Martello
In Martello, a row of dipoles in the same horizontal plane is used at each vertical position and the out put of an entire row is fed to a receiver from which a coherent i.f. signal is produced.
The 60 (S713) or 40 (S723) i.f. outputs are fed to the b.f.n., figure 6. Within the b.f.n., any required phase shift is achieved simply by choosing resistors of suitable values. This is made possible by first splitting the i.f. signal into 4 fixed phases in quadrature and then using resistors to achieve intermediate phases by combining appropriate signal amplitudes. Furthermore, the system is not limited merely to a single elevation beam but can be extended to produce several separate beams simultaneously.
Before
deciding finally on the i.f. beam forming system, consideration was given to
the better known techniques of r.f. beam forming: these were rejected on the
grounds that they are not only more cumbersome and space consuming but once
installed are quite inflexible. I.f. beam forming, on the other hand, has the
advantage that the relatively compact self-contained and passive b.f.n., figure
7, may, if required, be replaced by another having different component values.
Thus if the operational role of the radar should change, a different elevation
beam pattern may be provided very simply, without touching the antenna itself.
Looking further ahead, the b.f.n. concept lends itself to the use of active rather than passive devices so that it is not difficult to achieve a rapidly adaptive beam forming system if required at some future time.
Generation
of the multiple beams is achieved by combining the outputs of the individual
receiver channels in the b.f.n. The beam shape and pointing angle is governed
by a matrix which couples selected amounts of each input to the various outputs
in the appropriate phase relationship. The synthesized beam contours are weighted
and the crossover points controlled for optimum performance.
The b.f.n. normally provides 8 beams for the S713 and 6 or 8 for the S723. Figure 8 shows the normal 8 beam pattern for 8713 and figure 4 (again) the receiving pattern in relation to the transmission pattern. Transmission occurs within an approximately cosec2 radiation pattern so that all targets on a particular bearing are illuminated by every radar pulse.
