Public Document Calculation of Wind Turbine - JRC of the Clearance Zone... · Public Document...

joint radio company Spectrum management services

Public Document

Calculation of Wind Turbine

clearance zones for JRC managed fixed services with particular reference to UHF

(460MHz) Telemetry Systems when turbine sizes and locations

are accurately known.

Issue 4.2 December 2014

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Page 2 of 18 Calculation of the Clearance Zone 4.2.1.docx © 2014 Joint Radio Company Ltd

Change History

Issue Date Change Author Editor Authorised

3.0.0 JUN 2006 Initial Issue PAS

3.0.1 30 OCT 2006 Clarification of low-frequency microwave link W/U and

maximum link losses for UHF Telemetry.

Bibliography updated. PAS SJP AAG

3.0.2 9 JAN 2007 Modification (relaxation) of the criteria for small turbines

(of less than 100m2 swept area).

Minor amendments to clarify text and inclusion of

Version Information Table. PAS SJP AAG

3.1 July 2009 Major Rewrite PAS SJP

4.0 Sept 2014 Updated PAS AL

4.2 Dec 2014 Revised: Editorials, formatting SJP

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Page 3 of 18 Calculation of the Clearance Zone 4.2.1.docx

© 2014 Joint Radio Company Ltd

Published by The Joint Radio Company Ltd.

JRC is a wholly owned joint venture between the UK electricity and gas industries specifically created to manage the radio spectrum allocations for these industries used to support emergency and safety critical operations. JRC also represents gas and electricity interests to government on radio

issues.

<www.jrc.co.uk/about-us>

© 2014 Joint Radio Company Ltd.

NOTICE: This is an uncontrolled copy. The definitive version is always available from www.jrc.co.uk/wind-farms/

Joint Radio Company Ltd. Dean Bradley House 52 Horseferry Road London SW1P 2AF

+44 20 7706 5199 +44 20 7222 0100 <[email protected]>

This document was Printed: 19 December 2014

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Contents

Contents .................................................................................................................... 4

1 Background ...................................................................................................... 5

1.1 Fixed radio infrastructure and interaction with wind turbines. ....................... 6

1.2 The differences between Mobile radio systems and fixed services with regard to wind turbine developments...................................................................... 7

2 Determination of the Clearance Zone ............................................................... 9

2.1 Determining the Diffraction Clearance Zone ................................................ 9

2.2 Calculation of the Reflection/Scattering zone. ............................................ 11

2.3 RCS Modifier with respect to the reflection angle. ...................................... 18

2.4 The Effect of Multiple Turbines. ................................................................. 19

2.5 Low Frequency Microwave Links ............................................................... 20

3 References ..................................................................................................... 21

4 Acronyms & Abbreviations ............................................................................. 22

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1 Background

(1) The current Government drive to find renewable sources of energy has resulted in the rapid development of wind farms. Wind energy is likely to be the single greatest contributor to the Government’s "15% by 2020" renewable aspiration. But there is a downside. Wind turbines reflect radio energy and because of their large moving surfaces the effect is difficult to predict and constantly changing. The effect has been described as similar to a dance hall mirror-ball, although the number of reflecting surfaces on a wind turbine is smaller.

(2) This ability to reflect radio waves means that wind turbines have some sort of effect on all radio communications, the systems most affected are those that rely on a stable propagation environment such as aircraft radars, television and fixed data systems.

(3) The turbine blades are not of metallic construction, but they can nevertheless reflect and diffract radio waves. Anti-icing, lightning and static electricity protection schemes built into turbine blades can further enhance their reflective radio properties.

(4) The interference effect of wind turbines on radar systems and analogue terrestrial TV systems have been investigated extensively. Ofcom, and its predecessor, the Radiocommunications Agency undertook some theoretical work on the potential for wind turbines to interfere with microwave fixed links but, because of the relatively small size of the market for utility telemetry radio systems, less research has been directed at this service.

(5) In 2002, the Radiocommunications Agency (now the Office of Communications, Ofcom) issued a paper that attempted to model the environment with respect to line-of-sight (LOS) microwave links and wind farms. The method described here is a modification of this LOS Microwave method that addresses the added complication offered by the fact that UHF links often run over obstructed paths.

