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Much has been bandied around the board about the history and intent of the FCC part 15 regulations regarding AM. In a recent thread started by Ermi Roos the subject once again was mentioned. It became clear to me that, other than bits and pieces posted by some (me included) there was really no good account of what happened regarding part 15 since the Communications Act of 1934. Ermi went to the source and published his account in another thread.
I thought his article which is based on facts from a credible source merits promotion to a new thread focused on this topic, so here’s copied article (had trouble with the link):
It had been my intent to connect the history of radio that I reported in this blog thread with the history of Part 15 AM. I had, at first, relied on previous comments by Neil on the part15.us Forum as being an authoritative source relating to the origins of Part 15 AM. In Neil’s own posts in this thread, however, he had cast some doubt about the accuracy of his account, and so I made additional efforts to find out what had really happened in the past. Not finding other sources, I wrote to the FCC for help, and I received the following, fairly complete, account of when various versions of the rules were enacted. The author is John A. Reed, Senior Engineer in the FCC Office of Engineering and Technology, Technical Rules Branch. Mr. Reed’s contributions have appeared elsewhere on this web site, and in print publications such as “Radio World.”
“The original regulation, Rule 25.02, adopted November 21, 1938, (and currently stated only in Section 15.221(a)) permitted any operation on any frequency provided the field strength did not exceed 15 uV/m at a distance of Lambda/ 2 Pi. Sometime after July 1, 1939, and before October 27 , 1939, this rule was renumbered Rule 2.102. On July 21, 1948, in FCC 48-1837, this rule was again redesignated as Section 15.02. On December 21, 1955, in Docked No. 9288, this rule was again redesignated as Section 15.7.
“On July 18, 1957, in Docket No. 9288, the Commission provided specific provisions for unlicensed operation in the AM band in what would be designated as Sections 15.202, and 15.204 of its regulations. Section 15.202 stated that the field strength limits shall not exceed 24000[/f(kHz)] uV/m at a distance of 100 feet. Section 15.204 was adopted as an alternative to the field strength limit to avoid the difficulties in making field strength measurements, provided that the input power to the final stage did not exceed 100 mW and the combined length of the antenna plus connecting lead did not exceed 10 feet. While the Commission originally proposed to permit an input power of 200 mW in the AM band (3rd Notice of Proposed Rule Making in Docket 9288 adopted November 8, 1956), the adopted limit was reduced to 100 mW to reduce the potential area of interference.
“In an Order adopted on November 12, 1974 (FCC 74-1221), the rules were again modified, redesignating 15.202 as Section 15.111 and Section 15.204 as Section 15.113. Section 15.113 was modified to include the length of the ground lead in addition to the length of the antenna and connecting lead in the 10 feet maximum. This change was added because the earlier rules had not contemplated anyone using an extended ground plane [lead] to extend the range. The change was made to stop this practice.
“On April 14, 1976, in Docket No. 20780, the Commission proposed several changes in Sections 15.111 and 15.113, but did not adopt any of these changes. Paragraph 16, however, made the Commission’s intentions clear that it implemented the alternative power measurements in lieu of a field strength limit to make it easier for home builders, that didn’t have the means to perform field strength measurements, to demonstrate that their products complied with the standards. However, this rule was never intended to provide a greater operating range than the original field strength limit. The operating ranges were expected to be about equal, but improvements in efficiency were starting to result in increased range, and increased potential interference, for systems operating under Section 15.113.
“Finally, on March 30, 1989, in ET Docket 87-389, the Commission adopted the current regulations. Section 15.111 was incorporated into Section 15.209 without changing the radiated emission limits except to specify the measurement range as 30 meters instead of 100 feet. Section 15.113 was redesignated as Section 15.219 with the only change to the limit being to specify the combined length of the antenna, connection lead and ground lead as 3 meters instead of 10 feet. Also in this order, the Commission established Section 15.221 to permit unlicensed operation in the AM broadcast band on the campus of any educational institute provided the emissions do not exceed the limits in 15.209 as measured from the campus boundary. On November 26, 1990, again in ET Docket No. 87-389, the Commission modified section 15.221 to permit carrier current systems to operate under the older limit of 15 uV/m at Lambda/2 Pi as an alternative to the field strength limits in Section 15.209.
