Could I Broadcast 0.5 Watts on FM?

Could I broadcast 0.5 watts on FM? Because they told me to broadcast 0.5watts is license free but I want to know if that's true.

Because I want to but my radio station to 0.5 watts, could someone tell me about this?

radio8z's picture

In the US the part 15 rules which apply to FM are written in terms of field strength at a distance limits and do not address the power of the transmitter.

Since field strength at a distance is determined by factors including power we can make certain predictions. I predict that it will be impossible to remain legal with a half watt transmitter connected to any antenna. You will most likely exceed the legal limits thousands of times over. So, with this in mind, the answer to your query regarding the legality of a .5 watt transmitter is for all practical purposes NO.

A FM transmitter and antenna operating legally under part 15 rules will yield a typical range of 200 feet. This is not a legal specification, it is just an example of the range to be expected. A system which has a mile of range is most likely not legal.


Hamilton's picture

The myth that 100 milliwatts can be used on FM really burns them up at the FCC.

radio8z's picture

That's why it is important to understand that for part 15 FM the legal limits are on field strength and not on power, antenna length, transmission lines, etc. These are for AM operations.

I see a lot of "conventional wisdom" exhibited across the boards regarding the "FM 25 milliwatt limit". This is misleading and is nonsense for anyone trying to adhere to the FCC rules.


Rattan's picture

It's easy to see where the 100 mw on FM myth makes a ton of enforcement headaches for the FCC. And, to be honest, probably results in a whole lot of people getting in trouble who honestly were *not* trying to be "pirates".

If the "250uV/M at a distance of 3 meters" wasn't something that was so nearly impossible for a hobby level person to check, it might be easier to clear up that misconception. But people who want to build an FM transmitter, whether to save money or for the learning experiences and "built it myself!" pride, are basically up the creek without a paddle. No way to test it with anything like common instruments to confirm compliance. So it's not hard to understand why they prefer to believe it's something that is enough that they *can* measure.

Even buying one, there are a ton of places on the net selling stuff as "part 15 FM" that isn't certified and where the stated power for what they're calling part 15 could get someone in a lot of trouble.

My usual advice to people interested in part 15 FM is buy a certified transmitter and also to check the FCC ID against the FCC database and look at pics and etc to confirm that it *is* the transmitter the ID # goes with. And use it just as it comes out of the box. If you want to build and tinker, build and tinker audio gear and cool accessories for your station. Much as I hate to say it, it just isn't worth the risk to build the transmitter in my opinion.

That isn't just talk, I started with a transmitter I built from a kit. As a result of conversations on this and other boards and asking questions, getting answers and learning more, I retired it permanently in favor of a commercial certified transmitter.

There is the occasional mention of the small but existent possibility that a certified transmitter might still, due to multipath effects, appear to put out as much as 2 or 3 times the allowed power if it was placed "just exactly wrong" in regards to local terrain and etc. But if you look through the FCC field actions, I doubt you'll see any actions involving stations putting out power in that range. Most of the NOUO, NAL and etc are for stations putting out at least several hundred times the amount that any actually legal certified Part 15 FM transmitter could. But those violations could easily happen with a "25 mw" or "10 mw" unit.

But a whole half a watt? Even into a random length piece of coathanger wire that would almost certainly put out far more than the legally allowed amount. Definitely enough to get busted for. If by some miracle it was actually low enough output with the antenna chosen that it was at legal power.. You'd still probably have spent a lot more for it than you could have for some certified units.

You can buy a C. Crane for under a hundred, and some certified FM part 15 transmitters can be bought (fully assembled) for as little as 20$.

Take any advice you find here or elsewhere as you will. But try to remember that it's *your* fanny and the people handing out advice aren't the ones who'd end up paying your fine if you get a NAL.


radio8z's picture


You raise some good points and offer good advice. Allow me to take this as a chance to express opinion regarding part 15 in general.

My first opinion is that part 15 AM and FM was never intended to allow the range necessary for community broadcasting. The experience which you have previously shared about your FM activities and your neighbors as listeners is great but what you are doing is a long way from serving a community. Please don't read this thoughts about a community extends beyond just a few neighbors and I do not mean to detract in any way from your efforts and success.

My next opinion is that the FM rules intended to provide a means to interface with existing receivers in the home. Just as I am convinced that part 15 rules for AM were to provide for "phono oscillator" applications I am also convinced that the FM rules were written with the same intent. In the era since 1937 when as best we can determine the part 15 rules were written for AM up to the late 1950s there were no kits or devices available to the hobbyists other than manufactured phonographs which included the oscillator. The KnightKit AM transmitter kit which became available around 1955 opened the door for the experimenter as did subsequent articles for roll your own AM transmitters published in Popular Electronics and other magazines. What was originally intended to be a novel addition to a home entertainment product became a neat way for a kid (me) to "broadcast" to the neighbors.

