Method for positioning a microwave antenna

Abstract

A method for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the antenna mounted on a metal monopole structure that periodically deforms due to amb

Description

Field of the disclosure A method for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure, wherein the metal monopole structure periodically deforms due to ambient temperature effects. Also described is a system for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure. Background to the disclosure Microwave antennae or transceivers for point-to-point line-of-sight communication between mobile base stations are commonly mounted on monopole metal (in particular, steel) structures. Such structures are sometimes known colloquially as ‘palm trees’. In hotter countries (such as those nearest the equator), the metal monopole structures often experience distortion or bending. Specifically, portions of the metal monopole structures that are facing the sun heat to a higher temperature than compared to other portions of the metal monopole structure. This causes the constituent metal in the higher temperature portions to expand by a greater amount than the portions less exposed to sunlight. In turn, the uneven expansion causes a bending of the metal monopole structure away from vertical, an effect sometimes known as “the sunflower effect”. FIGURE 1 illustrates the sunflower effect. In particular, FIGURE 1 (b) shows a distortion or bending of the metal structure due to an extra expansion, Ax, as a result of heating on the side of the metal structure facing the sun. In comparison, the metal monopole structure that does not experience unequal incident sunlight at different faces (and so does not exhibit a temperature differential at different portions of the metal structure), remains vertical and undistorted (as shown in FIGURE 1(a)). A consequence of the distortion or bending of the metal monopole structure is to cause a microwave antenna mounted on the structure to slightly shift or tilt. As such, the microwave antenna can be moved away from the optimum alignment for line-of-sight point-to-point communications, resulting in deterioration of signal power received at a receiving antenna. In some cases, the shift of the alignment of the microwave antenna as a result of the sunflower effect is so significant that it can severely affect the base transceiver station to base transceiver station transmission quality of service (causing dropped calls, reduced data throughput, other degradation in service, or even service outage). These complications become particularly prominent when using high frequency transmission bands (E-bands, for instance in the range from 60 GHz to 90 GHz) because the width of the beam (specifically, the width of the main lobe of the radiation pattern transmitted by the microwave antenna) is narrower and more sensitive to any deviation angle. In order to overcome problems associated with the sunflower effect in metal monopole structures upon which microwave antennae are mounted, sometimes cladding is used to improve the thermal isolation of the metal structures. However, this is costly, and requires extra installation time and manpower. Other techniques to mitigate the effect on point-to-point line-of-sight communications include lowering the height of the mounted microwave antenna, because the tip of the metal monopole structures exhibit the greatest angular deviation. However, this can also reduce the range of the point-to-point line-of-sight communications between two base stations, in view of the increased obstacles present at lower heights. As a consequence, a greater number of base stations are required to cover the same service area, with accompanying additional expense. A final option for mitigation is to reroute the point-to-point line-of-sight communication link between different base stations when a particular base station is suffering particular degradation in service. However, this again requires a greater number of base stations to be provided, with accompanying expense, and can be complex to implement. As such, there is a need for an improved method for positioning a microwave antenna on a metal monopole structure. Summary of the disclosure In a first aspect there is a method for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure, wherein the metal monopole structure periodically deforms due to ambient temperature effects, the method comprising: obtaining received signal measurements of the point-to-point communication link, taken at the microwave antenna having the transmission direction in a first direction and over a time period being a fraction or multiple of a total duration in which the deformation occurs; positioning the transmission direction of the microwave antenna in a second direction, in which an off-peak portion of a main lobe of a radiation pattern of the microwave antenna is aligned with an axis of the point-to-point communication link when deformation does not occur, the second direction being determined based on the received signal measurements and allowing operational communication over the point-to-point communication link when both deformation occurs and deformation does not occur. The method positions the transmission direction of an antenna in order to counteract a reduction of received signal power over a point-to-point communication link that is typically observed due to periodic deformation of a metal monopole structure on which the antenna is mounted resulting from ambient temperature effects (otherwise known as “the sunflower effect”). The disclosed method provides an arrangement or positioning of the antenna resulting in an improved transmitted signal power over the point-to-point communication link throughout a full arc of deflection of the transmission direction of an antenna, where the deflection is cause by deformation of the metal monopole structure. The axis of the point-to-point communication link may be considered as the axis of direct transmission between two antennae that are geographically spaced (e.g. the ‘line of sight’). The metal monopole structures are metal towers or pylons, sometimes known colloquially as ‘palm trees’, on which an antenna is mounted. The transmission direction of the microwave antenna is a primary direction in which transmission from the antenna is directed, and which aligns with the peak in the signal power of the transmitted signal of the main lobe of a radiation pattern for a given antenna. It will be understood that the radiation pattern conforms to known models, and describes a pattern of the radiation in terms of directivity versus angle (and is associated with signal power in a given direction from an antenna). An angle between the off-peak portion of the main lobe of the radiation pattern and a peak of the main lobe of the radiation pattern may be approximately half the magnitude of a maximum tilt angle of the transmission direction of the microwave antenna caused by the deformation. The maximum tilt angle (or arc of deflection) of the microwave antenna is the angle between the transmission direction of the antenna mounted on a given metal monopole structure without deformation and the transmission direction of the same antenna (without any intentional realignment) mounted on the same metal monopole structure after maximum deformation of the structure occurs. An angle between the off-peak portion of the main lobe of the radiation pattern and a peak of the main lobe of the radiation pattern may be half the magnitude of between 70% and 80% of a half power beam width of the radiation pattern of the microwave antenna. As a consequence, the line-of-sight of the point-to-point communication link is always or mostly maintained