US3399326A - Travelling wave tube having a graphite coating in the central region and the free end at least 10 wavelengths long and a qc of at least 0.4 - Google Patents

Description

Aug. 7. 1968 J. A. B. DECHERING ET AL 3,399,326

TRAVELLING wAvE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION AND THE FREE ENDS AT LEAST WAVELENGTHS LONG AND A QC OF AT LEAST 0.4 Filed Sept. 10, 1964 5 Sheets-Sheet 1 GRAPHITE COATING dB/cm INVENTION PRIOR ART T I I I I I I I I I JOHANNE 5 AB. DEEHEWG JACOB OBER ANNE MEIJER Act? Aug. 27, 1 968 TRAVELLING WAVE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION AND THE FREE ENDS AT LEAST WAVELENGTHS- LONG AND A QC OF AT LEAST 0 4 Filed Sept. 10, 1964 J A. B. DECHERING ET AL 3 Sheets-Sheet 2G 50dB Q6 43 40 36 33 30 26 23 20 dB o I I I I I 1 I I l" 0,01 0,05 0,2 5 20 100 500Lwa 10% Aug. 27, 1968 1 cH l ETAL 3,399,326

TRAVELLING WAVE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION AND THE FREE ENDS AT LEAST 10 WAVELENGTHS LONG AND A QC OF AT LEAST 0.4

Filedsept. 10, 1964 3 Sheets-Sheet 5.0 kMc/s United States Patent TRAVELLING WAVE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION AND THE FREE ENDS AT LEAST WAVELENGTHS LONG AND A QC OF AT LEAST 0.4

Johannes Antonius Bernardus Dechering, Jacob Ober, and

Anne Meijer, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Sept. 10, 1964, Ser. No. 395,383 2 Claims. (Cl. 315-35) ABSTRACT OF THE DISCLOSURE A travelling wave tube capable of amplifying an electrical signal substantially without amplitude distortion and 'A.M. to RM. conversionhaving attenuating material distributed about the center portion of the slow wave helix while portions at both ends are free of attenuating material. The attenuation per unit length at the center portion is constant while the length of the output portion free of attenuating material is at least ten wavelengths the value of the QC for the tube being at least 0.4.

This invention relates to devices including a travelling wave tube in which attenuating material is provided on or near the helical delay line so that portions at both ends are free from attenuating material. It also relates to travelling wave tubes intended for such devices.

In travelling wave tubes of the aforementioned kind, the attenuating material may be provided in various ways. For example, a very high attenuation per unit length over a small portion of the length at a certain distance from the output; a comparatively low attenu ation over a large portion of the length at some distance from the 'output, preceded by a short length of high attenuation at the input end; a low attenuation over a large portion of the length or an attenuation which linearly increases from the output to the input end.

In known arrangements, the first of which is regarded as unfavorable, the minimum distance of the attenuation from the output depends on the product QC for the relevant tube. The gain factor C, which is a measure of the coupling between the delay line and the beam, is defined in Formula 2.43 in the book Travelling-Wave Tubes (New York, 1950) by I. R. Pierce.'Q is the spacecharge parameter given by the Formula 7.15 on page 113 in the above-mentioned book by I. R. Pience. In all these cases the location of the attenuation between the free portions at the ends is determined inter alia by the requirement that reflections at the output must not cause interfering variations in the group delay times.

An object of the invention is to provide a location for the attenuation which, together with other parameters of the tube, affords advantages with respect to known devices.

According to the invention, in a device including a travelling wave tube which is provided with attenuating material on or near the helical delay line so that portions at both ends are free from attenuating material, the attenuation per unit length is constant, apart from the required transitions at the input and output ends, while the free portion at the output end has a length at least ten times the wavelength in the whole working range of the device and the value of the product QC for the tube is at least 0.4.

According to the invention, to reduce the length of the tube as far "as possible, it is preferable to give the attenuation a maximum value per unit length, Whilst as a matter of fact allowance has to be made for the known re- "ice quirement that the concentrated attenuation must exceed the net amplification of the tube :by at least 20 db.