(6) JRC has been assessing the potential for wind farms to cause interference to gas and electricity industry radio infrastructure for over ten years. JRC has co-ordinated over 10,000 wind farm applications in the last 18 months. UHF telemetry links in most cases are an integral part of the Supervisory Control and Data Acquisition (SCADA) systems used by utilities for monitoring and controlling their networks - including the infrastructure connecting the wind farms to the grid. Interruption to the reliable operation of these links compromises the integrity of the UK energy generation, transmission and distribution systems.

(7) UHF frequencies are particularly suited to this application as a single hop can provide a reliable link over a 25 km path (up to 50 km under ideal circumstances) and it is not necessary to have a line-of-sight path from transmitter to receiver. This ability of UHF telemetry systems to operate over obstructed paths is the feature that creates the greatest potential for incompatibility with wind turbines.

(8) Wind turbines frequently occupy the higher ground and protrude above the landscape they act as massive radio reflectors such that the reflected path via the wind turbine may be superior to the intended path. The reflected signal can thus be strong enough to reduce the performance of the link.

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(9) JRC may modify its criteria if more information becomes available due to practical experience and research into wind turbine interaction with radio links.

(10) JRC has undertaken tests on potential turbine effects to UHF links and in March 2009 Ofcom released a paper outlining practical tests carried out on wind turbines at various frequencies including 460 MHz, which suggested that under certain conditions the RCS of a turbine may be greater than we initially thought.

1.1 Fixed radio infrastructure and interaction with wind turbines.

(11) Fixed radio systems generally work from UHF (~450MHz) to multiple Gigahertz frequencies, although JRC do manage some fixed point to multipoint systems working in the middle VHF band (~140 MHz).

(12) Fixed links, including UHF telemetry point to multipoint systems, have fixed link ends. There are no percentages of locations to be considered with regard to coverage planning and interference, fixed links generally operate at 100% of time.

(13) Interruption caused by the signal dropping below minimum usable sensitivity or corruption to the bit pattern will cause the link to fail and be immediately noticed. This often causes the link to re synchronise and drop traffic for a length of time considerably longer than the interruption.

(14) Within the UK the transmit power for both point to point and point to multipoint systems are assigned to only just give enough receive level for the availability required. This is defined in OfW49 for UHF telemetry systems and OfW446 for point to point links.

(15) When very close to a radio path a turbine may cut the signal path or intrude into the Fresnel zone increasing the path loss or in some instances blocking the signal completely. The extent of this loss is dependent on the proximity of the turbine to the link ray, the size of the turbine and the Fresnel zone. The size of the Fresnel zone is dependent on the frequency of the link and the position on the link (the Fresnel zone reduces the closer you get to the link ends).

(16) Wind turbines, in addition, can introduce moving multipath which adds and subtracts with the wanted signal. This potentially reduces the worst case signal and degrades the fade margin of the link.

(17) A reflected signal may corrupt the bit pattern, this is more noticeable on high symbol rate links and more so on the higher multi-level modulation systems. Narrowband, lower bit rate schemes are less affected as the delay for the reflected signal to be destructive to the bit pattern is rarely achieved in practice.

(18) The turbine will also have more interference potential when it is in line with the path and the rotor is facing either end of the link, as the apparent turbine radar cross section is enhanced. This is known as forward scatter. Forward scatter occurs over a wider area on lower, frequency links. The wavelength with relation to the turbine blade width determine the beam-width of the enhancement.

(19) Any reduction in signal due to blocking or moving multipath or bit pattern corruption potentially caused by turbines, to a greater or lesser extent reduces the availability of the link.

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(20) As the positions of the link are fixed, signal degradation (fading) due to atmospheric multipath changes slowly. Any fade can exist for a considerable length of time, it is under these conditions that turbine interference caused by multipath is most noticeable. This is due to the reflected signal path length changing due to the movement of the turbine blade. Consequently the interfering signal will not suffer these fades over the same long period, it could in fact for short periods of time enhance the interfering signal.

(21) Point to multipoint systems have Omni directional or sectored antennas at one end of the link and directional antennas at the other. Point to point links have directional antennas at both ends. Lower frequency fixed link antenna systems have less directionality than higher frequency antenna systems. Lower frequency antenna systems tend to use Yagi or panel antennas. Higher frequency links tend to use parabolic dishes.

(22) The clearance requirement from a turbine to higher frequency links tends to be smaller than at lower frequencies. Apart from the Fresnel zone being smaller, the primary forward scatter zone is smaller and the antenna directionality greater, thus reducing the reflected signal and interference due to reflection/scattering.

