PROOF OF PERFORMANCE MEASUREMENTS
Daniel L. Davis, CPBE
February 28, 2013
It seems that quite a few radio station licensees have forgotten that the FCC requires performance measurements on radio stations. The Federal Communications Commission has set forth specifications for emissions limits of broadcast transmission systems. For AM stations, this is spelled out in §73.44 of the FCC rules and regulations. The requirements for FM stations are specified in §73.317. Another section of the rules, §73.1590, requires measurements to be conducted annually, on AM stations, to prove compliance with the commission’s rules. For both AM and FM stations, these measurements must be made upon initial equipment installation and whenever any modifications are made to the transmitter or antenna system, which might cause the facility to fall out of compliance. Modification includes replacement of any part of the transmission system, from the audio processor to the antenna. The purpose of these specifications is to keep interference among radio stations to a minimum, and to avoid interference to other radio services.
The FCC specifies radio frequency emissions limitations, based upon the licensed power of the station. For AM stations, separate measurements must be conducted for each mode of operation, with different limitations for each mode. A calibrated antenna and analyzer are necessary for acquiring the measurement data.
The station’s designated chief operator is responsible for keeping track of due dates for required measurements. He/she should then see that a qualified person of firm conducts the measurements in a timely manner. In the event that the measurements reveal noncompliance with FCC emissions specifications, necessary action must be taken to correct the problem. Then the measurements must be repeated, and a formal report must be placed in the station’s engineering files.
Having performed these measurements on many stations over the years, I have found that most stations have little difficulty meeting the standards, year after year. Some, however, have fallen short in certain areas. There are stations that had passed the test for several consecutive years, but they will not pass today, due to deterioration of some part of the transmission system.
A few years ago, I measured a station which used a 5kW transmitter for daytime operation and a 1kW transmitter for nighttime and standby operation. The daytime transmitter passed proof of performance measurements with no trouble at all. However, the nighttime transmitter looked terrible on the spectrum analyzer, and I could hear splatter on my automobile radio several channels removed from the station’s licensed frequency. It was obvious that the splatter was associated with modulation peaks. Upon further investigation, I determined that the transmitter was being frequently overmodulated on negative audio peaks. Once this was corrected, I performed another set of measurements, and the station easily passed the test.
In another case, I made measurements on a 10 kW station, which had been passing the test for a number of consecutive years. This time, though, I discovered excessive emissions on frequencies removed by 75 kHz or more from the carrier; the FCC requires a minimum of 80db of attenuation at these frequencies for stations of 5kW and greater. It was determined that the RF amplifier tube was very weak, causing excessive harmonic distortion. Once the tube was replaced, the facility passed performance measurements with flying colors. As an added bonus, the station's fidelity and coverage was significantly improved.
In yet another case, a broadcast station was notified by a broadcast engineer/amateur operator, in another region of the United States. Amateur operators were experiencing serious interference to one of the short wave bands, due to a strong third harmonic of my client's AM station. In this case, it was necessary to install a third harmonic trap in the antenna coupling unit network, to attenuate the harmonic to an acceptable level. This is one example of how a broadcast facility can cause harmful interference to other radio services.
Other possible causes of spurious emissions can be outside the transmitter or even away from the station property. Loose connections on the tower, broken guy wire insulators, poorly bonded metal fences, and other nearby metal structures, with loose or corroded connections, can cause serious problems.
The primary purpose of performance measurements is to ensure that each station occupies no more than its assigned slice of the radio spectrum, thereby minimizing interference to other radio services. More often than not, however, excessive emissions are the result of system weaknesses, which are degrading the quality and coverage of the station’s signal. Therefore, it is important for all stations to keep their facilities in top condition, in the interest of the broadcast community as a whole.