About the same time, FM started to catch on as a superior means of delivering high fidelity to listeners. My first FM transmitter was constructed from plans in Radio-Electronics and I spent $5.00 in 1960s $$ on a single Philco 2N499 transistor to make it work. It did.

Third opinion is that a clean and reliable FM transmitter is much more difficult for the hobbyist to build from scratch. For reasons I can only guess the part 15 rules specified measurements which are beyond the capability of the hobbyist perhaps because it was not anticipated that this would be a do it yourself endeavor but rather would be dependent on commercial producers of transmitters. Surprisingly to me I know of no commercial units offered for sale from the 1950s until about the mid 1980s other than the toy "Mr. Microphone" which probably would not pass certification. Anyone know if it did?. Now we have the utility units which are used couple to car radios for MP3 uses. We also have kits and assembled certified units available. I maintain that FM transmitter construction is not a do it yourself activity and folks should take your advice seriously when you recommend using a certified FM transmitter.

Few hobbyists can measure the output of their transmitters accurately even in the 10's of milliwatt range but the point is, and I shout for emphasis: THE LEGALITY OF FM TRANSMITTERS IS NOT DETERMINED BY WATTS!!!" so the measurement is meaningless. The best thought I can give to hobbyists such as you and I is to follow your advice and purchase a certified transmitter and if you use a kit then do what you need to to limit the range to about 200 feet.

The folks who try to achieve miles of range are the ones who cause and get into trouble. We all need to realize that community broadcasting beyond a few neighbors cannot be done within the rules.


ArtisanRadio's picture

I really take issue with this 200 feet limit. If you do the math, you should be able to get a field strength of a little less than 4uv/m at 200 METERS line of sight. That's more than enough to induce a 1-2 uv signal into a car radio with an approximately 1 meter antenna. Most car radios today can receive that kind of signal with some quieting if you are broadcasting in mono. Of course, most home receivers are far less sensitive, particularly the newer cheaper ones, and most people try to broadcast in stereo (which typically means you need 10 times the signal strength to get the equivalent quality that you get with mono). So if you know what you're doing, put that transmitter in a weatherproof box as high as possible and broadcast in mono, it IS possible to use FM legally.

And (not to be gloating, of course), in Canada the maximum field strength is 4 times that allowed in the U.S. (100 uv/m at 30 meters), so broadcast distances to a good car radio can approach 1/2 mile.

radio8z's picture


You are correct that it is possible to have a range greater than 200 feet legally. It depends on many factors and the one which I would choose as being dominant is the quality of the receiver and receiving antenna. A home theater receiver with a rooftop antenna is going to be able to receive a part 15 transmission from farther away than a portable with a short whip inside a house. Listeners reached would depend on their receiver system. Around here there are no rooftop antennas except for satellite dishes so I would not expect much of a potential home bound audience.

It is not likely that someone in a vehicle will tune in as they drive through the coverage range since the listening window will be short.

Limiting transmission range to 200 feet may very well result in a much lower field strength than allowed if this range is set using a good receiver. Your calculation suggests that the range could be more than 4 times farther. It is just a guideline which can be used to approximate a legal system and granted it could be in error up or down.

On AM, I heard a Talking House type transmitter 2.8 miles away with good copy but I was using my commercial grade receiver with an outdoor dipole. While moble, I could hear the station about 1/2 mile maximum away. This is just one example of how the receiving equipment affects range.


Rattan's picture

Hi Neil

I don't usually count what I do as "community" either. More like "near neighborhood". Definitely no offense taken.

Now somebody with a bit of budget in a fairly small town maybe *could* use part 15 FM with enough transmitters spaced up and down the streets to provide enough coverage for everyone in town to be able to tune in. That's not really a new idea, the Wellsville Ohio station did that on AM with multiple certified transmitters and probably some other places I don't know about have as well. Due to capture effect, I think it'd probably be easier with FM and certified FM transmitters cost less than AM, but that'd be balanced out by needing more of them. FM is more line of sight than AM, so geographic features alone in many places would make it require considerably more transmitters.

I do, however, strongly agree with ArtisanRadio on the much embattled topic of "range".. To a good car receiver where you can have at least close to direct line of sight and with the transmitter at least a few meters up off the ground and a ways out from any building, right around 200 meters on the average. But to a cheap portable "transistor radio" type receiver, yes, around 200 ft. There were some occasional spots up to almost a half mile away where one could *barely* make out the signal part of the time with the volume on the car radio way up (and a high tolerance for noise). That half mile isn't useful range by any stretch, but it did quite nicely demonstrate that the signal does keep going at feeble levels for a lot further than one might think.