in a region of the main lobe of the radiation pattern that changes substantially linearly (and having a relatively low gradient) with angle, even at the extremes of any tilt of the microwave antenna as a result of deformation of the metal monopole structure. The half power beam width (HPBw) is the angular width (or angular separation) in which the magnitude of the transmitted power decreases by 50% (-3dB) from the peak power of the main lobe. The angle between the off-peak portion of the main lobe of the radiation pattern and the peak of the main lobe of the radiation pattern may have an opposite direction to a tilt angle of the transmission direction of the microwave antenna caused by the deformation. In this way, the corrected alignment could also be termed a “reverse alignment” of the transmission direction of a microwave antenna, compared to the angular deviation observed due to deformation of a metal monopole structure due to the sunflower effect. The time period being a fraction or multiple of a total duration in which the deformation occurs may comprise: a daytime period and a night-time period; a 24-hour period; and / or two or more sequential 24-hour periods. The time period could be any period representative of the duration over which the deformation of the metal monopole structure occurs, in order that variations of signal measurements over the point-to-point communication link as a result of the deformation (and subsequent tilting of the antenna) can be quantified or assessed. The method may further comprise: determining a time within the time period, using the received signal measurements, at which the deformation causes a directional shift in the microwave antenna corresponding with movement from a peak of the main lobe of the radiation pattern being aligned with the axis of the point-to-point communication link to the off-peak portion of the main lobe of the radiation pattern being aligned with the axis of the point-to-point communication link; and wherein the second direction is a direction that maximises a received signal measurement of the point-to-point communication link at the determined time within a subsequent iteration of the time period. The step of determining may comprise: identifying a peak received signal measurement for a first time within the time period, the first time being a time at which the deformation does not substantially occur; and wherein the determined time is a second time or a time duration at which a received signal measurement of the received signal measurements is reduced from the peak received signal measurement at the first time by a predetermined amount or range. In particular, because the effect of the deformation of the metal monopole structure on the received signal power over the point-to-point communication link is cyclical (aligned with the cyclical nature of the sunflower effect), then the angle for the second direction can be translated to a particular time within the cycle. The determined time (or second time) can be identified as a time at which the antenna, having had a transmission direction aligned with the axis of the point-to-point communication link when no deformation of the metal monopole structure occurred, then experiences the received signal power having reduced from its maximum by a predetermined amount at the determined time during the deformation cycle. This method of determining the second direction for the transmission direction of the antenna may avoid some of the challenges associated with direct measurement of the maximum tilt of an antenna. The predetermined amount or range may be defined by one or more values from 5% to 40% on a logarithmic scale. In some examples, the predetermined amount or range may be defined by one or more values from 10% to 30% on a logarithmic scale. The predetermined amount or range may be set based on a maximum reduction in the received signal measurements from the peak received signal measurement over the time period. In other words, the magnitude of the predetermined amount depends on the magnitude of the maximum extent of the deviation of the transmission direction of a specific microwave antenna as a result of the sunflower effect. If the maximum angular displacement of the microwave antenna transmission direction is large, then the time is chosen as the time when the received signal measurement has dropped by a lower percentage (within the described ranges) from the maximum value, on a logarithmic scale. However, if the maximum angular displacement of the microwave antenna transmission direction is small, then the time is chosen as the time at which the received signal measurement has dropped by a greater percentage (within the described ranges) from the maximum value, on a logarithmic scale. Positioning the transmission direction of the microwave antenna in the second direction may comprise: determining a maximum deformation angle from the received signal measurements; and wherein the second direction is a direction based on half the determined maximum deformation angle. The second direction may be determined by direct measurement of the maximum deformation angle of the metal monopole structure (or the maximum tilt of an antenna). However, measurement or observation of such angles can be challenging in practice. The first direction may be set by positioning the transmission direction of the microwave antenna to maximise a received signal measurement of the point-to-point communication link at a time in which the deformation does not substantially occur. In other words, the antenna may be positioned to align a maximum amplitude of signal power of the main lobe of the radiation pattern of the antenna with the axis of the point-to-point communication link. Said alignment can be established by observing the received signal over the point-to-point communication link during an alignment adjustment process as part of maintenance and set-up of a particular microwave antenna. In a second aspect there is a system for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure, the system comprising a controller configured to position the transmission direction of the microwave antenna in accordance with the method of any preceding claim. The controller may be used to control one or more motors or other positioning devices, for physical movement or redirection of the microwave antenna on the metal monopole structure. The controller may also receive and / or process received signal measurements for the point-to-point communication link, in order to carry out the method. Brief description of the figures The disclosure may be put into practice in various ways, some of which will now be described by way of example only and with reference to the accompanying drawings in which: FIGURE 1 is a schematic illustration of deformation of a metal structure as a consequence of the sunflower effect. FIGURE 1(a) shows the metal structure at lower temperatures (for instance, at night) when no deformation occurs. FIGURE 1(b) shows the metal structure at higher temperatures (for instance, during peak sunlight hours) when deformation occurs; FIGURE 2 show plots of the radiation pattern of a microwave antenna. FIGURE 2(a) shows a horizontal projection of the polar radiation pattern. FIGURE 2(b) shows a rectangular radiation plot of directivity versus angle which describes the received signal power; FIGURE 3 is a vertical projection of a polar radiation pattern for a microwave antenna being part of a point-to-point communication link, the radiation pattern shown relative to the line-of-sight for the point-to-point communication link. The image shows the radiation patterns under ‘normal’ alignment, with and without deformation of the metal monopole structure as