Back in July, we posted Part 1 of our refresher course in dealing with wind farm and point-to-point telecommunications issues, which is available here. The article focused on point-to-point communications infrastructure, the assessment process and mitigation process. This article focuses on the interference mechanisms for the point-to-point links associated with the wind turbines and static structures.
The paper entitled “Fixed-link wind-turbine exclusion zone method”, by DF Bacon, version 1.1, released 28 Oct 2002 identifies three mechanisms in which wind turbines and static structures may cause signal degradation. These include:
- Near-field effects;
- Diffraction effects;
- Reflection effects.
Each of these are discussed in more detail below.
What are Nearfield Effects?
The basic theory for propagation of radio waves from point-to-point infrastructure works well for the most common scenarios where the object (wind turbine or other static structure) is a long way from the transmitting and receiving antennas (link ends), relative to antenna size, and wavelength of the signal. Close to the link ends, this theory is less accurate and the number of variables that can significantly affect transmission/ reception increases, therefore the near field may be more sensitive to interference from these objects.
The Ofcom paper notes that for simplicity, these effects are assumed in all directions from an antenna, as “it is believed that this will not result in impracticable restrictions”, suggesting that in practice the affected area will be small. In reality, nearfield effects are much more likely if an object is positioned in the general direction of link path i.e. if a wind turbine is located behind a link-end relative to a transmitter, nearfield effects are much less likely.
The Ofcom paper has the following equation where the antenna has no recognisable physical aperture (e.g. for a satellite television receiver, the aperture size might be the diameter of the dish) to calculate near field distance, Dnf:
- Dnf = Nnf λ g / π2
A worked example is given below where:
- Nnf is the degree of conservatism, recommended to take the value 3 (very conservative);
- λ is the wavelength in metres (as an example, in the 460MHz – 460,000,000 Hz – frequency band, the wavelength is approximately 0.65m). This is a typical frequency for an Ultra High Frequency (UHF) link;
- g is the boresight gain (= 100.1G, where G is the boresight gain in dBi – a measure of the ‘directionality’ of the antenna – in decibels, dBi), and assuming a conservative value of G for the antenna of 32dBi (as used in the Ofcom paper worked example), gives a value of g of 1,585;
- π has an approximate value 3.141.
This gives the result:
- Dnf = (3 x 0.65 x 1,585) ÷ 3.1412 = 313m.
This is the distance beyond which wind turbines and other static structures should be located. In the absence of any data, a 250m exclusion zone is initially suggested for microwave links and a 500m exclusion zone is initially suggested for UHF links.
As frequency decreases, boresight gain will tend to decrease as well, hence the value used is very conservative (it should be noted that a drop in boresight gain of 10dBi will reduce the value of Dnf by a factor of 10, so it reduces significantly with any reductions in gain).
What are Diffraction Effects?
Diffraction is the physical phenomenon of waves ‘bending’ around obstructions. It is one of the mechanisms that allow sound to be heard around corners. It is possible for wave-transmitted energy that would miss a receiver to be diffracted by an obstruction towards the receiver, causing constructive or destructive interference. Another diffraction effect is where the wave energy that bends around obstructions away from the receiver must come from the un-diffracted signal, reducing the strength of the main beam. These effects imply that the siting of a link end close to obstructions can weaken the signal reaching the dish.
The Ofcom paper recommends that for wind turbines, the 2nd Fresnel Zone is used as an exclusion zone for diffraction effects (the paper notes that this is considered conservative). The definition of a Fresnel Zone is described in the Ofcom paper as:
“The n-th Fresnel is the locus of all points for which, if the radio signal travelled in a straight line from the transmitter to the point and then to the receiver, the additional path length compared to the straight transmitter-receiver path equals nλ/2, where λ = wavelength.”
Therefore the 1st Fresnel Zone is the locus of points that will cause a signal to travel ½ a wavelength further than the direct path, and the 2nd Fresnel Zone will cause a signal to travel one wavelength further. Fresnel zones are ellipses with the transmitter and receiver at the focus points of the ellipse (which will normally be very close to the ends of the ellipse).
For wind turbines, the 2nd Fresnel zone is typically used for microwave links (1Ghz and above). The Ofcom paper has the following equation to calculate the 2nd Fresnel Zone, RF2, where d1 and d2 are the distances from each link end where the assessed wind turbine becomes tangential to the link path.
