Television digest with AM-FM reports (Jan-Dec 1951)

fications. In reviewing this proposal, the Commission has concluded that it is desirable to reduce station classifications to a minimum and that more than one class of station is unnecessary if provision is made for appropriate power ranges for the various sizes of cities and rural areas. Accordingly, only one class of television broadcast station is proposed, with provision for minimum and maximum effective radiated powers in accordance with the respective tables set forth below: 1. Minimum power Population of City ( excludes adjacent metropolitan areas) 1,000,000 & above 260,000 1,000,000 60,000 260,000 Under 60,000 2. Maximum power The maximum effective radiated power to be authorized on the respective channels is set forth in the following table: Maximum Effective Radiated Power Channels (in db above 1 kw) 2-6 20 db[100 kw]/600 ft. ant. 7-13 23 db[200 kw]/600 ft. ant. 14-83 23 db[200 kw]/600 ft. ant. 3. Any station may be authorized on appropriate application to increase its power to the maximum set forth above without the necessity of a hearing so far as interference to other stations is concerned. The use of antenna heights greater than 600 feet above average terrain is encouraged as a means for improving the quality of service. If an antenna height greater than 600 feet is used, the effective radiated power shall be limited to that value which will avoid interference within the Grade A service radius of any other station, either existing or provided for in the Table of Assignments, on the basis of operation of such station with the maximum power and antenna height of 600 feet as set forth above. Where antenna heights of less than 600 feet are utilized, the effective radiated power shall not exceed that listed above. E. Station Separations 1. Co-channel Separation The Table of Assignments contained in the Commission’s Notice of Further Proposed Rule Making, issued July 11, 1949, had as its objective co-channel separation of 220 miles in the VHF band and 200 miles in the UHF band. At the hearing on the general issues, testimony was offered that these separations could be reduced considerably by utilizing offset carrier operation. Evidence was also offered that more television service could be made available to the country if the separation objective were reduced to 160 miles for VHF channels. The Commission has carefully considered the above evidence and has concluded that some reduction in co-channel separation is possible because of the improvements which result from offset carrier operation. It is not deemed advisable to effectuate a reduction to 160 mile VHF separation as suggested at the hearing. In the first place, the evidence upon which the 150 mile separation is based is the theoretical computations of what coverage can be achieved. On the basis of the evidence in the record, it is clear that considerations of terrain and other propagation factors will materially affect many of the theoretical computations. In the second place, much of the propagation data — although the best available — upon which the Commission relies is necessarily quite meager. Postponing a decision in these proceedings would not materially aid this problem since it has been the Commission’s exper ® Or equivalent, based on the same Grade A service radius as with these values of effective radiated power and antenna height above average terrain. A chart showing this relationship is attached to Appendix B and identified as “Appendix IV.*' No minimum antenna height is specified. However, wherever feasible, hi&h antennas should be used to provide improved service. ience that substantial amounts of propagation data do not become available until stations are authorized on a regular basis. Hence, the Commission is faced with the practical problem that if it postpones assigning stations until sufficient propagation data are available, such data may never become available, while on the other hand if stations are assigned before sufficient propagation data are assembled, more interference may result in actual operation than was anticipated. In the Commission’s view, the best method of handling this problem is to assign stations as soon as a reasonably sufficient amount of data is accumulated but in doing so assignments should not be made on the barest minimum separation which exact calculations would indicate. Instead, a safety factor should be included. In this way, if as a result of actual experience more interference results than was indicated by the earlier calculations, the safety factor will prevent extensive damage to overall service. If actual experience shows that the amount of interference is approximately that predicted by the calculations, then the rules and standards can be amended to reflect the new data. In the Commission’s experience, it is much easier as a practical matter to reduce station separations which are somewhat larger than were originally thought to be necesary than it is to increase separations which are smaller than were originally thought to be necessary. With the foregoing in mind, the Commission has provided for a minimum co-channel separation between cities of 180 miles on Channels 2-13 and 165 miles on Channels 14-83. In order to provide for flexibility in the location of transmitters and in order to give communities within 15 miles of the city in question a reasonable opportunity to utilize channels as set forth in paragraph “II Bl” above, minimum separation between transmitters on the same channel is provided for at 170 miles for Channels 2-13 and 155 miles for Channels 14-83.® These separations are based on offset-carrier operation by the stations. 2. Adjacent Channel Separation Under the present television Standards, objectionable adjacent channel interference results when the ratio of the desired to the undesired signal falls below 6 db. The Commission’s proposals of July 11, 1949 did not recommend any change in this ratio. Considerable data presented to the Commission indicate that this ratio is too conservative and that it could be 0 db or — 6 db. In general, adjacent channel interference has not been of a serious nature and such problems as do exist can be solved to a very considerable extent by improvements in receiver design which are neither difficult nor costly. Experience has shown that many receivers are giving satisfactory adjacent channel performance in areas where interference is predicted under the present Standards. The Commission’s proposals of July 11, 1949, provided for a normal adjacent channel separation of 110 miles in the VHF band and 100 miles in the UHF band — one-half the distance provided for the normal co-channel separations. Since adjacent channel interference is so readily subject to being controlled by adequate design and production methods by manufacturers, the Commission believes that adjacent channel separations should be reduced, thus making possible a greater number of assignments. The Commission is of the opinion that these separations should be based upon receiver performance which may reasonably be expected of manufacturers and not on the characteristics of the poorer receivers. Separations have been based on the assumption of receivers having an adjacent channel rejection ratio of —6 db. Thus, a median field strength ratio of 0 db should provide service from one station or the other, at each receiver location for at least 90% of the time, irrespective of signal fading. The Table of Assignments has been based upon an adjacent channel separation between cities of 70 miles for ° In each case, the above figures are minimum separations. Greater separations are utilized in the sparsely settled areas of the country in order to secure a maximum amount of service. In addition, greater separations are also utilized in Gulf coast areas and in other areas where high levels of tropospheric propagation may be expected. This should be kept in mind by persons desiring to suggest changes in the Table of Assignments. Minimum Effective Radiated Power^ (in db above 1 kw) 17db[60 kw]/600 ft. ant. 10 db[10kw] /500 ft. 3db[2kw]/600ft. 0db[lkw]/300 ft. 9