Tuesday, January 20, 2015

Broadcast Wind presents the Gold Standard for Electromagnetic Interference Studies

Broadcast Wind is an engineering consulting company founded in 2010 by broadcast industry veterans to provide RF interference analysis for the wind and broadcasting industries and to help establish the standards needed to ensure non-interference of turbine blades with RF signals.  Clients include US broadcast station groups, the United States Department of Agriculture, Exelon, NextEra Energy Resources LLC, and Invenergy LLC.  Projects included AM, FM and television interference studies, and baseline RF field studies (taken prior to wind farm build-out).  Broadcast Wind personnel have presented to wind energy and broadcast industry trade groups, teaching the importance of identifying radio and television interference issues prior to wind farm construction.  Broadcast Wind specializes in Electromagnetic Interference Analysis (EIA) required by governing bodies from wind farm developers during the wind farm permitting process with special experience in interfacing with broadcasters to provide accurate information about a wind farm’s effects on their signals and to relieve concerns about signal loss.


TV Station Signals

Broadcast Wind provides wind farm developers and other interested stakeholders (permitting agencies, investors, etc.) with predictions of the effects of proposed wind farms on television signals.  Included in these predictions are estimates of number of viewers that would be affected – both directly off-air and via cable from providers that pick up local programming off air.  Estimates of impairment to cable off-air pick-up is an important, although frequently forgotten, component of an EIA because of the large number of viewers that may be affected.  There are no national databases of cable companies’ off-air receive sites.  Local research must be conducted to identify cable off-air signal pick-up points that may be affected by construction of a wind farm..
A computer simulation desktop study is used to assess potential for signal impairment due to a proposed wind farm.  The strength of signals from television stations whose coverage areas include the proposed wind farm site are simulated in the regions surrounding the proposed site for with and without the wind farm.  The software models electromagnetic wave propagation taking into account effects of terrain variation on the signal.  The individual wind turbines are modeled as additional path obstructions in the terrain.

 Our OET-69[1] analysis software divides the area within a station’s protected contour into a matrix of square grid cells (0.5 km on a side) and then uses the Longley-Rice propagation model to predict the field strength at each individual cell.  The output of the studies are analyzed to determine which cells (if any) show a critical reduction in the predicted field strength.

The cells in the Area of Potential Interference are where the simulated signal strength dropped to an unacceptable level, post construction. 

Probe Data – TV Dashboard

Broadcast Wind uses proprietary stationary remote RF probes to provide our wind farm developers and stakeholders with long term TV signal strength and signal quality data  critical locations prior to and following the construction of wind farms.  Similarly to the way meteorological gear is used by the wind industry, prior to and following wind farm construction to gather wind data over a period of time, our RF probes capture TV signal metrics through changes in seasons, and atmospheric conditions before, during, and after wind farm construction.  The probe data provides Broadcast Wind and the wind farm developers with reliable documentation of changes (if any) to signal quality consequential to construction. 

The probe data is forwarded to Broadcast Wind’s offices and optionally to a client’s office where it can be displayed on a concise graphical data dashboard and can be programmed to provide alerts if parameters go out of defined limits.  Dashboard data can be viewed remotely by hour, day, month and year.  A roll of the mouse allows the user to see what was going on with atmospheric conditions at the time of the probe reading.

In the following dashboard example we see correlation between UHF signal variation and precipitation.  Precipitation, high wind, time of day, and seasonal changes can all play a role in UHF DTV signal propagation. 

TV Probe Metrics:
The top three graphical metric lines are displayed on a common percent scale for convenience of visualization. The fourth, margin, is displayed in dBmv. The inset box provides the measurement data.
  • The TV signal “SEQ” tells us whether the video signal is watchable. An SEQ of 99% would mean that the viewer sees a ”hit” (I.e. pixilation) to the video 1% of the time that they are watching.   
  • Signal strength in dBmv and signal quality (MER) in dB are also captured. 
  • “Margin” describes how much signal strength buffer there is within the channel before the signal would start to fall apart.  More margin means a stronger, more stable video signal.

No single metric completely describes the quality of the signal reception.  The value of the dashboard is that it presents multiple metrics that can be correlated with each other and with external factors.