- RF2 = √ ((2 λ d1 d2) / (d1 + d2))
The value for RF2 will be a maximum where d1 = d2, i.e., in the middle of the link. As an example, this maximum value will now be calculated given set parameters.
The link path length is 20km, or 20,000m long: in the middle of the link path a wind turbine is to be sited, therefore d1 = d2 (10,000m); because this is a microwave link, λ = 0.3m (equivalent to a link frequency of 1GHz):
- RF2 = √ ((2 x 0.3 x 10,000 x 10,000) ÷ (10,000 + 10,000)) = 54.8m
This represents the largest value for the 2nd Fresnel Zone for this example link. This is the distance from the straight line between the link end and receiver (link end to link end) at the mid-point of the link.
For UHF links and any static structure, 60% of the first Fresnel zone radius is commonly used. From the first section, the wavelength at 460MHz is 0.65m. A worked example for a UHF link is shown below:
- RF0.6 = ( √ ((0.65 x 10,000 x 10,000) ÷ (10,000 + 10,000))) x 0.6 = 34.2m
This represents the largest value for 60% of the first Fresnel Zone for this example link. This is the distance from the straight line between the link end and receiver (link end to link end) at the mid-point of the link.
What are Reflection Effects?
Whilst link paths are typically designed so that the two link ends point directly towards each other, the actual signal that is propagated from the transmitter to the receiver travels outwards and disperses. This means the signal can be received in areas outside of the link boresight (the direct path) however these will be weaker than if the signal was received directly.
When a radio wave illuminates a wind turbine or other static structure, a proportion is reflected in multiple directions. If this reflected signal is then received at the opposite link end, there is a chance that significant effects could occur due to multiple receipts of the same signal (also known as multipath effect). Unless the level of the reflected signal is negligible compared to the direct signal (in terms of timing and power received), the combination of direct and reflected signals and the time differences between their modulation may cause performance degradation. This is a particular concern for UHF links where the receiving antenna is not in direct line of sight but the obstruction does have line of sight to both link ends. Therefore in instances where there is no line of sight, reflection effects are more likely. Microwave links are much less susceptible to reflection effects due to the higher frequency being more directional, which means direct line of sight is required. Essentially, the lower the signal frequency, the higher the chance of reflection effects occurring.
Modelling reflection effects is the most complex calculation which requires knowledge of the required signal (or ‘carrier’) to interference (CI) ratio, terrain data and signal propagation.
In general, the smaller the radar cross section (RCS) of a wind turbine, the less significant the impact, however onshore wind turbines are now getting bigger and bigger. Whilst the Ofcom paper correctly notes that RCS can be larger than the silhouette of the object as viewed from the direction of illumination, it also states the following:
“In the absence of more reliable information it is provisionally proposed that the optical silhouette of the complete blade set of a wind turbine, as viewed parallel to the axis of blade rotation, is used as the RCS.”
This means the visible area of the rotor blades with the disc facing the observer). This area generally reduces with the square-root of the turbine rotor diameter (i.e. halving the radius represents a quartering of the area – that is a 75% reduction).
Although the Ofcom paper example indicates effects up to 500m along the link path, they do not extend more than 15m to either side of the link path. Stakeholders are however, beginning to look more cautiously upon reflection effects, especially within initial high-level assessments.
Further information on communications issues surrounding wind farms can be found in the following articles:
- Refresher Course in Wind Farms and Point-to-Point Telecoms Issues – Part 1
- Communications planning issues for wind turbines
- What is a Fresnel zone
- Wind turbine terrestrial television interference
Click here to read the next article in this series, which will provide you with a photographic encyclopaedia of the relevant point-to-point communications infrastructure to help you identify the different types.
About Pager Power
Pager Power undertakes technical assessments on behalf of developers, which include assessing whether a proposed development infringes instrument flight procedures and would cause a problem for said technology. For more information about what we do, Please get in touch.
Thumbnail image accreditation: Sam Forson, March 30th 2017 on Pexels.com. Last accessed on 27th July 2022. Available at: https://www.pexels.com/photo/three-gray-wind-turbine-243138/