US2787734A - Broadband magnetron - Google Patents

Description

April 2, 1957 A. T. NoRDslEcK BROADBAND MAGNETRON Filed June 10, 1949 ARNOLD ATTORNEY BROADBAND MAGNETRoN Arnold Theodore Nordsieck, Champaign, Ill., assigner to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application June 10, 1949, Serial No. 98,194

1 Claim. (Cl. S15-39.3)

This invention relates to broadband amplifiers for microwaves, and more particularly to a novel broadband amplifier of the magnetron type.

It is an object of the invention to provide a multi-cavity magnetron type structure which is non-oscillating and which is capable of amplifying over a wide frequency range.

The multi-cavity magnetron principle for generating micro-wave power is readily convertible to broadband amplification in a way very similar to that in which the velocity modulation principle has been applied in the travelling wave amplifier. The most significant feature of the travelling wave amplier is that the coupling between the electron stream and the R. F. fields is maintained continuously with `a definite phase relation over a large number of complete R. F. cycles. From one point of view however the multi-cavity magnetron may be regarded as an amplifier which is likewise several R. F. cycles long around the anode circumference (n cycles long if n is the mode number). Under these conditions the magnetron functions as a self-excited oscillator because it is made re-entrant upon itself. Any noise signal present in any part of the tube is amplified indefinitely by circulating around the tube, and the tube oscillates spontaneously. A small fraction of the power circulating'in the device is bled ofi at one point in the circle, and this constitutes the output power. The phase relation between the electron stream and the R. F. field is maintained constant by adjusting the applied D. C. voltage and magnetic field so that the mean rate of circulation of the electrons around the tube is equal to the angular phase velocity of the R. F. field (the so-called Hartree condition). For a complete discussion of these ideas, reference may be made to the April 1946 issue of the Bell System Technical Journal, and to the article therein by Fisk, Hagestrum and Hartman, entitled The Magnetron as a Generator of Centimeter Waves.

In the mode of highest n value, the so called ir-mode, the excitation in an oscillating magnetron is not a circulating wave propagating around the anode structure but rather a standing wave pattern. ln this special case, widely used in oscillators, the qr-mode frequency is exactly the cut-off frequency of the transmission line forming the anode structure. ln modifying the magnetron to make it suitable for use as an amplifier, it is not desirable to operate near a cut-oft frequency of the transmission line forming the anode, since this condition would involve great frequency sensitivity. It may be noted however that even in a 1r-mode oscillation, where the excitation is necessarily a standing wave pattern, the electron stream is coupled appreciably only to that component of the pattern which represents a travelling wave circulating with the electron stream. The other component of the pattern, which represents fan equal wave travelling against the electron stream, has a negligible effect on the electronic motions, infiuencing their orbits somewhat but exchanging no net energy with them. This is because the relative phase of the electron stream and this latter field comnited States iPatent O 2387,?34 Patented Apr. a, 1957 ponent changes very rapidly. These results are known from theoretical investigations by Hartree and others. In magnetron oscillators operating in a non 1r-rnode, it has been established theoretically and confirmed experimentally by probe measurements in the resonator cavities that the excitation on the anode structure is a true rotating wave. In converting the multi-cavity magnetron oscillator for use as an amplifier, it is accordingly convenient to operate in a condition similar to the non wmode.

In accordance with a feature of my invention I provide a magnetron type amplifier structure having R. F. circuits electrically smooth and reflectionless over a wide band, in which the re-entrant character associated with the magnetron oscillator is not present and in which that part of the circuit which presents the radio frequency fields to electron stream provides a phase velocity which is small compared to the free space 'velocity of electromagnetic waves, and is independent of frequency over a Wide band.

The above-mentioned yand other object and features of the invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. l is a schematic diagram illustrative of certain features of the invention;

Fig. 2 is a sectional side view of an embodiment of I the invention; and Fig. 3 is a sectional end View of the embodiment shown in Fig. 2.

Referring first to Fig. 1, I show in schematic form a section of a linear magnetron having an anode 1 and a cathode 2. separated by a distance d centimeters. A D.C. voltage difference of V volts is applied between anode and cathode, and a magnetic field of B gauss is directed perpendicular to the plane of the section. A linear magnetron is shown for purposes of illustration because, since the magnetron is used as an amplifier, it cannot be re-entrant-and a linear magnetron is the simplest alternative to :a re-entrant circulating magnetron. Moreover, the formulae which express the behavior of the magnetron are simplest for the linear case.