AM and FM Broadcast Radio Stations

AM broadcast stations’ exclusion distance varies depending upon antenna type and broadcast frequency. Omni directional (non-directional) antennas, have an exclusion distance equal to 1 wavelength.  For AM antenna arrays (directional antennas), the exclusion distance is the lesser of 10 wavelengths or 3 kilometers.  AM broadcast coverage problems are only anticipated when AM broadcast antennas are located within the exclusion distance limit of wind turbine towers. 

The coverage of FM stations at distances greater than 4.0 km from wind turbines, is not subject to degradation.  FM transmitters with antennas closer than 4.0 km from proposed wind turbines can, under some conditions, experience a compromised signal. 

Similar to the TV signal analysis above, Broadcast Wind conducts a desktop study to determine potential areas of interference for FM radio stations.  Remote RF signal probes accessorized with long term FM signal quality logging capabilities are placed within areas with the highest risk for signal compromise prior to, during and following the proposed wind farm construction.

Probe Data – FM Dashboard
Broadcast Wind’s remote stationary probes can also monitor FM radio signals and forward them to our offices and to a client’s offices for display on a dashboard.  FM signal metrics are stored on the cloud where they are available for review by Broadcast Wind’s clients.

Point-to-Point Microwave Systems

Point-to-point microwaves that may be affected by the installation of wind farms operate over a wide range of frequencies (900 MHz – 23 GHz).  These microwave systems provide a wide range of telecommunication backhaul throughout the country supporting essential services such as land based telephone services, cellular networks, personal communication services; data and internet interconnects, network controls for utilities and railroads, and various video services.

Every point-to-point interconnect analysis must begin with solid geolocation information.  In some cases, a field survey may be recommended to validate tower coordinates found on governmental web sites to assure predictive model accuracy and to avoid issues of interference following construction. 

Once the accuracy of GPS data is assured, a two dimensional (2D) analysis is performed to determine whether any microwave signals intersect a proposed wind turbine’s footprint.

 An example of a 2D analysis showing the microwave beam apparently intersecting a turbine blade.

Once a 2D analysis is complete, a 3D analysis (pictured below) allows us to determine whether the signal is able to safely pass under or above the turbine blades without interference from the blades.



Point-to-Multipoint Microwave Systems

Wireless Internet Service Providers (WISPs) deliver Internet and other data services via radio transmission to business and/or residential subscribers.  WISPs  can use  frequency bands in both licensed and unlicensed spectrums.  Many rural community WISPs operate in the unlicensed spectrum since the initial capital outlay and ongoing operating costs are low.  The most common unlicensed bands used for this purpose are the 900 MHz, 2.4 GHz, and 5.8 GHz bands.  Since there aren’t any governmental databases containing local WISP information, site surveys and local town business research is needed to identify, and work with major WISP operators prior to construction and to help plan for and mitigate WISP user interference issues following construction.  Broadcast wind can identify the WISP service providers and receivers in the vicinity of a proposed wind farm for clients and determine by 2D and 3D analysis if the blades of any of the proposed turbines will interfere with the WISP subscribers’ signals.


Land Mobile and Emergency Services

Evaluation is needed for first responder entities: police, fire, emergency medical services, emergency management, hospitals, public works, transportation and other state, county, and municipal agencies. Generally land mobile and emergency radio systems are designed with multiple transmitters to provide redundancy so the service will not be interrupted as the receiving radio moves into and out of areas of signal blockage.  Never-the-less a thorough assessment of the effect of a proposed wind farm on emergency services radio signals is an important, yet frequently overlooked aspect of an electromagnetic impact analysis.  The building of a densely populated wind farm can be analogous to the placement of a city with hundreds of tall structures between an emergency transmitter and the emergency responder.  If signal levels are attenuated following construction, higher powered transmitters, repeaters or signal boosters may be employed to fill the compromised area.  The key to success in this area is the performance of a thorough RF field analysis prior to construction.  An analysis of this type will identify the industrial and business land mobile radio systems and commercial E911 operators near the proposed wind energy facility.

 Mobile Phone Systems

Modern mobile phones support a wide variety of personal communication services including telephony, text messaging, email, and internet access.  The major US mobile phone service providers currently support three digital technologies:  legacy 2G (voice and limited data), main stream 3G and newer, faster 4G.  Mobile phone services are divided into three categories, each operating in its own frequency bands. Advanced Wireless Service (AWS), Personal Communication Service (PCS), and Cellular (CLR).  They hold licenses on an area-wide basis which are typically comprised of several counties.

