FM Range

I know the fcc rule states a 200ft limit for fm but in some of the AM rules I also have seen a limit on feet. I say do it if they have a problem they will let you know.

If you were using the unit in a 100 story building on the 100th floor, it would be legal there as it would be from the first floor.

The height of the transmit antenna on Part 15 FM doesn't matter, but radiating the power that the EDM tx can produce will put you WAY over the peak FCC field allowed for Part 15 FM (no matter at what antenna height).

That peak FCC Part 15 FM field can be produced by about 11.4 nanowatts radiated from a 1/2-wave dipole, and not much more than that when using a short whip.
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So how do we arrive at something legal here without $50,000 worth of test equipment? Maybe do the math and put an attenuator on it? I have it on a 1/8 wave whip running through about 60 feet of old RG-8. It goes about 2-4 blocks. Regards,Lee

We know that AM transmitters can be synchronized. While I do not think FM can be synchronized ( due to the capture effect), this can work to your advantage.

To achieve really good range, one could ("simply") run a 1000 foot audio and power cable and every 250 feet have a FM transmitter connected to the wire. Due to the capture effect on FM, the problems of trying to sync FM should not be a problem as it is on FM.

Of course one can just have one FM transmitter on each end of their property, both connected by audio cable.

The problem I would worry about with putting transmitters for FM every 250 ft is that even if they were strictly legal power, there might be some areas where they'd overlap more than expected due to oddities of local terrain and the total field strength would still exceed what is allowed.

Besides, to cover a more or less linear distance of a mile would take about 11 transmitters? And that would only give a band of coverage about a mile long and 250 ft wide. Considering the audio to them would need to be synched very tightly, probably would have to use a cable that's nearly that long to feed them.

Not to say that it couldn't have it's uses. For example, along something like a nature trail to provide music and info. Or maybe at an outdooor event where a big PA would disturb too many people, have the audience bring portable FM recievers and headphones.

But for those sorts of applications the "leaky coax" part15 option might actually be more practical, since you'd be running some sort of cable in any case.

Actually using it for coverage of much of a city/town would probably run a fair bit of cash, considering the losses due to buildings, trees and so on.

Now so far as the FCC range estimations, a lot depends on the reciever and antenna you're being picked up on. The FCC's official range estimation on their site for CB radio (part95 aka 11meter) is only 1 to 5 miles. In practice, however, a *lot* of legal CB stations or mobiles with a properly tuned decent antenna regularly recieve and engage in 2 way communication with other legal stations over considerably greater distances. Type of antenna, location, how it's installed, weather conditions and etc all play a part in the actual range, even talking about just recieving.

I'm not sure about how the FCC does it's range calculations, but they do seem shorter than I'd expect for some services. As such at least for part 15 AM I'd say the 200 ft mentioned might be an expected minimum under adverse conditions or something.

Unfortunately, for FM there doesn't seem to be any answer that's definitive and not highly expensive for being sure the field strength is legal. Rich's oft-stated 11 nanowatts is no doubt an accurate alternate measure. I've seen enough of Rich's posts and charts to have a lot of faith in his accuracy on such matters. But it's similarly not very practical for a hobbyist with relatively primitive gear to measure.

So short of having an officially calibrated precision FSM, I haven't been able to figure out any better way than trying to stay in that 200 ft radius. Theoretically, yeah, if you had a good antenna with a good even radiation pattern and it was mounted high enough to be in the clear, I think legal range within the 250 microvolt at 3 meters limit would be possibly more than 200 ft. But it wouldn't be hugely better, like a mile or something. And it would take some expensive gear to be able to be sure you were staying in compliance.

One of the problems with what is actually best for range is it'd be an open field with no obstructions like buildings, trees and etc. What's the problem? Well, if there aren't any buildings and etc, there probably aren't many listeners, either.

Not to say there aren't plenty of uses for part15 FM. A "very near neighbors" station like mine, or maybe as the STL for an AM rig. But for "reaching the masses" by covering any sizable amount of terrain, I just don't think it's a practical option.

