Any 15.221 Operators Around?

I have not had the chance to play with Carrier Current but here is a few posts from part15.us i think would help with your situation.

Good Luck

http://part15.us/node/1411
http://part15.us/node/1477
http://part15.us/node/2295?page=1
http://part15.us/node/516
http://part15.us/node/book
the last link takes you to the part 15.us library.

15.221 is not very useful for a campus AM radio station unless the campus is very large. This is because the field srength limit of 15 uV/m at one radianlength from the border of the campus is very low, and it usually can't even be measured with a standard field strength meter. I saw a video advertisement about a prep school out in the country that has several square miles of mostly uninhabited land. This kind of of a campus might work with 15.221. Also, being out in the country, background noise may be low enough to make field strength readings according to the rules possible. An urban campus radio station would work better under 15.219 rules, particularly if multiple transmitters are used.

Are you basing the statement "15.221 is not very useful for a campus AM radio station unless the campus is very large" on theory or on practice? I need reports based on first-hand, in-the-trenches experience. And, while a hobbyist field strength meter might have trouble, pro gear like the Potomac FM-41 should prove more than adequate. Also, I'm not completely convinced that the noise floor is so horrendous that a precision tuned instrument couldn't differentiate RF signal from broadband noise. After all, if the Commission can measure to see if someone is staying within 15 µV @ 30M beyond the campus perimeter, I should be able to also. For that matter, if the noise floor were that high, it would likely swamp any efforts to produce a listenable signal anywhere other than adjacent to the transmitter (or is that what you're suggesting?).

Also, multiple transmitters is a non-starter for us; anyone who has ever had to deal with academic bureaucracy will attest that getting one of anything approved is challenge enough, much less multiples! I'll be happy if they let me mount one antenna without a box of paperwork and a team of electricians, physicists, aestheticians, arborists, etc., etc.

But look at it this way. A 15 uV/m signal is for all practical purposes completely useless. It is at or below the noise floor in most places. You need at least 10 times that much signal to be heard on a decent AM radio, which means that the range of a usable signal is going to have to be well within the boundary of your campus.

To look at it another way, a Part 15.219 installation can easily produce 15 uV/m at a distance of a mile. Compliance with 15.219 is intended to be relatively straightforward. So unless you have a campus that extends more than a mile, it seems as though 15.219 would be much easier to operate under. Plus, you can put compliant transmitters wherever you want-- and have as many as you want-- around the campus. That way you can have a decent signal right out to the edge of your property. I'm not sure why multiple transmitters would have to be a big deal from an administrative standpoint. They're small, and they take very little power. But it's your call.

My experience with carrier current AM comes from my work with a campus CC station as the "chief engineer". We would have liked to cover the entire campus with one transmitter but due to the noise and field strength limits mentioned by Ermi it was not workable.

The campus was served by installing a transmitter in each residence hall and linking them via leased telephone lines for the audio. The mode of signal delivery was radiation from the power wiring to the receivers in the dorms. There was so little radiation outside the buildings that the same frequency was used campus wide without mutual interference.

The signal in most of the dorm rooms was strong and listenable yet was not usable outside the buildings. The transmitters were coupled through tuned L networks to the hot (line) side of the 120 volt distribution system and despite the three phase distribution enough mutual coupling was present so only one phase needed to be fed to provide indoor coverage. This worked because the three phase feeds were dropped to each floor in an effort to balance the loads.

The dorms ranged in size from small three story buildings to large 10 story buildings. In one of the ten story buildings where the transmitter was installed in the basement coverage to the top floors was marginal. We could not get permission to move the transmitter to a middle floor so I don't know if this would have helped.

At the time there were no light dimmers or other non-linear loads other than fluorescent lighting to produce the hash which is now common inside buildings.

The biggest technical problem was keeping the audio feed to each transmitter at the proper level. It seemed that the line characteristics would change with time and I spent a lot of time going around campus adjusting the modulation of the transmitters. I later found out that unless the circuits were "red capped" as critical the telco would change the physical line routing from time to time. In modern buildings where the college owns the wiring this probably is not going to be a problem.

EDIT TO ADD: From my experience with CC and over the air part 15 AM I doubt that a single transmitter is going to work so despite the hassle I recommend you pursue the multiple carrier current transmitter approach.

