Commentary: Adjust Rules for NRSC Measurements

Mark Persons
Part of a Radio World series in which we share industry opinions about AM revitalization.

Normally I offer tech tips but this time I want to express a personal opinion on a topic important to me.

If it is a goal of the Federal Communications Commission to reduce regulatory and financial burden on AM broadcasters, then here is one way to help: Modify FCC rule 73.1590(a)(6) requiring annual occupied bandwidth and RF harmonic radiation measurements on AM stations, known as AM NRSC measurements.

You might remember that NRSC is the National Radio Systems Committee, which created standards for occupied bandwidth on AM and FM stations. The FCC turned those standards into rules.

I propose to exempt AM stations that use solid-state transmitters from the existing requirement to make annual measurements.

The rules were written when vacuum tubes were often used in transmitter designs. Tube performance degrades with time. The result is that tubes must be replaced every year or two in order to maintain operating specifications. Beyond that time in service, an AM transmitter may not be able to comply with FCC rules for occupied bandwidth, which results in interference to other stations on the dial. In the case of FM transmitters, a weak/soft tube does not normally cause bandwidth issues.

For AM stations continuing to use tube transmitters, this annual requirement should still apply. The requirement that all AM and FM licensees keep their equipment in compliance with FCC rules should stay in place, regardless of which transmitter design they use.

Changes in any AM or FM equipment that could alter the occupied bandwidth should automatically trigger the requirement to do measurements to assure compliance with FCC rules. A 30-day window to make successful compliance measurements sounds appropriate to me. The equipment change could be as simple as replacing an audio processor with a different model number.

This rule revision would bring AM stations into the same level of scrutiny that FM stations should meet; I feel my suggested changes will also assure FM compliance.


Note that any change in equipment lineup could unwittingly cause bandwidth problems. The only way to be sure the station is FCC legal is to do spectral analysis.

Further, the procedure for AM and FM stations should be written to require listing model numbers of studio transmitter links, audio processors and transmitters. (Serial numbers are not important.) In the case of FM stations, audio processors, stereo generators, RDS/RBDS generators, studio transmitter links and transmitters should be listed.

In essence, anything capable of controlling or changing occupied bandwidth should be documented.

The report would not be placed in a public file. Instead it would go into the station’s engineering file, which is not seen by the public. No change there. In that way, a station’s “custom lineup” of audio processing would not be revealed to competitors.

FCC FM rule 73.1590(d) regarding measurement data needs to be modified to strike the words “for a period of 2 years, and on request must be made available during that time to duly authorized representatives of the FCC.” This re-written rule would require measurement data to be kept on hand until superseded by a newer report. It is documented proof of legal operation.

An FCC inspector or mock inspector would request this report when visiting a station. He or she would check to see if the same equipment is employed, even if it is years later. Any report that did not have this data would be invalid.

Practically speaking, my proposal would mean that all or most stations would need to perform a new set of measurements based on these criteria in the immediate future. Again, any changes in the equipment lineup should require a new set of measurements to assure FCC compliance.


On another note, station technical consultants need to be vigilant in watching for new transmitter sites or changes to existing transmitter sites within five miles or so of the transmitters they are watching after. A new onsite or offsite transmitter in the AM or FM band might create mixing products that could render the station illegal under FCC rules.

Mandatory annual measurements are not needed, in my opinion. Just watching after the store, so to speak, is what is necessary.

For perspective, technology has improved to make equipment more reliable and trouble-free. FCC rules have been modified in the past to keep up, even if they did lag a bit from reality. Stations were required to read and log all operating parameters every half hour up until about 40 years ago. Monthly carrier frequency measurements were required until about 30 years ago, as were annual audio proof of performance measurements.

We don’t even have operator licenses anymore. Now is the time for the FCC to make the changes I outlined. It is good for the radio broadcast industry and makes perfect sense.

