MIT辐射实验室丛书V6-MICROWAVEMAGNETRONS资源-CSDN文库

3星 · 超过75%的资源需积分: 10 118 浏览量 2015-09-12 17:18:13 上传评论收藏 37.48MB PDF 举报

资源推荐

资源详情

资源评论

CHAPTER 1

INTRODUCTION

GEORGE B. COLLINS

A magnetron is a diode, usually cylindrical, with a magnetic field

parallel to its axis. Inmodern usage, ho~vever, theword implies a diode

that, with the aid of a magnetic field, produces short electromagnetic

waves, and it is with this meaning that the term is used in this volume.

Those magnetrons which produce radiation within the wavelength range

1 to 30 cm are here defined as microwave magnetrons. This class of

tubes is sometimes called cavity magnetrons from the fact that, in the

usual design, the resonant circuit is a number of closely coupled cavities

contained within the evacuated portion of the tube.

101. Early Types of Magnetrons.—k’f icrowave magnetrons and the

theory of their operation have their origin in contributions made by a

great many investigators extending back at least to 1921. A review of

this development will be given here ith the purpose of pointing out the

significant steps that have led to the present highly efficient sources of

microwaves. Editorial policy precludes the assignment of credit for

origination of ideas or inventions, and this question will be purposely

avoided as far as possible.

Nonoscillating Diodes with Magnetic Fields .—The basis for much of

the theory of magnetron operation was laid by Hulll who investigated

the behavior of electrons in a cylindrical diode in the presence of a mag-

netic field parallel to its axis. Such a diode is shown in Fig. 1.la. A

cylindrical anode surrounds a centrally placed cathode which is heated

to provide a source of electrons.

A nearly uniform magnetic field parallel

to the axis of the tube is produced by a solenoid or external magnet not

shown in the diagram. In the crossed electric and magnetic fields which

exist between the cathode and anode an electron that is emitted by

the cath;de moves under the influence of a force Fe =

Ee and a force

F~ = e/c(; X ~) (see Fig. 1.2), where

E is the electric field, B the

magnetic field, c the velocity of light, v the velocity of the electron, and

e is its charge. The solution of the resulting equations of motion, which

neglect space-charge effects, shows that the path of the electron is a

quasi-cycloidal orbit with a frequency given approximately

(1)

1A. W. Hull,Phgs. Re~.,18, 31 (1921).

9CC.11]

EARLY TYPES OF MAGNETRONS 3

When this orbit touches the anode, a condition of cutoff is said to

exist, and Eq. (2) holds

$=a[l-kYY

(2)

where

V is the potential difference between the anode and the cathode

and r. and

r, are their radii. The relation

is an important one from the standpoint

of magnetron operation. It implies that

for

V/B2 less than the right side of Eq.

(2), no current flows and, as

V/B’ is

increaeed through the cutofi condition, a

rapid increase in current takes place. For

obscure reasons the reduction of current at

cutoff, which is observed experimentally, is

not so abrupt as the theory outlined above

would indicate.

Cyclotron Frequenqi Oscillations.-The

type of diode shown in Fig. 10la can be made

to oscillate at very high frequencies if the

FIG. 1.2.—Forces on an

electronmovingina diodewith

a magneticfieldparallelto its

axis.

cathode and anode are made part of a resonant circuit with reasonably

high impedance and low losses. Conditions for oscillation are that V/B’

must be adjusted close to the cutoff condition given by Eq. (2) and that

the frequency of the resonant current be close to the transit frequency of

FIG.1.3.—Trajectoriesofelectrons:

(1)phasewithrespectto ther-ffieldis

unfavorablefor the supportof oscil-

lations;(2)favorable.

the electrons. An explanation of these

oscillations is given in terms of Fig. 1“3.

The dashed circle represents the path of

an electron in the interaction space and

modifies the trajectories of such an elec-

tron. Curve (1) represents the trajec-

tory of an ele~tron emitted at an instant

when the r-f field is in the same direc-

tion as the d-c field. Thus the effective

V acting on the electron is increased,

and from Eq. (2) it is seen that this

increases the cutoff radius with the

result that the electron strikes the

anode.

Curve (2) is for an electron emitted one-half period later when the

r-f fields are opposed to the d-c field.

The electron now misses the anode

and returns toward the cathode. Since the frequency of rotation as

given by (1) is made close to the r-f frequency, electron (1) will return

toward the cathode also retarded by the r-f field. This electron thus

INTRODUCTIO>V [SEC. 1.1

contributes energy to the r-f oscillation, and the process will continue as

long as the phase relationships with the r-f field persist or until the

electron is removed by some process.

As these phase relationships

cannot be maintained indefinitely, provisions are usually made for remov-

ing the electrons before they fall out of phase. One method is to tilt

the magnetic field slightly ]vith respect to the axis of the tube. This

causes the electrons to spiral out of the end of the anode before too many

revolutions occur.

A characteristic of this type of magnetron, which is important to

the operation of many magnetrons, is the quick removal from the r-f

field of the electrons whose phase is unfavorable to the support of oscilla-

tions and the retention in the r-f field of the favorable ones.

Split-anode magnetrons such as shown in Fig. 1lb will also oscillate

when the frequency of the resonant circuit (now connected to the two

segments) is close to the transit frequency of the electrons and the anode

voltage adjusted close to cutoff conditions.

h’o satisfactory analysis

has been made that gives the trajectory of the electrons in this case, but

it is probable that the unfavorable and favorable electrons are segregated

by processes similar to that illustrated in Fig. 1.3.

h’o large number of cyclotron-type magnetrons have been made,

but they have been used effectively as experimental sources of radio

frequency.’23 At 50-cm wavelength output powers of 100 watts have

been obtained; at 10-cm wavelength about 1 watt; and detectable

radiation has been produced at 0.6 cm. The efficiency of the split-

anode tubes is around 10 per cent for moderately long wavelengths as

compared with 1 per cent for the diode variety.