If you have specific questions I will try to help.

Neil

I have been assuming that your campus is a residential campus thus my reference to dorms.

Consider using a FM transmitter in each dorm. Though the legal range is limited, the resistance to interference, the stereo capabilities, and the capture effect of FM may make this workable. Even a 200 foot range inside a dorm building would cover a lot of potential listeners. Imagine a sphere with a radius of 200 feet centered within a building and the number of rooms within this sphere.

Leaky coax systems is another approach for FM but the field strength compliance can get tricky.

Neil

When I was an EE student, I tried this approach to see how well it would work. I was successful in matching the transmitter to the ac line, but what I found was that the first appliance cord that was plugged in downstream from the transmitter upset the loading and radiated most of the power, to such an extent that there was little signal left further down the pipe, so to speak. The reason carrier current works at AM frequencies is that the wavelength is so long that short cords plugged into the line do not absorb much power. Thus you can do a reasonable job of matching to the line and the signal can be heard for a long distance.

What I did not try on FM was "leaky coax". Commercially made leaky coax is available and is rather expensive. Plus, you would have to install it in fairly close proximity to the rooms you wanted to broadcast to. I've thought about ways to fabricate a leaky transmission line inexpensively but haven't worked with it lately. In any case, you're always going to be up against the problem of demonstrating compliance with the FCC's ridiculously low field strength limits. It's too bad that the public can't have more options for this use of this great band.

When I was an EE student, I tried this approach to see how well it would work. I was successful in matching the transmitter to the ac line, but what I found was that the first appliance cord that was plugged in downstream from the transmitter upset the loading and radiated most of the power, to such an extent that there was little signal left further down the pipe, so to speak. The reason carrier current works at AM frequencies is that the wavelength is so long that short cords plugged into the line do not absorb much power. Thus you can do a reasonable job of matching to the line and the signal can be heard for a long distance.

What I did not try on FM was "leaky coax". Commercially made leaky coax is available and is rather expensive. Plus, you would have to install it in fairly close proximity to the rooms you wanted to broadcast to. I've thought about ways to fabricate a leaky transmission line inexpensively but haven't worked with it lately. In any case, you're always going to be up against the problem of demonstrating compliance with the FCC's ridiculously low field strength limits. It's too bad that the public can't have more options for the use of this great band.

15 uV/m in the AM band can't be measured by The Commission, or by anybody else, regardless of the FSM used, except maybe in some rural areas. The Potomac instrument has a rated detection limit of 10 uV/m, but such a low signal can be detected only in a screen room. The normal atmospheric noise is just to high. The limit of 15 uV/m at one radianlength came about in 1938. Maybe the AM BCB was quieter back then. It's pretty silly that this limit, which is clearly too low, is still in the rules.

LPB lobbied for 15.221. But when John Devecka, former director of sales and marketing at LPB, had set up a Part 15 AM station at Loyola College in Maryland (WLOY), he did not use 15.221 on the Loyola campus in Baltimore. He used five synchronized Rangemasters to cover the campus and some of the surrounding community.

Added note: Field strength measurements in AM NOUOs are always MUCH higher than 15 uV/m. The FCC does not measure medium frequency field strengths anywhere near 15 uV/m.

Ermi,

Thanks for your comments. This is definitely one of the more interesting discussions I have seen lately.

Tnx, all, for the cogent info! I'm dealing with a commuter community college, so there are no dorms, thus carrier current isn't a strong candidate here. While I recognize that 15.221 also deals with carrier current, I was focusing on the subpart (b) provisions dealing with intentional radiators. Likewise, my comments re: administrative challenges are born of my frustrations after making the transition into the classroom after 35+ years in broadcasting. I'm used to just doing things rather than having to put in a requisition and then have various fiefdoms squabble over who will ignore the request. 😉 Long term, I would be willing to consider synced transmitters, but I just wanna get something going. After all, they were talking about a student station almost 10 years before I showed up.

