(57) Array antennas constructed from some forms of waveguide radiators with sidewall slots
exhibit cross polarisation and periodic errors. This problem arises from the difficulty
of obtaining phase reversal in the radiation from adjacent slots in such a radiator
other than by slot inclination. The present invention offers two solutions. Firstly,
a radiator 10 having a number of non-inclined sidewall slots such as 13 and 14 and
a septum 15 with inclined slots such as 25 and 26 which provide a phase relationship
in the radiation from the sidewall slots which ensures that no significant cross polarisation
errors nor grating lobes occur. A number of radiators 10 can be used to form an array.
Secondly, arrays using radiators with sidewall slots, in which the distance between
these slots, the dimensions of the radiators and the relative positions of individual
radiators are such that the phase relationship in radiation from the slots of the
array prevent significant grating lobes.
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