One thing however, you'll note that the FCC documentation, both in the "FCC Seizure Complaint" posted elsewhere here, and in the manual posted recently for FCC agents documenting proceedure, in all cases where readings are taken to determine violations they use the margin of error in FAVOR of the subject. So that may give us even a bit more of a fudge factor in the violation department.
The Z-Tech R-506 is the most accurate device out there for field strength readings, so we'll see how it goes. I believe field strength readings will be as accurate as possible, and as accurate as the FCC uses. I don't know how they determine actual output power from a transmitter, and I don't think they really care. As it's the field strength that counts in the case of FM. You could put out a dozen watts, but with a mismatched antenna probably still come in legal. And for AM it's 100mw on the final input and antenna design restrictions, or field strength limitations depending on which rule you invoke.
As for measuring power output from the transmitters directly, at least we'll have a reference as to what they're putting out. And even with a +/- 5% or thereabouts error, if, say a transmitter shows 100 mw out, it would be between 90 and 105 mw and for our purposes that's good enough for me. When it comes to being legal it's field strength that matters.
Tim in Bovey
Tim in Bovey
" ... The Z-Tech R-506 is the most accurate device out there for field strength readings, ...
Possibly true.
But even if the R-506 was 100% accurate at measuring VHF field intensity existing beyond 3 meters from the transmit antenna, that field can contain reflections of the transmitted signal that result in a net field at that measuring location that extrapolate to a field not meeting FCC §15.239 for a distance of 3 meters in free space from the transmit antenna -- even though that transmit system might be operating in compliance with §15.239 when measured 3 meters away from it.
Conversely -- the field at such distant locations also might be reduced by signal reflections, extrapolating to a compliance with §15.239 when in fact, that system was non-compliant at a distance of 3 meters.
How the FCC applies/allows for this reality in their process of issuing Part 15 NOUOs has not been made public, AFAIK.
Very true indeed. I've pondered that myself, especially in cases where the FCC agent was quite a ways away from the transmitter. I would suspect that these variables could be used as a defense by a broadcaster. However, all of the NOUO's I've seen the broadcaster was grossly beyond the legal limit -- in the one case posted over 8,000 times the limit! It would be tough to blame that on reflections of a 250 Uv/m signal. My mission is to take readings at 3 meters in an open area not likely to be affected by any reflections, etc. I should be able to get a pretty good idea of legal, anyway. So far I've only got one transmitter coming, the Decade MS-10 (If I remember right) but the Wholehouse and Sainsonics are worth some experimenting as well I think.
Tim in Bovey
Here is an analysis of the fields existing at 3 meters and 30 meters from a Part 15 FM system in and near the horizontal plane, for the stated conditions:
I made an assumption of -5 dBi as "most" of the gossip dealing with 2 meter duckys tend to point that way. Then I just plugged that into the Longley-Rice plot. When I asked the software to report the 70/60 dBuV using the same parameters the pattern shrunk greatly.
Me being legally Blind what did you finally end up with as Range in the Simulation?
The range is out to 1 mile but very spotty. This assumes a decent car radio but at that range it is very unlikley that a clock radio could pick it up. The area is very hilly suprisingly for Florida. When I ran the plot for the 70 dBuV City Grade (clock radio), the coverage area was much smaller.