(23) The blocking effect on reflected paths due to terrain or clutter (buildings/trees) is also more apparent at higher frequencies.

(24) 1400 MHz band links, although generically known as ‘microwave’, also inhabit the UHF band, which runs from 300 Hz to 3000 MHz. Due to forward scatter, antenna directivity and signal diffraction over terrain/buildings they perform more like UHF than higher frequency true 'microwave links'.

(25) At UHF frequencies, where the assignment criteria allows non line of sight paths, the interference due to reflections is enhanced when the wanted signal is non line of sight, with excess path loss and the reflected signal is line of sight to both ends of the link, or the path loss is considerably less than the main path. This is addressed in this document.

(26) In order that JRC can assess the potential effect of a wind farm on existing UHF telemetry links it was necessary to develop a methodology that, as accurately as possible, allows us to model the situation. A rigorous, practical, science-based approach to the subject is essential.

1.2 The differences between Mobile radio systems and fixed services with regard to wind turbine developments.

(27) A mobile radio system consists of a, or network of, transmitter/receiver 'base station' with an Omni directional/sectored antenna mounted on a mast often located on high ground.

(28) The base stations communicate with many vehicle based mobile radios or hand portable radios, which have omnidirectional antennas. Each individual mobile radio communicates for a relatively short period of time from anywhere over a relatively wide area. As each radio is not communicating continuously, the likelihood of interference is reduced by a percentage of locations, percentage of time ratio within the coverage area.

(29) Mobile radio systems tend to be narrowband, commonly with 6.25 kHz to 25 kHz channel spacing.

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(30) These systems generally use a different frequency for transmit to the one used for receive. Because of the dual frequency operation, and with the space to install suitable filtering, base stations normally transmit and receive at the same time (duplex). Due to the size of the filtering network required, mobile or outstation (OS) units generally cannot, even though the transmit and receive frequencies are different (simplex). This configuration, consisting of a duplex base station and two-frequency simplex mobiles/outstations is known as a half-duplex system, and is the commonest mobile radio system used.

(31) Mobile radio systems generally work in the VHF band between 138 and 200 MHz or the UHF band between 380 and 470 MHz. There are a few systems operating in VHF low band between 56 and 87.5 MHz, but JRC member companies tend not to use these. Mobile radio systems are generally analogue angle modulated or digital 4FSK. Tetra systems use a version of QPSK.

(32) These tend to be referred to as ‘Private/Professional Mobile Radio’ (PMR) or ‘Business Radio’ systems.

(33) JRC do not normally coordinate wind turbines with PMR systems except when the turbine is very close (<500m) to the transmitting site, and only if it

causes significant blocking, affecting the service area.

(34) A PMR system is designed to take into consideration multipath, both static, due to buildings, and moving, multiple diffractions caused by a moving end point (moving vehicle or user walking about). Wind farms will simply modify the rate of the peaks and nulls to the signal.

(35) As the RF powers are fixed this only becomes a problem at the extremity of reception when the signal is close to system design threshold. A properly designed wide area radio system should have handed the mobile off to another stronger site before the signal gets this low, unless it is moving out of its normal area of operation.

(36) Turbines will have more effect on a PMR system the closer they are to the path end, which includes the mobile unit end as well as the fixed base station, so any turbine, no matter where it is in the system’s service area, has the potential to degrade the overall system performance.

(37) The effect of the turbine will be enhanced when it is in forward scatter mode and a turbine will cause more multipath when the wanted signal has high path loss due to obstructions/terrain with the reflected signal having a much better path from the turbines to both ends of the wanted path. This becomes a much larger problem if the reflected signal is equal in amplitude and opposite phase to the wanted signal and totally cancels it out. No amount of signal headroom can compensate for that. This would only happen in extreme (very low W/U) cases and for very small percentages of time.

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2 Determination of the Clearance Zone

(38) There are various issues to be examined when considering the potential for wind farms to cause interference to radio links:

Near field effects- where a transmitting or receiving antenna has a near-field zone where local inductive fields are significant, and within which it is not simple to predict the effect of other objects.

Obstruction – the physical obstruction to the radio path by a turbine structure that is attenuating the received and/or transmitted signal.

Diffraction – although not directly obstructing the radio signal, because of the wave-like nature of a radio signal, large structures close to the radio path can cause interference patterns to be generated.