Basically, the reason I was attracted to the .221 provisions had more to do with being able to futz around with a somewhat more efficient and/or easier antenna system rather than being limited by the <3M proscriptions. I may be all wet here, but if the AM restrictions seem onerous, the FM ones seem preposterous. By following .221, I had hoped to mount a station without having to jump through as many of the hoops usually required of micro-power broadcasters; e.g., why mount the transmitter in a weatherproof housing at the base of the antenna when 15.221 seems to allow a little leeway with the antenna system. Coax anyone? If the bottom line is that the field strength 100 feet past the edge of the campus has to be no more than a "conventional" (15.209) station mounted along that perimeter, why not goose things up a bit at the transmitter site at the center of the campus? While I now have a fresh appreciation of the challenges inherent with trying to measure such puny signals (tnx, Ermi), I didn't want to get too bogged down with trying to measure the immeasurable (how many angels can dance on the head of a pin?) so much as take advantage of what little breathing room seemingly 15.221(b) affords.

Ultimately the main focus will likely end up being 'net streaming, but as a curmudgeon and somewhat of a purist, it would be nice to have a "real radio" component to the operation. We shall see.

Again, thanks to all for the good stuff!

Addendum: A re-re-reading of 15.221(c) points up why the almost-impossible-to-measure measurements are important: you have to keep a copy of these unicorn/leprechaun sitings on file at the studio or transmitter. The light dawns! It's the Catch 22 of .221!

Your mention of a Community College campus helps put this into perspective for me. I taught at a CC and for one of our Electronic Communication labs I brought in a Ramsey AM-25 and a FM 25 transmitter. After the exercise of examining the outputs with a spectrum analyzer and so on at the finish of the lab I placed the FM transmitter with the whip extended in a window (2nd story) and dropped a 3 meter wire out the window for the AM. Both transmitters were grounded to the lab power system. I used a CD player to provide audio and invited the students to tune in when they reached the parking lot and report the next lab period.

The student lot edge was about 300 feet line of sight from the window. The reports varied but most could hear the FM and none could hear the AM. It was night so the AM was probably buried in skywave noise and I did not use a loading coil so the radiation was probably pretty weak.

I think what this is going to come down to is to try it and evaluate what happens. For AM, I would operate under part 15.219 and try it during the day. An installation with a loading coil and ground radials might work.

As a Professor at the college I had no problem with the "bureaucracy" and I and a fellow faculty member easily obtained permission to install ham antennas including a beam on a rotor on a building roof. What I am suggesting is if you get a faculty member on board it could be much easier, especially if there is educational value offered. At my college there is a multimedia communications program and that's where I would start for a campus station.

Neil

Thanks for the input, Neil. I am a faculty member (in Radio & TV), but at my state-run school it appears there are a lot of administrative folks with kingdoms to protect. The "doing" of something requires knowledge not only of whom to approach, but which faction really has the power to act. The old joke goes: "Why is there so much in-fighting in academia?" "Because the stakes are so low." Not to tell tales out of school ("out of school"--that's funny!), but it took a month of gamesmanship between two administrative contingents to get an area's air conditioning repaired; it's all about whose budget will be affected and who gets to regally deign to fix something. Mind you, I really am NOT bitter; I have a blast teaching and am looking forward mightily to getting our peanut-whistle station going; I just know that "easily obtaining permission" in my setting can be a challenge.

Campus politics aside, I will take your and others' experiences into consideration as we go forward, and I appreciate the information.

73,
Bob

Radio Bob you have stirred up one of my favorite subjects in "Stories Out of School," a new series in which you have full credit for inspiring.

As an AV service provider during 40-years I have seen covert corporate and faculty wranglings of all kinds. I shall quickly describe one.

The recording staff, with me as technical support sub-contractor, were told that an outside firm had already been appointed to build an entirely new recording facility. We were all annoyed by the fact that we had not been asked to design and plan our own workspace, something we were fully qualified to do. The management decided to appease us with a friendly meeting so we could "get to know" the important studio genius from the east, who bragged about his private plane. We politely made comments about what was needed and everybody acted as though full accord had been reached. The next day I called an executive and asked for a copy of "the minutes of the meeting," so I could keep track of the happy results as they came, but was told "no minutes had been kept."

The new recording studio was built directly above a loading dock where, all day, a giant noisy door was raised and eighteen wheel trucks unloaded cargo. Recording had to be suspended during all the noise. There was no satisfaction in saying, "I told you so."

There was no proof of under the table payoffs.