Abstract :
The properties of waveguide modes (phase velocity greater than velocity of light) are investigated for a helix line of finite wire diameter and with an arbitrary surrounding medium. It is assumed that the guide wavelength is always much greater than the pitch of the helix. Use of the helix waveguide as a mode filter in transmitting the H01 mode is studied in particular. It is shown that the magnitude of the losses and the angle of pitch of the helix determine whether the waveguide modes are capable of existence. For a given angle of pitch the losses must be larger than a definite value to make waveguide modes possible. If the losses are less than this value, the helix line has only the character of a delay line (phase velocity smaller than velocity of light). On helix lines with an angle of pitch ¿ ¿ 0.1 the copper losses in the helix wire alone are sufficient to allow the existence of the H0n modes. For the Hmn modes, which interfere with H01 mode transmission, a maximum attainable loss exists which can be achieved by correct selection of the wire diameter in conjunction with the surrounding medium. This maximum loss is inversely proportional to the square of the helix diameter and to the frequency. For instance, with a helix diameter of 5 cm and a frequency of 50 Gc/s the maximum attenuation of the H12 mode is ¿ = 0.0158 nepers/cm, and hence exceeds by a factor of 3 180 the attenuation in a homogeneous circular copper waveguide having the same dimensions.