Reflection/Scattering – where the radio waves are reflected or scattered off a large structure and interfere with the wanted signal.

(39) At UHF frequencies and with the sort of antennas deployed for telemetry links, the near field clearance zone is always less than the other clearance zones and is therefore not usually used in the determination of the Clearance Zone.

(40) When protecting link availability approaching 99.9%, the wind turbine must be profiled in the worst case, i.e. With maximum horizontal profile, maximum radar cross section, maximum Doppler shift, etc. It is accepted that all of these conditions will not be fulfilled at all times, and in practice may only be for a small percentage of time. However, it must be remembered that the total tolerance for loss of service to such a link is no more than 0.1% of the time.

(41) The Clearance Zone is not an amalgam of the Diffraction and Reflection/Scattering Zones but simply the largest one of these zones.

2.1 Determining the Diffraction Clearance Zone

(42) The Diffraction Clearance Zone used by JRC for evaluating wind farms on links below 1000 MHz when the path is line-of-sight, has been modified to the following:

For structures with non-moving elements or small turbines having moving elements with less than 380m2 swept area, the clearance is determined by the swept area of the turbine with respect to the Fresnel zone (<20%) providing the turbine blade does not cut the link ray.

For structures with moving elements above 380m2, 0.6 Fresnel zone clearance is presently used.*

For links above 1000 MHz, 2.0 Fresnel zones are still used.*

* These figures are under review.

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(43) To the above Diffraction Clearance Zone is added a buffer zone to allow for

location uncertainty of the link ends and turbine construction, and also ellipsoid conversion anomalies.

Initially, or where 8 digit NGRs, or 6 digit Landranger NGRs are used, 150 metres is added;

where 12 digit NGRs have been supplied and verified by JRC, we initially add 25m; and

An additional allowance is also added for turbine micro-siting. If this is unknown, then 100m is assumed.

NOTE: The above figures may be reduced with a JRC site survey.

(44) No part of the turbine shall enter the Diffraction Clearance Zone.

(45) For example, the worst case 0.6 Fresnel zone clearance on a typical 15 km path would be ~38m plus uncertainty; on a 4 km path this would drop to 20m plus uncertainty. The Fresnel zone reduces the closer to the link ends you get.

(46) When a link is 'line of sight' (LOS), turbine height and ground height in relation to the ray path and Fresnel zone are taken into consideration when calculating the clearance required for a detailed coordination.

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2.2 Calculation of the Reflection/Scattering zone.

(47) The David Bacon Method of determining the effects of reflection scattering on LOS microwave links as defined in the Ofcom document "Fixed-link wind-turbine exclusion zone method" [1], whilst not appropriate for determining clearance zones for non-line-of-sight UHF paths, can be used as a basis to determine the clearance required (with some modifications, see below).

(48) UHF telemetry links, unlike links that operate at higher frequencies (above 3 GHz), do not always operate over a clear line-of-sight path with no intrusion into the Fresnel Zone. This feature adds further complication to the David Bacon calculation and these additional considerations are discussed below.

Extract from the document.

A1.3 Reflection/scattering clearance zone

This zone sets a lateral distance from the radio path Ds to ensure that any multipath effects due to reflection or scattering from the wind turbine ‘W’ are negligible, as shown in figure A1.2. All distances are in km.

Figure A1.2: Reflection/scattering clearance zone

The ratio, expressed in dB, of the wanted signal level received from the direct T-R path divided by the worst-case signal level received from the indirect T-W-R path, is given by:

Rci =71 - S + 20 log (s1 s2) -

22

21, sDd 20 log (Dp) + G1(0) + G2(0) - G1(1) - G2(2) (dB) (A1.3)

where: s1, 2 = (km) (A1.3a)

S = 10 log( ) (dB) (A1.3b)

= Worst-case radar cross section of turbine (m2)

G1, 2 (0) = Antenna boresight gains (dBi) (A1.3c)

G1, 2 (1, 2) = Antenna gain at off-boresight angles (dBi) (A1.3d)

1, 2 = angle ( Ds , d1, 2 ) (A1.3e)

where the function ‘angle’ represents a generalised form of arctan (Ds / d ) with protection against zero-divide for d=0, and returning a result in the range zero to 180 degrees.

End of extract from document.

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2.2.1 Additional considerations required when link path loss is greater than free space loss.

