Understanding the Requirements for
DTV Construction Permit Application
by Oded Bendov
The FCC’s 5th Report and Order (5th R&O), adopted April 3, 1997, states (section 72) that a construction permit will be granted within days of filing an application provided that "yes" is checked on items 1(a), (b) and (c) of Form 301 (Appendix E.)
Item 1(c) on Form 301 states that the proposed DTV facility "...operate with an effective radiated power (ERP) and antenna height above average terrain (HAAT) that do not exceed the DTV reference ERP (emphasis added) and HAAT for this station as established in 47 C.F.R. Section 73.622."
Appendix B of the FCC’s 6th Report and Order (6th R&O), adopted April 3, 1997, contains the table of DTV Allotments, Assignments, Assignment Pairings with Analog Stations and Service Replication and Interference Evaluation (DTV-TA). The DTV-TA also includes the maximum ERP. The DTV-TA does not include reference ERP which, in general, varies on each of the 360 radials used to define the replicating contour.
Broadcasters may assume that a DTV facility can operate at the ERP specified in the DTV-TA provided that the DTV antenna’s pattern is essentially the same as that of their NTSC antenna’s. For example, the common assumption is that a station now operating with an omnidirectional NTSC antenna could operate with an omnidirectional DTV antenna with the ERP shown in the DTV-TA. However, every licensee has been assigned a new, directional pattern for its DTV antenna and the ERP in the DTV-TA refers to the maximum ERP of that directional pattern. The variation between the present NTSC pattern and the newly assigned DTV pattern depends on the local terrain and is a product of the Commission’s replication process using F50,90 to match the F50,50 Grade B contour.
The Commission used this replication DA pattern to compute interference and service and the values are noted in the allotment table. Thus, if one specifies the ERP on each radial (360) to be identical (omnidirectional), the interference fields to other stations will be increased. One interpretation of this is that Question 1(c) cannot be answered affirmatively unless an appropriate directional antenna is employed. Alternatively, an interference study must be included with the application showing no new interference to other stations (since OET-69 is not yet available, this may be difficult to accomplish).
What’s more, the radiation center of the DTV antenna cannot, in general, be located at the same point as that of the NTSC antenna. The 5th and 6th R&O do not allow for any increase in height (stacked antennas, for example) without filing interference analysis. Nor is there any ERP compensation (power increase) permitted when lowering the height of the radiation center (side-mounted antennas, for example) without interference analysis.
Following is a description of the FCC method of deriving the new, directional, patterns.
A. Redefinition of the NTSC Grade B Contour
The FCC has replaced the original Grade B contour of each station, based on 8 radials spaced at 45-degree intervals, with a new Grade B contour, based on 360 radials spaced at 1-degree intervals. This new contour was further expanded or shrunk (only at UHF) by application of the so-called "dipole factor".
Except in areas where the terrain is flat, the new Grade B contour (the contour subject to "replication") will be different from the original Grade B contour. Additionally, due to the dipole factor adjustment, NTSC channels above 38 have their existing contours shrunk (by up to several kilometers at channel 69) and channels below 38 have their existing contours expanded (by up to several kilometers at channel 14).1
B. Assignment of a New Directional Azimuthal Pattern
By redefining a new Grade B for the purpose of "replication" and by using F50,90 rather than F50,50 curves to calculate the reference ERP, the ERP required to replicate the new Grade B in 360 directions, the FCC assigned every channel a new, directional antenna pattern for DTV. This is true even for stations now operating with a omnidirectional antenna pattern. The FCC’s assigned directional patterns cannot, in general, be matched by practical antennas, resulting in further loss of DTV service.
Only the maximum ERPs of the directional patterns are given in the DTV-TA. One interpretation is that an application for a DTV license based on a station’s present antenna pattern, directional or omnidirectional, based on the published maximum ERP, could be rejected because, in some directions, the ERP may exceed the reference ERP derived from the new Grade B contour.
Under such
assumptions, the available filing choices may be:
- Submit a comprehensive analysis to show that no interference is caused by exceeding the reference ERP on any radial.
