There are a few threads active here where operation on 87.9 MHz is mentioned. Rather than try to comment in each of these it seems best to start a new thread on this topic.
First, is operation on 87.9 MHz legal? Yes [EDIT Maybe not, See posts 5 and 9], according to Part 15.209 which states that the field strength limit is 100 uV/m at 3 meters, however if a frequency of 88.1 MHz up to 107.9 MHz (accounting for occupied bandwidth) is used then per Part 15.239 the field strength permitted is 250 uV/m at 3 meters. What this means is if a transmitter is operating legally from 88.1 to 107.9 MHz at 250 uV/m at 3 m. and it is switched to 87.9 MHz it is not legal unless the field strength is reduced. There is no mention at all about power in the pertinent rules so cites to transmitter power for FM are not meaningful.
Another item is references to AM transmitter output power. Under Part 15.219 AM band operation reference to power is for the DC input power to the final stage, not output power. Picky point, perhaps, but accurate.
Please keep in mind that this site wants to promote legal activities and talk about improbable range on FM, antenna changes with certified transmitters, and "secret adjustment pots" and such raises some questions about this. Experimentation and sharing of results is good and very helpful but we need to try to keep things within the realm of legal operation. One good example is a recent report of the field strength change resulting from adjusting the "secret pot" which showed that this can produce a field strength 23 times the U. S. limit. This should be interpreted as a caution about the resultant rules violation and not as a way to boost range. Such good information should be posted but it should not be misused.
Neil
It could prove why the Scoche transmits on 87.9 Mhz. And the C. Crane did transmit a field strength of 100 and something (I'd have to re read the strength) at 3 meters. This is another reason transmitters are doing the 87.9 Mhz now. That makes some sense. Well then guess I can use 87.9 as long as I don't go over the field strength. As for the other TX and adding to the antenna by long cable? No one knows what the actual field strength is for the regular just plug it in. I'm sure its like in the 100's not 250 uV/m @ 3 meters so even though you add the AUX cable its probably still below the max limit. Since 87.9 is so quiet it could explain the greater range because of the noise floor being lower. And the frequency is lower thus it takes less power to get the same range. So I'm still betting that the transmitter I was talking about and the extra cable still won't go over the limit. Thanks for the info on 87.9 Mhz.
I could not agree more with Neil's post above.
The adjustment control on the C.Crane, for example, is stashed inside for a reason. Clearly all the people who post on Youtube and write reviews on Amazon and such who talk about cranking this up for better sound quality and coverage are not people who have any clue that they are making their transmitters illegal. Clearly the transmitter I tested was putting out far less than the legal limit (although it did deliver the range promised which was 30 - 75 feet line of sight). Someone with the proper test equipment could indeed adjust this control for maximum legal output. The percentage of people equipped to do so is rather limited. My mission was to see if full legal output coule be obtained and rather than stand in the field turning the knob a titch, remounting the transmitter at height, testing, taking it down and doing it again until I reached full legal limit was clearly too time consuming. But simply turning it up all the way, we know that it can in fact reach the legal limit, and much more. I certainly do not condone using it over the legal limit. My mission entirely was to see if the transmitter was legal as arrived (it was) and what it's potential may be.
Bottom line to FM'ers is that if you're getting over 250-300 feet there is simply no way you're legal. I've now tested three certified transmimtters. Two of which had over the limit output as shipped, and even at this higher than legal output they didn't get to 300 feet in clear path line of sight transmission. And remember, I'm doing this range testing in a very small town in the boonies away from 90% of the noise, interference, and crowded bands in a major city, and I'm testing line of sight -- nothing at all between me and the transmitter.
ANY change to the antenna on a certified transmitter makes it illegal. Clipping a 6" piece of wire to the telescoping antenna cancels it's certification.