We consider the behavior of the magnetron of Fig. l when amplifying a wave having a free space wave length A0. The symbol )tu (shown in Fig. l) is used to denote transmission line wave length, or the distance along the anode corresponding to one complete cycle of the R. F. fields. The number` ha 7k is then determined by the anode geometry, and is much less than unity. The formulas which are used in the following description are taken from the paper previously gauss The unit current I0 is given by 8 I 17 amperes per space charge spoke where W is the width of the anode.

The maximum electronic efficiency is given by In order to obtain an overall efficiency of approximately 50%, au electronic eciency of. 6.0% is.` sufficient, since losses in the structure are small. This corresponds to a value for V/ V of about 8. Assuming that Vo=160 volts, the required voltage V is.1280'volts,

and

For a midband free space wavelength o=6 cm., the guide wavelength Aa becomes 6X.025=0.15 cm.

The anode-cathode spacing d may be found from the empirical rule that `i=07 approximately which has been found to give'good results in other magnetrons. Thus d=0.67(0.15),=0.10 cm.=.04 inch The unitmagnetic. tield` B0 is To find B, it is noted. thatY B/B0=1/z (V/.V0-I-1)=4.5

hence B: (4.5)(425.)=1920 gauss The current per space charge spoke is p amperes= 15 ma.

where W has been assumed to be. 0.15 inch. Assuming that unit current tioWs, the maximum generated R. F. power per spoke at the outputV end is For an output power level of 200 watts, the gain per spoke is Since more than one unit of current canbe drawn, a gain of 0.5 db per spoke is reasonable. For a total gain of 20 db, therefore, 40 spokes are required. This requires that the anode have a length of The anodemay conveniently be` formed offa helix` having a pitch of 1A to 1/6 ha; as an example 11)\I=0.04cm. The circumference of the helix is'. the Miko-:1.5 cm., so that the helix diameter is In Figs. 2 and 3, I show an illustrative embodiment of my invention. The R. F. structure is enclosed in a suitable evacuated vessel. 3. The input signal is applied i is supplied to cathode 6 via lines 7. The' D. C. electric eld is applied between-one of lines 7 and anode 5, as shown. The amplified output energy is coupled via impedance matching devices` (not shown) to coaxialline 8.

In Fig. 3 a section of the amplifier, along lines 3-3 i high, is shown in which correspondingl parts are given like reference numerals. In addition magnetic poles 9 and 10 which supply the transverse magnetic field are shown.

While I have described `above the principles of my in vention in connectionV with' a particularembodiment, it is to be clearly understood that this description is made only by way of example and not as a limitation tothe scope of my invention; In particular the invention is not limited to a linear magnetron, as it is apparent that other structures might be used. For example, a circular magnetron which is not `re-entrant and in which the input and output circuits are isolated from one another may be used. Again, the invention is not limited to the helical type of anode, as other types of known transmission lines on which waves propagate at reduced velocity may be used.

What I claim is:

Inl a broadband' microwave amplier, a two element substantiallyl rectilinear transmission line forl propagating electromagnetic waves, comprising two spaced conductive elements, one of said elements comprising an inductively loaded anode consisting essentially ofk a helically shaped metallic structure, and the other of said elements comprising an electron emissive cathode extending along the length of said line, means for producing a unidirectional electric eld between said two elements, means for producing between said elements a magnetic field perpendicular to said electricfeld, and to said linemeans for applying radio frequency energy to one end of said transmission line and output means coupled to the other end of said transmission line.

References Cited in the tile of this patent UNITED' STATES PATENTS 2,112,538 Potter July 5, 1938 2,241,976 Blewettet al May 13, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,367,295 Llewellyn Ian. 16,` 1945 2,414,121. Pierce Ian. 14, 1947 2,428,612 Blewett Oct. 7, 1947 2,439,401 Smith Apr. 13, 1948 2,511,407 Kleen et al June 13, 1950 2,566,087 Lerbs Aug. 28, 1951 2,582,185 Willshaw Jan. 8, 1952

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