- Submit an application based on the directional antenna specified by the FCC.
- Submit an application based on the minimum ERP derived by the FCC in one or more of the 360 radial directions.
C. Example
Consider KFCC2, a VHF channel is Los Angeles. The new NTSC Grade B contour is shown as a dashed line in Figure 1. Now using the maximum DTV ERP as published in the DTV-TA, an omnidirectional pattern and the F50,90 curves, the contour of the UHF-DTV channel was plotted. The new contour either matches or exceeds the new Grade B contour. The match point is near the 60-degree azimuth. In order not to exceed the new reference ERP and to operate with an omnidirectional antenna, the DTV ERP has to be reduced by almost 3.5 dB. The resulting DTV contour is now inside the new Grade B contour with a match point near the coast line at the 150-degree azimuth.
Assuming KFCC would not wish to reduce the maximum allowable ERP, the directional antenna pattern required for filing without being subjected to an interference analysis is shown in Figure 2. It cannot be matched with practical antenna patterns.
If this interpretation is correct, every station filing on the maximum DTV ERP will have to accompany the filing with an exhaustive interference analysis. Unfortunately, neither the FCC’s software nor its terrain and demographic databases are publicly available in ready-to-use formats. Once access to the directional patterns and the software becomes readily available, a processing bottleneck could result.
D. Power vs. Height
Most stations will have to install their DTV antennas below their present NTSC antennas.
Appendix E of the 6th R&O specifies a decrease of power with increased antenna height for a station that operates on a channel allotted subsequent to the initial DTV-TA. What has not been made clear is whether the formulae of Appendix E apply to the stations listed in the DTV-TA.
It would seem equitable to allow those stations who must, of necessity, operate at lower heights an increase in power based on similar formulae that will insure that same level of interference.
Similarly, stations that wish to provide maximum DTV service by installing a DTV antenna on top of a present NTSC antenna are required to submit interference analysis simply because the radiation center of the DTV antenna may be higher than that of the NTSC antenna (the height specified in the table).
E. Interference
Appendix E of the 6th R&O specified that service and interference analyses be based on OET Bulletin 69. As noted above, that publication is not presently available.
Therefore, neither the instructions nor the software is available for those who would like to proceed with their system design in order to meet the FCC’s station construction timeline.
F. Resolution
The problems described in this paper were presented to the staffs of the Mass Media Bureau and the Office of Engineering and Technology at a meeting held on May 27, 1997. The presentation was made by members of the DTV Committee of the Association of the Federal Communications Consulting Engineers (AFCCE). The resolution of the problem is not immediately clear. The Commission staff has taken these comments under advisement and may issue a clarification very soon. However, it may be necessary to file a Petition-for-Reconsideration to suggest appropriate modifications to the rules; this, of course, will not be accomplished before the now specified effective date of June 13 for the new rules. AFCCE is planning to file such a petition.
G. Unresolved Critical Issues
In its reply comments to the Sixth Further Notice of Proposed Rule Making, AFCCE raised a number of issues that, in AFCCE’s opinion, needed review. These issues were not addressed or commented on in the 6th R&O. To reiterate:
The field strength of DTV cannot be directly measured. Nor can it be used as a predictor of coverage. Underlying the FCC’s derivation of required field strength contours is the wrong assumption that the DTV power is not spread over the 6 MHz bandwidth.
Ideally, what the broadcast industry would prefer is a DTV contour definition that is based on a parameter that is measurable and can serve as service predictor.
Selective use of two different propagation models, which do not provide similar answers, may be at the root of the problems now being encountered.
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1 The FCC has, for reasons unexplained, ignored the loss variation with frequency of the 50’ downlead in Appendix A. the correct "dipole factor" effect, downlead cable loss included, is ±3.3 dB instead of ±2.3 dB.
2 A real station, call letters changed.
Comments may be transmitted to the AFCCE DTV Committee by E-mail at afcce@cuenet.com (automatic distribution to all committee members).
June 2,
1997