Daniel

Daniel gave a good overview of this topic.

The reality is that part 15 FM limits result in a typical range of 200 feet. This is not a rule. It is just a guideline for what to expect. It is also a guide, with no legal basis, with which a hobbyist who wants to approximate compliance can adjust their transmitter/antenna without the need for expensive and difficult field strength measurements.

If one checks the FCC field notices and citations one will see that the FCC is acting against many each week who exceed the part 15 limits on FM.

Be careful with FM broadcasting.

Neil

You can read those recent enforcement actions here:

www.fcc.gov/eb/FieldNotices

Certainly an interesting question.

On one hand Rich's post about 11 nano-watts being enough to exceed the limits is certainly verifiable by calculation, but doesn't include real world effects like internal losses, antenna matching and the effects of your physical location.

On the other hand I've run an EDMTX10 with a quickie dipole and no SWR tuning and the range I observed was closer to 400 feet on a good quality pocket radio and a few hundred yards on a car radio. That's more that 250 uV/Meter at 3 Meters. How much more I can't say, but certainly two or three times the limit.

Ramsey Kits seems to feel that a 2mW unit is a good compromise between the calculated 11 nano-watt transmitter guidelines and the real world outcomes of a hobby kit. Ramsey has been cited by the FCC a number of times, but never for these 2mW FM kits.

Perusing the FCC enforcement actions, ignoring the obviously outrageous pirate operations that are cited for levels like 95,000 mV/M at 150 Meters (MY GOD!!!), shows quite a few cites for power levels on the order of several thousand uV/m at various distances in the 100 to 200 Meter range.

Almost every enforcement action seems to start with a report or complaint being received, too, which indicates the FCC doesn't just go looking, someone needs to start the process.

I guess the questions you need to answer for yourself are, first, will my transmissions generate any complaints to the FCC? If not, well...

Next, if a complaint is received, will a field agent investigate? If the field agent investigates, will he or she observe field strength over 250 uV/M at 3 Meters (very likely yes, here)?

And now the award winning question - if you're over the limit, but not by thousands of uV/M, what will the agent do? Hard to imagine an agent just heading home and forgetting about it though. I mean, after they've gone to the trouble of going to your location and all, plus, they'll want to show responsiveness to whomever filed the original complaint.

So, what will happen? If the enforcement database is any indication, and absent super high power levels, obscene or seditious content, or interference with another service, the first notice you would receive could be a "CITATION." This seems to be a notice that a person or business is engaged in an activity that is generally permitted, but is being done improperly. The Iglesia de Dios Ebenezer church with the AM operation that had a 90 foot ground lead received a citation.

In the FM band, it looks like the first step would be a "NOTICE OF UNLICENSED OPERATION" or NOUO. An FCC version of "KNOCK IT OFF!" It doesn't look like any fun, but it appears that all you need to do is stop and be otherwise cooperative and that seems to be the end of the story.

If there were other issues regarding your transmissions it looks like the next step would be a "NOTICE OF VIOLATION" or NOV. This is not cool. I was the unhappy recipient of one of these babies because I was trying a leaky cable antenna with a transmitter that was way too powerful - 6 watts (I know, I know, but it looked like it might work...). I was also advertising my station in the newspaper - oops. With an NOV you've got 10 days to stop, plead your case, beg forgiveness and clearly demonstrate you will not do that again. I was forgiven.

Had I not responded quickly and appropriately, I would have received the same thing Howard Stern gets - a "NOTICE OF APPARENT LIABILITY FOR FORFEITURE" or NAL (shiver). This is heavy business, indeed. The NAL seems to say "You're busted, and we're going to fine you unless you talk us out of it, and by the way, you probably won't talk us out of it." Not good.

I doubt there is anything binding the FCC to this order of action, and it might be possible to get an NAL right out of the box, but the encforcement database doens't show any examples of that. My experience with enforcement was "firm but fair."

