STRUCTURE FOR ANTENNAE FIELD OF THE INVENTION The present invention relates to the field of antennae, and structures for incorporating antennae. BACKGROUND PRIOR ART In radar systems, there is a constant need for improvement in range and tracking capabilities. In the past, rotating antenna systems with reflectors have been used to direct the beam at a target. The introduction of more advanced electronic systems allowed for the individual control of antenna elements, and this allows for electronic beam steering in one or two dimensions. Electronic beam steering is much quicker than mechanic beam steering which allows for the tracking of multiple targets simultaneously. Besides advanced electronic beam steering technologies higher signal powers allows for a significant increase in range which is desirable when manufacturers are trying to meet more stringent demands of lower signatures of targets and higher velocities. Developments in Gallium Nitride (GaN) power amplifiers show a clear trend to more and more RF output power, by virtue of improved heat transfer in the radio frequency (RF) integrated circuit (IC) and in IC packages to a cooling manifold. Obviously, the generated output power has to pass the antenna. Printed circuit board (PCB)-based antennas such as patch antennas use transmission lines that cannot tolerate higher power levels due to dielectric and conduction losses. Cooling the antenna is not always enough, especially when components are thermally isolated. At very high power levels, air filled waveguides have low losses. One downside of open-ended waveguides versus patch antennas is that large arrays of open-ended waveguides are much more difficult and thus expensive to fabricate. But due to recent developments of metal 3D printing technologies, these fabrication costs can be reduced significantly. When constructing phased array antennas, one ideally wants to maximize the element spacing to maximize the gain without introducing grating lobes. Here grating lobes are extra unwanted directions of transmission. The antenna gain is directly proportional to the surface area of the antenna. This means that a larger antenna spacing directly allows one to increase the surface area and thus gain without increasing the cost due to additional electronics. The large size of some antenna geometries such as the open- ended waveguide antennas forces one to increase the antenna grid spacing correspondingly. Phased array antennas are arrays of antenna elements on a two-dimensional grid. The phase and amplitudes of each antenna can be controlled electronically or digitally which allows the operator to control the shape and direction of the beam at will. A characteristic of certain types of antenna, including phased array antenna types such as patch antennae and waveguide matrix antennae is the scan angle. Figure 1 provides a representation of an antennas scan angle. A radar antenna will have a characteristic angular radiation pattern 110 for a given overall primary orientation, with a maximum efficiency at a central lobe 111. The primary orientation may be adjusted e.g. mechanically or in a phased array by phase shifting the signal of each array element, as known to the skilled person. On this basis, an overall efficiency curve 130 may be compiled for the antenna as a whole on the basis of the sum of the efficiency of each element multiplied by the complex excitation and the antenna return loss at the respective angle. The scan angle may be defined as the angle ^scan within which the overall efficiency exceeds a minimum threshold 140. As such, ^scan is the maximum angle at which the scan performance is degraded beyond a certain limit defined for the system. This may either be due to grating lobes or blind spots that appear sooner (as can happen with Patch antenna arrays on dielectric media for example). In many contexts, multiple separate antenna arrays may be installed in the same structure, and where this is the case interference may occur between respective antennae. These interference problems are related to propagation of EM surface waves over and around the surface of the structure from a transmitting antenna to a receiving antenna integrated in the walls of the same structure. A number of approaches to mitigating EM interference between two phased array antennas placed in same structure or between a phased array antenna and other transmitting or receiving antennas that are integrated in the structure are known. Arrays may be situated with a view to interference considerations, for example by locating arrays and other equipment at sufficient distance and/or by locating them at different sides of the structure. The structure itself may by shaped so as to limit interference. The structure itself may be formed from RF absorbing materials The structure may be provided with specific periodic structures, also known as meta materials, placed on or integrated in the walls to mitigate surface waves. The structure may be provided with surface formations comprising pins in a dielectric medium for suppressing waves in general, see the paper “Electromagnetic Characterization of Textured Surfaces Formed by Metallic Pins”, by Mário G. Silveirinha, Carlos A. Fernandes and Jorge R. Costa, in IEEE Transactions On Antennas And Propagation, Vol.56, No.2, February 2008 The use of Sievenpiper mushrooms in a patch antenna, see the paper “Elimination of Scan Blindness in Phased Array of Microstrip Patches Using Electromagnetic Bandgap Materials” by Yunqi Fu and Naichang Yuan in IEEE Antennas And Wireless Propagation Letters, VOL.3, 200463 General solutions such as placing antennas at different sides of the structure or at a sufficient distance therefrom, or selectively shaping the structure do not always provide sufficient mitigation of surface waves and are not always practical to implement. Other proposed solutions in the prior art including meta-materials and absorbent materials are often impractical, difficult or expensive to implement. It is accordingly desired to develop new radar structures better addressing the foregoing considerations. SUMMARY OF THE INVENTION In accordance with the present invention in a first aspect there is provided an antenna structure having a radiating surface in a first plane and having a maximum scan angle with respect to the normal of said first plane. The maximum scan angle defines a primary radiation region, where the antenna structure further comprises a first plurality of conductive pin elements arranged in a first row. Each conductive pin element has a proximal end and a distal end, the proximal end of each pin element being closer to the first plane than the distal end of each respective pin element, the proximal end of each pin element being