Review of Revised OET Bulletin No. 69, "Longley-Rice Methodology for Evaluating TV Coverage and Interference" - The Repacking Formula

There are eight issues targeted in the Commission’s review of the OET Bulletin No. 69 (Longley-Rice) software.  They are as follows:

1.      Population (census) data.  The DTV allotment channels assigned by the Commission (circa 1998) were based on 1990 Census data.  A number of years after that, they began to incorporate the 2000 Census data in coverage and interference studies.  They now propose to migrate to the 2010 Census data.  I think this is a good thing, since the 2010 Census represents reality more so than use of older census data.

2.       Terrain data.  Currently, most Longley-Rice software (including the Commission’s) uses 3-arcsecond terrain data.  The Commission now wants to use 1-arcsecond terrain data in order to provide more accurate results of coverage and interference.  This is also a good idea, in my opinion, because it will yield a result which closer resembles actual field strength in a given area.

3.       Correction of inaccurate engineering data in records of some stations.  This absolutely needs to be fixed.  I suggest that the Commission request a review by every station of its engineering record and submit any and all corrections.  I can’t tell you how many times I have seen obviously miss-rotated antenna data in CP records (and then carried through into licenses) and use of ERP in dBk instead of kW in license applications (flip-flopping the values in the 302 Form).

4.       Use of actual station elevation patterns in calculations.  Right now, the software assumes a standard elevation pattern for all stations’ antennas.  Since elevation patterns can be quite diverse, use of actual elevation patterns should yield a better depiction of actual signal level, at least on a theoretical basis.  I support the effort, but with reservation.  Based on results of field studies I have seen and participated in, calculated signal level bears little resemblance to actual signal level at particular locations.  This is because RF doesn’t always behave as predicted.  When an antenna is side-mounted on a tower, significant scalloping of the pattern can occur, resulting in peaks and valleys in the amount of radiated signal in a particular direction.  In addition, attenuation effects from manmade structures and vegetation is not factored into the equation.  There are dozens of reasons why we tend not to find a good correlation between predicted signal strength and actual signal strength, except in very general ways.  I also want to mention that nulls and maxima in elevation patterns affect signal levels relatively close to the transmitter site, where receivers tend to have line-of-sight and relatively high signal strengths (even in the elevation pattern nulls).  Finally, many stations use mechanical beam tilt in addition to electrical beam tilt, which significantly changes the power radiated in certain directions and out to significant distances.  I think that if the FCC truly wants a more accurate picture of what is going on in the real world, they need to develop a way to incorporate the use of mechanical beam tilt into the calculations.

5.       Depression angle calculation.  The Commission accidently accepted a mistake in the original software whereby calculation of signal at a given receive site used a depression angle to the transmitting antenna based on the latter’s height above ground rather than the difference between the transmitter site and receive site in terms of height above mean sea level.  This can cause a serious anomaly when the calculations are derived for mountainous areas.  This should be fixed, in my opinion.

6.       Precision of geographic coordinates.  The software presently rounds transmitter and receiver locations to the nearest second.  They now propose to round to the nearest 0.001 second.  Since most antenna sites have not been surveyed to that precision, I don’t really think this is going to make a difference in the real world.  But, hey, go for it if you think it will make things better.

7.       Establishment of a calculation grid.  I don’t really understand what they are talking about here and I am concerned that this might be the way they can get away with repacking stations into tighter spectrum confines.  So, unless somebody can explain to me why this is a good thing for broadcasters, then I am opposed to it.

8.       Treatment of error codes.  Right now, when the software goes bonk along a certain azimuth under calculation (usually along azimuths where the terrain is going through rapid fluctuations), it just goes ahead and assumes that there is coverage by the station in all of the cells where this error code comes up.  The FCC is asking if they should continue to allow the program to do this.  In a perfect world, I would ask the FCC to fix the problem with the software, but in the absence of that, I think it is a conservative (and potentially beneficial) approach to keep things the same (i.e. assume coverage in the error cells).  I say keep it the same.

The NAB came out with its initial comments on these potential changes to the software.  While they are not opposed to the concept of updating the software, they are unhappy that it is being done now, just before the auction.  Many stations have begun conducting coverage and interference analyses on their stations in order to determine whether or not to participate in the auction or to make sure they are not getting screwed by the repack (in terms of interference-free coverage they now have versus what they given on their new channel).  The results of all those studies would be useless if the changes to OET 69 was implemented now.  The NAB is lobbying for the Commission to make the changes after the dust settles with the auction and the spectrum repack.

The concern I have is that there will not be enough time between now and the auction to adequately evaluate the differences that the new and old software have on station coverage and incoming interference.  And, I would be afraid that the changes the Commission makes to the software would disadvantage the broadcaster in some way that would only be discovered after it is too late.  If I could cherry-pick the items that I could support now in the changed software, they would be: use of 2010 Census data, use of 1-second terrain data, correction of station data in the FCC engineering database, correction of the depression angle problem, and use of actual antenna elevation patterns in the calculations.  Numbers 6, 7, and 8 above are not worth consideration at this time.  And I am very leery of the calculation grid thing.

If you have any questions regarding my comments, please feel free to share.