In short, it's all fun and games until you're holding a certified letter from the FCC with your name on it...

I'm not an FCC lawyer, and this ain't legal advice, by the way!

Experimental broadcasting for a better tomorrow!

SCWIS wrote: "On one hand Rich's post about 11 nano-watts being enough to exceed the limits is certainly verifiable by calculation, but doesn't include real world effects like internal losses, antenna matching and the effects of your physical location."
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To clarify, that ~ 11.4 nanowatts generates the peak legal FCC field in free space when actually radiated by a 1/2-wave dipole.

The issue here is knowing what percentage of the available transmitter power actually is radiated by the antenna. But a 1/2-wave dipole is a very efficient radiator, so much more than 11 nW will be radiated with the 10/25/35 mW "Part 15" FM transmitters typically used. Note that the radiation efficiency of a 1/2-wave dipole is independent of any reference to an external r-f ground connection, such as needed for a MW monopole used on Part 15 AM.

Once that FM field is launched and travels more than ? meters from the antenna, it is increasingly subject to the effects of the propagation environment -- which include obstruction loss (buildings, etc), and reflections. The reflections either can reinforce the signal, making it greater than its value over a free space path, or reduce it.

I'd guess that the FCC wouldn't get too excited if the Part 15 FM field they measured some hundreds of meters away was not more than 6 dB greater than the free space value generated by a legal field at that distance, as that amount of increased field might be produced in the presence of a 100% reflection of the legal field. But of course that would be up to the individual FCC inspector.
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To add to the puzzles and uncertainties, while most of the time the violations read like this:

"Your operation on frequency 95.9 MHz was measured at 5,552 microvolts per meter (uV/m) at 82 meters. This exceeds the allowable unlicensed limit of 250 uV/m at 3 meters established in Part 15. Thus, this station is operating in violation of 47 U.S.C. S 301."

every once in a while you'll see one that goes:

"Your operation on frequency 88.1 MHz was measured at 72,816 microvolts per meter (uV/m) at 3 meters. This exceeds the allowable unlicensed limit of 100 uV/m at 3 meters established in Part 15. Thus, this station is operating in violation of 47 U.S.C. S 301."

Now, admittedly the measurement in these cases was clearly very high (especially in the second case) and so maybe the question of that 150 uV/m difference is pretty academic. But for anyone actually trying to run legal, 100 is less than half of the 250 usually discussed as being the legal limit for the field strength, and I'd assume would be a difference in allowed FM range?

Daniel

SCWIS,

Thanks for the very well written discussion of this topic and for your "horror story".

Daniel,

I don't follow your point in the last paragraph of the post above. Remember that the rules do not address range, though field strength at a certain distance does manifest as range.

Compared to 250 uV/m at 3 m. , up or down by 100 uV/m at 3 m. can have a pretty big effect on the range. Is this what you meant?

Neil

What I was referring to Neil, is that second notice I pasted seems to say the allowed field strength is 100 uV/M at 3 M rather than 250.

Unless I'm reading it wrong or missing some key point of it..

Daniel

Hmmm, 88.1 MHz

It might be a typo. Every now and then you'll see an erratum note in the list, too, correcting things like that.

Several NOUOs contain this language

For example, Section 15.239 of the Commission's Rules permits
non-licensed operation of broadcast stations that transmit
on a frequency between 88 and 108 MHz to operate at a field
strength of 250 µV/m as measured at a distance of three
meters from the transmitting antenna.

88.1 might also be a protected freq. I don't have time to check that this moment, but some freqs have extra protection because of treaties with Canada and Mexico (I saw that in Part 73 once, I think) and that cite was in Texas.

Experimental broadcasting for a better tomorrow!

Daniel,

That explains that. I didn't pick up on that difference. I thought that there was a different limit for frequencies at the low end of the band, but I just checked 15.239 and didn't see it. Maybe it was dropped in the Sept. 05 revision. I could be wrong but I don't think it is worth pursuing.

Neil