Plasma antenna of traveling wave with axial radiation

The dispersion equations for azimuthally asymmetric slow waves propagating along the plasma column, located in the dielectric cylinder are derived and solved. These waves induce currents that lead to the radiation of electromagnetic waves into the surrounding space. The antenna, which is a hollow dielectric cylinder of finite length, filled with an isotropic cold collisionless plasma is considered. The formulas for the angular distribution of the radiation in the far-field for azimuthally asymmetric and azimuthally symmetric slow current waves propagating along the antenna are derived. They can be used for case when the antenna length is much greater than antenna transverse dimensions and the radiated wave length. At that antenna radius, the thickness of the dielectric tube and dielectric permittivity, plasma density, the phase velocity and the wave current frequency can be arbitrary. Practically interesting case of a linear antenna of a traveling wave, which phase velocity is close to the speed of light is considered. Asymmetrical current wave with m=1 has a particular place. For it the maximum of directional diagram (pattern), unlike the asymmetric waves (m>1) and the symmetric wave (m=0), accounts for the direction of the antenna axis, and the side-lobe level is much lower than for the symmetric wave. The width of the pattern is decreased with increasing the ratio of the antenna length to the radiated wave length L/λ and is 17° for L/λ=10 and 12° for L/λ=20. Thus, the plasma antenna operating in the regime of the asymmetric wave current with m=1, can serve as a source of strictly axial radiation. It is shown that a sufficiently thin dielectric shell does not influence on the normalized radiation pattern.