For me personally the concept of FM Part 15 "broadcasting" simply isn't viable. FM is best used as a way to get audio to various parts of your studios, etc and perhaps for in house or yard casting. I can guarantee you that any FM transmission going 300 feet or more would test way over the legal limit if I were to set up the FIM at 3 meters and test it.
Many are clearly hoping to do what is not possible within the laws.
TIB
Write this down. The parties responsible for equipment compliance are encouraged to employ the minimum field strength necessary for communications.
That is a direct quote from the FCC Rules Part 15.15 General Technical Requirements (c).
What it means is that although other parts of the rule give top end limits to radiated power, as does 15.239, you aren't required to stay at full legal power.
Here at KDX Worldround Radio we do our best to know where maximum legal power is and how to achieve it, but then we roll back the field strength to the minimum required to achieve our communications objective.
As far as 87.7 and 87.9, there are FCC Field Notices floating around referring to a WAIVER allowing 50 mW at these and other frequencies for WIRELESS MICROPHONES. And while I have pondered whether a Part 15 radio transmitter is, in some stretch of definition, a form of wireless microphone, it isn't. It could be, but it depends on how the equipment is used. A continuous fully produced broadcast is not something one does on a wireless microphone.
Every once in awhile I wonder how to say what Neil said above, but I don't want to come across like a Bible thumper pointing out the 10 Commandments. Neil did an excellent job reminding that this website is, after all the discussion, devoted to compliance with the Part 15 Rules.
It is not always possible to have the woman you want, the bank account you want nor the effective radiated power we fantasize about.
You can build an 11-foot tower, but you can only use 10-feet of it.
Under 15.209, the carrier should not be below 88MHz. See the ** note on https://www.law.cornell.edu/cfr/text/47/15.209 (You can go to the CFR and read the same content, but the cornell.edu site is much easier to navigate.)
So under 15.209, someone could use an 88MHz carrier and modulating it could put energy below 88MHz. But they could not use a carrier below 88MHz, (87.9MHz or 87.7MHz as examples). The (b) is saying if someone were using a carrier right at 88MHz, then their field strength would be limited to 100uv/m@3m rather than 150uv/m@3m because 88MHz is the band edge.
It would be possible for a manufacturer to design a transmitter that obeyed 15.239 power limits for 88.1-107.9MHz (250uv/m@3m), and obeyed 15.209 power limits for 88MHz (100uv/m@3m), and get that certified. (15.205 prevents operation above 108MHz.)
Nothing anyone says on an online forum will physically prevent someone from using a transmitter on 87.9MHz. If someone on a forum somewhere said it was legal to operate on 87.9 and an FCC field agent visited, what the poster on the internet said means nothing, all that matters is how the FCC field agent interprets the relevant rules. If you read NOUO’s for 87.9MHz, there is an incontinency in how the FCC handles them, so counting on any FCC agent to read the rules the same way you do may be a bad idea.
Also keep in mind if there is an IBOC station near you on 88.1, they are the licensed user of 87.9-88MHz (KBBG, KCEP, KCRY, KDHX, KHID, KKJZ, KLBR, KLFO, KMPQ, KMUE, KNSQ, KPGS, KQHR, KQOC, KRSD, KUHU, KUSW, KVSC, WCQS, WDIY, WDPR, WJSP-FM, WMAW-FM, WNCH, WUTC, WUWF, WVPE, WYPR, WZXM). If there is a real CH6 TV station near you, they are the licensed user of 82-88MHz. http://www.rabbitears.info/allocationmap.php?circoc=&ciradj=&channel=6&adj=N&all=Y
The text in this post is my opinion and how I read the relevant content in Part 15. I would never operate a transmitter on 87.7 or 87.9.
TiminBovey stated "Bottom line to FM'ers is that if you're getting over 250-300 feet there is simply no way you're legal."