spaced away from the first plane by a surface feature, whereby the proximal end of each pin element engages a surface of said surface feature and the distal end of no said pin element major axis impinges on said primary radiation region. In a development of the first aspect, the distance between each adjacent pair of pins in the first row is a first predetermined distance. In a further development of the first aspect, each pin meets the surface of the surface feature at an angle of between ninety and twenty degrees. In a further development of the first aspect, proximal extremity of each said pin meets the surface of said surface feature substantially at right angles. In a further development of the first aspect, the antenna structure comprises a further plurality of conductive pin elements arranged in one or more further rows parallel to the first row, where each successive said further row is spaced away from an adjacent row along the surface of said surface feature by a second predetermined distance. In a further development of the first aspect, there are provided at least ten rows. In a further development of the first aspect, the surface feature defines a varying cross section in a plane normal to said radiating surface such that the primary axis of each pin in each successive further row is oriented at a respective pin orientation angle ^ _n with respect to the edge of said primary radiation region in said plane having a value less than corresponding angle of the adjacent said pin in the preceding row ^ _n-1. In a further development of the first aspect, the varying cross section of the surface feature defines a continuous curve rising from the plane. In a further development of the first aspect, surface feature describes a slope at an angle with respect to the normal of said plane equal to or larger than said scan angle such that the normal of said surface feature with which said proximal end of each said pin element engages points towards said radiating surface. In a further development of the first aspect, the surface feature describes a platform substantially parallel to said plane and spaced therefrom in the mean direction of the radiation of the radiating surface. In a further development of the first aspect, the surface feature is formed of a conductive material and each conductive pin element is formed contiguously with the surface feature. In a further development of the first aspect, the surface feature is formed of a conductive material and each said pin is formed monolithically with the surface feature. In a further development of the first aspect, the antenna structure comprises a conductive substrate in which the radiating surface is mounted, and wherein each conductive pin element and the radiating surface are electrically coupled. In a further development of the first aspect, each conductive pin element and said radiating surface are electrically coupled to a common ground connection. In a further development of the first aspect, the length of the pins is longer than the speed of light in the medium in which the pins are embedded, divided by four times the lower frequency limit (f1) of the waveguide and shorter than the speed of light in the medium in which the pins are embedded, divided by two times the lower frequency limit (f1). In a further development of the first aspect, at least the first plurality of conductive pin elements (313) conforms to the periphery of the radiating surface along the entire periphery of the radiating surface. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and its various features and advantages will emerge from the following description of a number of exemplary embodiments provided for illustration purposes only and its appended figures in which: Figure 1 provides a representation of an antennas scan angle; Figure 2 shows an antenna structure in accordance with a first embodiment; Figure 3 shows an embodiment with a surface feature having a first alternative form. Figure 4 shows an embodiment with a surface feature having a second alternative form; Figure 5 shows an embodiment with a surface feature having a third alternative form; Figure 6a shows a first continuous disposition along the edge of a radiating surface; Figure 6b shows a second continuous disposition along the edge of a radiating surface; Figure 6c shows a third continuous disposition along the edge of a radiating surface; Figure 7 presents a variant of the embodiment of figure 3 in which the length of the pins is adjusted to fill the available space; Figure 8 shows an embodiment with multiple antennae; and Figure 9 shows an embodiment with multiple antennae in multiple planes. DETAILED DESCRIPTION OF THE INVENTION It is desired to provide an antenna structure imposing less electromagnetic interference on neighboring antenna devices, and preferably less susceptible to electromagnetic interference from such neighboring antenna devices, e.g. in the same installation or more generally between an antenna and other transmitting or receiving antennas that are integrated in a given installation. Such interference problems may be related to propagation of electromagnetic surface waves over and around the surface of the installation from a transmitting antenna to a receiving antenna integrated in the walls of the same installation. In accordance with embodiments, the propagation of electromagnetic surface waves to and/or from the antenna panels is mitigated by a matrix of pin elements disposed in a propagation path as discussed below. Figure 2 shows an antenna structure in accordance with a first embodiment. Figure 2 shows in particular an antenna structure 200 having a radiating surface 210 in a first plane 211 and having a maximal scan angle ^ _scan with respect to the normal of said first plane defining a primary radiation region. The first antenna may comprise any radiating device as may occur to the skilled person. In particular, the first antenna may comprise a two dimensional array of radiating elements for example in a beam steering configuration, comprising for example multiple waveguide or patch antenna elements. Such a two dimensional array may form an antenna which is substantially circular, oval, ridged or rectangular in plane 211 for example. As shown, the antenna structure further comprises a first plurality of conductive pin elements (213) arranged in a first row. Each said conductive pin element has a proximal end and a distal end, the proximal end of each said pin element being closer to said first plane than the distal end of each respective pin element, the proximal end of each pin element being spaced away from said first plane in the general direction of primary radiation of said antenna, e.g. away from a substrate defining plane 