However that was with some "heroics" in mounting that would be impractical without a lot of rethinking, and as such I settle for less range and the convenience of having my transmitter right in my studio (which is in a corner of the kitchen). It still covers my few listeners quite nicely and can be picked up a little over 120 meters away with the car receiver. With the cheap portable, well it barely makes it across the street. But that's enough to be able to use a cheap portable out in the yard, so I'm cool with that. The few neighbors that listen use home stereos with at least some sort of receiving antenna. They've all been within at the most maybe 300 ft or less.

Calling it "community broadcasting" would be quite a stretch of credulity though. I'd say community broadcasting *can* be done, but it would take basically a network of transmitters and my budget doesn't go that far. They'd pretty much have to be commercial certified anyway because of the limit on homebrew part 15 devices to 5 units at a time. For a single transmitter the only way it might manage to be community would be if it was in a trailer park or maybe a campground or something. And that probably still wouldn't be enough to be what most people think of as "community" sized operations.

I can't really speak much about AM. I built an SSTran, but listener interest among my current listeners was very low for the AM efforts. I did some experimentation with loading coils this summer but I felt the sound quality was not as good when I'd finally get the system tuned for the different locations around the house and yard I tried. I *like* how the SStran sounds in it's "10 ft wire
mode", though I use it with the vertical I made for the loading coil experiments of wood and 6 gauge copper wire, which is only 7 ft and some because it had to fit in some rooms that didn't happen to have 10 ft ceilings. It does an admirable job of sounding real good on radios anywhere in the house and also sounding great on the vintage tube radio of the one neighbor interested in listening to my station via AM. His receiver is less than 100 ft from where my transmitter/antenna for AM are currently sitting, *and* he has an outdoor longwire antenna for his receiver. I'm not sure about exact dimensions, but I'd guess it as 40 or 50 ft of wire. Lately I've only actually been putting the AM on the air if he calls and asks for it. Or if I'm running some sort of holiday programming or special event.

I had fun building it, I have fun tinkering with it sometimes, it's fun when the one neighbor wants to tune in.. But my enthusiasm got a bit damped for AM because none of my regular listeners (including my own household) have much interest in listening on AM.

But I'd definitely agree that 200 *meters* is closer to the range to a car radio if some pains were taken in mounting the transmitter outdoors and reasonably high and in the clear. In my experience the "200 ft range" often mentioned would be to a cheap handheld portable or maybe a boombox,and it'll only reliably be that much with direct line of sight. A good car radio or a home stereo with an antenna can pick up a certified part 15 FM at more than 200 ft. It still wouldn't be like a mile though, or a half mile, probably not even a quarter. *Maybe* a tenth of a mile, if they use something like an old roof-mounted TV antenna and a pretty decent stereo? And that'd be assuming the station went to some pains to do a good outdoor mounting of the transmitter/antenna.

That's my evaluation based strictly on practical experiments with consumer level gear anyway, and as such would be considered anecdotal.


Rich's picture

Daniel wrote: But I'd definitely agree that 200 *meters* is closer to the range to a car radio if some pains were taken in mounting the transmitter outdoors and reasonably high and in the clear.

Good post. Just to note in a helpful way that Part 15 FM rules do not specify anything that would mean that the transmitter would have to be installed physically close to the transmit antenna. That is just a physical reality that has been applied to Part 15 AM systems under 15.219, which specify the radiating length of the antenna system to include the "feedline."

Part 15 FM limits the peak field radiated by the transmit antenna to 250 µV/m in any direction 3 meters away from it. It is silent about the feedline, the antenna gain, transmitter output power, and radiated power.

The Part 15 FM radiation limit can be met even when the FM transmitter is located several hundred feet from the antenna, as long as the output power of the transmitter, the loss in the coax transmission line to the antenna, matching loss, and peak antenna gain acting together produce the maximum legal field for Part 15 FM systems.

Evaluating all that can be a problem, granted. But probably in all practical cases it does not require an FM transmitter capable of 10 to 35 milliwatts (or more) of r-f output power to meet the FCC limit for Part 15 FM.


radio8z's picture

Rattan (aka as Daniel),

You wrote:

I do, however, strongly agree with ArtisanRadio on the much embattled topic of "range".. To a good car receiver where you can have at least close to direct line of sight and with the transmitter at least a few meters up off the ground and a ways out from any building, right around 200 meters on the average. But to a cheap portable "transistor radio" type receiver, yes, around 200 ft.

I would hope that if you again study my reply to ArtisanRadio you will see that I am in general agreement with him/her. Let's be really careful about the difference between 200 meters and 200 feet, a difference of about 4 to one. From a practical point of view a 200 meter range based on a car radio check has nothing to do with the real range to be expected given a typical listener's situation. No one is going to listen on a car radio since the signal will only be available for a short time as the vehicle passes through the area. Unless someone is sitting in their driveway or garage this is nonsense to assume that usable range is so defined.

