Err...that is Rich is a resource. He used to have his hand on the throttle of a 50 kW blowtorch AM station and he has knowledge and experience with antenna applications for commercial broadcasters. He is a lifetime Society of Broadcast Engineers member.
A proposal to the FCC will need to address political as well as technical matters. Rich, could you give this some thought and maybe educate us as to what might be reasonable from a technical perspective. I don't know where to start, but it would seem that it could be based on the desired range of an FM signal and build from that.
Neil
At one time the FCC considered allowing LPFM stations with radiated powers of 1 to 10 watts, giving a useful coverage radius of about 1.5 miles. But then in their 6th Report and Order ( https://www.fcc.gov/document/lpfm-fifth-order-reconsideration-and-sixth-report-and-order) they eliminated that class per their reasoning on pp 76-77 of that document. Here is a quote of that section:
A number of LPFM proponents urge us to retain the LP10 class of service, arguing that it
is needed to ensure that LPFM opportunities are available in urban areas. 517 Other commenters advocate eliminating the LP10 class. 518 They point out that, from an engineering standpoint, the LP10 class is spectrally inefficient. 519 We agree that the existing LP10 class is an inefficient utilization of spectrum. LP10 stations offer more limited service but are more susceptible to interference than LP100 stations. Given the increasingly crowded nature of the FM band, we find it appropriate to take this into account. 520 We also are concerned that the reach of LP10 stations would be too small for the stations to be economically viable. As the Media Bureau recently noted, even higher-powered LP100 stations have small service areas and are constrained in “their ability to gain listeners” and “appeal to potential underwriters.” 521 Because we find that licensing LP10 stations would be an inefficient use of available spectrum and are concerned that LP10 stations would have an even higher failure rate than LP100 stations, we eliminate the LP10 station class.
From what I'm reading of the views of posters on Part 15 websites, it appears that this coverage radius of about 1.5 miles is approximately what they want from a Part 15 FM setup. But it seems unlikely that the FCC would permit that for unlicensed FM transmit systems when they didn't permit that for licensed stations -- where they would have control of the frequencies and powers they permitted.
The FCC considers all uses of the FM spectrum in its actions, and the potential for mutual interference among them. Whether or not such spectrum use is licensed makes no difference to the interference such setups can produce.
Maybe ALPB members and/or "FM Manifesto" supporters could discuss this, to see if it gives them perspective on their best course of action.
My title is a twist of the expression "grass roots" which is often used to describe an effort by ordinary people to move the government.
We appreciate the exhibit from member Rich on recent FCC discussion on management of the FM band.
Only one line of Rich's text doesn't fit the "FM Initiative" and its companion "FM Manifesto":
"...where they would have control of the frequencies and powers they permitted."
We are not seeking control of the frequencies we use. We accept the current FCC allowance that we are obligated to search for and use the most open and non-interfering frequencies available, if any. And we are not seeking powers of the range 1 to 10 Watts, therefore our interest lies in a much smaller power range than the Report and Order spoke about.
Except for those differences the 6th Report and Order and Rich's combined comments appear to address the same concerns we are presently discussing.
It seems to me that we need to establish some idea of the technical requirements, ignore legal for the moment, which are realistic and needed to give a certain range desired. Rich's post suggests that a radiated power of 1 to 10 Watts would be a starting point to achieve a 1.5 mile range.
One problem we face is that when we talk about range we talk in terms of using a car radio as our test receiver. This is probably the optimum situation and gives an optimistic maximum range estimate, but if the goal is to reach listeners who are not mobile then account need be made of the typical in home receiving system which likely gives much less range.
Amateur radio operators achieve their range on VHF by using good antennas and sensitive receivers, not necessarily by using high power transmitters and gain antennas. Running my 4 Watt 2 meter transmitter into a roof mounted antenna I can operate through a repeater 25 miles away and with 25 Watts mobile the repeater range is up to 70 miles. This is because the repeater receive antenna is usually mounted on a high structure and a very sensitive receiver is used. Such will not be the case for the typical FM listener's equipment. The difference needs to be accounted by increasing the radiated power and this is why I don't believe we are talking about hundreds of milliwatts to get out a mile or so.