a result of the sunflower effect; FIGURE 4 is a plot of time versus received signal power over a point-to-point communication link in which the metal monopole structure on which the transmitting microwave antenna is mounted is affected by the sunflower effect, the transmitting microwave antenna being under ‘normal’ alignment; FIGURE 5 demonstrates the angular adjustment of the transmission direction of the antenna in normal operation (‘AW), after deformation of the metal monopole structure as a result of the sunflower effect and without the disclosed method having been implemented (‘Nd’) and after implementation of the disclosed method but at a time when deformation does not occur (‘Cwd’); FIGURE 6 is a schematic image of the vertical projection of the main lobe of the radiation pattern of microwave signals transmitted from a microwave antenna; FIGURE 7 shows vertical projections of a polar radiation pattern for a microwave antenna mounted on a metal monopole structure. FIGURE 7(a) shows a radiation pattern for a microwave antenna under normal alignment, before and after deformation of the metal monopole structure as a result of the sunflower effect. FIGURE 7(b) shows a radiation pattern for a microwave antenna under corrected alignment according to the disclosed method, before and after deformation of the metal monopole structure as a result of the sunflower effect; FIGURE 8 is a vertical projection of a polar radiation pattern for a microwave antenna being part of a point-to-point communication link, the radiation pattern shown relative to the line-of-sight for the point-to-point communication link. The plot shows a comparison of the radiation patterns of the antenna under normal alignment and under corrected alignment at a time when deformation does not occur; FIGURE 9 shows a graph of modelled received signal power versus time (on the 24-hour clock) for a microwave antenna. A first trace illustrates the received signal power for a microwave antenna positioned with normal alignment and a second trace illustrates the received signal power for a microwave antenna positioned with corrected alignment according to the disclosed method; FIGURE 10 shows a plot of received signal power versus time (on the 24-hour clock) for a microwave antenna with normal alignment and for a microwave antenna with corrected alignment according to the disclosed method, measured at a first site; FIGURE 11 shows a plot of received signal power versus time (on the 24-hour clock) for a microwave antenna with normal alignment and for a microwave antenna with corrected alignment according to the disclosed method, measured at a second site; FIGURE 12 shows a plot of received signal power versus time (on the 24-hour clock) for a microwave antenna with normal alignment and for a microwave antenna with corrected alignment according to the disclosed method, measured at a third site; and FIGURE 13 shows a plot of received signal power versus time (on the 24-hour clock) for a microwave antenna with normal alignment and for a microwave antenna with corrected alignment according to the disclosed method, measured at a fourth site. It will be understood that like features are labelled using like reference numerals. The figures are not to scale. Detailed description of specific examples As noted above, FIGURE 1 illustrates “the sunflower effect”, which describes a bending or distortion to a metal monopole structure in view of uneven heating, for instance as a result of intense sunlight incident on only one or some faces of the metal structure. In particular, increased heating of metal at a face of the metal monopole structure at which sunlight is incident experiences greater expansion than a face at lower temperature because it has not experienced the same sunlight exposure. The additional expansion (Ax in FIGURE 1) causes a bending (or camber or tilt) of the metal structure. The extent of the bending is greater at the top of the tower than compared to the middle or lower portion of the tower. In order to provide for point-to-point line-of-sight communications (for instance between base stations in a telecommunications network), microwave antenna or transceivers may be mounted on the metal monopole structure. A microwave antenna (such as a microwave dish) has a transmission direction, being a primary direction in which transmission from the antenna is directed, and a radiation pattern. The radiation pattern can be simulated according to known models (see for instance, see “Antenna Theory Analysis and Design”, Constantine A. Balanis, 3rd edition, John Wiley &Sons, INC, 2005, chapters 2, 12, 15). An example of a horizontal polar projection of the three-dimensional radiation plot (the “radiation pattern”) is shown in FIGURE 2(a). An example of a rectangular radiation plot, an alternative presentation method to a polar projection, is shown in FIGURE 2(b). Specifically, FIGURE 2(b) shows a rectangular radiation plot of directivity versus angle which describes the received signal power. As demonstrated at FIGURE 2(a) and FIGURE 2(b), the radiation pattern exhibits a “main lobe”, a number of “side lobes” and a “back lobe”. The main lobe is aligned in the desired direction of propagation (here denoted the “transmission direction”) and the side and back lobes represent radiation in an unwanted direction. The transmission direction can be considered to extend along a central axis of the main lobe of the radiation pattern, wherein the peak of signal power of the transmitted signal is aligned with the central axis. Ideal antenna designs maximise radiation in the main lobe compared to the side or back lobe. As evident from the radiation pattern in FIGURE 2(a), the signal power of the transmitted signal has a maximum at the central axis of the main lobe of the radiation pattern (in other words, in the transmission direction). In a small angular segment spanning either side of the central axis of the main lobe of the radiation pattern, there is a region in which the gradient of the signal power (in other words, the rate of change of the signal power per angular increment, dD / dcp in FIGURE 2(b)) is relatively low. However, at larger angles from the central axis of the main lobe, the gradient of the signal power (the rate of change of the signal power per angular increment, dDW in FIGURE 2(b)) is greater. In view of the shape of the radiation pattern, it will be understood that the maximum received signal of a point-to-point communication link between two microwave antennae is achieved when the line-of-sight between the two microwave antennae is aligned with the peak of the main lobe of the radiation pattern of the transmitting antenna (in other words, when the transmission direction of the transmitting antenna is aligned with the line-of-sight). Where the antenna is initially positioned to align its transmission direction with the line-of-sight to another microwave antenna, when a metal monopole structure on which the antenna is mounted experiences distortion due to the sunflower effect, then the transmission direction of the antenna is slightly deviated at an angle away from the line-of-sight. This causes the line-of-sight to no longer align with the peak of the main lobe and instead to align with an off-peak portion. As a consequence, the power of the signal received by the receiving antenna is reduced compared to the maximum. The amount of the reduction is dependent on the angular deviation of the microwave antenna. By observation of the shape of the main lobe, it will be understood that larger angular deviation causes the power to be more significantly reduced from the maximum. The effect of the tilt or deviation of the microwave antenna (and so its radiation pattern) can be further