COUPLED HELICES 159 



pitch and length of such a stepped coupler are rather critical, the re- 

 quirements are indicated in the equations in Section 2.4. 



The useful bandwidth of the stepped coupler is not as great as that 

 of the tapered variety, but may be as much as an octave. It has however 

 the advantage of being very much shorter and simpler than the tapered 

 coupler. 



3.2 Low-Noise Transverse-Field Amplifier 



r One application of coupled helices which has been suggested from the 

 very beginning is for a transverse field amplifier with low noise factor. 

 In such an amplifier the EF structure is required to produce a field which 

 is purely transverse at the position of the beam. For the transverse mode 

 there is always such a cylindrical surface where the longitudinal field is 

 zero and this can be obtained from the field equation of Appendix II. 

 In Fig. 3.1 we have plotted the value of the radius f at which the longi- 

 tudinal field is zero for various parameters. The significant feature of 

 this plot is that the radius which specifies zero longitudinal field is not 

 constant with frequency. At frequencies away from the design frequency 

 the electron beam will be in a position where interaction with longitudinal 

 components might become important and thus shotnoise power will be 

 introduced into the circuit. Thus the bandwidth of the amplifier over 

 which it has a good noise factor would tend to be limited. However, this 

 effect can be reduced by using the smallest practicable value of b/a. 



Section 2.12 indicates that the impedance of the transverse mode is 

 very high, and thus this structure should be well suited for transverse 

 field amplifiers. 



3.3 Dispersive Traveling-Wave Tube 



Large bandwidth is not always essential in microwave amplifiers. In 

 particular, the enormous bandwidth over which the traveling-wave tube 

 is potentially capable of amplifying has so far found little application, 

 while relatively narrow bandwidths (although quite wide by previous 

 standards) are of immediate interest. Such a relatively narrow band, if 

 it is an inherent electronic property of the tube, makes matching the 

 tube to the external circuits easier. It may permit, for instance, the use 

 of non-reciprocal attenuation by means of ferrites in the ferromagnetic 

 resonance region. It obviates filters designed to deliberately reduce the 

 band in certain applications. Last, but not least, it offers the possibility 

 of trading bandwidth for gain and efficiency. 



A very simple method of making a traveling-wave tube narrow-band 



