http://transition.fcc.gov/eb/FieldNotices/2003/DOC-308327A1.html
This NOUO from July states that "the field strength of the signal on frequency
1650 kHz was measured at 550 microvolts per meter (uV/m ) at 353 meters,
which exceeded the maximum permitted level of 14.5 uV/m (24,000/1650) at
30 meters established in section 15.209(a) of the Rules (See 47 C.F.R.
S:15.209(a))"
550 microvolts per meter at 1158 ft (353 meters) sounds like the threshold for scrutiny just dropped significantly. It seems like we are getting into areas where a ground mounted single transmitter's signal strength might possibly fall. This is hardly what I would consider a killer signal. (Anyone with a NEC program?)
It later states that the operator was using "five antennas mounted on the roof, each with a vertical whip approximately 3 meters long, and the ground leads totaled over 15 meters in length." Does this mean each antenna was 3 meters long, but the five ground leads were each 3 meters long? Could this have been five Rangemasters sync'ed together? I would think that sort of installation would have provided a higher signal strength than what was stated above. I suppose the determining factor is the 3 meter (or 15 meter) ground length, either way over the 15.219(b)limit.
More questions than answers are provided by this NOUO.
"Does this mean each antenna was 3 meters long, but the five ground leads were each 3 meters long?"
Since it specifically states:"five antennas mounted on the roof, each with a vertical whip approximately 3 meters long", I read that as what it says, the whips were 3 meters each.
"but the five ground leads were each 3 meters long?"
It does not say the ground leads were 3 meters each. They could have been a set where 5 were a meter each, all connected to a final 10 meter lead that actually went to ground, or something like that, which would total 15 meters.
It doesn't say there were 5 transmitters and whips, but it does look like they might be a set of 5 Rangemasters ... although I think that even if they were sync'd they would tend to cancel each other out in that close proximity.
Awhile back there was an inquiry here from someone who wanted to mount 4 Rangemasters on a roof. That was a couple years or so ago. I'll try to find it later if I have time.
This July NOUO is closer to the limits than the previous NOUO in this thread. The 550 microvolts per meter (uV/m) at 353 meters measurement is very close to what would be expected from a legal ground-mounted part 15.219 transmitter.
I did a 4NEC2 (NEC-2D) run for a ground-mounted 3 meter antenna to get a base line for a legal Part 15.219 system.
Parameters:
Perfect Ground
3 meter antenna mounted at ground level
Transmitter coil resistance: 20 ohms
Ground resistance: 10 ohms
RF input power to coil: .075W (corresponds to 75% eff. into coil)
Field strength at distance = 360 meters: 603 uV/m
My conclusion:
It is clear from the two NOUOs referenced in this thread that the FCC is leaning toward a valid field-strength based approach, in addition to citing violations of the 3 meter rule, to bust part 15.219 operators rather than just simply citing a violation of the 3 meter rule.
The elaborate 5-antenna roof installation this operator used wasn't very effective. He got about the same signal that a good ground-mounted system would produce. With all those visible antennas, he was just asking for a 15.219 violation, regardless of the measured field strength.
Of course, we can only wonder why someone complained to the FCC. The property belongs to an insurance company. That leads me to believe they were advertising their services. I wouldn't think that would be a threat to any local licensed broadcasters, unless they are pathologically paranoid about losing a few bucks of advertising revenue.
"That leads me to believe they were advertising their services. I wouldn't think that would be a threat to any local licensed broadcasters, unless they are pathologically paranoid about losing a few bucks of advertising revenue."
That makes more sense and adds even more weight to the whole thing being just some move by another station. If it indeed was exactly that, then that could mean businesses liberty to do their own advertising, even under legal situations such as setting up a Part 15 system, is threatened.
And what will that mean for Part 15 broadcasters in general when the rules do not prohibit a Part 15 AM station (209 or 221) from selling commercial airtime for revenue?
Yep..this one will be very interesting to see develop.
RFB
550 uV/m at 353 m is equivalent to 6472 uV/m at 30 m. This corresponds to 0.42 mW of radiated power from an electrically short vertical monopole over ground, which might be possible with a non-elevated antenna if an efficient antenna system is used. I don't know of any presently-existing unmodified commercial Part 15 AM transmitter has 75% efficiency into the loading coil. Neil, radio8z, modified a Ramsey that can perhaps do that.
Here is the relevant information copied from the model output file. Results from the model are in ballpark agreement with your calculations.
INPUT POWER = 7.5350E-02 WATTS
RADIATED POWER= 2.6369E-04 WATTS
STRUCTURE LOSS= 7.5087E-02 WATTS
NETWORK LOSS = 0.0000E+00 WATTS
EFFICIENCY = 0.35 PERCENT
- - - NEAR ELECTRIC FIELDS - - -
- LOCATION -
X VOLTS/M DEGREES
30.0000 7.1686E-03 123.29
360.0000 6.0289E-04 -66.43
Regarding transmitter efficiency, I am referring to the "RF transistor efficiency". Transistor efficiency is a measure of how much of the 100mW input power is dissipated in the transistor as heat. A good Class C design can achieve 75% transistor efficiency, but Class C mode doesn't really exist in any part 15 transmitters that I know about. An efficient Class C design requires a carefully controlled current waveform to the transistor base with a duty cycle such that the transistor will be ON for only about 30% of the time. Most part 15 transmitters that look like they might be Class C actually hammer the transistor base with a high current square wave at 50% duty cycle. This can actually produce good efficiency (about 80%). It ends up being more like a sloppy Class E mode.
A well designed and properly tuned Class E circuit using a low on-resistance MOSFET can easily produce 98% transistor efficiency at MW frequencies.
The NEC model doesn't include the transistor. It simply uses an "input power" source applied to the antenna load. In real life this power source comes from the RF transistor output, after the RF transistor efficiency reduces the 100mW power from the power supply.
The 0.35% EFFICIENCY and 263.69 uW RADIATED POWER reported above by NEC represents the overall performance of the antenna/ground structure. Most of the power loss is in the coil resistance (20 ohms) and the ground resistance (10 ohms). The approximately 75mW input power was a value I entered as input data based on a 100mW transmitter operating at 75% transistor efficiency. This can be easily changed to any value if the actual transistor efficiency is known for a given circuit.
Perhaps you can tell us the measured efficiency of the AMT 3000, which is not a Class C design, before the loading coil.
The AMT 3000 uses a tuning circuit that uses a combined tuning inductor for a capacitive input L network, and the loading coil, in series (but you know that, since you are the designer). The composite inductor would be replaced for efficiency testing purposes by an inductor that has the few uH of only the L-network portion of the tuning inductor. The dummy load for an efficiency test would have the total supposed resistance of the loading coil loss resistance and ground loss resistance (30 ohms in the example given by you above; 27 to 33 ohms is OK). I'm pretty sure that an accurate measurement would give a calculated efficiency significantly less than 75%, but I leave that up to you. If you actually do the test, you would be the first Part 15 AM transmitter manufacturer to report actual measured output stage efficiency results (sans loading coil).