understood with reference to FIGURE 3. A first trace, Nm, of FIGURE 3 shows the radiation pattern of an antenna at a first time, the antenna positioned having a transmission direction - meaning the peak of the main lobe - aligned with the line-of-sight between the two illustrated antennae used in a point-to-point communication link at a time when the metal monopole structure on which the antenna is mounted experiences no distortion due to the sunflower effect. As such, the signal power received at the receiving antenna is at a maximum. The second trace, Nd, of FIGURE 3 shows the radiation pattern of the same antenna (without any intentional change to its alignment) but at a different, second time. At this time, the metal monopole structure on which the transmitting microwave antenna is mounted has experienced bending due to the sunflower effect. The bending of the metal monopole structure is sufficient to tilt the microwave antenna to change the transmission direction such that the line-of-sight between two the antennae used in the point-to-point communication link is aligned with an off-peak region of the main lobe. It can be seen that the off-peak region aligned with the line-of-sight exhibits a significantly lower power. Consequently, comparing the antenna at the first and second times there is a noticeable reduction in the received signal power of the point-to-point communication link. FIGURE 4 shows a plot of measured received signal level (Rx signal power) versus time (on the 24-hour clock) for a real-life point-to-point communication link where the transmitting microwave antenna is mounted on a metal monopole structure and initially positioned, at a time when deformation does not occur, such that its transmission direction is aligned with the line-of-sight between two antennae of the point-to-point communication link. In FIGURE 4, it can be seen that there is a significant reduction in the received signal level during the hours of around 7am and 12 midday, which are the hours when the sunlight incident on the metal monopole structure has the greatest heating effect. The significant drop in the received signal power during this period results from the deformation of the metal monopole structure on which the microwave antenna is mounted, and the subsequent tilt of the microwave antenna to an angle where the line-of-sight is aligned with an off-peak portion of the main lobe, as described above with reference to FIGURE 3. The inventors of the present disclosure have recognised that the deterioration in the received signal power caused by the sunflower effect can be reduced by adjusting or correcting the alignment of the transmitting antenna to take into account the deviation of the transmission direction of the antenna as a result of the sunflower effect. More specifically, the inventors have recognised that the majority of the reduction in received signal power of the point-to-point communication link can be mitigated by positioning the transmission direction of microwave antenna at an angle (a ‘corrected’ alignment) from the line-of-sight between two antennae of the point-to-point communication link at a time when no deformation of the metal monopole structure occurs. In a first example, the ‘corrected’ alignment, which takes place at a time when no deformation of the metal monopole structure occurs, requires the positioning of the antenna to have an angle, a, between the transmission direction and the line-of-sight between two antennae of the point-to-point communication link. The angle, a, is approximately half of the magnitude of the maximum tilt angle, Q, of the transmission direction of the microwave antenna caused by the deformation of the metal monopole structure at its fullest extent. In other words, with reference to FIGURE 5, the maximum tilt angle (or maximum deflection angle) of the microwave antenna, 0, can be determined as the angle between the transmission direction of the antenna under ‘normal’ alignment (as described above with respect to FIGURE 3) and without deformation, Nwd, and the transmission direction of the antenna under the same ‘normal’ alignment but with maximum observed deviation due to deformation, ND. The ‘corrected’ alignment, Cwd, then has a transmission direction at an angle, a, from the line-or-sight of the point-to-point communication link at a time when no deformation of the metal monopole structure occurs, wherein that angle, a, is half the maximum tilt angle, 0, and in the opposite direction (i.e. the angle, a, of the corrected alignment beam to the normal beam (or to the LOS) being a = -1 / 20). In this way, the corrected alignment could also be termed a “half-way reverse alignment” of the transmission direction of a microwave antenna, compared to the angular deviation observed due to deformation of a metal monopole structure due to the sunflower effect. In a second example, the inventors have recognised that the best results (being the greatest average received signal power over a full time duration in which deformation of the metal monopole structure occurs) are achieved when the ‘corrected’ alignment positions of the antenna have a transmission direction at an angle, or, from the line-of-sight between two antennae of the point-to-point communication link, wherein angle, or, is half the magnitude of between 70% and 80% of a half power beam width (HPBw) of the microwave antenna. In other words, angle, or, is 35% to 40% of the half power beam width (which is an angular width defined in the art, as described further below). As a consequence of this corrected alignment, the line-of-sight of the point-to-point communication link is always or mostly maintained in a region of the main lobe of the radiation pattern that is linear with a relatively low gradient (a low sensitivity region, as discussed further below), even at the extremes of any tilt of the microwave antenna as a result of deformation of the metal monopole structure. As such, although a small reduction in the received signal power is observed at certain times during the course of a day (or the course of a period of a cycle of deformation of the metal monopole structure), the reduction of the received signal power at any point in the cycle is significantly less than the reduction in received signal power due to the tilt of the antenna under ‘normal’ alignment (as per FIGURE 3) when the metal monopole structure is deformed. Overall, the inventors have recognised that a better average received signal power (across a whole day) is achieved when the transmission direction of the microwave antenna is positioned at a ‘corrected’ angle from the line-of-sight of the point-to-point communication link when no deformation of the metal monopole structure takes place. This goes against the expectation that the peak of the main lobe of the radiation pattern should always be aligned with the line-of-sight of the point-to-point communication link when no deformation of the metal monopole structure takes place. In particular, the inventors have identified that using this ‘corrected alignment’, although there is a small reduction in received signal power during the periods when no deformation of the metal monopole structure takes place, there is no time during a deformation cycle at which the received signal power is severely reduced. This is because the line-of-sight of the point-to-point communication link is always or mostly maintained within an approximately linear and relatively low gradient (i.e. low sensitivity) region of the main lobe of the radiation pattern, even at the greatest extents of the deformation of the metal monopole structure and resultant tilt of the microwave antenna. To consider the appropriate regions of the main lobe of the radiation