Like the title of this thread, that statement needs to be clarified. It is true for a legal, Part 15 transmitter antenna within several meters of ground. The NEC 4.2 software, for a field strength of 250uv/m at 3 meters, gives a minimum field strength of under 1uv/m at 100 meters, not very useful. However, if that antenna is installed 10 meters above ground, then the field strength is between 6.5-10uv/m at 100 meters, varying with the elevation of the receiving antenna (again from the NEC 4.2 software). A signal with that field strength CAN be heard by a sensitive radio and well beyond that 100 meters.
I recognize that few are going to mount an antenna that high for logistical reasons, but we have to be careful when we make blanket statements that an FM signal is illegal if it gets more than x meters range. In this case, it is unlikely that you're going to get more than 200-300 feet range, but given the right conditions (transmitting antenna height, receivng antenna height, receiver sensitivity), it is possible. For something like 1/4 mile range (around 1300 feet), well, that's different, and you can comfortably say with 99.9% certainty that it is illegal (I always like to leave that 0.1% open because you really never can tell).
I've verified that an antenna mounted high up gets much greater range, not with test equipment or software, but with real world results. Not scientific at all, but still compelling nonetheless. Currently, my antenna is mounted about 1 meter high, within a window well. With a transmitter tuned to Canadian regulations, I get about 100 meters of clear signal line of sight, and maybe another 50-100 meters of noisy signal on a car radio (the difference mainly due to weather, believe it or not). With a transmitter mounted 10 meters high (well above a 3 story commercial building), and that building at the top of a hill, I was able to get up to 1000 meters on a car radio (with a weak and noisy but still listenable signal). Quite a difference. Same transmitter.
... So under 15.209, someone could use an 88MHz carrier and modulating it could put energy below 88MHz. ...
As noted, frequency-modulating an 88.0 MHz carrier frequency will produce sideband energy below 88.0 MHz.
However according to this text quoted below from the link below, no radiation of those lower-frequency FM sidebands is permitted for a legal Part 15 FM transmitter operating at a 88.0 MHz.
§15.239 Operation in the band 88–108 MHz.
(a) Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88–108 MHz.
The only practical solution for this is not to use a carrier frequency below 88.1 MHz.
Also to note, many newer FM radio receivers in the US don't tune to 88.0 MHz.
http://www.gpo.gov/fdsys/pkg/CFR-2011-title47-vol1/pdf/CFR-2011-title47-vol1-sec15-239.pdf
It's very, very good to see this site stand by legal operation on AM and FM. I just wanted to show some appreciation for Neil and the other moderators for keeping it in order.
But it's also good to see conversations and experimentation about those legal limits - having been a moderator, I know the difficulty in balancing order with creativity of thought and expression.
[Some content deleted because it may be inaccurate.]
[edit Well, things are getting sticky. The link in post 5 is 15.209 and if you follow the asterisk you get the comment "Except as provided in paragraph (g), fundamental emissions from intentional radiators operating under this section shall not be located in the frequency bands 54-72 MHz, 76-88 MHz..." Following to para (g) we find that operation with a carrier below 88 MHz is restricted to perimeter protection, periodic transmissions, and bio-telemetry. This would not include broadcasting below 88.1 MHz. See also post #7 and this thread http://www.part15.us/forum/part15-forums/general-discussion/part-15-fm-outside-88-108mhz-band. ]
... If you operate with 100 uV/m the 15.209 applies and you can use frequencies below 88 MHz. ...
For consideration - as I read §15.239 and §15.209:
- all r-f emissions permitted to "Part 15 FM" stations must lie within a 200 kHz r-f bandwidth contained wholly within the 88-108 MHz band, and
- the carrier frequency of an unlicensed FM station may not lie in the r-f spectrum 76-88 MHz (and other spectra), regardless of the field intensity radiated.
Taken together this doesn't appear to permit legal operation of unlicensed Part 15 FM setups with carrier frequencies below 88.1 MHz, or above 107.9 MHz.
