If in an application to the Commission a vacuum tube of a type number and power rating not given in the foregoing tables is specified for operation in the last radio stage, it may be accepted provided there is also submitted to and approved by the Commission the manufacturer's rating of the vucuum tube for the system of modulation or class of service contemplated. These data must be supplied by the manufacturer. (Secs. 4 (i), 303 (e), 48 Stat. 1068, 1082; 47 U.S.C. 154 (i), 303 (e)) [Rule 127, as amended Apr. 20, 1938] 3.74 Absence of exact rating; nearest to apply. If the maximum rated carrier power of any broadcast transmitter, as determined by §§ 3.72, 3.73, does not give an exact rating as recognized in the Commission's plan of allocation, the nearest rating thereto shall apply to such transmitter.* [Rule 128] 3.75 Equipment changes; Commission approval necessary. No licensee shall change the number of vacuum tubes, change to vacuum tubes of different power rating or class of operation in the last radio stage, or change system of modulation without the authority of the Commission.* [Rule 129] 3.76 Equipment changes; Commission approval unnecessary. Other changes which do not affect the maximum power rating or operating power of the transmitter or the operation or precision of the frequency-control equipment may be made at any time without authority of the Commission, but in the next succeeding application for renewal of license such changes must be shown in full.* [Rule 130] 3.77 Radiating system; requirements. (a) All applicants for new, additional, or different broadcast facilities and all licensees requesting authority to move the location of the station shall specify a radiating system, the efficiency of which complies with the requirements of good engineering practice for the class and power of the station. (b) The Commission will publish from time to time specifications deemed necessary to meet the requirements of good engineering practice.15 15 It is the obligation of the licensee of every station to make efficient usage of the assignment granted by the Commission. It is not the intention of the Commission at this time to require all stations with questionable radiating systems to install antennas having the required efficiency, but it is the intention not to grant additional facilities to licensees of broadcast stations unless they are making efficient usage of the assignment already granted. That is, the licensee of a broadcast station requesting more power, change in time of operation, different frequency, or more of the transmitter, must have an antenna for the assignment requested that meets with the minimum requirements before favorable consideration will be given. A graph which may be obtained from the Commission shows the minimum physical height of antenna proper or minimum effective field intensity that stations must have before additional facilities will be granted. These minimum actual physical vertical heights of antennas permitted to be installed are shown by curves, A, B, C, and D of Figure 1 as follows: A. Local Channel Stations, 100 watts night and day or 100 watts night and 250 watts day, or a minimum effective field intensity at one mile of 40 mv/m for 100 watts. Page 138 **For statutory and source citations, see note to §3.1. (c) No broadcast station licensee shall change the physical height of the transmitting antenna, or supporting structures, or make any changes in the radiating system which will measurably alter the radiation patterns except upon written application to and authority from the Commission. (d) The antenna and/or supporting structure shall be painted and illuminated in accordance with the specifications supplied by the Commission pursuant to section 303 (q) of the Communications Act of 1934 (48 Stat. 1083; 47 U.S.C. 303 (q)).† (Secs. 4 (i), 303 (e), 48 Stat. 1068, 1082; 47 U.S.C. 154 (i), 303 (e)) [Rule 131, as amended Oct. 15, 1935] 3.78 Transmitter requirements. (a) The transmitter proper and associated transmitting equipment of each broadcast station shall be designed, constructed and operated in accordance with good engineering practice in all phases not otherwise specifically included in the regulations in this part. (b) The transmitter shall be wired and shielded in accordance with good engineering practice and shall be provided with safety features B. Regional Channel, limited time, day etc., Stations 250 watts to 1,000 watts night and day, or a minimum effective field intensity at one mile of 150 mv/m for 1 kilowatt. C. All stations other than Dominant Clear Channel Stations having an operating power night or day greater than one kilowatt and less than 25 kilowatts, or a minimum effective field intensity at one mile of 175 mv/m for one kilowatt. D. All Dominant Clear Channel Stations and all other stations having a maximum operating power night or day of over 10 kilowatts, or a minimum effective field intensity at one mile of 200 mv/m for one kilowatt. The heights given on the graph for the antenna apply regardless of whether the antenna is located on the ground or on a building. Except for the reduction of shadows locating the antenna on a building does not necessarily increase the efficiency. In applying these curves the maximum operating power shall determine which curve is applicable. In case it is contended that the required antenna efficiency can be obtained without antennas of the height specified, a complete field intensity survey must be supplied to the Commission showing that the field intensity at a mile without attenuation fulfills at least the minimum requirements. This field survey must be made by a qualified engineer using equipment of acceptable accuracy. To obtain the maximum