. The tension collector 2-cathode is approximately half of the tension collector 1-cathode.

• It follows an improvement of the output (which passes typically from 30 to 40%) when the tube functions with saturation and a considerable reduction in the power dissipated in the zone “collector” of the SIGNAL when those work with small signal.
• For example, the dissipated power, for SIGNAL of 750 W, passes from 2400 W for a SIGNAL mono-collector to 1300 W for Bi-collectors.

The envelope with vacuum

A very thorough vacuum must be maintained permanently during all the life of the SIGNAL.

The enclosure must thus be able to preserve this vacuum (10^-8 Torr) without which appear risks of arc and/or a pollution of cathode.

The most fragile points of the enclosure are the accesses ultra high frequency to the entry and the exit where “windows” are inevitable. According to the frequency of the signal, the connectors are of type Coaxial or Waveguide.

Degradation of the SIGNAL

Filament

Resistance tungsten of the filament is dependant on the temperature and present a plus coefficient. It is thus necessary to limit the call of cold current (generally with approximately twice the face value).

Propeller

It is the most fragile part of the SIGNAL, and it can be instantaneously molten in the event of error of implementation or bad polarization.

The electron beam must remain perfectly rectilinear, of constant section, the exit of the gun to the collector. So electrons fall on the propeller (what results in an increase in the current propeller), the food propeller and anode must be crossed. The maximum energy intercepted by the propeller must remain lower than the value specified by the manufacturer of the tube (5 to 20 Joule S according to the tube).

The leading causes of defocusing are: • Fall of tension collector - cathode; • Tension propeller - cathode apart from the specified limits; • Tension propeller - anode apart from the specified limits; • Tension filament • Power of entry excessive ultra high frequency; • Excessive ROS of exit; • Overheat; • Presence of a magnetic field near the tube.

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