(49) Result of A1.3 – (PL [Dp]-FS [Dp]) + (PL [s1]-FS [s1]) + (PL [s2]-FS [s2])

FS = Free Space Path Loss: where FSL = 32.4+20logF[MHz]+20logD[km]

PL = Path Loss: derived from radio planning tool using ITU-R. 525/526/Delta Bullington propagation model and a k factor of 4/3.

(50) This loss is computed for path loss using antenna heights and turbine hub height. The path loss is normally less to the turbine tip but more to the bottom of the rotor, but in most instances the differences cancel each other out. In addition, the majority of the RCS for the blades is concentrated close to the hub as is any static reflection from the nacelle. Any static reflections due to the tower will also be less at the bottom of the rotor resulting in this method erring on the side of caution.

(51) As it is impractical to measure the signal at hub height of the proposed turbines we have to rely on predictions for this path, consequently the predicted signal level is used for the wanted signal as well, the theory being that if the paths are similar in horizontal angle then any inaccuracies will appear on both paths and cancel. If the angles are vastly different then a more in-depth investigation into local clutter is required.

(52) The wanted/unwanted (W/U) ratio used by JRC for UHF telemetry links is 38 dB using the JRC method for estimating turbine RCS.

(53) JRC acknowledge that this figure appears conservative when taken in isolation. We originally estimated the turbine RCS using the physical profile, as suggested in the David Bacon Method but experimentation using large turbines determined 38 dB was a figure that resulted in minimal performance degradation to the link. The algorithm JRC have traditionally used for calculating monostatic RCS could to be up to 16 dB inaccurate on a 80m hub 90m rotor turbine at UHF frequencies in light of tests carried out by JRC and using the collected data from an Ofcom sponsored report.

(54) This means the real W/U ratio is likely to be closer to 22 dB, the same as used in radio planning for co channel interference and also the same figure as used if the ITU-R BT 805 method is used to calculate turbine interference.

(55) Until more meaningful tests are carried out on different turbine types to determine an accurate maximum static and moving bistatic Radar Cross Section (RCS) across all pitch and yaw situations at JRC link frequencies (specifically 460 MHz) and long term tests are made on individual equipment types under these conditions, then JRC propose to continue using the original RCS estimation method and 38 dB W/U ratio in these calculations.

(56) Utility telemetry systems generally use an omnidirectional antenna with isotropic gain (dBi) between 2 and 8 dB at the scanning site. Yagi antennas with a gain between 11 and 17 dBi are generally used at the outstation.

(57) Owing to the relatively flat response around the boresight of a Yagi antenna; the deep nulls in the polar pattern (up to 8 dB per 5°); and distortion of these nulls due to the effect of the mounting structure on their electrical characteristics, the accuracy of the nulls of Yagi-type antennas cannot be

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guaranteed. Thus a mask is used in preference to the actual polar pattern for response off bore-sight.

(58) The H-plane mask of one of the more popular outstation antennas in use compared with the RPE for a standard antenna as defined in VNS2111 is indicated in the figures below:

Figure 2.1: Horizontal polar pattern used in calculations, of a Jaybeam 7018 12 element vertically polarised Yagi antenna. Gain is 14.2 dBi

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Figure 2.2: Horizontal polar pattern for a standard antenna as defined in VNS2111 (Formerly MPT1411). Gain is 11 dBi

(59) JRC will now use the RPE of the installed antenna or of the VNS2111 defined standard antenna in calculations, whichever has the better response.

(60) Generally, the scanner is on a high site, with the antenna installed on a mast in the region of 25m to 60m above average ground level (AGL). Outstations normally have the antennas mounted at 4-10m AGL, usually on the side of a low building or a free-standing mast or pole.

(61) This means that when the turbines are close to the scanner end, the path advantage via the turbine is generally less than when the turbines are close to the outstation end.

(62) As the paths are not always line-of-sight with no Fresnel zone incursions, there are instances where the full turbine profile can be seen from both ends of the link and the link has an obstructed path.

(63) When a turbine is close to an outstation it is not unusual for the additional loss compared to free space loss to be 10 to 15 dB worse on the direct path than via the turbine; in extreme cases this can be in excess of 30 dB.

(64) As the maximum allowed path loss in OfW49 is 143 dB, a 10 km link could have as much as 37 dB loss above free space and still be acceptable.

(65) If the path loss of a link exceeds 143 dB and/or the received signal strength is below that specified in OfW49, the link W/U will only be protected as if these parameters are met.