Wind turbines present no significant threat to mobile phone services.  The mobile phone system architecture is based on low-latency packet switching and redundant cellular geographic coverage.  Packets are dynamically routed among cells as mobile phones change location and as network traffic changes.  A given mobile phone conversation is typically made up of packets that travel different routes and are assembled seamlessly at their destination.  If a given cellular link is unavailable for any reason – interference from a wind turbine or other - the packet is automatically switched to another cell with no interruption of service.  The user of the phone is unaware of cellular transitions, so will not be affected by any that may be triggered by a wind turbine.

Government Radar Systems

Wind Farm siting can potentially affect government radar systems.  The Department of defense offers a Preliminary Screening Tool[2] that can be used by wind farm developers to determine if there will be interference to air defense and homeland security radars (long range radars), to weather surveillance radar – Doppler radar (NEXRAD), or to military operations radars.  Broadcast Wind can assist clients with preparation and use of this preliminary screening tool. 
The FAA requires that all developers proposing a wind farm with turbines that exceed 200 feet above ground level file a Notice of Proposed Construction or Alteration form[3].  For each turbine in the farm the form must specify the turbine ID number, latitude and longitude in degrees, minutes and seconds (NAD 83), site elevation, height above ground level (AGL), overall height above mean sea level (AMSL), and preferred marking and lighting.  Upon approval of the proposal, the FAA will issue a Determination of No Hazard.  Broadcast Wind can assist clients with preparation and submission of the Notice of Proposed Construction or Alteration form to the FAA.

Commercial Doppler Radar

Commercial Doppler radars are located on television towers to take advantage of their height.  They are either operated by the broadcaster to support the station’s local weather forecasting service or leased to other commercial parties for private weather monitoring or forecasting.  A wind developer needs to know if his turbines will be in the line of sight of any Doppler radars.  The curvature of the earth determines the minimum separation distance below which a wind turbine is in the line of sight of a distant radar.  If the separation distance is less than the sum of the distances to the horizon of each object the turbine will be in the radar’s line of sight.
The separation distance below which a turbine is in the line of sight is given by the following equation.
Dseparation = 3.57 x ((Hturbine)1/2 + (Hradar)1/2) x 1000
Where all dimensions are in the same units.
For example, a turbine with blade tip height 150 meters above ground will be within the line of sight of a radar on a tower 300 meters above ground if the distance between them is less than 106 kilometers. 
Broadcast Wind can locate and identify the commercial Doppler radars within line of sight of the turbines in a proposed wind farm for clients. 

Telecommunication Towers

 A comprehensive survey of all communication
towers within range of the wind farm can provide a valuable check for completeness of the possible impact of a proposed wind farm on the individual telecommunication services discussed in the earlier sections of this paper.  Data obtained from FAA and FCC data bases, from county and township planning and zoning boards, and from other sources is compiled and analyzed to ensure that all RF signals that may be affected are accounted for and identified. 

The survey can be extended to on-site verification of tower locations in case there are errors in the location information on file in registered data bases.  This verification will ensure the accuracy of the predicted impact of the wind turbines on microwave, television, and radio signals.  Broadcast Wind can identify and confirm the precise location of all communications towers in the vicinity of a proposed wind farm for clients.


Accurate pre-construction identification and characterization of potential interference to electromagnetic transmissions is vital to the success of a wind energy project, both for permitting and for avoidance of post-construction problems.  Television and radio broadcasting signals are especially critical because they directly impact the public.  At the permitting stage an energy developer can encounter local resistance based on fear of loss of television or radio reception and after construction it can be confronted with costly claims for remediation of (real or imagined) service loss.
Broadcast Wind can provide the full spectrum of electromagnetic interference assessment.  In addition we bring special expertise and experience in areas of television and radio broadcasting.  This experience includes detailed characterization of the predicted effect of proposed wind farms on broadcasting signals with the added benefit of remote probes for long term in-the-field monitoring of signal strength prior to and after construction.  Long term monitoring provides the developer with forensic evidence that can protect him from invalid claims for restoration of service.   Our experience also includes interfacing with broadcasters at the beginning of a project to relieve possible exaggerated or misplaced fears that could lead to avoidable obstacles to successful permitting.