pattern in which the line-of-sight of the communication link is maintained under corrected alignment, we turn back to FIGURE 2. In particular, the inventors have recognised that a small angular segment spanning either side of the central axis (and peak) of the main lobe of the radiation pattern, as described above, can be considered a ‘low sensitivity’ region of operation (shown as angular width ‘LS’ in FIGURE 2(b)). In this low sensitivity region of operation, the change in the signal power per increment of angle, ¢, is comparatively flat or linear. In comparison, at larger angles, there is demonstrated a ‘high sensitivity’ (or fast damping) region of operation (shown as angular regions ‘HS’ in FIGURE 2(b)), in which the signal power changes more rapidly with angle ¢. The low sensitivity ‘LS and high sensitivity ‘HS regions are also marked on FIGURES. The low sensitivity region, LS, in FIGURE 2(b) is defined as a region having a total angular width being 75% of the half power beam width (HPBw). The low sensitivity region, LS, is symmetrical around the peak of the main lobe. The high sensitivity regions (‘HS’ in FIGURE 2(b)), are those regions having an angle greater than the boundaries of the low sensitivity region. With respect to this definition, the half-power beam width (HPBw) is a standard measure known in the art as the angular width (or angular separation) in which the magnitude of the transmitted power decreases by 50% (-3dB) from the peak power of the main lobe (see, for instance, section 2.4 of “Antenna Theory Analysis and Design”, Constantine A. Balanis, 3rd edition, John Wiley &Sons, INC, 2005). The HPBw can be defined as: HPBw = — D where for an ideal microwave antenna k = 57.3, but for a typical microwave antenna k= 70, D is dish diameter and A is the operating wavelength. Overall, the corrected alignment put forward in the present disclosure looks to maintain the line-of-sight of a point-to-potin communication link within the low sensitivity region of the amin lobe of the transmitting antenna, even at the maximum extent of any deflection of the transmission direction of the antenna as a result of the sunflower effect. The angle, a, for the transmission direction of the corrected alignment to the LOS according to the definitions above can be identified by observation of received signal power over a time period being a fraction or multiple of a total duration in which deformation of the metal monopole structure occurs. More specifically, as the deformation of the metal monopole structure occurs due to incident sunlight at a face of the metal monopole structure, the deformation is overall cyclical and linked to the daylight hours and strength of the sun. In particular, the deformation begins as the sun rises, is most significant during the hottest hours of sunlight, and then reduces as the intensity of the sunlight lessens prior to sunset. During night hours, the metal monopole structure does not demonstrate the deformation described. In this way, the deformation is periodic and fairly repeatable. It should be noted that, as the sunflower effect is typically only exhibited to a significant extent in hotter countries nearer to the earth’s equator, the hours of sunlight are fairly consistent throughout the year. It will be understood that the specific pattern of deformation, which is reflected in the changing received signal power of the point-to-point communication, will be specific to a particular geographical location. In view of the above, to identify the angle for positioning of the microwave antenna under corrected alignment, the received signal power of the point-to-point communication when the antenna is in ‘normal’ alignment (as per FIGURE 3) is first observed over a duration representative of the cyclical deformation of the metal monopole structure. In particular, the transmission direction for the microwave antenna is aligned with the axis of the point-to-point communication link at a time when no deformation of the metal monopole structure occurs, and the received signal power of the point-to-point communication is observed over one or more cycles of the deformation. In other words, the effect on the received signal power of the so-called sunflower effect is recorded for a given metal monopole structure for a particular time duration. From this observation, the angle, a, for ‘corrected’ alignment of the microwave antenna can be established. The duration may be, for instance, a daytime period and a nighttime period, a 24-hour period, or two or more sequential 24-hour periods. Ideally, the behaviour of the deformation is determined across a number of cycles, in order to more accurately assess the change in the received signal power associated with the deformation effects. The displacement of the microwave antenna may be different according to the temperature profile throughout the day and on different days throughout the year. With this in mind, the received signal power is preferably monitored during the given duration on several days to provide an average received signal power reflecting the sunflower effect on a given microwave antenna. To better illustrate the improvements provided by the ‘corrected’ alignment, FIGURE 7 illustrates the alignment of the transmission direction of the microwave antenna, in particular showing the alignment of the main lobe of the radiation pattern for the antenna, compared to the line-of-sight (LOS) for the point-to-point communication link. FIGURE 7(a) shows a first trace (solid line, NWD) being the main lobe under ‘normal’ alignment (as per FIGURE 3) and when no deformation occurs to the metal monopole structure. Here, the peak of the main lobe is aligned with the line-of-sight axis of the point-to-point communication link. FIGURE 7(a) further shows a second trace (dotted line, Nd) being the main lobe under ‘normal’ alignment but when the microwave antenna is at the maximum tilt angle caused by the deformation of the metal monopole structure. In contrast, FIGURE 7(b) provides an example of the ‘corrected’ antenna alignment according to the present disclosure. Here, a first trace (solid line, CWD) shows the main lobe under ‘corrected’ alignment and when no deformation occurs to the metal monopole structure. The peak of the main lobe is aligned at an angle to the line-of-sight axis of the point-to-point communication link. A second trace, (dotted line, Cd) shows the main lobe under ‘corrected’ alignment but when the microwave antenna is at the maximum tilt angle caused by the deformation of the metal monopole structure. By comparison of FIGURE 7(a) and FIGURE 7(b), it can be seen that, although under ‘normal’ alignment the maximum signal power is received over the point-to-point communication link at times when deformation does not occur, whereas under ‘corrected’ alignment the signal power at the same times is slightly reduced from the maximum, the effect on the signal power due to the sunflower effect is overall much less. In particular, when deviation of the microwave antenna under ‘normal’ alignment occurs due to the sunflower effect, then the axis of the point-to-point communication link is aligned with a region of the radiation pattern of the deviated beam in which the rate of change of the signal power is relatively steep (a high sensitivity, HS, region of the main lobe). As a consequence, the signal power reduces dramatically when at the maximum tilt angle, 0. In contrast, the antenna with ‘corrected’ alignment does not experience the same drop in signal power even though the same maximum tilt, Q, of the antenna occurs due to deformation. In particular, at both the times when deformation does not occur and the times when maximum deformation occurs, the point-to-point