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Figure 2.3: Extreme example path profile of a non LOS link with a wind turbine close to the outstation where the path via the turbine is significantly better than the direct

path.

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Table 2.1: – Simple example of reflection Clearance Zones for links close to an outstation (OS)

against a turbine with an 90 m rotor (Peak RCS of +27.7 dBsm);

for 38 dB W/U ratio at the outstation end;

with paths of 20 km and 5 km where all the paths are free space;

with the main path having 10 and 15 dB more loss with respect to free space loss than the reflected path;

using a standard antenna as defined in VNS 2111

assuming the antennas are in perfect alignment.

Path RCS Path Distance from Distance Discrim. Antenna

A-B m

dBsm Difference dB

OS along link m

from link m

T-B m

A-T m

Angle OS degrees

Discrim. dB

W/U dB

20000 27.7 0.0 560 0 560 19440 0.0 0.0 38.0

20000 27.7 0.0 212 122 245 19788 30 7.0 38.0

20000 27.7 0.0 144 144 204 19857 45 8.6 38.0

20000 27.7 0.0 83 144 166 19918 60 10.3 38.0

20000 27.7 10.0 1900 0 1900 18100 0.0 0.0 38.0

20000 27.7 10.0 690 398 797 19314 30 7.0 38.0

20000 27.7 10.0 460 460 651 19545 45 8.6 38.0

20000 27.7 10.0 265 459 530 19740 60 10.3 38.0

20000 27.7 15.0 3750 0 3750 16250 0.0 0.0 38.0

20000 27.7 15.0 1260 727 1455 18754 30 7.0 38.0

20000 27.7 15.0 840 840 1188 19178 45 8.6 38.0

20000 27.7 15.0 480 831 960 19538 60 10.3 38.0

5000 27.7 0.0 620 0 620 4380 0.0 0.0 38.0

5000 27.7 0.0 220 127 254 4782 30 7.0 38.0

5000 27.7 0.0 147 147 208 4855 45 8.6 38.0

5000 27.7 0.0 84 145 168 4918 60 10.3 38.0

5000 27.7 10.0 2500 0 2500 2500 0.0 0.0 35.2

5000 27.7 10.0 780 450 901 4244 30 7.0 38.0

5000 27.7 10.0 500 500 707 4528 45 8.6 38.0

5000 27.7 10.0 275 476 550 4749 60 10.3 38.0

5000 27.7 15.0 2500 0 2500 2500 0.0 0.0 30.2

5000 27.7 15.0 1720 993 1986 3427 30 7.0 38.0

5000 27.7 15.0 970 970 1372 4145 45 8.6 38.0

5000 27.7 15.0 510 883 1020 4576 60 10.3 38.0

NOTES: 1: Discrimination angle of interfering signal with respect to wanted

signal 2: “Path difference” is the Main/Reflected path difference with

respect to the free space loss paths. 3: “With in excess of 7.2 dB path difference on a 5 km link, 38 dB

W/U cannot be achieved anywhere along the link without antenna discrimination.

(66) When a turbine is close to the scanner, the difference in additional loss compared to free space loss on the two paths is less (generally in the region of 0 to 10 dB). However this depends on the mast height, hub height and local topography, although generally offset by the omnidirectional antennas used at the scanner end, which have no attenuation of the unwanted signal.

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Figure 2.4: The same extreme example path profile of a non LOS link with a wind turbine close to the scanner. Where the path via the turbine is only

marginally better than the direct path.

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Table 2.2: – Simple example of reflection Clearance Zones for links, close to a

scanner:

against a turbine with an 90 m rotor (Peak RCS of +27.7 dBsm);

for 38 dB W/U ratio at the outstation end;

with paths of 20 km and 5 km where all the paths are free space;

with the main path having 3 and 6 dB more loss with respect to free space loss than the reflected path; and

using an Omnidirectional antenna. (all distances in metres)

Path A-B m

RCS dBsm

Path Difference

dB

Distance from

Outstation along link

m

Distance from link m

T-B m

A-T m

Discrim. angle OS degrees

Antenna discrim.

dB

W/U dB

20000 27.7 0.0 560 0 560 19440 0.0 0.0 38.0

20000 27.7 3.0 800 0 800 19200 0.0 0.0 38.0

20000 27.7 6.0 1150 0 1150 18850 0.0 0.0 38.0

5000 27.7 0.0 620 0 620 4380 0.0 0.0 38.0

5000 27.7 3.0 950 0 950 4050 0.0 0.0 38.0

5000 27.7 6.0 1580 0 1580 3420 0.0 0.0 38.0

NOTES 1: Discrimination angle of interfering signal with respect to wanted

signal 2: “Path Difference” is the Main/Reflected path difference with

respect to free space loss.

