Martello
Height
accuracy is independent of whether the radar is operating at a spot frequency
or in an agile mode and the azimuth beam position is unaffected by frequency
changes within the operating bandwidth, because of the 'squintless' design of
the antenna r.f. distribution system. Thus there is complete freedom to exploit
the operating frequency, as required, as an anti-jamming measure.
THE SQUINTLESS FEED
At this point a brief diversion into 'squintless feeds' is appropriate to explain their relevance to Martello.
As mentioned earlier, two basic requirements in surveillance
radars are that side lobes should be as low as possible to minimize the effects
of jamming and that the system should be capable of frequency agility. With
the widely used conventional arrangements of feed and reflector, the reflector
may either be curved in both planes, figure 9, in which case a 
feed
at the focus can cope with a broad bandwidth or it may be contoured in the vertical
plane only, with a full-length horizontal feed, figure 10.
It has been shown that the latter can give superior side-lobe performance if the relative powers of the feed outputs are properly tailored but it requires special design for the feed to operate over a band of frequencies without causing an azimuthal shift of the beam with frequency.
It was to this problem that Marconi engineers(3) addressed themselves in the 60's and production examples of the waveguide squintless feed were first shown on the Marconi 5600 series of transportable 2D surveillance radars at the Farnborough Air Show of 1968, figure 10 again. In essence, the feed is a waveguide distribution system in which equal path lengths are maintained between the input output port and each of the several radiating receiving horns. Changes of frequency cause the same change of phase to all horns and therefore the beam remains normal to the reflector. Further, several transmitters in frequency diversity may feed a common reflector with absolute beam alignment, another useful feature for long range defence surveillance radars.
Extensive design data has now been established enabling instructions for tape controlled milling machines to be raised readily for waveguide feeds to suit a wide range of reflector sizes and frequency bands. Squintless feeds have by now been used in many radars, both ground-based and naval, and, in the present context, for the vertical distribution of the transmitter power to the S713 Martello antenna. Moreover, a derivation of the same principle, but constructed in tri-plate is used in the horizontal planes of both S713 and S723 antennas to feed the rows of radiating dipoles.
SIGNAL PROCESSING
Although wartime radars achieved excellent long range detection under favourable conditions, they were generally poor in their ability to separate wanted from unwanted targets. Probably the major achievement of post war radars has been the ability to separate targets of interest from the clutter caused. for instance, by ground returns, birds, clouds and rough seas. The requirements for reliable detection of small targets in clutter have become progressively
more and more stringent and improvements have been achieved by better signal processors. It has been a long but exciting technical evolution from the original analogue m.t.i. systems of the 50's, which sensed the Doppler frequency change in returning signals to distinguish between fixed and moving targets, to the present day computer-like comprehensive digital processors, as used in Martello. But at every stage of refinement in processing, improvements in stability have become necessary in all circuits in the signal chain: there are now stringent specifications controlling timing, transmitter spectral purity and noise content.
For example, the noise content of a pulse transmitter may have to be 60 dB below the carrier throughout the entire operating band of the radar or perhaps more than -110 dB within a very narrow band. These are requirements which did not have to be considered for wartime radars.