[1] FCC OET BULLETIN No. 69, Longley-Rice Methodology for Evaluating TV Coverage and Interference, February 6, 2004
[3] FAA Form 7460-1

Friday, May 17, 2013

Broadcast Wind DTV RF Probe

Broadcast Wind DTV RF Probe

One bit of technology coming out of our current USDA Phase II project was the development of automated probes to be used to capture DTV RF signal parameters over a long period of time in diverse locations, distances, weather conditions, in the vicinity of wind farms.
We developed an inexpensive, easily deployed field monitoring probe to record key television signal metrics, over a one year period, at remote, unattended measurement sites and transmit the data back to our central location for logging, processing and analysis. The metrics captured include date, time, 8VSB lock, RF frequency, signal strength, signal modulation error ratio (MER), receiver AGC gain, frequency offset, and transport stream (TS) data,  packet error rate (PER), reception margin over threshold of visibility (TOV) and detailed multipath (equalizer tap) data showing signal reflections (or echoes) from the wind turbines and other objects in the field.  The probe is able to capture all of these metrics at a rate of 2 times per second.  It is remotely programmable via broadband wireless modem and a Windows GUI. 

Probe Components

Programmable Attenuator

The electronically-controlled programmable attenuator enables determination of the signal margin at the probe’s receive site.  Determination of margin is accomplished by increasing the attenuation to lower the signal to a level just above the threshold of visible errors (TOV) and audible errors (TOA).

We chose the Vaunix model LDA-102  (3.86” x 2.52” x 1.35”) 50-Ohm relay-switched step attenuator that covers a frequency band from DC to 1 GHz (i.e., the entire broadcast television band).  It can be easily varied in 1-dB steps over a 63 dB range via a USB interface to the probe computer (it actually has 0.5 dB steps, but only 1-dB attenuation steps will be used for this field test). Attenuation accuracy is 0.3 dB or 5% of the selected attenuator value (whichever is greater), which is more than adequate for determining margin in the field. The switching speed of this relay step attenuator is about 70 nsec, which is significantly more than adequate in speed for this particular project.

The attenuator is powered (5 Volt) by on-board computer USB port so no separate, external power supply is needed.

Low Noise Amplifier

The low noise RF amplifier provides signal amplification within RF probe not only to provide enough level to the reference DTV receiver (after experiencing insertion loss from the bandpass filter and programmable attenuator) but it also determines the sensitivity of the entire probe. The primary concern with using any active device before the DTV receiver is overload from interfering signals such as other DTV signals, particularly from nearby full-power stations. The single RF channel bandpass filter in front of the pre- amplifier, and selection of test frequency without a high power station transmitting on first or second adjacent channels help avoid overload interference, but it is good practice to also use of a robust) amplifier with good overload performance; i.e., large third order intercept points, (IP3).

DTV Receiver

A good performance commercial DTV receiver that simulates at least fifth generation (5G) consumer receivers sold since 2006 was desired. The Silicon Dust model HD HomeRun Tech3 DTV receiver provides a set of diagnostic parameters that are retrievable from the 100Base-T Ethernet connection (RJ-45) on the back of the receiver under computer control. While not needed for this project, transport stream analysis is possible as well.

Computer Controller

The computer controller, while not directly handling the RF signal, has control over the programmable attenuator (via a USB port) and the DTV receiver (via an Ethernet port), and gathers all the required field test data from the receiver for subsequent transmission over the Internet (via cellular modem) for final data processing at our central location. It needed to be rugged and robust for various field environments and easily restart and pick up where it left off during power surges or failures.

A small form factor computer was found to meet all of these requirements.  The system features an Intel Atom D2500 processor with 4 GB DDR3 RAM, 120 GB solid state drive (SDD), dual Gigabit LAN ports with Ethernet controller (to communicate with the DTV receiver and an Internet modem), 6 USB ports, dual external serial COM ports, and a single internal parallel port. While operating autonomously in the field, the computer needs no mouse, keyboard, or monitor. However, upon setup, there are ports for all three of these devices (mouse and keyboard via USB ports and monitor through either VGA or DVI-I ports).