The shortcomings of this class of magnetron are low efficiency, low

power, and generally erratic behavior, but extremely high frequencies

can be generated by these oscillators.

Negative Resistance or Habann Type.—If the magnetic field of a split-

anode magnetron is greatly increased over what is required for the

cyclotron-type oscillations, a new type can occur which has been called

negative-resistance or Habann-type oscillations.

The frequency is

determined almost wholly by the resonant circuit, and the magnetic

field is not critical as is the case with cyclotron oscillations. These

oscillators have been investigated by Kilgore4 who observed in a mag-

netron containing gas at low pressure, luminous paths corresponding to

electron trajectories of the form shown in Fig. 1.4.

The form of the r-f field is shown, and this combined with the d-c

1A. Zarek,Cos.Pro. Pest Math. a FTYS.JPrague,53, 578 (1924).

2H. Yagi, l%oc. IRE, 16, 715 (1926).

3C. E. Cleetonand N. H. Williams,

Phys. Ren., 60, 1091(1936).

4G. R. Kilgore,

Proc. IRE, 24, 1140(1!)36).

SEC.1.1]

EARLY TYPES OF MAGNETRONS 5

field and the high magnetic field causes the electrons to spiral out to

the anode segment that is at the lowest (most negative) potential.

The magnetron thus has the characteristics of a negative resistance neces-

sary to produce oscillations. It is observed that the efficiency of this

type of oscillation is enhanced if the electron moves out to the anode

making tenor more spirals. The frequency of thespiraling isdekmnined

by Eq. (l), and thus magnetic field strengths are needed that are ten

times those required to produce the same frequency by cyclotron-type

oscillations.

Providhg sufficiently high magnetic fields to satisfy this

requirement for very high frequencies is one of the principal objections

to this type of oscillation asa practi-

cal source of microwaves.

+150

An important modification in

the design of split-anode magne-

trons was made when the resonant

circuit was placed entirely within

the vacuum system. This step

was the result of efforts to increase , I ,

both the frequency and power out-

put. Figure 1.lc shows such a

design. This type of tube has pro-

duced power outputs of 100 to 400

watts at 50 cm and 80 watts at 20

cm.

Traveling-wave Oscillations.— +50

Electronpath

This third type of oscillation also

FIG.1.4.—Trajectoriesof anelectronin

occurs in split-anode magnetrons

a split-anodemagnetronwhenused as a

Habann-typeoscillator.

and is related to the negative resist-

ance type. The two differ only in the ratio of the angular frequency of the

traveling wave to the cyclotron frequency. In the negative-resistance

magnetron the magnetic field is so high that on the cyclotron time scale

the traveling wave remains nearly stationary. There is no sharp dividing

line between the two. For the same frequencies the magnetic field

required is much lower than that needed to produce negative-resistance

oscillations; and although the magnetic field may be close to the value

necessary to produce cyclotron-type oscillation, its value is not critical

and the anode potential is lower, so that oscillations occur below cutoff

conditions.

Traveling-wave oscillations have also been observed’ in four-segment

and even eight-segment magnetrons.

Figure 1.ld illustrates a four-

segment magnetron and shows in particular the manner in which the

alternate segments are connected together within the vacuum envelope.

1K. Posthumus,Wireless

Eng., 12, 126(1935).

剩余805页未读，继续阅读

评论收藏

内容反馈

quick2015

2017-11-11

很值得一看

和谐社区

粉丝: 9
资源: 16

MIT辐射实验室丛书 V6-MICROWAVE MAGNETRONS

最新资源

MIT辐射实验室丛书 V6-MICROWAVE MAGNETRONS

MIT辐射实验室丛书 V15-CRYSTAL RECTIFIERS

MIT辐射实验室丛书 V22-CATHODE RAY TUBE DISPLAY

MIT辐射实验室丛书 V18-VACUUM TUBE AMPLIFIERS

MIT辐射实验室丛书 V20-ELECTRONIC TIME MEASUERMENT

MIT辐射实验室丛书 V16-MICROWAVE MIXERS

MIT辐射实验室丛书 V11-TECHNIQUES OF MICROWAVE MEASURMENTS

MICROWAVE MAGNETRONS.rar

Artech-Microwave.Circuit.Modeling.using.Electromagnetic

MIT辐射实验室丛书 V17-COMPONENTS HANDBOOK

MIT辐射实验室丛书 V21-ELECTRONIC INSTRUMENTS

MIT辐射实验室丛书 V26-RADAR SCANNERS AND RADOMES

MIT辐射实验室丛书 V1-RADAR SYSTEM ENGINEERING

MIT辐射实验室丛书 V2-RADAR AIDS TO NAVIGATION

微波工程第四版-Microwave Engineering

Microwave filters for RF-Microwave application

Artech - Broadband Microwave Amplifiers

Solid-State Microwave High-Power Amplifiers-2009

MIT辐射实验室丛书 V4-LORAN

MIT辐射实验室丛书 V3-RADAR BEACONS

MIT辐射实验室丛书 V13-PROPOGATION OF SHORT RADIO WAVE

Deep-learning-enabled self-adaptive microwave cloak.pdf

fs-level laser–RF synchronization with a fiber-loop optical-microwave phase detector

Microwave Office 中文版入门教程

A New Class of Broad-Band Microwave 90~De~ree

01-Digital Microwave Communication Principles-A .ppt

Radio-Frequency And Microwave Communication Circuits 2nd

UL923-2019 Microwave Cooking Appliances.pdf

最新资源