2.3 RCS Modifier with respect to the reflection angle.

(67) The RCS figures used so far in this report are peak monostatic figures, that is, the transmitter and receiver are co-located and the return comes directly back off of the turbine. When considering turbines we are more interested in the bistatic RCS where the transmitter and receiver are not co-located. This varies with reflection angle. There are three additional zones:

Forward scatter where the wanted and unwanted signal are from the same direction with the turbine directly between the A and B end, which enhances the monostatic RCS. For this, JRC use the RA figure as defined in ITU-R BT805. The peak enhancement being 10 dB at 0 degree reflection angle. The thicker the blade the narrower the enhancement sector. The higher the frequency the smaller the enhancement zone. As an example on a 90m turbine at 461 MHz some enhancement occurs over a +- 11 degree reflection angle.

Side Scatter this occurs between the forward scatter zone and a reflection angle of 180 degrees. This will reduce the monostatic figure depending on reflection angle. A turbine is a difficult structure to assess as it is a mixture of flat, round spherical and tapered surfaces that all have different responses with respect to reflection angles. JRC has produced a compromise for the reduction used.

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Angles close to the forward scatter zone there is an area where there is some forward scatter that is below the monostatic RCS value but above the reduction due to reflection angle.

Figure 2.5: An example; the polar response of a 90m rotor diameter turbine, used by JRC when analysing reflections. Peak RCS is 37.7 dBsm. Monostatic RCS is

+27.7 dBsm.

(68) JRC now also cross references ITU-R BT805 the recommendation used for impairment caused to analogue television reception by a wind turbine (That uses similar frequencies and antennas as scanning telemetry), but with the addition of JRC side scatter reduction using 22 dB as the Wanted/Unwanted ratio when the delay is not greater than 0.25 of symbol time. Reference is also made to the normal radar formulas as a cross check with the basic JRC method.

Table 2.3: Comparative figures used for a 90m turbine in analysis.

Frequency 461.00 MHz

Turbine Blade Diameter m 90.00

Turbine Blade width m 2.54

Turbine Blade Area m^2 85.56

RCS Monostatic dBsm JRC 27.67

RCS Factor Monostatic dB JRC 42.41

RF-BT 805 dB 17.62

Equivalent JRC RCS using BT805 43.38

Equivalent JRC RCS using BT805 Worst FS 53.38

Diff Mono RCS JRC - BT805 15.70

(69) The link maybe horizontally polarised. In these cases JRC are of the

opinion the forward scatter will be less as the maximum scatter will also occur when the turbine blade is horizontal.

2.4 The Effect of Multiple Turbines.

(70) Considering the unwanted signal as interference, then the RCS would be equal to the sum of the RCS values. Two turbines with the same calculated W/U will degrade the resultant W/U by 3 dB so long as the interfering signals arrive in phase. Four turbines will degrade the W/U by 6 dB and eight

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turbines by 9 dB (10 log no of turbines). There are however normally predominant turbines when analysing turbine interference. In the case of a development with two turbines where the second turbine’s predicted W/U was 10 dB better than the first, the resultant W/U degrade would be ~0.5 dB. In the case of a four turbine scheme where the other three turbine’s predicted W/U were 10 dB better than the first, the resultant W/U degrade would be ~1.2 dB. In an eight turbine scheme with the other seven turbines predicted W/U was 10 dB better than the first the resultant W/U degrade would be ~2.3 dB.

(71) In analysing scenarios, JRC now initially considers the addition of all turbines within 1000m of a link below 1000 MHz (500m above 1000MHz). The resultant W/U is calculated as if the interfering signals were in phase.

(72) With multiple turbines and rotating elements all with different path lengths to and from the link ends it is unlikely that all reflected signals will arrive in phase with each other and out of phase with the wanted signal, JRC also considers the addition of the worst case of 50 % of the turbines operating in forward scatter or the addition of the worst two if operating in side scatter.