IDK how these certified transmitters are able to include 87.9 Mhz (Scoche, the $7 transmitter on Ebay I think its iSound). Now as far as the new white space device rule as far as I can understand it was for wireless microphone and other white space devices. Now if you take that at face value and the fact that the FCC does not care about content but cares more about the RF energy I'd say its still legal for 87.9 Mhz. Even some Radio engineers on my Facebook who got there from the ALPB facebook page are using 87.9 Mhz on FM. Its sticky and there is a phrase about part 15 operators not causing interference to licensed stations either near or far. The only way for me to do this is 87.9 Mhz. Again this is why I'm so strong in the initial thumping the need for the FM initiative and the inclusion of making 87.9 legal in blinding light open face and up front close and personal. If the FCC does this it will clear up the need to do the only thing one can do to not cause harm to other stations on the dial and that would be to use 87.9 Mhz on FM. OK but wait i'm not done I have some more discussion to try and answer a point about.
Early myth was busted about FM only going 150 Feet Legally!!
Not to sound like a Pirate Radio thumper or devil's advocate but here it is again. We've seen how if you have a good car Radio or sensitive Radio and lets not forget how far above sea level you are when you transmit. I'm still not a believer that moisture and temperature don't change things for low power broadcasting. One more point that seems to go past everyone and that is the fact that the ground or shield can effect your range for FM as does for AM. And lets talk about that carrier current effect that my SainSonic AX-05B did exhibit on the -48dbm (legal power setting). Again for those who just tuned in at the -48 dbm setting my signal did not leave the confines of the house but if I got close to the power lines I heard my FM signal sometimes extremely clear. Go a few inches not feet away from the line and it was gone! I suspect that my audio cable from my computer was causing this effect. I had my Laptop plugged into a 3 prong power strip. My mini refrig was also less than 3 feet away from the FM Transmitter and it is made of Metal so is the FM TX. Now as we learned about the man who had his Talking house AM transmitter's wire antenna a few feet away from the power box and he was able to pick up his AM station all over the town so too I suspect that somehow an inductance effect was picking up my signal and carrying it along. Could that $7 transmitter do this being the shield is the antenna? Why certainly. What about if one had the cable a wave length away from a power cord? Well capacitance inductance could occur. Is it illegal? Well since the power line now is the antenna and the 250 uV/m law applies (from the antenna) you are allowed 3 meters from the antenna. If that power line just so happens to travel to the entire city and does not have a transformer to block it that entire line is your capacitance antenna. So is it possible to legally transmit without cranking up the power? I say yes it is. Doing a 468/87.9 you get 5.32 so at 5 feet away from the power lines you have your capacitance antenna for FM. But if that transmitter is injecting a signal into the ground of the power line? It could still carry. The lead from the power strip to the wall can indeed be part of the antenna on that little $7 transmitter. And we didn't even get to the fact this transmitter has a built in USB battery charger. Now since the shield is the antenna and if I needed a longer USB cord to reach from the USB port of my phone charger to the TX that could again make as an antenna. The legality could be questionable but again if an inspector were to come I'd admit that yes I need to have my cord reach the TX and since if I plug the TX directly into my computer it covers up the other jacks hence the need for the cable. This is actually a true statement. So now we have a transmitter injecting into the neutral side of the power line and the audio cable and the USB cable all having a shield and acting as an antenna. So now I see how this TX could reach .5 miles because we don't know how far the TX was from Winter4w's power cables. I'm sure there will be all sorts of no this and that and some say that carrier current is not likely to work for FM, but we have not ever tested the fact however I seen this with my own experience not a book, Internet forum this was my own experience. So I'm just stating what is possible based on a few factors. Could this explain how the transmitter in Frankinmuth, Mich was going ½ mile all day every day 365 and for years not weeks and no NOUO'S? I'd say its a possibility. And yes I heard that high pitched whine between songs as Carl mentioned. So again I'm sure it was not from an illegal TX but one that somehow was inducting into the power lines.
