Meeting the SPLIT-CHANNEL STANDARDS

Meeting the SPLIT-CHANNEL STANDARDS

In general, a screwdriver adjustment of deviation control by technician will comply with FCC regulations until 1963

—Courtesy Dumont Photographic Dept.

Typical kit of modification parts. While not required by FCC, receiver conversion may be necessary to insure proper reception of narrow-band signals

IN AN EFFORT to break a log-jam of increasing clamor for additional radio frequencies and reduce interference to and by present users, the Federal Communications Commission recently ordered that all radio systems operating in certain bands of the spectrum reduce transmitter bandwidth. An FCC report and order of June 26, 1958, in effect split the present 25 to 50 me channels in two, and nearly tripled the 152-162 me channels. This action holds promise of eliminating many fire service radio problems presently existing. By this means new frequencies became available for use by the services, although they have not yet been assigned.

In so doing, the FCC placed in effect certain technical standards for radio operation which had been adopted in Docket 11253, dated September 19, 1956. These affect all users concerned including the fire service. Because of these new changes, three modifications must be made in all transmitter installations now operated by the fire service. The first of these must be accomplished by present users on or before August 1, 1960. A second report and order on Docket 12295, dated December 17, 1958 and effective February 1, 1959, specifically exempts those frequencies below 42 mc from meeting any of the new standards until November 1, 1963.

New fire radio systems within the United States, excepting Alaska and the territories, licensed by the FCC after August 1, 1958, must comply at the outset. In addition, all users who modify their licenses to change frequencies or occupy an area which is not a slight expansion of their existing coverage, will be considered new systems and must immediately comply with the new standards.

What is required

The first step to be accomplished is to reduce the transmitter “deviation from the present ± 15 kc to ± 5 kc. Deviation may be defined as the total amount of spectrum occupied by a radio signal each side of the “center” or identifying frequency. It is sometimes referred to in the trade as “swing.”

This may be done only by a licensed technician and most units which have been manufactured since 1949 require only a screw driver adjustment which may be made during normal servicing. Once the deviation has been changed, the system is then transmitting “narrow band” and meets the rules until 1963. Some older models may require replacement of certain circuit components. In many cases these units will be difficult to convert to full compliance by the 1963 date.

The final steps necessary to correct the transmitting equipment to the new technical standards require that where power output exceeds 3 watts, a frequency stability of .002 per cent or better must be maintained in the 25-50 me band and .0005 per cent in the 152-162 me band. For less than 3 watts output, .005 per cent is required.

In addition, a low-pass, or audio rolloff filter is necessary on all transmitters over 3 watts output. The filter attenuates sharply all voice frequencies above 3000 cycles per second. While such a requirement may not appeal to a hi-fi fan, it is necessary for effective narrow-band operation.

The necessary range of the speaking voice is generally within 100 to 3000 cycles. The radiated signal must be wide enough to carry the intelligence impressed on it. Therefore the use of a filter is required if the frequency of the transmitter is to be maintained within the allotted bandwidth of 10 kc ( ±5 kc) and the speech is to remain undistorted.

Most of the recently manufactured radios already meet the previous requirements and units purchased during the past five years can, in general, be quickly adapted to the standards. The addition of oven-type crystals and the inclusion of an audio filter by the technician is usually the only necessary task.

In some older models temperature compensation of the oscillator circuit in the transmitter is dicated in order to prevent “drift” or movement off the designated frequency. This generally requires the addition of certain circuit components by the teclmician depending on the make and model. Some older transmitters cannot be economically converted to the new standards.

Further steps necessary

All of the foregoing apply only to the transmitter operating in the affected bands. Unifortunately these changes also create additional problems having a direct bearing on good reception in a converted system.

Presently operating wide-band receivers may work reasonably well in systems employing narrow-band transmitters providing the receiver stability is sufficient to prevent message loss. Some reduction of volume may be noticed, but a radio technician may be able to increase the audio gain to overcome this deficiency.

The wide bandwidth can prove disadvantageous when newly created channels are assigned by the FCC and become occupied. The receiver may then pick up unwanted signals from nearby stations on the adjacent frequencies.

Successful receiver conversion to eliminate possible headaches depends heavily on the original design. In some cases it will be necessary to install heated crystals and additional oscillator temperature compensation to provide the necessary stability for reception of the narrow transmitter signal. A new filter or “IF” (intermediate frequency) assembly may be required for proper selectivity. These changes will so affect the receiver performance that revision of the “discriminator” and “squelch” operation most likely will be necessary. This may require from 30 to 40 minutes time on the part of the technician, or several hours of difficult work on each individual unit.

In large radio systems the length of time involved in completing a systemwide changeover may run into many man hours. In order to prepare for such a program it will be wise for the fire chief to hold a conference with the communications supervisor, the radio technician and the manufacturer’s or distributor’s representative. Competent opinions can then be obtained on the advisability of converting present units or purchasing a complete new system.

If full conversion is decided upon, a program may then be scheduled to include the removal from service of the individual mobile and station units; an inventory of the necessary parts; budgetary allowances; shipping and receiving dates and the expected completion date. In this manner the changeover may be accomplished in an orderly manner with the least inconvenience to the department as a whole, and with reasonable certainty that this project will be completed within the mandatory time limit set by the FCC. At the same time, allowance may be made for normal maintenance and service for operating units.

Factors governing decision

Serious consideration should be given at this time to complete replacement of older radio units with new models wliich must automatically meet the FCC standards and specifications. It has been a rule-of-thumb that radio equipment has an expected life of approximately 10 years. In some instances systems have been amortized on a five-year basis. In many cases the life span has been exceeded, but due to the new requirements a hard look must be given to the wisdom of authorizing a difficult conversion for equipment which may reach its life expectancy figure before or shortly after the final conversion date in 1963.

In the event full replacement is contemplated within a few years after the cut-off date, a complete cost evaluation should be made and compared to the figures for new units. Any comparison should include the costs for complete replacement versus the time required for conversion in man-hours, new parts and continuing depreciation. This may produce convincing arguments for justifying an increased capital expenditure budget for a complete new radio system.

In this regard, the use of Civil Defense “matching funds” should be explored. The Federal Office of Civil and Defense Mobilization may provide funds for radio systems which are necessary for civil defense responsibilities “over and above” normal everyday requirements. It is suggested that before any conversion or replacement plan is considered, all possibilities of receiving financial assistance in this manner be fully explored with the proper civil defense officials. If the original equipment was purchased with matching funds, it is probable there will be little difficulty in obtaining assistance.

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SPLIT CHANNELS

Continued from page 117

The benefits expected from the recent FCC orders most likely will result in more efficient radio operations for some time to come. Temporary hardships to existing radio systems may be expected. In the long run, the advantages to be gained from the new technical standards should far outweigh any short-term disadvantages and improved fire service communications will result.

Acknowledgment: In addition to the manufacturers whose photographs and diagram are credited in the text, the editors wish to acknowledge the technical asistance of the Radio Corporation of America and Bendix Radio, Division of Bendix Aviation Corporation, in the preparation of this material.

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