211 and into space, by a surface feature 214, whereby the proximal end of each said pin element engages a surface of said surface feature 214 and the distal end of no said pin element major axis impinges on said primary radiation region. The surface structure and conductive pins may be seen as together defining an interference mitigation structure. PTSW (Plante trapped surface wave) modes can be excited by electric fields due to apertures adjacent to the wire medium. It is therefore expected that the wire medium supports propagation of PTSW modes below its cut-off frequency. Above the first cut off frequency, there are no PTSW modes. One would predict that coupling below cut- off due to the presence of a strongly bound PTSW mode can aid in coupling over larger distances. This mode becomes more and more confined closer to the surface because ^ ₀ gets larger and thus a stronger exponential decay. Besides the Transverse Magnetic (TM) modes in the pass band regions, there are also Transverse Electromagnetic (TEM) and Transverse Electric (TE) solutions if both the electric field and magnetic field is polarized perpendicular to the wire medium. Certain implementations of the embedded wire medium may support TE modes. Implementations may act in a manner similar to a line of trees besides a highway reducing noise pollution. The medium seen as a homogenized medium supports an evanescent mode through it is above the cut-off frequency of the wire medium. The pin medium may be selected such that the plasma frequency is sufficiently far above the operating region of interest. The PTSW modes that are improper inside the stop band are solutions assuming that the medium operates below the plasma frequency. The plasma frequency is controlled via the wire spacing p and the wire radius r. The plasma phase constant k _p = 2^f/c ₀ is always a fraction of the lattice vector k _a = 2^/p. This fraction decreases as the radius of the wire decreases for a given lattice spacing. For a ratio of r = 0.1p the plasma phase constant k _p is approximately 0.4k _a . The choice of wires in practice depends on fabrication limits and the difficulty of performing simulations on certain mesh sizes. Very small wires are difficult to manufacturer robustly. Regarding simulations, designs with a small wire radius and lattice spacing will increase the complexity of the simulation model which makes them computationally expensive to solve. The precise method at which the PTSW bandgap interferes with the coupling is difficult to determine as the geometries are complex and the wires need to be simulated with a significant size in order to limit the computational complexity which means that the homogenization model is less accurate. Depending on desired system characteristics and operating conditions, suitable wire radius values may be found to fall for example between 0.1mm and 5mm. The spaced conductive pin elements are grounded, for example by connection to a conducting surface. The wire lattice acts as a low frequency anisotropic metal. Metals reflect electromagnetic waves below a certain frequency called the plasma frequency. The mobility of the electrons in a metal enables them to oppose incoming waves and reflect them. Because of the limited mobility of the electrons, electromagnetic waves with a frequency higher than the plasma frequency can partially pass through the metal. The properties of these materials can be modelled via a frequency dependent dielectric permittivity model called Drude’s model for metals. Above the plasma frequency, the dielectric constant 0 < ^ _r < 1 and waves can pass through. For metals, the plasma frequency is typically above the visible spectrum. For a lattice constant smaller than the wavelength, the wire medium can be thought of as an anisotropic metal as it behaves as a metal for incoming waves that have the electric field polarized parallel to the wires. The plasma frequency however can be tuned at will by means of the lattice spacing and wire radius. For waves with the electric field polarized perpendicular to the wire direction, the wire medium is transparent and a normal TEM mode is allowed to exist. In any embodiment, the conductive pins may be formed with respect to the structure in a variety of manners. For example, each pin may be formed contiguously with the respective the structure, that is to say, that while each pin is formed separately from the structure, it is placed in electrical contact therewith. Alternatively, each pin may be formed monolithically with the structure, that is to say, that each pin is formed of the same material and in a single piece with the structure. Still further, pins may be physically and/or separated from the structure, and connected to ground by a separate conductive matrix. The pins may by physically discrete elements which are inserted into corresponding sockets in the structure. Alternatively, the pins might also be separated from the structure by an isolating sheet such that they are electrically connected at RF frequencies, but not at DC. The structure may comprise a conductive material, and may be connected to electrical ground. Where this is the case, each pin may be formed contiguously with the structure, e.g. having been screwed, riveted, push fitted or otherwise brought into mechanical and electrical engagement therewith. Still further, each said pin may be formed monolithically with the structure, for example by welding or soldering thereto, or by being stamped, moulded, cast, printed by additive manufacturing methods, sintered, machined or otherwise formed from a single substrate. Still further, a plurality of pin elements may be formed on or as part of a thin substrate that may be glued, soldered, riveted or otherwise mounted on the structure. This might conveniently tape the form of a flexible tape, which may be conductive, or not, in line with the foregoing embodiments. For example, pin elements might be formed as part of a continuous aluminium alloy band that may easily be applied to the surface of the structure as generally presented herein. Accordingly, the surface feature may be formed of a conductive material. Each said conductive pin element may be formed contiguously, and possibly monolithically with the surface feature. The antenna structure may further comprise a conductive substrate 201 on which the radiating surface 210 is mounted, where each conductive pin element and the radiating surface may be electrically coupled to said substrate. Each conductive pin element and the radiating surface may be electrically coupled to a common ground connection. When the operating frequency of the antenna is sufficiently far below a material’s plasma frequency, a grounded slab of this medium can be shown to allow Plane