communication link is aligned with a region of the radiation pattern of the deviated beam in which the change in the signal power is relatively flat (a low sensitivity, LS, region of the main lobe). As such, although a small drop in the signal power may be experienced during periods including where no deformation occurs, it is negligible than when compared to the drop in signal power for an antenna under ‘normal’ alignment when at the maximum deflection of the microwave antenna. A further illustration of the corrected alignment is shown in FIGURE 8. In particular, FIGURE 8 compares two traces for the alignment of the main lobe of the radiation pattern for the antenna compared to the line-of-sight (LOS) to the second receiving antenna. A first trace, Nwd, shows the main lobe under ‘normal’ alignment and when no deformation occurs to the metal monopole structure. Here, the peak of the main lobe is aligned with the line-of- sight axis of the point-to-point communication link. A second trace, Cwd, shows the main lobe under ‘corrected’ alignment and when no deformation occurs to the metal monopole structure. The peak of the main lobe is aligned at an angle, a, to the line-of-sight axis of the point-to-point communication link. In a first technique, the angle a, between the transmission direction of the corrected alignment beam without deformation, Cwd, and the transmission direction of the beam under normal alignment and without deformation, Nwd, could be determined by observation and measurement of deviation or displacement of the microwave antenna as a result of the sunflower effect. For example, the direction and magnitude of any deflection or tilt of the microwave antenna can be detected through measurement devices installed on the metal monopole structure on which the microwave antenna are mounted. Such devices can be employed to measure the ‘verticality’ of the metal monopole structure, for example to monitor the maximum displacement of the top of the structure in order to confirm that any lean of the structure is within a safe and acceptable range. However, such measurements require specific devices and technical expertise to install and interpret the results, adding time, cost and effort. Therefore, although conceptually the measurement of the displacement of the microwave antenna due to deformation appears straightforward, in practice the measurement of such a tilt of the microwave antenna to cause the deviation in the transmission direction is difficult and may be impractical. In particular, both the displacement and calculation of the angle of the transmission direction under ‘corrected’ alignment must take into account displacement of the microwave antenna in three dimensions. In view of the challenges associated with the first described technique, the inventors have identified an alternative, second technique for determining the angle a, for use in the ‘corrected’ alignment. This second technique makes use of knowledge and / or simulation of the received signal power at a receiving antenna of the given point-to-point communication link over time. As the effect of the deformation of the metal monopole structure on the received signal power is cyclical (aligned with the cyclical nature of the sunflower effect), then the angle, a, can be translated to a particular time, t, within the cycle. The time, t, is a time at which, when the antenna is under ‘normal’ alignment, the received signal power has reduced from its maximum by a predetermined amount. Said predetermined amount may be anywhere between 5% and 40%, and is more preferably between 10% and 30%. Once the time, t, has been identified from measurement or simulation of the received signal power under ‘normal’ alignment, then the corrected alignment can be implemented by aligning the transmission direction of the microwave antenna (being the peak of the main lobe of the radiation pattern) with the line-of-sight axis of the point-to-point communication link at the identified time, t. This equates to the microwave antenna being aligned having a transmission direction at an angle, a, from the line-of-sight of the point-to-point communication link when no deformation of the metal monopole structure occurs, according to the second example above (i.e. where the angle is half of between 70% and 80% of the HPBw). As noted above, the predetermined amount by which the received signal power has reduced from its maximum at time, t, is between 5% to 40% (and more preferably, between 10% and 30%) on a logarithmic scale. The magnitude of the predetermined amount depends on the magnitude of the maximum extent of the deviation of the transmission direction of a specific microwave antenna as a result of the sunflower effect. For instance, if the maximum angular displacement of the microwave antenna transmission direction is large (for instance, resulting in a deviation in the received signal power of over 20dBm between minimum and maximum under normal alignment), then the time, t, is chosen as the time when the received signal measurement has dropped by 20% from the maximum value, on a logarithmic scale. However, if the maximum angular displacement of the microwave antenna transmission direction is lower (for instance, resulting in a deviation in the received signal power of less than 20dBm between minimum and maximum under normal alignment), then the time, t, is chosen as the time at which the received signal measurement has dropped by more than 20% from the maximum value, on a logarithmic scale. Finally, if the angular displacement of the microwave antenna transmission direction is extremely severe (for instance, resulting in a reduction in the received signal power of much greater than 20dBm between minimum and maximum under normal alignment), then the time, t, is chosen as the time when the received signal measurement has dropped by less than 20% (for instance, 18%) compared to the maximum value, on a logarithmic scale. It should be noted that severe displacement, resulting in a reduction in the received signal power of much greater than 20dBm under normal alignment, is highly unusual. An example of corrected alignment via the second described technique (correlating angle to time) is discussed with reference to FIGURE 9. FIGURE 9 shows a plot of a simulation of the received signal power versus time (on the 24 clock) for an antenna received signal transmitted by a particular microwave antenna to be aligned. Although simulated data is shown, measured data could equally be used. It can be seen that for the antenna according to its ‘normal’ alignment (trace N) there is a significant reduction of the received signal power, reaching a minimum between 10am and 11am. In the example of FIGURE 9, the minimum received signal power under normal alignment is around -80dBm, compared to -35 dBm at the maximum of the received signal power. Looking to FIGURE 9, a time, can be determined at which, under normal alignment, the maximum received signal has dropped by a predefined amount. For instance, when the time, ti, is approximately 8:25 (am), the received signal, Raiign, has dropped by around 18% of its maximum (being around -42 dBm, which is around 18% less than 35 dBm on a logarithmic scale). It will be understood that a second time, fe, can also be established when the received signal is 18% less than the maximum, this second time being present during the increase of received signal after the minimum has been reached). In FIGURE 9, the second time, tz, is approximately 12:45 (pm). According to the present disclosure, alignment of the transmission direction (i.e. peak of the main lobe of the radiation pattern) of the microwave antenna with the line-of-sight of the point-to-point communication link at the