If a plane electromagnetic wave satisfies the equation $${{{\partial ^2}\,{E_x}} \over {\partial \,{z^2}}} = \,{c^2}{{{\partial ^2}\,{E_x}} \over {\partial \,{t^2}}},$$ the wave propagates in the

A transmission line is distortionless if

In a uniform linear array, four isotropic radiating elements are spaced $$\lambda /4$$ apart. The progressive phase shift between the elements required for forming the main beam at 60° off the end-fire is

The phase velocity of waves propagating in a hollow metal waveguide is