FCC Sets Deadlines For LPTV Conversion To Digital and Operation in Core Spectrum

If you own an analog or digital LPTV station (or television translator or Class A station) that operates on an out-of-core television channel (Channels 52 – 69), you will no longer be able to stay on the air after December 31 of this year. In addition, if you want to move to an in-core channel (Channel 2 through Channel 51), you have to find an available channel and file your displacement application by September 1, 2011, which is just around the corner. After that date, the Commission will no longer accept displacement applications from out-of-core stations. All of these dates and edicts can be found in the FCC’s recently released Second Report and Order on the subject: http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0715/FCC-11-110A1.pdf

The September deadline is reason enough to get hopping on finding an in-core home for your station, if you have any interest in extending the operation of the facility beyond the end of the year. However, there is another reason to consider submitting a displacement application to the FCC as soon as possible. A large number of applications for new facilities have been filed in the past two years. These filings, once granted, will reduce the available spectrum for a number of potentially displaced stations such as yours. A displacement application trumps an application for a new facility, but only while if the pre-existing application remains pending until the Commission grants your application (in which case, the application for the new LPTV station is dismissed).

For these reasons, you should consider filing a displacement application with the FCC and seek operation on an in-core channel. The application must specify digital operation, and you will have at least 36 months to construct on the new channel. In addition, if your station meets certain rural coverage requirements, you may be eligible for reimbursement of a significant portion of your construction costs by the NTIA. More information on that program is located on the NTIA web site (http://www.ntia.doc.gov/lptv/index.html).

In the 2nd R&O, the Commission also indicated that all analog LPTV stations must cease operation and switch to digital operation by September 1, 2015. An analog station can either file an on-channel digital flashcut application or a digital companion channel application on a different channel. If a companion channel is desired, it should be filed as quickly as possible for the same reasons stated above. However, it is important to note that a companion-channel application does not take precedence over an application for a new LPTV facility.

Smith and Fisher can provide services such as channel searches and the preparation of the engineering portion of the FCC displacement or companion-channel application. We also can determine if your other analog in-core stations would be eligible for a digital on-channel flashcut facility and assist in the FCC filing for such a facility, if eligible.

If you have any questions regarding this issue, please don’t hesitate to call me at 703-494-2101, or send me an e-mail (Kevin@SmithandFisher.com). You can find out more information about our firm on our web site, Twitter and Facebook.

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RF Exposure Calculations For License Renewals

When I am asked to perform an RF calculation for an FM or television station, I gather some key pieces of information before I start. I need to know the effective radiated power of the station, the polarization of the signal (horizontal, elliptical or circular), the make and model of the transmitting antenna, its orientation (if directional) and its height above ground (or roof level, if mounted to a structure on a building). If I know the make and model of the antenna, I can usually find its elevation pattern either in my files or on the manufacturer’s web site. The elevation pattern allows me to determine the intensity of each side-lobe of the antenna pattern as well as its distance from the base of the tower.

The other very important fact that plays a part in the situation is whether or not the area surrounding the transmitter site can be considered a “controlled” environment. The Commission has separate RF exposure guidelines for controlled sites and uncontrolled (usually public) areas. The exposure guidelines are five times more strict for uncontrolled areas than they are for controlled environments.

The FCC considers a site to be controlled if two requirements can be met. First, the site must be secure from public access. This usually means a fence with a locked gate surrounding the transmitter building/tower compound or locked doors to all entrances to a building’s roof if your antenna is so mounted. As I mentioned in my previous blog, a locked gate across an access road to a site is not sufficient to consider the site as controlled, if hikers, bikers or ATV users can go around the gate or access the site from another path. The second requirement for a controlled site is that every person that enters the site must be trained in RF safety awareness. This means everyone, from station personnel to contractors to equipment/site maintenance people. If even one person accesses the site without such training, the site will be considered to be “uncontrolled” by the FCC and the stricter exposure standard will apply.

Once I am convinced as to which standard to apply in the case at hand, I conduct a study to see if there are other FM and television broadcast facilities operating at the same site as my client’s facility. Usually, stations within a few hundred feet of my client’s are considered. Although there can be RF contributions from other types of antennas (cell phone, two-way, microwave, etc.) at the site, their individual contributions are usually less than 1% of the FCC’s public exposure guidelines and can therefore be ignored.

I then perform my RF calculations and find the maximum power density value that would occur two meters (six feet) above ground from my client’s facility. I also compute the distance from the base of the tower where this maximum value is calculated to occur (and the direction, if a directional antenna is used). I divide the result by the applicable exposure standard for that frequency and type of environment in order to get a percentage of the maximum allowable value (MPE). There are three basic factors that result in high RF numbers. They are: high effective radiated power, low antenna height above ground (or roof), and frequency. With respect to frequency, FM and VHF television stations have the strictest exposure standard, because their wavelength is closest to the height of an average human being. Therefore, these frequencies are absorbed by the body much more efficiently than is the RF from a UHF station, which has a much smaller wavelength.

If there are no other co-located or nearby FM or television station antennas in operation, I am unconcerned as long as my client’s contribution to the RF environment is within 80 to 90 percent of the applicable maximum guideline value. If there other FM or television stations operate from the site, I am finished with my study if my client’s maximum calculated contribution to the RF environment is less than 5% of its guideline value. That’s because the FCC considers a station to be liable for fines and employment of RF mitigation techniques only if its RF contribution in an area of concern is greater than five percent. Since calculations are based on some very conservative assumptions and since I tend to find lower RF numbers in the field when I make measurements, I feel comfortable in claiming that my client’s RF contribution can be excluded from consideration if I calculate it to be less than five percent of the applicable standard.

If I calculate a number greater than 5 percent and there are other stations at my client’s site, I have to perform a similar calculation for each of the other stations. If I assume that each station’s RF maximum value occurs at the same point (which they don’t) and the total calculated RF is significantly less than 100 percent of the MPE, then everything is peachy.

If the number is close to or greater than the MPE in any of the above situations, I usually suggest that a power density survey of the site be conducted. In my next blog, I will talk about surveys and some things I have learned from doing more than 100 of them.

If you have any questions or would like us to help with the RF portion of your renewal, please call or send an e-mail to Kevin@SmithandFisher.com