Neil
Up to this point the members of the FM Initiative here and at the ALPB have been uniform in our agreement that upgrade to the Canadian BETS level would be satisfactory. Not even a milliWatt, I'm guessing.
However I have proposed in my FM Manifesto that the FCC provide a measurement method actually accessible to part 15 hobbyists. Rounding things off to an allowed power of 1 milliWatt would be convenient because it could be measured for less than $20,000.
The FCC is interested in limiting the possibility of interference to licensed FM stations by unlicensed ones. Very likely this is the reason they use field intensity as the "yardstick" in §15.239.
Knowing the exact output power from an FM transmitter does not translate directly to the field intensity that power can produce at any given receive location. That also depends on the transmit antenna system used, the heights and polarizations of the transmit and receive antennas, path length, obstructions/reflections on and near the propagation path, earth conductivity, and frequency.
And as Tim's measurements show, the radiation characteristics and fields from FM transmitters certified as meeting §15.239 are hugely increased just by plugging an audio cable into that transmitter.
This definitely is a case where a simple question cannot produce a simple answer, if accuracy is needed.
This may seem a little strange, since I was so positive about increasing FM range, however that was done. However, I'm wondering if more energy should perhaps be put into what CAN be done with Part 15, rather than what can't.
There are other scenarios to increasing the reach of a Part 15 station other than increasing power (or field strength). One such way would be multiple transmitters, spread over a geographic area (realistically AM, although FM would still be possible for a smaller area). Another is streaming, combined with an aggressive campaign to get Internet Radios (or some other means of playing the stream) into local businesses.
I suspect that attempting to change the Part 15 rules would be very difficult. And time consuming. And political. Whereas, these kinds of options just have technical problems (some admittedly significant) such as how you distribute the signals and synchronization all the way through to marketing and copyright licensing concerns. And maybe financial issues.
I just wonder if it's worth it beating your head against the FCC brick wall, when the relatively few resources could be better spent in other areas.
Here is what we do now based on Tim's tests:
1. Out of the box the Whole House FM Transmitter 3.0 puts out an RF Power of 2 mW into a rubber duck and goes around 300 Ft to a portable M Radio. To double this range you would have to square root the 2mW (which is 4mW) this will get you 600 Ft. Now for 900 fT you need to squuare root the 4mW which is now 16 mW. So for 1,200 Ft to a portable Radio you need 32 mW to do this into a rubber duck antenna similar to the original antenna on the Whole House FM Transmitter 3.0. If the SainSonic was truly putting out 500 mW and all the energy was not wasted on spurs and harmonics at 500 mW you would have to keep square rooting the amounts of power till you get to 500 mW and that will tell you what you should expect from 500 mW. So 1.500 Ft is 32 mW, 1,800 Ft is 64 mW and 1,100 Ft is 128 mW. So if you want over 1,000 Ft it will take you 128 mW into a rubber duck. Asking for 100 mW on FM and maybe 87.9 Mhz for areas with no channel 6 should be allowed. So knowing about the 2mW output for the Whole House FM Transmitter 3.0 was a key to asking for mW. 100 mW would get you some nice range if that transmitter was clean and its energy went where it was supposed to go.
In Reply 23 above, Thelegacy posted (in part)...
Out of the box the Whole House FM Transmitter 3.0 puts out an RF Power of 2 mW into a rubber duck and goes around 300 Ft to a portable M Radio.
What verifiable proof exists for the values reported by that statement?
To double this range you would have to square root the 2mW (which is 4mW) ... etc
In any case, the square root of 2 (mW) is about 1.414 (mW), not 4 mW.
The same relations apply to the other changes you stated in Reply 23 above.
Since 1,000 mW equals a watt we should then ask for 1 Watt. As far as Verified proof check out teh range reports per Amazon.com search for Whole House FM Transmitter 3.0. Tim also said he was able to hear the transmitter 300 Ft on a portable Radio. So knowing this we need 1 Watt to get close to square rooting the power just to get 600 Ft.