Moisture and temperature and wet earth acting as a conductor
On 2 meter Ham Radio with as little as 100 mW my friend N8DEF at the time has shown me how even that little power level could change during fog, temperature. At times he could hit a repeater at 50 mW that was 2 miles away but not all the time. So that in mind 2 meters is from 144-147 Mhz. 144-145 is usually SSB (yes 2 meters has side band activity). FM is lower so if this happens on chanels 2-6 and 2 meter ham it certainly can happen on FM giving a range for your TX that normally may travel a few feet. If you live near salt water as we know that makes for super conductors here for ground. Your little TX going 150 Ft on a normal day could follow the fog and moisture and travel ¼ to 1 mile maybe MORE. You see where I'm going with this. Without a FIM 71 you don't know what your TX will do till you have real world experience. And what works for one may not work for the other. I've seen the range for my AM and my FM (when I used it) TX change a few times. This is why its hard to swallow one size fits all for FM. And I've seen what would be innocent folks burned only because of these notions for FM. Hope this helps a little.
I'd like to see some numerical analysis that it's possible for a legal Part 15 FM signal to travel a mile under ANY conditions.
200 meters and maybe a bit more? Possible, under the conditions I described previously. Most likely, with the typical radio, a few hundred feet.
From what I understand, that whitespace document is just a proposal, and is not yet law. Plus, it's unclear whether Part 15 broadcasting would be considered a legal use. Just because 87.9 is clear doesn't mean you can legally use it, at least right now.
... I'd like to see some numerical analysis that it's possible for a legal Part 15 FM signal to travel a mile under ANY conditions. ...
The EM field of any transmit system will travel (propagate) an infinite distance away from the source.
However its field strength at a distant location may not be sufficient for useful reception by a receive system at that location.
In the near-ish future I'll look at this with NEC for a "typical" unlicensed system complying with §15.239, and post further if anything new is discovered with respect to my earlier posts on this topic.
Proposed FM Improvements to Part 15 Service (formerly FM Manifesto)
Given all developments up to this point here is what I believe in principle:
Foremost, and of absolute necessity, the FCC is hereby requested to provide the means accessible to the common person for determining compliance with the rule for intentional radiation on the FM band 88 to 108 MHz. As it presently stands, 15.239 is absolutely meaningless to the ordinary radio enthusiast and compliance cannot be reasonably verified. Given the discovery that certified FM transmitters fall either short of or significantly above permissable strength, the individual is placed in the untenablke position of either being deprived of the right to the full strength allowed, or in the disturbing position of possibly unwittingly violating the rule. Such an unobservable rule is useless and should be refined at the earliest possible time.
Further, whereas the extremely deficient signal strength allowed by 15.239 may be reasonable and suitable for congested population districts suffering from FM bloat, there are many more thousands of square miles within the United States within which the FM band is sufficiently unpopulated to allow a more generous und useful level of RF field strength. We therefore propose a tiered scheme, designed for self-application by Part 15 users within their given districts, with power levels as high as 1 Watt (debatable) in least populated areas.
In some locations the frequencies 87.7 and 88.9 MHz appear to be entirely unutilized and given the reception of those frequencies by average FM radios, we request they become availble in locations to be specified.
This 1st Draft Document is submitted for consideration by readers of this website and discussion at the ALPB Meeting of Nov. 7, 2015
Consider Rich's previous post where he states that the signal goes an infinite distance and what limits communication is the signal to noise level at the receiver.
With this in mind, it is not accurate to compare 144 - 148 MHz 2 meter ham experience to Part 15 FM systems because the bandwidth on 2 meters is narrow (5 kHz or so) and the bandwidth for FM broadcast is wide (150 kHz or so). This matters because the noise generated in a receiver is proportional to the square root of the bandwidth (among other things). A narrow bandwidth system will have less receiver noise and hence better range than a wide bandwidth system under similar conditions. The key to range is improve the signal to noise ratio at the receiver by increasing the signal or decreasing the noise or both.
Neil