efficiency of which any antenna is capable, a good ground or counterpoise system must be employed. At the present state of the art, it appears that where a vertical radiator is employed the ground system should consist of radial wires at least 1/4 wave length long. There should be as many of these radials as practicable and in no event less than 70. These wires should be buried only deep enough to provide mechanical protection (not greater than 12 inches). However, they should not be permitted to rest on the surface. In many cases a counterpoise or combination counterpoise and ground system may be superior to a ground, especially where a good ground canont be obtained. It should be borne in mind that the above specifications are the minimum and where possible better antenna and ground system should be installed. Before any change is made in the antenna, details should be submitted to the Commission for approval in order that it may be definitely determined that the installation will meet the requirements of § 3.77 and that it does not constitute an objectionable hazard to air commerce. These data may be submitted by letter setting out full details. [FCC, BD, Oct. 15, 1935] *For statutory and source citations, see note to § 3.1. Page 139 in accordance with the specifications of article 37 of the current National Electrical Code as approved by the American Standards Association.16 16 Referring to paragraph (a) of the above section, at present good engineering practice shall be interpreted as follows: In general the transmitter must be constructed either on racks and panels or in totally enclosed frames protected as required by the sections of article 37 of the National Electrical Code quoted at the end of this footnote. The final stages of high power transmitters may be assembled in open frames providing the equipment is enclosed by a protective fence. Means must be provided for making all tuning adjustments, requiring voltages in excess of 750 volts to be applied to the circuit, from the front panels with all access doors closed. Proper bleeder resistors should be installed across all condenser banks to remove any charge which may remain after the high voltage circuit is opened. All meters which have more than 1,000 volts potential to ground on the movement shall be protected by a cage or cover in addition to regular case even if bakelite. All plate supply and other high voltage equipment including transformers, filters, rectifiers, and motor generators, must be protected so as to prevent injury to operating personnel. This protection should include commutator guards on all high voltage rotating machinery. The transmitter panel or units shall be wired in accordance with standard switchboard practice, either with insulated leads properly cabled and supported or with rigid bus bar properly insulated and protected. Wiring between units of the transmitter with the exception of circuits carrying R. F. energy shall be installed in conduits or approved fibre or metal raceways to protect them from mechanical injury. Circuits carrying low-level R. F. between units shall be of either concentric tube, two-wire balanced lines or properly shielded to prevent the pickup of modulated R. F. energy from the output circuits. Each stage (including the oscillator) preceding the modulated stage shall be properly shielded and filtered to prevent feedback from any circuit following the modulated stage. An exception to this requirement will be made in the case of high-level modulated transmitters of approved manufacture which have been properly engineered to prevent reaction. The crystal chamber, together with the conductor to the oscillator circuit, must be totally shielded. The crystal chamber must be so constructed, insulated and temperature-controlled that the maximum temperature variation at the crystal shall not be greater than 0.1 degrees Centigrade. An exception would be made in the case of transmitters employing so called "AT" or zero coefficient crystals wherein the maximum allowable temperature variation at the crystal is 1.0 Centigrade. A thermometer must be installed in such a manner that the temperature at the crystal can be accurately measured and the temperature logged each half hour in accordance with § 3.101. It is preferable that the tank circuit of the oscillator tube be installed in the temperature-controlled chamber. In case an excessive shift in frequency is found during warmup periods the crystal oscillator must be operated continuously. The commission will take special precautions to ascertain that composite crystal chambers and oscillator units meet the requirements of "good engineering practice" before the station is considered as having satisfactorily complied with § 3.78. The radio frequency energy operating the monitor must be obtained from some stage in the transmitter prior to the modulated stage and the monitor circuits must be such that the monitor can be operated continuously without heterodyning the carrier. In addition, the monitor and the radio frequency line from the transmitter must be thoroughly shielded to prevent regeneration in the transmitter. The transmitter power supply shall be so constructed that the maximum plate voltage regulation between no modulation and 100% modulation shall not exceed 5%. Adequate provision shall also be made for varying the transmitter power output between sufficient limits to compensate for excessive variations in line voltage, or other factors which may affect the power output. A complete set of spare tubes for the transmitter and frequency monitor should be on hand at all times, the spares to include thyratron tubes, when used. Page 140 (c) The station equipment shall be so operated, tuned, and