Radar System Engineering

SEC. 9.4] NONSCANNING ANTENNAS 277
perpendicular polarization, posts scatter the radiation badly and are not
generally used; but fortunately the wavefronts are not disturbed by
gentle variations in the spacing of the sheets. Both polarizations are
used in practice.
FIG. 9,9.—Photograph of PPI scope of 3-cm airborneradar: Cape Cod area.
Another type of linear source of radiation is the linear array of dipoles.
If these are so excited as to emit energy in phase, the resulting radiation
is a fan in the broadside direction. Further discussion of line sources of
radiation will be found in Sees. 9.12 and 9.14.
9.4. Nonscan.ning Antennas.-There are certain types of radar
antenna which are not ordinarily required to scan. In this category are
the end-fire antennas which are sometimes attached to airborne guns in
order to permit radar range-finding. As an example, one end-fire linear
array (Fig. 9.11) is a series of dipoles excited in such a phase relationship
that their energy is beamed in the direction of the array. Another endfire
array is the Yagi antenna. Commonly, this is an array of dipoles,
only one of which is excited by the transmission line; behind it is a parasitic
reflector dipole and in front are several parasitic director dipoles. The