Bifilar Helix for Backward-Wave Oscillators

A bifilar helix used as the slow wave circuit for the backward-wave oscillator is investigated in this paper. Calculations show that the bifilar helix has a higher impedance than that of the ordinary single-wire helix. The propagating characteristics of the bifilar helix are analyzed and some experimental results of cold measurements are presented. It is also shown that with a bifilar helix, a periodic electrostatic field which can be used to focus the electron stream is readily obtained by applying a proper dc potential difference between two helical wires. The same bifilar structure may thus be used both for the propagating and for the focusing circuit. A tube of this kind has been constructed and the experimental results are described.