Comment on this or any article. Write to

Mark Persons, WØMH, is a Certified Professional Broadcast Engineer and has more than 30 years’ experience. His website is

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To the contrary, I have found that the bulk of the AM Transmitters,that are having difficulty meeting the AM NRSC Standards,are Solid State Transmitters. The "efficient" modulation techniques that are used,on Solid State AM Transmitters,tend to produce sideband products that are related to the PWM switching frequency. When the TX Mod Filter Networks are properly designed and functioning, as they should,the "by-products" normally meet the NRSC Standard. When you factor in some age on the components,the "Trash" that is radiated can yield a "Failed NRSC" status,as well as interference to other stations. Harmonic content can also become a issue of change from year to year. One particular (unnamed) "Brand" is notorious for "Failing". Obviously I am biased.
By Frank Hertel on 2/7/2014
A failed electrolytic in a solid state FM amplifier will also cause a spur; that's not exclusive to AM. One doesn't know unless there's a complaint or inspection - just ask my local FCC inspector.
By Lindsay Collins on 2/7/2014
I think it is a travesty to hold an analog station to the NRSC 2 standard when the "improved" HD digital station is allowed to create garbage as much as 20 to 30 dB higher all the while complying with NRSC 5. If NRSC 5 is adequate protection then it should be appropriate for ALL kinds of Medium Wave Broadcast facilities. HD is being used as a weapon to prevent listeners from hearing adjacent channel stations that are quite solid and easily listenable in the presence of a mere analog 2nd and 3rd adjacent station that complies with NRSC 2.
By Tim Cutforth on 2/7/2014
Mark, although ideally I'd like to agree with you, I have experienced something firsthand that makes me think the NRSC measurements are a good thing for the industry. I have a client that found issues with an Energy Onix Pulsar 1000 during the NRSC measurement. It turns out the guy that does the meausements found multiple other Pulsar stations with the same abnomilty. He was able to talk to Berine and get E-O to modify our PDM frequencies to stop the problem. Not only did he cure our station, his mesurments and dillgence cured a bunch of stations. I've also seem caps in ATUs or phasors that display spurious garbage. The pattern stays in where there isn't much indicator of the problem. Lastly, most AMs are either on a shoestring or neglected as the lesser of a cluster. Need meausements...
By Patrick on 2/6/2014
In 20 years of doing AM NRSC measurements on as many as 50 stations a year, I found was rare when a solid state transmitter had a problem. Yes, the Harris SX and Gates series transmitters sometimes develop spurs. FM transmitters can have similar problems where frequent measurements are not even required. Annual testing does not “assure compliance” for the 365 days between measurements. Remember, every AM and FM radio station is required to stay within FCC limits regardless of when it was last measured. Mark Persons CPBE
By Mark Persons on 2/6/2014
I agree with Steve and Burt. I think the annual NRSC tests should stay in place, even for solid-state transmitters. Just a few short years ago, a failed NRSC measurement alerted me to an undetected problem. It wasn't very bad yet, there was no audible distortion, and no interference was being caused, but it was just bad enough to fail the NRSC test. After "shotgun" replacing all of the capacitors in the modulator section, (they ALL tested good) the problem went away, and the transmitter once again passed with flying colors. In this case the NRSC test brought to light a problem before it became bad enough to have caused any harm. Isn't that the whole point of this test? ...and yes, it was an SX-2.5.
By Bart Jones on 2/6/2014
I have had a lot of experience with the newer solid state transmitters. Their complex modulation schemes as well as broad RF amplifier stages can produce spurs and intermodulation products. I can provide spectrographs of these if anyone's interested. Instead of replacing tubes we are now replacing electrolytic type capacitors and adding filters to antenna systems. These are mostly great transmitters, but they have their own set of problems. You cannot simply turn them on and walk away. They, like tube type transmitters, require emission monitoring and regular care and maintenance. Burt Weiner Glendale, California
By Burt Weiner on 2/6/2014
Mark, I agree with you concerning the degradation of performance of tube type transmitters, but what about the solid state AM transmitters like the Harris SX and Gates series that use large value capacitors in the modulator sections? When those caps go bad and produce the dreaded "Harris slosh" the 60 KHz switching frequency supression goes away and spurs of substantial amplitude can pop up above the required AM RF mask. I don't have much experience with AM transmitters newer than those vintages, so I'm not sure if newer designs by Broadcast Electronics, Nautel or Harris have that problem, but it would be pretty difficult for the FCC to seperate things just on the basis of tubes or solid state. Steve Brown Radio Rangers Minneapolis
By Steve Brown on 2/5/2014

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