Martello
Needless
to say, the intrusion of metallic supports within the radiating field of the
dipoles must be avoided and the dipoles and the stripline feed protected from
the weather to avoid deterioration. Add to this the requirement to withstand
high explosive pressures of over 400 lb/sq. foot plus ice loading up to a distributed
weight of around 2 tons without deformation and it will be appreciated that
this was a design challenge somewhat out of the ordinary. Dipoles and feed are
shown in figure 13.
Once the overall dimensions of the planar arrays had been decided by taking into account the horizontal and vertical radiation patterns needed for radar performance in conjunction with the constraints of transportability, thought was directed to the various ways in which the radiating elements could be grouped.
For the S713 the solution chosen was to use 12 modules, of common design and therefore interchangeable, each having 5 rows. The modules are easily removable from the main spine of the antenna structure for transportation on a separate vehicle.
For the S723, it was decided that it was impractical
to break the 40-foot lengths and maintain the accuracy of assembly in the field
and so the entire array is divided into four 40 foot modules each having 10
rows. Again they are interchangeable, which eases
erection
and logistic problems, but this feature was only achieved after careful design
of the r.f. feed system whereby the relative phasing of individual rows is carefully
tailored.
The design specification required the equipment to have a limited 'off road capability', yet comply with UK road transport regulations and to fulfil the requirements for roll-on, roll-off sea ferries and C130 aircraft.
The concept of the open slatted antenna array, divided into modules mounted on an upright equipment enclosure (spine) was conceived as the most cost- and weight- effective solution.
The decision to use a slatted type array with one row
of dipoles per slat, rather than one
which
enclosed more elements within larger radomes, was taken carefully after studying
the problems associated with ice loading and weather sealing in relation to
wind and blast loading effects.
Both the S713 and S723 employ a stiff but light-weight equipment enclosure known as the 'spine' which forms the support for the antenna modules and houses electronic equipment as shown in the general block diagrams, figure 14 and figure 15. At the base of the spine the turning gear, rotating joint and azimuth data take-off are located, and the antenna turns on a massive single cross-roll bearing into the outer race of which the main drive gear teeth are cut, giving a compact but lightly stressed arrangement to ensure long life. The bearing diameter of some 4 feet ensures adequate clearance for the rotating joint, which passes through it. This comprises multiple slip rings together with, in the S713, a rotary waveguide joint for coupling from the external transmitter.
(Note: Larger copies of figures 13 and 14 can be viewed by clicking on the images - Ed.)
