question about Alf Lite 15.209 vs 15.219 vs 15.221

[pianoplayer88key]

So, if I understand correctly, 15.219 applies when you're using a 3 meter antenna and 100mW transmitter power, and no legal field strength limit. (btw, I mentioned the Alf Lite because I want to broadcast in full-bandwidth C-Quam AM.)

I'm not sure how the 15.209 rule or the 15.221 rule works, though. Most of the time that I would be operating under one of those rules would be under the following conditions:
in a building, or outdoors in a small area
an audience of up to about 300 to 500 people, all in close proximity. (the transmitter would be slightly off to one side of the crowd).
I would prefer to have a solid signal in the local area, like about 10mV/m or so (or whatever on an SRF-42 would have full receiver background noise/static quieting), where there might be people listening (in the LOCAL area). Most of them would probably be within about 100 to 150 feet of the transmitter.
Because of portablity requirements, I might not be able to hook up much of an antenna (like maybe a couple inches), and no ground, so transmitter power might have to be cranked up to several watts or more. (but, how do I figure out how to get the coverage I'm looking for, and stay legal? In these cases I have to go by a field strength limit because I wouldn't be complying with the antenna/power rule (the antenna would be super small so the power would be boosted to compensate.)

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Also, how do I make sure that any devices that may be interfering with my AM receivers (wherever I may be listening to whatever) comply with the following: for unintentional radiators, maximum field strength is 1 x 10^-600 picovolts/meter at 1 x 10^-1200 picometers from the exterior surface of the offending device between 500 and 1800 kHz? I'm getting tired of hearing the power lines and computers and other electronic devices when I use my Select-A-Tenna.

 

[tregonsee]

Section 15.209 is the general criterion for how much "oomph" your signal can have, on any frequency, in terms of microvolts (uV) of signal strength at a given distance from the radiating element or device.

As you indicated, Section 15.219 is one of the alternative criteria to Section 15.209, where you operate in the AM broadcast band and adhere to limits on antenna length and power input to the final RF amplifier instead of having to measure the actual signal strength. Tunable calibrated field strength meters are test-grade items and not cheap.

Section 15.221 is another alternative to 15.209, but it applies to "carrier current" systems, where RF is (intentionally) carried over the power lines or over a "leaky" coaxial cable, instead of radiated by an antenna. The idea is that the receiver is close enough to the conducting line to pick up the very low-level signal radiated by the line. The terms of this alternative are somewhat different for a college campus. Either way, you seem to be talking about antenna-radiated RF, so you go under either 15.209 or 15.219.

Referring to what I said about tunable calibrated FSMs above, unless you can find one cheap, or have a very accommodating and accordingly equipped friend, you should not be looking to vary your power or antenna length. You should have at least 7-foot ceilings in any building. I would have a second antenna, say about 6.5 feet high, with a "capacity hat" on top with a hat radial length of at least 6 inches and longer if possible (3 feet max). This maximizes your radiating surface while still giving you a shorter antenna. Outdoors, you can and should use all of those 3 meters you have.

I wouldn't even think about a "two-inch" antenna. It is hard enough to match impedances between the antenna and transmitter in the AM band with a 3-meter antenna. You can't push the power unless you can match the impedances, and matching the impedances doesn't do much if your match unit is chewing up most of the power you push. Building a matching unit (coil and/or capacitor) to match an antenna much shorter than 6 feet might well be more difficult than working with an antenna closer to the limit.

If your listeners will be within 300-400 feet of the transmitter, how about using a Part 15 FM transmitter? These come with the antenna attached (18-24 inches and usually collapsible), as the manufacturers have to make a unit to the 15.239 signal strength specs and then stamp out copies. A lot easier for a portable-type setup, and almost all of these units are stereo, plus there are many Walkman-type FM receivers.

As far as AM interference, I can't say much except to suggest that you go to www.part15.us, then left-click on the Library link on the left side, then left-click on "Receiving Part 15 Transmitters" on the page that comes up. There are other resources but not at my fingertips now...