first time h (or second time fe) causes the antenna to have a ‘corrected’ alignment as identified by the inventors. This ‘corrected’ alignment provides a transmission direction at an angle, a, to the line-of-sight of the point-to-point communication link when no deformation of the metal monopole structure occurs. The second trace, C, in FIGURE 9 shows the received signal power during the same daytime period when the antenna has undergone ‘corrected’ alignment at first time ft or second time t2. It can be seen that, although a change in the received signal power still occurs as a consequence of the sunflower effect, the difference between the minimum and maximum received signal is much less than when the antenna is under ‘normal’ alignment. The improved received signal power of the antenna with ‘corrected’ alignment is provided because the line-of-sight of the point-to-point communication link remains in the low sensitivity, LS, region of the main lobe throughout the whole deviation of the microwave antenna as a result of the deformation of the metal monopole structure. As will be understood, the low sensitivity region represents a region of the main lobe of the radiation pattern which has a flat or small rate of change in the radiation power versus change in angle. The ‘corrected’ alignment has been demonstrated to provide significant improvements to the service quality of a communication link. In particular, positioning of a microwave antenna according to the described method has been shown to significantly reduce the magnitude of a change in the received signal power as a consequence of the sunflower effect, thereby preventing severe deterioration in service across the duration of a whole cycle of the deformation. A number of examples of the results of the described method achieved for four different transmitting microwave antennae are shown at FIGURES 10 to 13. Each of FIGURES 10 to 13 show plots of the signal power of a received signal at a microwave antenna from a different microwave antenna. Each plot shows a received signal power recorded before application of the described positioning method (i.e. the antenna having ‘normal’ alignment), and after positioning of the microwave antenna according to the described positioning method (i.e. the antenna having ‘corrected’ alignment). In the plot of received signal recorded before application of the described positioning method (in other words, with ‘normal’ alignment) it can be seen that there is a significant reduction in the received signal power during the hours of greatest sunlight incident on the metal monopole structure. In the plot of received signal recorded after application of the described positioning method (in other words, with ‘corrected’ alignment) it can be seen that there a much-reduced decrease in the received signal power during the hours of greatest sunlight incident on the metal monopole structure. Reviewing the figures in detail, in the data shown in FIGURE 10, the reduction in the deviation of the received signal level is 91% across a 24-hour period. In the data shown in FIGURE 11, the reduction in the deviation of the received signal level is 85% across a 24-hour period. In the data shown in FIGURE 12, the reduction in the deviation of the received signal level is 85% across a 24-hour period. Finally, in the data shown in FIGURE 13, the reduction in the deviation of the received signal level is 76% across a 24-hour period. Across all four sites, a resultant impact on communication traffic has been seen to be almost entirely avoided. It will be understood that positioning a transmission direction of the microwave antenna according to the present disclosure could be achieved by a manual positioning or alignment once a suitable direction (or ‘corrected’ angle) for the alignment has been identified. For instance, the manual positioning of the transmitting antenna may be by an onsite visit by an engineer, who may access and position the microwave antenna on the metal monopole structure as required. In an alternative, a system for positioning the transmission direction of the microwave antenna mounted on the metal monopole structure may be used. The system may comprise a controller configured to position the transmission direction of a microwave antenna in accordance with the method of the present disclosure. For instance, the controller may be used to control one or more motors or other positioning devices, for physical movement or redirection of the microwave antenna on the metal monopole structure. The controller may also receive and / or process received signal measurements for the point-to-point communication link, in order to carry out the method. In some cases, the controller may be remotely controlled. Such a system may reduce the workload of a site engineer, by removing or reducing the need for site visits. Although examples according to the disclosure have been described with reference to specific illustrative examples (in particular, microwave antennas), approaches according to the disclosure may be applied to other types of communication and / or antenna subject to the sunflower effect. Certain features may be omitted or substituted, for example as indicated herein. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. In this detailed description of the various examples and / or embodiments, for the purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the examples and / or embodiments disclosed. One skilled in the art will appreciate, however, that these various examples and / or embodiments may be practiced with or without these specific details. Furthermore, one skilled in the art can readily appreciate that the specific sequences in which methods are presented and performed are illustrative and it is contemplated that the sequences can be varied and still remain within the scope of the various examples and / or embodiments disclosed herein. As used herein, including in the claims, unless the context indicates otherwise, singular forms of terms are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as "a" or "an" means "one or more". Throughout the description and claims of this disclosure, the words "comprise", "including", "having" and "contain" and variations of the words, for example "comprising" and "comprises" or similar, mean "including but not limited to", and are not intended to (and do not) exclude other components. Also, the use of “or” is inclusive, such that the phrase “A or B” is true when “A” is true, “B” is true, or both “A” and “B” are true. The use of any and all examples, or exemplary language ("for instance", "such as", "for example" and like language) provided herein, is intended merely to better illustrate the disclosure and does not indicate a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure. The terms “first” and “second” may be reversed without changing the scope of the disclosure. That is, the terms are relative such that an element termed a “first” element or position may instead be termed a “second” element or position and an element termed a “second” element or position may instead be considered a “first” element or position. Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise. Moreover, where a step is described as being performed after another step, this does not preclude intervening steps being performed. It is also to be understood that, for any given component, example or embodiment described herein, any of the possible candidates or alternatives listed for that component 5 may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. It will be understood that any list of such candidates or alternatives is merely illustrative and not limiting, unless implicitly or explicitly understood or stated otherwise. 10