adjusted that emissions are not radiated outside the authorized band which cause or are capable of causing interference to the communications of other stations. The spurious emissions, including radio frequency harmonics and audio frequency harmonics, shall be maintained at as low a level as required by good engineering practice. The program distortion, audio frequency range, carrier hum, noise level, and other essential phases of the operation which control the external effects shall at all times conform to the requirements of good engineering practice. (d) Whenever, in this section, the term "good engineering practice" is used, the specifications deemed necessary to meet the requirements of good engineering practice will be published from time to time. (e) This section shall be effective upon its adoption provided, however, that existing broadcast stations shall be allowed 1 year in which to meet the requirements herein.† (Secs. 4 (i), 303 (e), 48 Stat. 1068, 1082; 47 U.S.C. 154 (i), 303 (e) [Rule 132, as amended Nov. 12, 1935] TECHNICAL OPERATION 3.91 Determination of operating power. The operating power of a broadcast station will be determined either by direct measurement or indirect measurement by means of the plate input power to the last radio stage.*t [Rule 134] 3.92 Indirect measurement; normal method. Unless specifically authorized by the Commission to do otherwise, the licensee of a broadcast station shall compute its operating power by indirect method by means of the plate input power to the last radio stage.*† [Rule 134] No requests for new broadcasting facilities will be granted unless the equipment proposed to be installed conforms with the definitions of "good engineering practice" as outlined in this part. It is the obligation of the licensee of each existing station to take the necessary steps to assure that the transmitting equipment complies with § 3.78 and these definitions of "good engineering practice.' Any changes in the transmitter for which a construction permit is necessary by other sections, application therefor must be made in the regular manner. The purpose of this section is to improve broadcast reception and to protect the lives of the station operators. Many frequency deviations are caused by poor equipment. The mutual interference caused by such deviations will thus be reduced as the deviations are reduced. The continuity of service and fidelity of transmission will be improved. This section is for the good of the licensees as well as the listeners and the cooperation of all licensees is requested in assisting the Commission in the administration thereof. [FCC, BD, Jan. 10, 1936] The pertinent sections of article 37 of the National Electrical Code read as follows: j. The transmitter shall be enclosed in a metal frame, or grill or separated from the operating space by a barrier or other equivalent means, all metallic parts of which are effectually connected to the ground. k. All external metallic handles and controls accessible to the operating personnel shall be effectually grounded. No circuit in excess of 150 volts should have any parts exposed to direct contact. A complete dead front type of switchboard is preferred. 1. All access doors shall be provided with interlocks which will disconnect all voltages in excess of 750 volts when any access door is opened. [FCC, BD, Jan. 10, 1936] *For statutory and source citations, see note to §3.1. Page 141 3.93 Changes. Any licensee who has at any time been authorized by the Commission to compute operating power by any other method (e. g., by antenna input direct measurement, or radiated power measurement computed from field intensity measurements) shall, upon making any change in the antenna system or in the antenna current measuring instruments, or any other change which may change the characteristics of the antenna, revert to the use of the indirect measurement of antenna input until further order of the Commission.*† [Rule 134] 3.94 Indirect measurement. The operating power shall be determined by indirect measurement from the plate input power of the last radio stage by multiplying the plate voltage (E,) by the total plate current of the last radio stage (I,) and by the proper factor (F) given in the following tables according to the power and system of modulation used; that is Operating power =E, XI, X F (a) Factor to be used for stations using high-level modulation— (b) Factor to be used for stations of all powers using low level modulation Maximum percentage of modulation: 75 to 85-.. 86 to 100-‒‒‒ Factor (F) to be used in determining the oper ating power from the plate input power 0.40 .33 (c) Factor to be used for stations of all powers using grid bias modulation in the last radio stage Maximum percentage of modulation: 75 to 85 86 to 100 *+ [Rule 135] Factor (F) to be used in determining the operating power from the plate input power 0.27 .22 3.95 Application of factor (F). In computing operating power by indirect measurement, the above factors shall apply in all cases, and no distinction will be recognized due to the operating power being less than the maximum rated carrier power.*t [Rule 136] 3.96 Direct measurement; Commission approval. The antenna input power determined by direct measurement is the square of the antenna current times the antenna resistance at the point where the current is measured and at the operating frequency. Direct measurement of the antenna input power will be accepted as the operating power of the station, provided the data on the antenna resistance measurements are submitted under oath giving detailed description of the method used and the data taken. The antenna cur Page 142 **For statutory and source citations, see note to § 3.1. |