Due to the comparatively large distance of the attenuation from the output of the tube, it is primarily achieved that the output power of the tube becomes considerably higher than in the case where the attenuation would be nearer to the output. A second, very important advantage is that, due to the combination of the comparatively great attenuation-free length at the ouptut and the selected minimum value for the product QC, the amplitude-phase distortion, the A.M.P.M. conversion of the signal, is considerably limited with respect to known tubes and devices.

The latter may perhaps be made acceptable as follows: Two phenomena determine the nonlinear behavior of a travelling wave tube. Firstly the velocity of the electrons decreases towards the output due to energy given off by the beam to the field of the delay line and, secondly, the fundamental harmonic OLf the density modulation in the beam is decreased due to the occurrence of higher harmonics. The first phenomenon implies an increase in phase length of the delay line. The second phenomenon becomes manifest in a decrease in phase length of the delay line, but only of the nonattenuated portion of the output end. Thus the effect of the repulsion of electrons increases with the length of this portion so that the said effect may compensate for the influence of the decrease in velocity of the beam for tubes having comparatively strong space-charge fields (high value for QC).

It is to be noted that high values for QC up to 1.0 are included in calculations, it is true, but that the practical values lie in the vicinity of 0.3. Also output portions of delay lines which are free .from attenuating material and longer than 10 wavelengths are not impossible in known constructions. However, the combination of a high QC and a large free length with the consequent advantages is not mentioned in literature.

In this connection it is to be noted that no attention or hardly any attention has been paid to the A.M.P.M. conversion at least in literature and in patent specifications. Still the A.M.P.M. conversion of a travelling wave tube is important. Such tubes are frequently used as output tubes in microwave connections. The amplitude amplification of the signal may become dependent upon frequency if the adjustment of the filters in the amplifier, in front of the output tube, is incorrect, for example, due to variation. Such amplitude variation over the frequency range becomes manifest, due to the A.M.P.M. conversion of the travelling wave tube, as an interfering modulation in the frequency-modulated signal if the A.M.- P.M. conversion exceeds a given value. Consequently builders of transmitters impose certain requirements in this regard.

The invention will now be described, in detaiL'by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 is an elevational view of a helical delay line of a tube according to the invention, with attenuating material provided on the ceramic carriers;

FIGURE 2 shows several attenuation profiles for this tube;

FIGURE 3 illustrates the results of measurements carried out on such tubes;

FIGURE 4 illustrates the manner in which the output power of a tube of FIGURE 1 varies as a function of frequency dependent on the free output portion, and

FIGURE 5 illustrates for the same arrangement the output power as a function of the input power and also the amplitude-phase distortion.

In FIGURE 1, a helicallywound delay line 1 is a molybdenum wire of 0.250 mm. thick, having a diameter of 2.2 mm. and a pitch of 0.54 mm., while the wound length betweenholders 2 at the gun side andholders 3 at the collector side is 130 mm. Theline 1 is supported by three smallceramic rods 4 which fit into the widened ends of theholders 2, 3 and are held together bymetal bands 5. Agraphite layer 6, which forms the attenuating material of the tube, is sprayed from a colloidal suspension onto the assembly comprising the helical line and the ceramic rods.

FIGURE 2 illustrates the variation in the height of the attenuation along the delay line in db per centimeter in which the origin for the longitudinal axis lies at the cathode side, that is to say curve A .for a conventional tube and curve B for a tube according to the invention. The tube is intended for the frequency range between 3800 rnc./s. and 5000 mc./s. so that, as can be seen from the figure, in the case of curve B the free output portion has a length of 10 wavelengths in the whole working range of the tube. The total attenuation is in each case substantially the same and is about 70 db. The tube is intended for beam accelerating voltage between 1050 volts and 1200 volts at a beam current of 50 mamps whereby a maximum amplification of approximately 45 db at low level is obtained. The QC value calculated for this tube is 0.43.

