Part 15 FM...

[edarmsttrong]

Just curious what, if any, antenna restrictions there are for Part 15 FM transmitters. I understand the 250 uV/m @ 3 meters part, but is there any restriction on height or length of the antenna? Does coax count as part of the overall antenna length? I have a plan for a distributed system of many Part 15 FM transmitters, kind of like what some folks are doing with Part 15 AM, but the whole thing is contingent on the technical rules.

Thanks for the help!

 

[Nostalgia]

Yes, unfortunately, there are restrictions on the antenna. Essentially it says there should be a integral whip antenna that should only be replaced with an exact replacement should it break. My FM100B works all up and down my block with the supplied antenna. That said, most transmitters also offer an external antenna connector which is supposed to be so unique that a typical consumer could not find one at the Shack. Mine as a BNC!! A type N female would have better met the intent of the rule, and easier for me to hook up my 1/2" heliax should I get brave enough to put an external antenna outside. A friend has done just that and covers several blocks of his neighborhood with his Comet 5/8th wave ground plane. Even though we live in a metro area we have a couple of good size gaps that local part 15 and LPFM stations have jumped on. Only the tropo kills us from time to time,

I read all the posts and it is clear that most part 15'ers want distance vs quality which is why most of the distributed systems are AM and not FM. Check the Ramsey website. Under their FAQs they do have a link to some good part 15 rules and regs and a fact sheet on the FCC website. Good luck with your project.

 

[Neil E.]

If the transmitter is FCC certified then there are restrictions on the antenna used including the unique connector rule as was mentioned above and you are usually required to use the antenna supplied by the manufacturer and are not permitted to modify the antenna or transmitter.

But if it is a non certified kit such as the Ramsey series there are no such restrictions. There are no restrictions on the antenna, transmission line, or ground length such as we find with part 15 AM.

The restriction for part 15 FM is in terms of field strength at a distance (250uV/m at 3 m). This effectively restricts the system to producing this field as a maximum in any direction. Since the radiated field depends greatly on the antenna this amounts to a practical restriction on the antenna. For example, a transmitter using a whip may be in compliance but if it is driving a beam or Yagi antenna it can very well be out of compliance since these antennas have gain. There is no restriction for using a beam or Yagi physically but the resultant field strength due to the antenna gain is the problem.

There is no advantage to be had by using a directional gain antenna since one can produce the same maximum compliant field with an omnidirectional whip and actually have a larger coverage area. The key is the FCC field strength limit applies to the maximum permitted field in any direction. A directional antenna just puts more energy into one direction than another yet the energy in the preferred direction still has to meet the field strength limits so there is no advantage in signal strength in this direction over using a simple whip.

 

[LibertyNT]

Maybe One Day part 15 FM will be loosened up a bit.

 

[edarmsttrong]

Maybe One Day part 15 FM will be loosened up a bit.

Maybe, but I don't want to wait until I'm 70 years old...

So, if I were to build a kit (and I believe I can legally build 5 such kits), and feed the RF from each of them into a J-Pole antenna on the roof of a couple of 10 story buildings or even some mountains around a valley, I should get decent coverage. I'm guessing I could cover a small-market town, with a few well placed transmitters, although I would not expect the signal to penetrate some of the buildings. I'm not even gonna get into mutlipath and phase relationships, at this point just yet. The 250 uV/m @ three meters applies to the field strength off of the antenna at three meters distance (ERP), if I'm not mistaken. What I'm curious to know, is how height affects this limitation. If I'm not mistaken, the higher the antenna is, the more coverage area one can achieve. I know physics will actually determine the coverage area, but I'm thinking in legal terms, at this point.

I'm not expecting a flame-throwing signal, but enough of one to offer a choice or alternative to the stations in my area that are all owned by one company.

 

[stormy01]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

 

[Nostalgia]

I think everybody is reading a little too much into the "home built" provision of part 15. "Home-Built Transmitters that are Not for Sale: Hobbyists, inventors and other parties that design and build Part 15 transmitters with no intention of ever marketing them may construct and operate up to five such transmitters for their own personal use without having to obtain FCC equipment authorization. If possible, these transmitters should be tested for compliance with the Commission's rules." I read this to mean ham radio style home brew radios, not simply putting together commercially produced kits. I welcome a discussion on this but I think I am right on this one. Also, even though you could built 5 of these things you still need to be in compliance with part 15 even if you dont get them certified.

And as far as the antennas go: "each Part 15 transmitter must be designed to ensure that no type of antenna can be used with it other than the one used to demonstrate compliance with the technical standards. This means that Part 15
transmitters must have permanently attached antennas, or detachable antennas with unique connectors." I do not see where it specifies either AM or FM transmitters, it is for both. Regardless, if the FM transmitter was certified with a giant 20db gain 6 bay antenna, power would have to be further reduced to still meet the 250 uv/meter @ 3 meters rule.