Thelegacy posted: we need 1 Watt to get close to square rooting the power just to get 600 Ft.
Kindly note that "square rooting" radiated power does NOT increase radiated fields at a given receive site.
It decreases them.
One of the things that Rich has shown us with some of the antenna analysis he's done is that predicting the range of an FM transmitter solely based on power cannot easily be done. It's not simply just linear.
Range (or field strength) for a given power level depends on a huge number of factors, the most important being transmitter antenna height (and the type of antenna), receiver antenna height (and the type of antenna), obstructions between the two antennas, and of course, distance.
And then you have the disparity in receivers. Most people won't use car radios to listen, and there is a large difference in the sensitivity and selectivity of modern portable and handheld receivers.
So, if you have a lousy portable radio and a transmitter with a lousy antenna (i.e., a rubber duck) at ground level getting a range of, say 600 feet, that range could be many times more that with a transmitter mounted at 10 meters, a good antenna and a much better radio. Milliwatts with a 1/4 wave vertical, elevated, can get you several miles of range under good conditions (I know, I've experimented).
Johny C in New Zealand stated in one of the ALPB meetings that line of sight, his 1 watt in New Zealand could probably get 20 km to a good car radio. The FCC isn't going to look at that 600 feet - they're going to look at the maximum range you could get, as that will dictate potential interference.
And that's also the danger with specifying the rules as output power. Field strength maximums put hard, measureable (admittedly with expensive equipment) range limits on your signal. With output power, not so much.
Artisan has just described the dilemma faced by serious Part 15 FM operators.
Earlier Artisan stated the obvious common sense that may spell the end of the FM Initiative, namely that moving the FCC to grant our wishes is probably a lost cause.
But I think we should hold tight for Tim in Bovey's next round of tests, especially with the C.Crane FM2. If it comes out compliant, we will have a handle to hold.
If one of us invented an accurate inexpensive field strength meter we would become celebrated radio heroes.
We already know what 1,000 uV/M @ 3 meters or 100 uV/m @ 30 meters will do as this is what is allowed in Canada so that should be what we stride towards. One reason is that the Radio's for home use are starting to be crude and can't receive as far as they use to and it should be reason to raise the limits in the first place. I do realize the FCC is going to look at a car Radio in their judgment for rather or not the limits will be raised. As more tests are done via Time we'll know what we're dealing with. I'm hoping that the C. Crane is under the legal limits and when you adjust the secret power control to max its legal and at least goes 300 Ft to a portable Radio and 800+ to a Car Radio. Either way I'm in need of a TX that went as far as my SainSonic but without the spatter and harmonics that are caused by not having a good low pass or band pass filter.
Here is a way to sort of make a poor man's FIM: Its an idea but don't know if it will work. Take a 2 meter Ham Radio field strength meter or one that goes from 54-148 Mhz or higher. Once we have a Well known Legal FM Transmitter like a C. Crane FM2 and it did exactly 250 uV/M @ 3 meters, here is what we could do: These meters I'm talking about are the ones for testing the field of an antenna. Once we have a 250 uV/M transmitter by the Potomac FIM 71 we simply pad the simple cheap field meter to equal the same readings as the FIM. It may not be exact, but what you could do since often these antenna meters won't receive past a few inches we simply make the 250 uV/M @ 3 meters read on our cheap meter 250 at 3 inches. So if your over 250 @ 3 inches you'll know your over the limit. Again its crude and probably can't be used at 10 feet from the transmitter but would be a close estimate as to what the strength is and would be something a hobbyist could build and sell as a close guess. At least it would be something in a closer ball park to use to make a transmitter compliant and won't cost anything close to a grand.
Rich Quoted and said:
To double this range you would have to square root the 2mW (which is 4mW) ... etc
In any case, the square root of 2 (mW) is about 1.414 (mW), not 4 mW.
I said: I think he should have said squared.