Claims

1. A method for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure, wherein the metal monopole structure periodically deforms due to ambient temperature effects, the method comprising:obtaining received signal measurements of the point-to-point communication link, taken at the microwave antenna having the transmission direction in a first direction and over a time period being a fraction or multiple of a total duration in which the deformation occurs;positioning the transmission direction of the microwave antenna in a second direction, in which an off-peak portion of a main lobe of a radiation pattern of the microwave antenna is aligned with an axis of the point-to-point communication link when deformation does not occur, the second direction being determined based on the received signal measurements and allowing operational communication over the point-to-point communication link when both deformation occurs and deformation does not occur.

2. The method of claim 1, wherein an angle between the off-peak portion of the main lobe of the radiation pattern and a peak of the main lobe of the radiation pattern is approximately half the magnitude of a maximum tilt angle of the transmission direction of the microwave antenna caused by the deformation.

3. The method of claim 1, wherein an angle between the off-peak portion of the main lobe of the radiation pattern and a peak of the main lobe of the radiation pattern is half the magnitude of between 70% and 80% of a half power beam width of the radiation pattern of the microwave antenna.

4. The method of claim 2 or claim 3, wherein the angle between the off-peak portion of the main lobe of the radiation pattern and the peak of the main lobe of the radiation pattern has an opposite direction to a tilt angle of the transmission direction of the microwave antenna caused by the deformation.

5. The method of any preceding claim, wherein the time period being a fraction or multiple of a total duration in which the deformation occurs comprises: a daytime period and a night-time period; a 24 hour period; and / or two or more sequential 24 hour periods.

6. The method of any preceding claim, further comprising:determining a time within the time period, using the received signal measurements, at which the deformation causes a directional shift in the microwave antenna corresponding with movement from a peak of the main lobe of the radiation pattern being aligned with the axis of the point-to-point communication link to the off-peak portion of the main lobe of the radiation pattern being aligned with the axis of the point-to-point communication link; andwherein the second direction is a direction that maximises a received signal measurement of the point-to-point communication link at the determined time within a subsequent iteration of the time period.

7. The method of claim 6, wherein the step of determining comprises:identifying a peak received signal measurement for a first time within the time period, the first time being a time at which the deformation does not substantially occur; andwherein the determined time is a second time or a time duration at which a received signal measurement of the received signal measurements is reduced from the peak received signal measurement at the first time by a predetermined amount or range.

8. The method of claim 7, wherein the predetermined amount or range is defined by one or more values from 5% to 40% on a logarithmic scale.

9. The method of claim 8, wherein the predetermined amount or range is defined by one or more values from 10% to 30% on a logarithmic scale.

10. The method of any one of claims 7 to 8, wherein the predetermined amount or range is set based on a maximum reduction in the received signal measurements from the peak received signal measurement over the time period.

11. The method of any one of claims 1 to 5, wherein positioning the transmission direction of the microwave antenna in the second direction comprises:determining a maximum deformation angle from the received signal measurements; andwherein the second direction is a direction based on half the determined maximumdeformation angle.

12. The method of any preceding claim, wherein the first direction is set by positioning the transmission direction of the microwave antenna to maximise a received signal measurement of the point-to-point communication link at a time in which the deformation does not substantially occur.

513. A system for positioning a transmission direction of a microwave antenna for a point-to-point communication link, the microwave antenna mounted on a metal monopole structure, the system comprising a controller configured to position the transmission direction of the microwave antenna in accordance with the method of any preceding claim.10

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