The computer resides in a small well-shielded, fan-less enclosure that operates silently, and includes both an external 60 Watt 12V AC-DC adapter and an 80W internal DC-DC power board that provide more than enough power for the computer.

The computer uses Windows 7 in 32-bit mode.  The probe’s control software was written in C.


The RF probe is contained in a PVC enclosure that easily allows all of the components to fit cleanly inside. There is easy access to the components for trouble-shooting and calibration as needed.  It is weather-proof so it can be placed in inhospitable settings.

Probe enclosure with internal components

Probe System Output
The RF probe design contains several measurement features utilized in the DTV reception analysis and characterization of a receiver due to wind turbines located near DTV transmitters. Each of these measurement features is described below.

Signal Level Measurement:  (dBm)

The probe design enables indirectly measuring signal field strength at its antenna input. The receiver captures signal power level at its RF input, and calculates the field strength at the antenna input knowing pre-measured gains and losses as well as using the well-known dipole factor for antenna conversion between field strength and power.

The overall system gain, GS, is defined as the signal level increase between the antenna output (same as coaxial downlead input) and the DTV receiver input (i.e., accounting for the coaxial cable loss, the filter insertion loss, the attenuator insertion loss plus extra programmed loss, the splitter loss, and the pre-amplifier gain). Once this system gain is known for a given RF channel, the field strength can be calculated with the following formula:

F.S. (dBµV/m)  =  S GS + A + K G )

where              F.S. is the DTV field strength (in dBµV/m) at the antenna input

S is the DTV signal power level at the DTV receiver input (in dBm)

GS is the overall system gain (in dB) at the RF test channel center frequency

A is the selected attenuation level (in dB)

GA is the forward antenna gain (in dBd) at the RF test channel center frequency

K is the dipole factor (in dBµV/m-dBm) at the RF test channel center frequency

The speed of the receiver’s signal level measurement is important (e.g., AGC speed) since dynamic signal levels are measured from the wind turbine’s rotating blades.

Signal Quality Measurement:  MER (dB)

The RF probe provides a signal quality measurement at the output of the DTV receiver’s equalizer referred to as Modulation Error Ratio, MER (in dB). This measurement is similar to the conventional signal-to-noise ratio (SNR), except that it uses both the demodulated in-phase (I) and quadrature (Q) baseband channel signals for the calculation. This parameter, when accurately determined in the DTV receiver’s 8-VSB demodulator chip, provides an indication of the signal quality at the equalizer’s output (i.e., the error correction circuitrys input). Typically, good modern-day receivers have a 15 dB value for MER at TOV.

It is important to note that the “noise referred to in the signal quality metric is not only the traditional additive white Gaussian noise (AWGN) found at the input to the receiver’s analog tuner component, but also any other undesired signals that might be present,  such as co-channel DTV interference signals, adjacent channel cross-modulation and/or inter-modulation signals due to circuit non-linearities, and multipath signals (delayed replicas of the noise-like desired DTV signal) that are not completely canceled. All of these “noise” components are represented in the MER value provided by the RF probe.

Multipath is considered a linear distortion, and can therefore be removed by a linear equalizer in a DTV receiver (assuming that the equalizer is robust enough and the receive loops are locked). However, if the echo delay is longer than the correction range of the internal equalizer or the speed of the equalizer is not fast enough to follow dynamic multipath (e.g., from a moving airplane or from moving turbine blades), then the MER value will reflect this lack of perfect correction in the form of a higher MER value. If the receiver becomes unlocked (e.g., carrier recovery or symbol clock recovery or data frame recovery fails), then no valid MER value is available from the receiver. It should be noted that if the various recovery loops remained locked, even though the unit may be below the error threshold (even down to 4 or 5 dB SNR in some ATSC receivers), it is still possible for many 8-VSB demodulators to provide a reasonably accurate value of MER since they often use the periodically-transmitted binary data frame sync as a known reference signal to calculate MER.

Data Quality Measurement:  Accumulated TS Packet Errors

The RF probe provides a data quality measurement in the form of transport stream (TS) packet errors. While this particular ATSC receiver used in the RF probe produces a signal quality number (value between 0 and 100 that varies logarithmically with the number of packet errors), the actual numeric value of packet errors is a better parameter to use for this application. The actual implementation of the error counter in the DTV receiver has been determined to be just a raw counter of packet errors that is never reset at a particular time interval, but only during a channel change. To determine if new packet errors have occurred since the last polling of this counter, subtraction of the two error counter numbers is required.