The rules are the rules, anyone who feels their interpretation of them is correct must live with the consequences should they be proven wrong. I do not believe the FCC is spending much time searching for part 15 transmitters or even pirates who are blasting more power. But the more power you have, or more coverage you achieved with multiple transmitters and outside antennas, the more chance you have a drawing a complaint if you interfere with someone trying to hear the Prairie Home Companion on some distant PBS station. Thats when you make the evening news!

 

[edarmsttrong]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

I thought I read somewhere that some Ham operators put a two-meter transceiver in a high-altitude balloon, and were able to work most of the US on a few milliwatts...kinda cool! ;D

 

[stormy01]

That's true, but...ham operators have very sensitive receivers, something like -160dBm or along those lines. That's more than 100 times the sensitivity of the best FM tuners out there for consumer use. Amateur Radio/Hams also use narrowband FM which is much more efficient than wideband FM, or CW, or perhaps a digital mode and they use really sharp IF circuits (meaning the bandpass is only on the order of a few kHz at most), and they are used to listening for signals right above the atmospheric white noise - the 'rushing' sound you hear on a frequency that has no signal on it.

I don't suppose your average Joe or Sally has the patience to listen to that- only DXers would put up with barely receivable signals. I would add that when the signal runs out, it runs out, no height advantage will get the signal out any further. The 500 foot distance I gave earlier is just that, an example, in round numbers. When the signal fizzles out at say, 2 miles, it's no longer there, and no amount of amplification or selectivity will cause a signal to be audible that does not exist at the 'front end' or tuner of a receiver!

I am not trying to discourage anyone from experimentation - please regard the FCC regulations. I am not fond of these rules either, but rules they are. Perhaps work with organizations that have been petitioning the FCC for more power, antenna length, etc. would be the way to go in the long term, but have fun with what we've been given to work with for the time being.

 

[LibertyNT]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

FM Doesnt Work That Way Stormy.
Line Of Sight Remember. Putting it up higher would only
increase its range. a Transmitter normally only going 500 FT and put it on
a 1000 ft pole you would get the station very clear for maybe a mile or so.

Its Amazing really.

 

[radioman148]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

FM Doesnt Work That Way Stormy.
Line Of Sight Remember. Putting it up higher would only
increase its range. a Transmitter normally only going 500 FT and put it on
a 1000 ft pole you would get the station very clear for maybe a mile or so.

Its Amazing really.

So in other words if I connect a part 15 transmitter to an FM antenna at 60 ft I'll get out at least several blocks.

 

[rfry]

So in other words if I connect a part 15 transmitter to an FM antenna at 60 ft I'll get out at least several blocks.

The link below leads to a chart showing signal strength vs. distance for an FCC-compliant Part 15 system (the red trace) and several other systems. These are based on a free-space, unobstructed path.

The power radiated by a 1/2-wave dipole to produce the maximum field allowed by Part 15 FM is very small: 0.000 000 01143 watts, or 11.43 nanowatts. Most "Part 15" FM transmitters produce thousands of times this amount of power.

The signal from a compliant system drops below 10 microvolts/meter at a distance of around 60 meters, or ~ 200 feet.

So legally serving a radius of several blocks is unlikely, regardless of the height of the transmit antenna (unfortunately).

http://i62.photobucket.com/albums/h85/rfry-100/6417f684.gif

//

 

[radioman148]

So in other words if I connect a part 15 transmitter to an FM antenna at 60 ft I'll get out at least several blocks.

The link below leads to a chart showing signal strength vs. distance for an FCC-compliant Part 15 system (the red trace) and several other systems. These are based on a free-space, unobstructed path.

The power radiated by a 1/2-wave dipole to produce the maximum field allowed by Part 15 FM is very small: 0.000 000 01143 watts, or 11.43 nanowatts. Most "Part 15" FM transmitters produce thousands of times this amount of power.

The signal from a compliant system drops below 10 microvolts/meter at a distance of around 60 meters, or ~ 200 feet.

So legally serving a radius of several blocks is unlikely, regardless of the height of the transmit antenna (unfortunately).

http://i62.photobucket.com/albums/h85/rfry-100/6417f684.gif

//

That's kind of what I figured. Thanks for the confirmation.

 

[stormy01]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

FM Doesnt Work That Way Stormy.
Line Of Sight Remember. Putting it up higher would only
increase its range. a Transmitter normally only going 500 FT and put it on
a 1000 ft pole you would get the station very clear for maybe a mile or so.

Its Amazing really.

I believe that almost no one is really reading what I am really saying here! (Many of you have a pipe dream that your neighborhood is gonna discover your little 25mW FM transmitter and listen on in - yeah, if you tell them it's there on the dial).

Again, even if the signal completely disappears at 500 feet and its Light Of Sight at 500', someone like Richard Fry, please explain to us how on earth you're gonna a hear station more than 10 times the distance away at about a mile (5283') also LOS when it was already completely gone [not receivable by the radio tuner] at 500'...something is wrong here...either you have more than the legal power for a Part 15 FM device and/or your antenna has gain which produces a signal whose field strength greater than the legal limit!

If you can't hear a signal that no longer exists at a lesser distance, then in turn at a greater distance given
that there were no obstructions at either distance,you are not going to be able to hear the signal either!