Trapped Surface Wave (PTSW) modes at periodic intervals depending on the height of the medium. This is a preferred property because now its frequency dependent behaviour is no longer a function of its geometry in the direction of propagation but rather orthogonal to it. Grounding conducting pins into arrangements as described above yields a certain set of PTSW modes. An approximate model for the horizontal propagation constant k _^ of a grounded wire medium in air can be made which assumes an infinite plasma frequency: ^{^^} ^ ^{^^^^} _{^ ^ ^^^^ ^ ^ ^ ^ ^ ^ ^} Where k ₀ = ^/c and k _p the plasma wave number k _p = 2^f _p /c ₀ and L the length of the wires. The solution for the horizontal propagation constant emerges as the plasma frequency approaches infinity: _{^^ ^^^^^^^^ ^ ^^ ^^ ^ ^^^^} ^{^} ^ _{^ ^} ^{^} ^ Here ^ ₀ is the damping factor of the wave above the wire medium in the vertical Z direction. It may be borne in mind that the model of the preceding equation calculates the solutions in the passband only, as it also predicts solutions in the stop band that are incorrect. The following equation meanwhile specifies this same relation but without the inclusion of solutions in the stop band. Mathematically the cut-off regions of the wire medium can be written as: _{^^ ^} ^{^ ^^^^} ^ _{^ ^ ^ ^^^ ^ ^ ^^^^^^^^^^^ ! ^} Which can be simplified to find the stop band regions for values of n = [0,1,2,...,n], ^{^^} _{^^ ^^ ^} ^{^} ^ _{^ ^ ^ ^} ^{^^} ^ _{^ ^^ ^ ^^} Bearing in mind that while n = 0, ±1, ±2 etc. is the entire set of solutions mathematically, in practice only the set of solutions for n=1,2,3,4 and onwards is important because negative frequencies are not physically interesting but rather more mathematical. From this it is possible to write the first stopband for n = 0 as ^{^^} _{^ ^ ^} ^{^^} _{"^ ^^} Where f_1 is the lower edge of the desired stop band and f_2 the upper edge of the desired stop band. f_2 cannot be more than twice f_1 as the stop band only spans one octave. On this basis, the length L of the conducting pins may be selected as ^{^^} _{^ ^ ^} ^{^^} _{"^# ^#} Where the fs is a selected cut-off frequency in the stopband. For example, in the band of 2.8GHz to 3.5GHz the available lengths according to the preceding equation would be 26.7mm to 42.8mm. In case where a dielectric material is present around the pins the length is decreased ( ^{( by factor of $ of the ) ) %&' dielectric material. More simply one can write ^ - ^ .+/where} _{01 is the group velocity of TEM electromagnetic waves in that dielectric medium, or 01} = c ₀ /n where n is the refractive index of the material. The pins are not necessarily in air, but can be located in a dielectric medium. Encapsulating the pins in a dielectric medium may facilitate the manufacturing process and improve the robustness of the device. Accordingly, in certain embodiments the length of the pins may longer than the speed of light in the medium of the surface feature, divided by four times the lower frequency limit (f1) of the waveguide and shorter than the speed of light in the medium of the surface feature, divided by two times the upper frequency limit (f2). The described configuration may be seen as defining a “forbidden zone” 220 set by the primary radiation region boundary 212 corresponding to the projection from the edge of the antenna at the scan angle as discussed above, whereby the interference mitigation structure is placed and dimensioned so as to be present solely outside of the primary radiation region as defined by the transmitting antenna. In case the antenna has a scan angle in the plane at which the described structure is placed, the pins may not impinge into this “forbidden zone” 220. In case the described interference mitigation structure is placed in between two or more antenna systems, the interference mitigation structure may preferably respect the “forbidden zone” of each antenna system, so as to stay within the region defined by the edges of the radiation regions. For example, two adjacent antenna systems may define a triangular region defining the physical limits of the interference mitigation structure. The interference mitigation structure may be defined as an array of pins on a conducting surface. All pins may be connected electrically to the surface. While the pins of figure 2 are shown at approximately 90 degrees to the surface structure, the pins may assume angles ^ up to 70 degrees ^ _posmax, ^ _negmax away from the surface normal ^ ₀ . Angles beyond that may see significant reduction in the performance of the pins. This angle can be in any direction around the surface normal. It can be away from the radiation direction, towards it or lateral to it. A majority of pins will preferably be substantially parallel to their adjacent pins. In embodiments presenting a curved surface the angle constantly changes slightly, although adjacent pins in the same row will remain parallel, and adjacent pins in adjacent columns may vary only slightly in angle from one column to the next. Exceptions may occur for example where pins comply with a discontinuity in the underlying substrate, e.g. at an edge or corner, or at a point where differently oriented groups of pins intersect. Accordingly, in certain embodiments the proximal end of each said pin element may engage the surface of the surface feature at an angle parallel to the normal of the surface of the surface feature at the point of intersection, or within a predetermined range of deviation angles from parallel to the normal of said surface feature at the point of intersection. Any angle in between is also possible. However, pins around any specific pin should preferably tend to have the same orientation. Gradual changes throughout the medium in orientation is possible. Each said pin may meet the surface of said surface feature at an angle of between ninety and twenty degrees. Preferably, the proximal extremity of each said pin may meet the surface of the surface feature substantially at right angles. Preferably no tilt is applied to the pins in the direction perpendicular to the vertical plane radiating from the edge of the antenna structure. The distance between each adjacent pair of pins in the first row may be equal to single common first predetermined distance. As shown in figure 2, the antenna structure comprises an optional further plurality of conductive pin elements 215a, 215b, 215c arranged in one or more further rows parallel to said first row, where each successive said further row is spaced away from an adjacent row along the surface of the surface feature by a second predetermined distance 216a, 216b, 216c. The pins of adjacent rows need not be aligned from one row to the next. For example, the pins of a given row may be