In FIGURE 3 the output power of a tube W in watts is plotted along the vertical axis and the input power W in milliwatts is plotted along the horizontal axis. The dashed curves indicated by 1050A, 1100A, and 1200A illustrate the manner in which the output power varies as a function of the input power for a tube having an attenuation profile as shown by line A in FIGURE 2, at accelerating voltages of 1050 volts, etc. The dot-anddash curves 1050B, etc., illustrate the manner in which the output power at these voltages varies for a tube having a damping profile as shown by curve B in FIGURE 2. The set of curves indicated by 1050A, etc., and the set of curves indicated by 1050B, etc., have envelopes W and W respectively, which lines indicate the output power at a given input power for an optimum accelerating voltage adjusted between 1050 and 1200 volts. From the figure it can be seen that in the case of curve B the output power is higher by a factor of approximately 1.5 than in the case of curve A. The figure also shows the aligned straight lines. K and M and the inclined straight line L. In the case of curve A and to the left of the straight lines L and K the amplitude-phase distortion, the A.M.P.M. conversion, is smaller than 3 per db whereas in the case of curve B the value smaller than 3 per db to the left of the lines K andM. The 3 per db limit for the A.M.P.M. conversion is a practical limit below the value of 5 /db which is adhered to by many builders of transmitters for the usability of a tube adjustment. From the figure it appears that in the case of curve B the tube may be given an arbitrary adjustment at an input power which is limited to 2 milliwatts, whereas in the case of curve A this range is much more limited and especially the output power may be smaller by a factor up to 2. The amplification may be read from the numbers writteen with the inclined lines. All'the measurements hold good for the frequency of 4000 mc./s. I

In FIGURE 4, the lines I, II, III and IVillustrate th output power W in watts of the tube as a function of the frequency in kmc/s with an attenuation profile according to Him FIGURE 1 if the free output portion varied from 47 mm. in line I, from 37 mm. in line II, from 27 mm. in line III and from 17 mm. in line IV. It will readily be seen that 6 high output power which is substantially independent of frequency is obtained only in the case of curve I. i

In FIGURE 5 are plotted, on the left-hand vertical axis, the output power in watts and, on the right-hand vertical axis, the A.M.P.M. conversion in degrees per db as a function of the input power in milliwatts, plotted on the horizontal axis, all this at a frequency of 4000 mc./s. The dot-and-dash line illustrates the output power at the optimum accelerating voltage adjusted, the drawn. out line illustrates the A.M.P.M. conversion at an accelerating voltage of 1060 volts, and the dashed line illustrates the A.M.P.M. conversion for an accelerating voltage of 1100 volts. FIGURES 5-1 to 5-IV correspond, as to the location of the damping, to curves I to IV of FIGURE 4. A striking fact is the low value of the A.M.- P.M. conversion in the case I, namely less than 2 degrees db up to saturation.

What is claimed is:

1. An electron discharge device for amplifying an electrical signal with reduced amplitude distortion for given power output comprising means to project an electron beam along a given axis and a slow-wave propagating structure 'of substantially tubular form coaxial therewith and surrounding the electron beam, said slow-wave structure comprising a helical delay line having a central portion covered with an attenuating material and the ends of which are free of attenuating material, the portion of the delay line covered with attenuating material having a damping per unit length which is constant, the ends free of attenuating material having a length of at least 10 wavelengths for the entire operating range of the device, said device having a value of the product QC of at least 0.4, Q being a space-charge parameter and C a gain factor.

2. An electron discharge device as claimed inclaim 1 in which the attenuating material is graphite.

References Cited UNITED STATES PATENTS 2,636,948 9/1953 Pierce 315-16 X 2,771,565 11/1956 Bryant et al. 315--3.5 2,790,926 9/1957 Morton 3l53.5 3,005,126 10/1961 Cutler 31539.3 X

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, ]R., Assistant Examiner.

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