2.5 Low Frequency Microwave Links

(73) This document is primarily intended for the evaluation of 460 MHz UHF links. The formulae used are also relevant for 1400 MHz links although the W/U required is 42 dB, for a class 2 link (4 state modulation) or 49 dB for a class 4 (16 state and above modulation).

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3 References

[1] Radiocommunications Agency - Title: "Fixed-link wind-turbine exclusion zone method" Author: D F Bacon Status: released 28 Oct 2002 Version: 1.1 (Current Version) URI: http://licensing.ofcom.org.uk/binaries/spectrum/fixed-terrestrial-links/wind-farms/windfarmdavidbacon.pdf [2] QinetiQ (for DTI)- Title: “Wind farms impact on radar aviation interests” Author: Gavin J Poupart (Prepared by) Status: Final report, September 2003 Version: DTI PUB URN 03/1294 Downloads: Part 1: http://webarchive.nationalarchives.gov.uk/20060216231311/http://www.dti.gov.uk/energy/renewables/publications/pdfs/w1400614part1.pdf Part 2 http://webarchive.nationalarchives.gov.uk/20060216231311/http://www.dti.gov.uk/energy/renewables/publications/pdfs/w1400614part2.pdf

[3] OFW49 Title: Frequency Assignment Criteria, Scanning telemetry radio services operating in the band 457.5 to 458.5 MHz and 463.5 to 464 MHz in which spectrum is managed by the Radiocommunications Agency. The previous RA version of the document is designated RA 375 and before that MPT 1411.

[4] VNS2111

Title: Performance Parameters for Scanning Telemetry and Telecontrol Systems Operating in the Frequency Band 457.5 MHz to 464.0 MHz

[5] ITU-r BT805

Title Assessment of impairment caused to television reception by a wind turbine

[6] Ofcom Independent Report on RF Measurement Assessment of Potential Wind Farm Interference to Fixed Links and Scanning Telemetry Devices, published in March 2009 http://licensing.ofcom.org.uk/radiocommunication-licences/fixed-terrestrial-links/guidance-for-licensees/wind-farms/rf_measurement/ (Checked: 12 December, 2014)

http://licensing.ofcom.org.uk/binaries/spectrum/fixed-terrestrial-links/wind-farms/windfarmdavidbacon.pdf

http://licensing.ofcom.org.uk/binaries/spectrum/fixed-terrestrial-links/wind-farms/windfarmdavidbacon.pdf

http://webarchive.nationalarchives.gov.uk/20060216231311/http:/www.dti.gov.uk/energy/renewables/publications/pdfs/w1400614part1.pdf






http://licensing.ofcom.org.uk/radiocommunication-licences/fixed-terrestrial-links/guidance-for-licensees/wind-farms/rf_measurement/



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4 Acronyms & Abbreviations

dBsm decibel square metres, an alternative way of expressing RCS, equivalent to 10LOG(RCS) in m2 .

Fresnel Zone (pronounced FRA-nel zone), named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture. More: http://en.wikipedia.org/wiki/Fresnel_zone

Harmful Interference Interference which endangers the functioning of a Radionavigation Service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with the ITU-R Radio Regulations. (International Telecommunication Union Radio Regulations)

Interference The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, or loss of information which could be extracted in the absence of such unwanted energy. (International Telecommunication Union Radio Regulations)

Microwaves Microwaves are electromagnetic waves with wavelengths longer than those of Terahertz (THz) wavelengths, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, Terahertz radiation, microwaves, and lower frequency radio waves are fairly arbitrary and are used variously between different fields of study. For instance, in the David Bacon Report the term effectively referred to frequencies above 3 GHz where radio links are increasingly line-of-sight with respect to increasing frequency. More: http://en.wikipedia.org/wiki/Microwave

RCS A term that represents the radar "size" of an object, the Radar Cross Section in square metres. Note that it gives an equivalent area that represents how much power is reflected and does not correspond to physical size. Sometimes expressed as dBsm, decibel square metres (10LOG RCS in square metres). More: http://en.wikipedia.org/wiki/Radar_cross_section

UHF Ultra High Frequency – officially defined as the range 300 MHz to 3000 MHz and includes part of the region often referred to as “Microwaves”. More: http://en.wikipedia.org/wiki/UHF

W/U Wanted to Unwanted (ratio). The ratio of the wanted signal with respect to the unwanted. Usually expressed in decibels.

http://en.wikipedia.org/wiki/Radar_cross_section

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