offset with respect to an adjacent further row parallel by an amount between 0.5 times (180 degrees out of phase) the first predetermined distance and zero (in phase). It may be appreciated that electro-magnetic energy to which a first row of pins is exposed will interfere with the incoming field and cancel in the direction of propagation. The next row of pins will take the leftover energy and redirect it in a similar fashion. Therefore each subsequent row of pins will yield more isolation and therefore increase the effectiveness of the medium. It is for that reason that more rows of pins are preferred over fewer rows. On this basis there may be provided more than 2 rows, and more preferably more than 5 rows, and still more preferably more than 9 rows. The number of rows may not be constant, for example where antenna structures get closer to each other, or to other structural features, there may simply not be enough space to provide a full complement of rows whilst respective the “forbidden zone” constraints as discussed above. Where this is the case, all available space in the constrained region may advantageously be filled with rows as far as possible within the “forbidden zone” constraints as discussed above. The surface feature 214 as shown in figure 2 is a flat step, substantially parallel to the surface of the substrate in which the antenna is mounted. In other embodiments, the surface feature may take other forms, always respecting of the “forbidden zone” constraints as discussed above. Figure 3 shows an embodiment with a surface feature having a first alternative form. As shown in figure 3, there is shown an antenna structure substantially identical to that of figure 2, apart from the configuration of the interference mitigation structure. As shown, the surface feature 314 presents a curved surface in a vertical plane 302 normal to the nearest edge of the antenna structure. This curve may be any curve, such as a circular or elliptical arc, parabola, hyperbola, or any other curve, including rational curves, transcendental curves, piecewise constructions, etc. As shown in figure 3, the surface feature defines a varying cross section in a plane 302 normal to the nearest edge of the radiating surface (i.e. the plane of the page of figure 3) such that the primary axis of each said pin in each successive said further row (when added at a position further from the radiating surface) is oriented at a respective pin orientation angle ^ _n with respect to the boundary of the surface feature 314 with which it intersects. As shown, the angle ^ ₁ of a first pin 313, ^ ₂ of a second pin 315a and ^ ₃ of third pin 315b are all substantially 90 degrees, however in other embodiments a difference in angle from one row to the next may be defined, for example as a constant step change from one row to the next, or may become progressively larger, or progressively smaller. As such, in certain embodiments, the varying cross section of said surface feature may define a continuous curve rising from the plane 211. This curve may preferably always curve in the same direction. This curve may preferably have an inverted U shape. This curve may preferably not be hollow. In a sloped realization part of the EM energy curves over the invention and interacts with the last pin row. This energy may not be obstructed by further rows because this pin row is the last row. That energy will diffract around the interference mitigation structure. With a curved realization each subsequent row of pins is in the shadow region of the previous row. The curve also naturally follows the diffraction curve of the electromagnetic energy leftover from the preceding pins. In other words, the curve is designed such that it maximizes the “contact surface” of the electromagnetic energy naturally diffracting around the invention. It also naturally avoids sudden abrupt changes in the pin structure thus avoiding extra diffractions. In other embodiments, the cross section of said surface feature may define a non- continuous surface. For example, the profile 330 comprising a sequence of flat sections at progressive angles would provide the same angular configurations of pins 313, 315a, 315b etc. as described above. Nevertheless, a smooth continuous curve has been found to provide optimal performance and is preferred. Figure 4 shows an embodiment with a surface feature having a second alternative form. As shown in figure 4, there is shown an antenna structure substantially identical to that of figures 2 or 3, apart from the configuration of the interference mitigation structure. As shown, the surface feature 414 presents a flat profile in the vertical plane radiating from the edge of the antenna structure. As shown, the surface feature 414 describes a slope 416 at an angle ^ _slope equal to or greater than the scan angle ^ _scan such that the normal 440 of the surface feature 414 with which the proximal end of each pin element engages points or leans towards the radiating surface 210 when viewed from above the point at which the normal 440 engages the surface feature, with respect to said plane 211. . The slope 414 may preferably be angled towards the radiating source. The angle of the slope may be chosen such that the electromagnetic energy grazes the slope. In other words, the pins may be normal or nearly normal to the primary radiation region boundary 212 of the EM energy. A very steep slope may cause the energy to expose the pins head on in which case the behavior of the pins in inhibiting interference may be degraded. In other words, the surface in which the proximal ends of the pins are located may be parallel/tangential or nearly parallel/tangential to the local direction of the electromagnetic energy or more preferably be parallel or nearly parallel to the direction of the electromagnetic energy More particularly, the primary radiation region boundary 212 is in fact the boundary of the area intentionally and strongly illuminated by the antenna. However, when considering the unwanted propagation of EM waves from a source 210 antenna to another antenna the shortest and dominating propagation path will usually be the shortest way and that is for many cases more or less parallel and as close as possible to the surface of the surface structure. So the preferred orientation of the pins may be perpendicular to the (overall slope of) the surface. This is not in all cases 1:1 related to boundary 212 of the intentionally illuminated area. As shown in figure 4, the surface feature defines a varying cross section in a plane normal to the radiating surface (i.e. the plane of the page of figure 4) such that the primary axis of each said pin in each successive said further row (when added at a position further from the radiating