You can't get a signal out of nothing! It has to be there!

 

[rfry]

(Many of you have a pipe dream that your neighborhood is gonna discover your little 25mW FM transmitter and listen on in - yeah, if you tell them it's there on the dial).

However that 25 mW FM transmitter with even a very simple whip antenna can produce fields far in excess of those allowed by Part 15 FM. Many of the NOUOs (violations) listed on the FCC's website refer to measured fields produced by radiated powers of about this value.

For example, the 25 mW curve in the chart I posted shows about 700 microvolts/meter at 1.61 km (one mile). That is a fairly strong signal that could be listened to on almost any FM radio, including a Walkman. But it is also illegal at that distance for a Part 15 FM station.

Again, even if the signal completely disappears at 500 feet and its Light Of Sight at 500', someone like Richard Fry, please explain to us how on earth you're gonna a hear station more than 10 times the distance away at about a mile (5283') also LOS when it was already completely gone [not receivable by the radio tuner] at 500'

When LOS propagation loss causes the signal to fall below the acceptable noise threshold of a given receiving system then moving it further from the transmitter does not improve the signal - it is reduced even further.

//

 

[stormy01]

However that 25 mW FM transmitter with even a very simple whip antenna can produce fields far in excess of those allowed by Part 15 FM. Many of the NOUOs (violations) listed on the FCC's website refer to measured fields produced by radiated powers of about this value.

For example, the 25 mW curve in the chart I posted shows about 700 microvolts/meter at 1.61 km (one mile). That is a fairly strong signal that could be listened to on almost any FM radio, including a Walkman. But it is also illegal at that distance for a Part 15 FM station.

Mr. Fry just wants to protect you guys from a NAL...unless you don't mind parting with $10,000.00 of your ca$h should you get caught by the FCC... If all of you really want to do over the air radio right, get your investors together and buy an existing station or build a new one. Part 15 is meant to be "yardcasting" - it's a convenience so that you can listen to your music on your property without moving your component stereo system outdoors... Instead you can listen out on the patio with a boombox or on a "walkman" while you mow the lawn... Have a nice weekend....

 

[LibertyNT]

Signal strength decreases logarithmically as a function of the distance. So if your transmitter/antenna has a maximum 500 foot range, and you placed the tx/antenna combination on a 1000 foot tall building, you wouldn't hear it on the ground!

FM Doesnt Work That Way Stormy.
Line Of Sight Remember. Putting it up higher would only
increase its range. a Transmitter normally only going 500 FT and put it on
a 1000 ft pole you would get the station very clear for maybe a mile or so.

Its Amazing really.

So in other words if I connect a part 15 transmitter to an FM antenna at 60 ft I'll get out at least several blocks.

Yup. Although i Doubt it would be legal. Your Best bet would be going AM.

 

[Nostalgia]

I was discussing quality (FM) vs distance (AM) with someone wanting to put up a part 15 distributed system covering several miles of a local community. He asked me about my system. Allow me to quote myself in my reply to him:

"Most, like you, opt for AM to get the distance. I wanted quality so mine is FM. FM has even more antenna restrictions than AM so unfortunately it has less commercial appeal for ventures like yours even though it is in stereo and sounds better. I am happy with the extended neighborhood coverage I have. It is only meant to allow me to hear my own website through FM radios throughout the house and yard, all beyond that is ego!" (emphasis added)

It is fun to hear my station as I turn onto my street but I'm sure I am the only one listening! Stormy01, as well as others, have made some good points but as part 15 goes, it is what it is, and that is hobby radio. True, some are making a commercial venture out of it, and some appear to actually be making a go of it but those are rare. I had an offer in on a small AM last year but the seller backed out at the last minute. The whole market has been turned upside down since then so perhaps it was a blessing he got sellers remorse but I will continue to be on the lookout for a "real" radio station. Meanwhile my little FM100B is playing oldies in the background as I type this.

 

[rfry]

Mr. Fry just wants to protect you guys from a NAL...unless you don't mind parting with $10,000.00 of your ca$h should you get caught by the FCC...

Thanks, but a review of this subject on the FCC website shows that normally they first issue a Notice Of Unlicensed Operation (NOUO) to the operator of an unlicensed "station" that is non-compliant with Part 15.

That FCC NOUO by itself does not require a monetary fine.

Further FCC actions/sanctions depend on the response(s) of the person(s) receiving the original NOUO.

//

 

[stormy01]

Mr. Fry just wants to protect you guys from a NAL...unless you don't mind parting with $10,000.00 of your ca$h should you get caught by the FCC...

Thanks, but a review of this subject on the FCC website shows that normally they first issue a Notice Of Unlicensed Operation (NOUO) to the operator of an unlicensed "station" that is non-compliant with Part 15.

That FCC NOUO by itself does not require a monetary fine.

Further FCC actions/sanctions depend on the response(s) of the person(s) receiving the original NOUO.

//

Thanks for the clarification. There are some individuals out there that need to be aware and do the right thing from the get-go, instead of seeing what they can get away with, pushing the envelope (and I don't mean the modulation envelope, either...