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The traveling-wave slot antenna is similar to a traveling-wave wire antenna, but it is far more versatile because the phase velocity and rate of radiation of the fields in the antenna can be controlled. Four types of traveling-wave slot antennas have been identified. These are: (a) the conventional transverse electric, TE (no tangential E field parallel to the slot length); (b) transverse magnetic, TM (no tangential H field parallel to the slot length); (c) a hybrid with negligible transverse E, and (d) a hybrid with negligible normal H. Only the hybrid types are capable of producing maximum radiation in the direction of the slot axis (i. e., end-fire radiation). The complex propagation constant which is characteristic of uniform traveling-wave slots has been measured for a variety of waveguide geometries and is presented in the form of graphs. The radiation pattern of a traveling-wave slot can be controlled to give low side lobe or "cosecant squared" type patterns by appropriate variation of slot width with distance z along the axis. An approximate formula for the variation of attenuation Â¿ with z required to give a specified pattern can be derived. This in turn gives the required variation of slot width with z. An examination of the principle of superposition shows that the conventional technique of array design is an approximation which has proved inadequate in the design of certain slot arrays.