surface) is oriented at a respective pin orientation angle with respect to the normal 440 of the slope 416, similarly to the configurations discussed above. Accordingly, as shown, the pins 413, 415a, 415b, 415c are substantially parallel, and the angles substantially equal, with each pin being parallel to the normal 440 of the slope 416. It will be noted that the approaches of figures 3 and 4 may be combined to as to impose a progression on the angles of the pins for example as described with respect to figure 3), while retaining a planar surface feature for example as shown in figure 4, for example by progressively adjusting the angle at which the proximal end of the pins of successive rows meet the upper surface of the surface feature. Still further surface feature configurations may be envisaged. Figure 5 shows an embodiment with a surface feature having a third alternative form. As shown in figure 5, there is shown an antenna structure substantially identical to that of figures 2, 3 or 4, apart from the configuration of the interference mitigation structure. As shown, the surface feature 514 presents a stepped profile in the vertical plane normal to the nearest edge of the edge of the antenna structure. As shown, the steps of the surface feature 514 describe a slope 516 at an angle ^ _slope equal to or greater than the scan angle ^ _scan such that the normal 440 of the surface feature 414 with which the proximal end of each pin element engages points or leans towards the radiating surface 210 when viewed from above the point at which the pin element engages the surface feature, with respect to said plane 211, e.g. as shown by arrow 230 for pin 213. As shown in figure 5, the surface feature defines a varying cross section in a plane normal to the radiating surface (i.e. the plane of the page of figure 5) such that the primary axis of each said pin in each successive said further row (when added at a position further from the radiating surface) is oriented at a respective pin orientation angle ^ _n with respect the normal 540 of the slope 516, similarly to the configurations discussed above.. Each pin may be aligned at a given angle with respect to the normal 540 of the slope 516As shown, the pins 413, 415a, 415b, 415c are substantially parallel, and the pin angles are substantially equal, with each pin being parallel to the normal 540 of the slope 516. A sloped surface has a clear advantage over a flat surface, e.g. as described with reference to figure 2. Since the height of the pins and the number of rows both ought to be maximized within the aforementioned constraints, a sloped surface can fit more pins within the same region than a flat surface if the primary radiation region extends diagonally over the invention. As is illustrated above, a large number of pins have to be omitted in the flat realization in comparison to the sloped version. The sloped version extends as high as the flat realization but it comprises a larger number of pins. It will be noted that the approaches of figures 3 and/or 4 and 5 may be combined to as to impose a progression on the angles of the pins for example as described with respect to figure 3, while retaining a stepped surface feature for example as shown in figure 5, for example by progressively adjusting the angle at which the proximal end of the pins of successive rows meet the upper surface of the surface feature. Similarly, an angled planar surface as described with reference to figure 4 may form one of a series of such angled surfaces, with interceding step features as described with reference to figure 5. The angle of each planar part may be constant, or may vary progressively for example as described with reference to alternative profile 330. It will be appreciated that the embodiments of figures 2, 3 4 and 5 have been described in the context of the profile of the surface feature in a plane normal to the radiating surface. In certain embodiments, the surface feature profile, and/or disposition of rows of pins, may be expected to remain substantially equivalent along the length of the edge of the antenna structure. Figure 6a shows a first continuous disposition along the edge of a radiating surface. As shown in figure 6a a quadrilateral radiating surface 610a is provided with a surface feature 644a defined by a continuous extrusion of a profile 614a corresponding substantially to the profile 414 described with respect to figure 4. The surface feature 644a corresponds to a continuous extrusion of a profile 614a along the whole length of one edge of the quadrilateral radiating surface 610a. In some embodiments the surface feature may correspond to a continuous extrusion of a profile 614a along only a part of length of one edge of the quadrilateral radiating surface 610a. While figure 6a presents a continuous disposition the surface feature profile that is substantially equivalent along the length of the edge of the antenna structure, as mentioned above, the disposition of rows of pins may also be expected to remain substantially equivalent along the length. As shown in figure 6b, an exemplary group of pins 660 is presented. This group of pins comprises 12 pins arranged in three rows and four columns. There may be any number of columns, in line with the foregoing discussion, including pin extending along the entire length, or periphery of the surface feature 644a. Any number of rows may be present, and in line with the foregoing discussion, as many rows may be provided as compatible with the available space and the respect of the “forbidden zone” constraint. As shown, the Rows are separated by a distance b (corresponding for example to 216a and/or 216b and/or 216c as discussed above), and the pins in a given row are separated by a distance p. Each pin has a length L _n. While presented in figure 6a, these general considerations relating to the disposition of the pins may apply equally to figures 6b, 6c, and all embodiments presented herein. It will be appreciated that this band 660 need not necessarily be continuous, and that in accordance with certain embodiments the band of pin elements may correspond to one side of the antenna, two sides of the antenna, three sides of the antenna, or continuous or broken sections corresponding to any number of sides, or intermittently on any given side. The distance a between the edge of the antenna e.g. 610a and the inner row of pin elements may be any distance. As shown, the distance is relatively small, in the same order of magnitude as distance b and p for example, however in other embodiments the distance may be substantially greater, up to the edge of the surface in which the first antenna and associated pin elements are provided. The antenna e.g. 610a may be designed to