This error counter value indicates the number of uncorrected transport stream MPEG data packets (as determined by the Reed-Solomon decoder) at the output of the 8-VSB demodulator, regardless of whether the cause of the errors is low signal level, multipath, interference, or overload. This error value can be used to determine reception capability at a given site for various propagation conditions at the time of testing.
Reception Margin:  M (dB)

The RF probe provides a means to measure the actual reception margin (in dB) at a given location at a particular time. Margin, in this context, refers to the amount of signal level reduction under actual site propagation conditions that can be tolerated before TOV is reached. It represents an indication (or amount) of safety” in terms of signal level. For a situation where there is essentially no multipath or interference, the measured signal level can be used to directly compute the SNR value at the input to the receiver’s tuner and compare it to the known SNR value at TOV (15 dB for a good receiver) to determine margin. In our field tests multipath is present from the wind turbines so the margin is measured utilizing the programmable RF attenuator for signal reduction and the ATSC receiver for packet error detection, under the control of the local probe computer which implements an iterative algorithm that methodically finds TOV and therefore the margin value.

Sample Probe TOV Data Output:

Sample TOV data from the probe. Four measurements separated by 13 seconds each indicate TOV between 19 and 20 dB.

The presence of multipath or interference can cause the SNR value at TOV to be something greater than 15 dB (i.e., TOV degradation), and therefore this measurement can indicate a quantitative degradation amount that propagation effects have on the signal threshold at a test site. From this quantitative margin value, statistical analysis can be performed on the data obtained from various test sites in the region with multiple transmit antenna heights above ground level compared to the turbine height above ground level.

Multipath Measurement
Absent from the vanilla DTV receiver’s capabilities was the ability to measure and log signal multipath information.  The Broadcast Wind team worked together with the consumer receiver’s manufacturer and the holders of the 8VSB chip’s IP to develop a program to query the receiver’s equalizer and draw the multipath information out of its hardware’s registers.  We then wrote software giving the probe the capacity to display and to log equalizer multipath data, thus rounding out the probe’s data reporting and collection capabilities to a level exceeding most of what’s available on the market today.

A screen shot from a remote probe’s GUI showing on-site multipath measurements.  The probe displays the full measure of pre and post echoes being received at the location in microseconds. The direct signal from the transmitter is in the center at 0 µs.  Small reflected pre and post multipath echoes can be seen to the left (pre) and right (post) of the main signal.

The multipath data is also stored in its numeric form on the probe and uploaded to the Broadcast Wind database for evaluation.  A slice of the data collected is shown below:

A screen shot of the probe’s raw tap weight data stored on the cloud by the probe.  The tap weight in hardware registers nos. 500 – 509 are shown for four consecutive measurements separated by 5 seconds each. The direct signal from the transmitter is at tap no. 505 with pre and post multipath echoes represented by their relative weights in the registers to the left and right.

Self Check / Self Heal Capabilities
Since the probes are operating in remote, unmanned environments, the operating software has been designed to monitor operational health and to take corrective action in the event of failure.  In the event of communication loss, the probe will log the event, and continue to record all of the RF data locally until the system automatically resets.  The probe emails health status and diagnostic metrics to the Broadcast Wind offices twice daily.

Sample of diagnostic metrics emailed from the probe twice daily

When a communication outage occurs, the system self heals, and notifies Broadcast Wind of the exception via email.  No data is lost during communication outages since all data is stored locally, at the probe, and copied to the cloud once communication is restored.

Screen shot of notification of communication outage

Solar Capability
When grid power is available, the probe is powered by an external 60 Watt 12V AC-DC adapter.  For those locations where grid power is not available, we have developed a solar / battery solution using the lowest monthly average sun hours (December) in sizing the system so that it will continue to run 24/7/365.  The design calls for 5 days of backup power (for cloudy days) and a conservative 70% depth of discharge.   

Field data from the probes will continue to be collected and analyzed over the course of our project and will be used to gain a much better understanding of the dynamics between wind turbines and digital television signals.  These analyses are also being used to enhance and refine the predictive capabilities of our company’s RF interference modeling software: “WINTIP”.