COUPLED HELICES 161 



of the dielectric between the helices in the actual tube, and to the neglect 

 of power propagated in the form of spatial harmonics. 



Nevertheless, the tube operated satisfactorily with distributed non- 

 reciprocal ferrite attenuation along the whole helix and gave, at the 

 center frequency of 4,500 mc/s more than 40 db stable gain. 



The gain fell to zero at 3,950 mc/s at one end of the band and at 

 4,980 mc/s at the other. The forward loss was 12 db. The backward 

 loss was of the order of 50 db at the maximum gain frequency. 



3.4 Devices Using Both Modes 



In this section we shall discuss applications of the coupled-helix princi- 

 ple which depend for their function on the simultaneous presence of both 

 the transverse and the longitudinal modes. When present in substantially 

 equal magnitude a spatial beat-phenomenon takes place, that is, RF 

 power transfers back and forth between inner and outer helix. 



Thus, there are points, periodic with distance along each helix, where 

 there is substantially no current or voltage; at these points a helix can be 

 terminated, cut-off, or connected to external circuits without detriment. 



The main object, then, of all devices discussed in this section is power 

 transfer from one helix to the other; and, as will be seen, this can be ac- 

 complished in a remarkably efficient, elegant, and broad-band manner. 



3.4.1 Coupled-Helix Transducer 



It is, by now, a well known fact that a good match can be obtained 

 between a coaxial line and a helix of proportions such as used in TWT's. A 

 wire helix in free space has an effective impedance of the order of 100 

 ohms. A conducting shield near the helix, however, tends to reduce the 

 helix impedance, and a value of 70 or even 50 ohms is easily attained. 

 Pro\'ided that the transition region between the coaxial line and the 

 helix does not present too abrupt a change in geometry or impedance, 

 relatively good transitions, operable over bandwidths of several octaves, 

 can l)e made, and are used in practice to feed into and out of tubes em- 

 ploying helices such as TWT's and backward-wave oscillators. 



One particularly awkward point remains, namely, the necessity to lead 

 the coaxial line through the tube envelope. This is a complication in 

 manufacture and reciuires careful positioning and dimensioning of the 

 helix and other tube parts. 



Coupled helices offer an opportunity to overcome this difficulty in the 

 form of the so-called coupled-helix transducer, a sketch of which is 

 shown in Fig. 3.2. As has been shown in Section 2.3, with helices having 