operate at a predetermined wavelength, in which case the distance between rows b may preferably be a distance greater than the operating wavelength of the antenna. Deviations may occur for example where the geometry of the structure or the shape of an antenna imposes a sharp turn in a row, e.g. at the corner as shown in figure 6b below. In any case, this distance b may preferable by greater than twice the operating wavelength of the first antenna. Still more preferably this distance may be greater than 3 times said wavelength. The distance p between each adjacent pair of pins in a given row may be a predetermined distance. This distance may be generally equal for all conductive pin elements in the same given row. Deviations may occur for example where the geometry of the structure or the shape of an antenna imposes a sharp turn in a row, e.g. at the corner as shown in figure 6b. This distance may be equal for all conductive pin elements. The distances b and p are determined substantially on the basis of the same underlying principles as discussed further below, and in some embodiments these distances may be equal. In other embodiments b and p may have different values, while still both satisfying the criteria described below. The distance between each adjacent pair of pins in a given said row p is a predetermined distance as described in more detail below. As discussed above, the same calculations may be used in determining the distance between adjacent rows b, however for the sake of simplicity the following discussion will refer only to b. As discussed below, for optimal operation the plasma frequency f _p is preferably higher than the operating band of the antenna. In the definition as presented in the equation below both p and p/r (radius r =d/2) play a role in the value of f _p . For proper operation f _p >>f _operating (=c ₀ /^) with p/r > 4. In preferred embodiments having more than 2 rows of pins this implies that p < 0.25^. The plasma frequency of this material in air can be approximated via a simple formula if the assumption is made that the ratio of the lattice spacing a to the wire radius r is at least 10 or larger (p/r > 10), bearing in mind that the formula is only exact for infinitely thin wires but sufficiently accurate as long as p/r > 10. ^ ^{^ ^ ^^ ^ ^ ^ 3 2 2} 4 _{^ 5} ² ^ _{^^6 ^ ^78^98} The spacing p between adjacent pins in the same row, and between adjacent rows may be constant, or may be varied across the array. Generally, it is advantageous for a strong coupling reduction that the amount of rows of pins is maximized. This implies that p is small compared to lambda. Each pin has a diameter d. The diameter of each pin is preferably less than the predetermined distance p and/or b as defined above. For example, the diameter of each pin is preferably less than the distance by which the axis of one row is separated from the next row. If the wire radius is made smaller compared to the lattice spacing (p and or b) the plasma frequency will decrease and when the radius is made larger, the plasma frequency will approach c ₀ /p. Accordingly, the diameter of each pin is preferably less than the predetermined distance between elements in the same row, p. The length of the pins is preferably longer than the speed of light in the medium of the surface feature, divided by four times the lower frequency limit (f1) of the antenna and shorter than the speed of light in the medium of the surface feature, divided by two times the lower frequency limit (f1). Figure 6b shows a second continuous disposition along the edge of a radiating surface. As shown in figure 6b a quadrilateral radiating surface 610b is provided with a surface feature 644b defined by a continuous extrusion of a profile 614b corresponding substantially to the profile 414 described with respect to figure 4. The surface feature 644b corresponds to a continuous extrusion of a profile 614b along the entire periphery of the quadrilateral radiating surface 610b. In some embodiments the surface feature may correspond to a continuous extrusion of a profile 614b along the only a part of the periphery of the radiating surface 610b (for example two or three sides). Figure 6c shows a third continuous disposition along the edge of a radiating surface. As shown in figure 6c a circular radiating surface 610c is provided with a surface feature 644c defined by a continuous extrusion of a profile 614c corresponding substantially to the profile 414 described with respect to figure 4. The surface feature 644c corresponds to a continuous extrusion of a profile 614c along the entire periphery of the circular radiating surface 610c. In some embodiments the surface feature may correspond to a continuous extrusion of a profile 614c along the only a part of the periphery of the radiating surface 610c. In certain embodiments, the profile may change at different points along the periphery of the radiating surface. For example, it will be appreciated that in the case of the example of figure 6b, maintain a constant base width implies a wider profile 654b at the corner, when considered in terms of the plane radiating from the radiating surface 610b, as indicated by the arrow associated with the profile 654b. In other cases, the profile may change along its length to use any of the various profiles described herein at different points along its length. Still further, the dimensions and disposition may vary depending on the available space, so as to provide as many rows as possible as discussed above. Accordingly, the surface feature may describe a platform or other solid of extrusion or prismatic form, substantially parallel to the plane and spaced therefrom in the mean direction of the radiation of said radiating surface. Similarly, at least the first plurality of conductive pin elements may conform to the periphery of the radiating surface along the entire periphery of the radiating surface. As stated above, the length of the pins L is preferably longer than the speed of light in the medium of the surface feature, divided by four times the lower frequency limit (f1) of the antenna and shorter than the speed of light in the medium of the surface feature divided by two times the lower frequency limit (f1). Preferably, the height/shape of surface feature together with length of the respective pins may be chosen such that available space is filled. In certain embodiments, the length of the pins may vary. For example, although some embodiments, for example based on the description of figure 4 or 5, may allow the use of pins of a single fixed length to fill the available space, it will be appreciated that the various profiles discussed above for example with reference to figures 2 to 5 may not necessary conform exactly to the