The coupling of three coaxial helices

The conventional 2-helix coupler, such as that proposed for travelling-wave tubes and backward-wave oscillators, can provide a good match over a wide frequency band. If the ratio between the external and internal helices exceeds two, the coupler may be inconveniently long and will have poor matching properties. The necessity for a small diameter ratio is obviated by introducing a third intermediately placed helix, which is physically unconnected and contrawound with respect to the inner and outer helices. The analysis of the 3-helix structure developed in the paper shows that the structure can support three fundamental forward waves with phase-change coefficients ß(1), ß(2) and ß(3). The phase velocity corresponding to ß(1) is approximately equal to that on a single helix. The phase velocities of the remaining two waves are determined by ß(3) < ß(1) < ß(2). When ß(2) ¿ ß(1) = ß(1) ¿ ß(3) and there is equal coupling between the helices, the power will be periodically transferred between the innermost and outermost helices. By analogy with a 2-helix coupler, the unidirectional power transfer can be achieved by discontinuing the intermediate helix after a distance L = ¿/[ß(2) ¿ ß(1)]. The diameter of the intermediate helix is a function of the ratio between the diameters of the outermost and innermost helices. Experiments have in general confirmed the theoretical predictions. A 3-helix coupler was designed from the above theory and tested in the range 8¿20cm. The diameter ratio of outermost/innermost helix was 3.56 and the input standing-wave ratio over most of this range was less than 2.