“That’s the basic history. Let me know if you have any further questions.”
Here’s the link to the orginal thread which also contains some very interesting history and information:
http://www.part15.us/node/1712
Neil
The most significant information given in Mr. Reed’s letter is that the FCC considers Section 15.209, which specifies very restrictive field strength limits in the AM BCB, to be equivalent to Section 15.219, which specifies the 100 mW power input limit to the final transmitter stage, and the 3 meter total length limit of the antenna, transmission line, and ground lead. As Mr. Reed explains, Section 15.219 exists only to permit hobbyists, who do not have access to a field strength meter, to comply with the rules without having to perform field strength measurements. Section 15.219 is not intended to permit longrer range operation than Section 15.209 allows. Mr. Reed states that the FCC did not anticipate the improvements in efficiency under 15.219, which, he says, produce “increased potential interference.”
In my blog, which is linked in Neil’s post above, I report results of experiments I performed using a car radio to receive Part 15.209-compliant signals. The signals are hopelessly noisy at 100 feet. It was necessary to bring the radiator of the 15.209 signal to about 15 to 20 feet from the car radio antenna to get good reception. This increased the field strength by about 20 dB. In contrast, using even an inefficient 15.219 transmitter, good reception is easy to get at 100 feet and more.
People sometimes suggest changes to Section 15.219 rules to increase range. An example is a proposed petition to the FCC found in Dan Jackson’s blog:
http://www.part15.us/node/1575
According to Mr. Reed’s letter, the range possible with 15.219-compliant transmitters already exceeds the FCC’s intent. So, it seems unlikely that the FCC will accept further enhancements to Section 15.219.
It should be noted that Part 15 does not offer an alternative to field strength measurements for FM. Apparently, the FCC did not wish to repeat what they seem to consider to be a mistake with respect to Part 15 AM.
Total posts : 45366This informative thread asking about the history of the Part 15 Regulations is one branch of a fuller look at the past which includes the history of Part 15 radio stations, however small or short lived.
Just today I was wondering how many years my Part 15 station, begun in 2007, has been carrying the Annual New Orleans Jazz and Heritage Festival? I’m guessing three or four, but there’s no effort to collect and maintain the many programs and experiences of my activities, and I guess it’s that way with most stations.
The exact same thing is true of online streaming stations and podcasts. Who’s keeping track when stations start and end?
Even licensed broadcasting loses its past in fragmented and very incomplete attempts to “remember” the past, some of which can be re-assembled by picking through FCC files or old newspapers.
For that matter the history of people gets buried along with passing generations…
Even the formal history of countries ends up being disputable as revealed by Howard Zinn’s “The Real History of the United States,” carefully researched, in which he reveals events as they actually happened, not the patriotically glorious stuff taught in schools.
The fade of life.
Total posts : 45366Here the AM is strictly field strength…..nothing about 100mW into final, limits like 3 meters for the antenna and ground length combined or anything like that.
The FM field strength here is 1000uV/M@3 meters and the AM is 2.5 times that, 250uV/M@30 meters or 2500uV/M@3 meters.
If I were to be on AM here I could string an antenna anywhere at any length, have the ground connected to anything with any length and have any power of the transmitter anything as long as the field strength at any point isn’t more than stated.
Don’t know how that compares with the FCC rules with transmit range but consider the range in the USA for FM is say 200ft with a portable radio average quality. Here the legal range may be 4x that. Then take that range and multiply that by another 2 and 1/2 times and that’s the coverage you can legally get here with AM….with the advantage of having your transmitter inside and not having to worry about weather proof boxes and someone stealing your outdoor equipment because of the feedline length restrictions.
I could string an antenna all around a town(in theory) and get even coverage everywhere and that wouldn’t be illegal.
Thought I’d mention this USA/Canada comparison for anyone’s interest as everything here mostly is about FCC rules.
Mark
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