periphery of the “forbidden zone”, and that the length of the pins may be adjusted to as to fill the available space (preferably subject to the considerations and calculations presented above). Figure 7 presents a variant of the embodiment of figure 3 in which the length of the pins is adjusted to fill the available space. Figure 7 is substantially identical to figure 3, apart from the fact that while the pins 313, 315a, 315b and 315c of figure 3 are shown as being of equal length, the corresponding pins 713, 715a, 715b, 715c of the structure 700 of figure 7 are progressively longer for successive rows further from the antenna 210, such that the distal end of each pin terminates just outside the “forbidden zone” defined by primary radiation region boundary 312. As mentioned above, certain implementations may comprise a plurality of antennae. Where this is the case, the disposition of the interference mitigation structure may take into account the characteristics of each antenna, with a view to limiting the interference of each antenna on each other antenna. Figure 8 shows an embodiment with multiple antennae. Figure 8 extents the embodiment of figure 7 to a multiple antenna context. As shown, the surface feature 314 presents a curved surface in a vertical plane radiating from the edge of the antenna structure 210. Meanwhile, a second antenna 810 is provided, with its own respective scan angle ^ _scan2 , defining a boundary to the scan primary radiation region 812. It may be seen that the boundaries 312 and 812 intersect to define a substantially triangular region, outside of which subsists an extended “forbidden zone”. On this basis, the pins 813, 815a, 815b, on one hand and 815c, 815d of the structure 800 of figure 8 on the other are generally progressively longer for successive rows further from the respective nearest one of the two antennae, as far as the intersection of the two boundaries 812, 312, such that the distal end of each pin terminates just outside the “forbidden zone” defined by the scan angle of the nearest antenna, such that the pins extend to the limit of the extended forbidden zone formed by the intersections of the respective scan angles of the antennae 210, 810. It will be appreciated that this principle of respecting extended forbidden zone formed by the intersections of the respective scan angles of multiple antennae may be extended to any number of antennae. It will be appreciated that this approach may be adapted to any of the interference mitigation structure dispositions presented herein, such as based on the profile types of any of figures 2 to 5, or otherwise. Where adjacent antennae have different operating characteristics, such as their respective characteristic wavelengths, the corresponding characteristics of the interference mitigation structure such as pin length, spacing etc., may be calculated as a function of whichever antenna is closest to, whichever antenna it is oriented towards, whichever antenna it receives the strongest signal from, or any combination of these. The corresponding characteristics of the interference mitigation structure such as pin length, spacing etc., may be calculated on a per pin basis as a function of a weighted combination of whichever antennae that pin is closest to, whichever antennae it is oriented towards, whichever antenna it receives the strongest signal from, or any combination of these, such that these characteristics may progress continuously from pin to pin. In will be appreciated that intermediate approaches, for example where the length of the pins becomes progressively longer for successive rows further from the antenna 210, without necessarily reaching the extremity of the “forbidden zone” defined by scan angle defining primary radiation region 212 are also envisaged. It may be noted that the preceding embodiments relate primarily to antennae disposed in a single plane. Nevertheless, as mentioned above, embodiments extend to situations where some or all of the antennae are disposed on separate planes. Figure 9 shows an embodiment with multiple antennae in multiple planes. Figure 9 extents the embodiment of figure 8 to a multiple plane context. As shown, the surface feature 914 presents a curved surface in a vertical plane radiating from the edge of the radiating surface 210. Meanwhile, a second antenna with radiating surface 910 is provided, in a second respective plane 911 with its own respective scan angle ^ _scan2 , defining a primary radiation region boundary to the scan region 912. It may be seen that the boundaries 312 and 912 intersect to define a region, which in this case is a quadrilateral, outside of which subsists an extended “forbidden zone”. Preferably pin lengths are subject to the considerations and calculations presented above, in which case the scenario presented in figure 9 may be seen as not to scale. On this basis, the pins 913, 915a, 915b, 915c, 915d 915e, 915f of the structure 900 of figure 9 are generally progressively longer for successive rows further from the nearest of the two antennae, such that the distal end of each pin terminates just outside the “forbidden zone” defined by the scan angle of the nearest antenna, such that the pins extend to the limit of the extended forbidden zone formed by the intersections of the respective scan angles of the antennae 210, 910. It will be appreciated that this approach may be adapted to any of the interference mitigation structure dispositions presented herein, such as based on the profile types of any of figures 2 to 5, or otherwise. It will be appreciated that intermediate approaches, for example where the length of the pins becomes progressively longer for successive rows further from the antenna 210, without necessarily reaching the extremity of the “forbidden zone defined by an angle 212 are also envisaged. Accordingly, an interference mitigation structure may be situated adjacent an antenna, or between members of a group of antennas on a surface. The structure may comprise one or more rows of pins of predetermined dimensions and spacing mounted on a surface structure protruding from the antenna surface, such that the mitigation of the PTSW mode propagation due to the pins interferes with the coupling between antennae. That is to say, the pins serve to reject the PTSW mode at the characteristic frequencies. The dimensions and profile of the surface structure and the length and angle of the pins may be selected such that the pins do not protrude into a “forbidden zone” defined by the scan angle of the or each antenna. The examples described above are given as non-limitative illustrations of embodiments of the invention. They do not in any way limit the scope of the invention which is defined by the following claims.