So we are left with the portable or home theater receivers. Yes, as I proclaimed, the range with listeners so equipped might exceed the magic 200 foot limit, but in my humble home, except for my ham activities, all receivers are either miserable $15 portables or the home theater which is connected to cable. I could not recieve my neighbor's part 15 FM transmission even if I knew it existed and gave a rat's a** about their programming and wanted to tune in without modifying my equipment.

As I recall you posted a while ago your experiences and you mentioned providing your neighbor with a high end receiver so they could hear you. That is great but the reality is that people no longer huddle around their receivers seeking information or entertainment as is fantasized in the idillic Norman Rockwell paintings of yesteryear.

Part 15 FM is a hobby and also provides for personal use, not for broadcasting. It is a lot of fun, it is a technical challenge, it is a learning opportunity, but it is not a broadcasting activity.


Rattan's picture

"Just to note in a helpful way that Part 15 FM rules do not specify anything that would mean that the transmitter would have to be installed physically close to the transmit antenna."

Culpa mea, Rich. Quite correct. Since with my own FM transmitter the antenna is a part of the unit, I spoke of them as a connected unit to indicate the system in general. But agreed that there is nothing mandating that certified designs might not use a feedline and etc.

Neil: I apologize if I worded things badly enough to give you the impression I was disagreeing with you on some of the points of the discussion on range. What I intended was to second your and ArtisanRadio's points with my own thoughts and anecdotal experiences. My apologies.

I will disagree at slightly on a few points as you've stated them in the more recent post though..

"So we are left with the portable or home theater receivers."

Actually, there are a wide range of possible receivers, especially if you consider used stereo equipment from the 70s and 80. Some of that is very inexpensive when encountered at yard sales of secondhand stores and some of those units have amazing reception and sound.

On the other hand I agree that the "car radio" range is mostly useless when talking about part 15 FM especially. Even if you put up a sign (like realtors do with the talking house) and someone tried to tune in, they'd be out of range within seconds or possibly before they even were *able* to tune in. It wouldn't make much sense to think of that as a possible "listening audience".

I will also agree that people don't huddle around the radio in the living room like in Norman Rockwell paintings anymore.. At least not most people most of the time. But once in a blue moon I've seen it happen, even here in my own household. In all fairness though, my household is atypical. A radio station or mp3 player or the stereo or even someone picking up a musical instrument is more likely than the TV being on here.

Radio doesn't hold the place it once did for news and entertainment. But lets face it, radio programming isn't what it used to be, either.

However, on your own part15 station, it can be pretty much however you wish radio was. With some work, you can maybe make it into something you enjoy listening to more than what you can usually find on the air.

I must admit I'm not sure I quite get your point on one matter though...

"Part 15 FM is a hobby and also provides for personal use, not for broadcasting."

I would usually think of broadcasting as one-way transmission of music and etc for entertainment purposes. That is not permitted on the Ham or CB bands (for example). But it is not prohibited on either FM or AM part 15, so far as I am aware? Now I would agree that originally, many years ago, part 15 on the AM band consisted of "phonocaster" type circuits. The history has been discussed here and elsewhere and I'll mention that I've found it fascinating..

But in this time frame, units expressly designed for "one-way transmission of music and etc for entertainment purposes" have been submitted and certified and the manufactured units are available on the market. If I recall correctly, the Rangemaster (for example) has a 600 ohm balanced input? I do not recall ever seeing many phonographs that had an output of that sort. So it was obviously not designed as a phonocaster. For that matter, phonographs aren't real common these days either. So are we saying that part 15 on the AM and FM broadcast bands should now be retired because the phonographs they may have been originally allowed for are no longer common items in homes and as such there is no reason to have something to use radio to get the signal to one's "hi-fi?"

I don't recall seeing any prohibition against putting together broadcast style programming (provided one does not use call letters of an actual licensed station and etc) and transmitting it via part 15 FM or AM BCB devices.

Agreed, it may not be what was originally intended by the rules back in the day. But if it was somehow utterly unacceptable to the FCC then what about the "Wild Planet Radio DJ" toy? I don't know for sure if the "Mr Microphone" was certified, but the Wild Planet unit was and for that matter it was crystal oscillator and had a built in cassette player and microphone I believe. Rather obviously not intended strictly for playing phonograph records?

Admittedly, the range to a listener with gear likely to be able to tune in isn't large in any case.. But if one finds that acceptable, then the hobby can include "very tiny radio stations".

Running a very small radio station within the limits of the part 15 regulations for the AM or FM broadcast bands would be, I think, just as acceptable a use of the potentialities allowed by part 15 BCB rules as getting a Glenn Miller song from the phonograph in the den to the Hi-Fi in the living room was? In either case, it is entertainment.

Sorry to have gone so verbose Neil, and I hope I haven't been too impossibly dense or come across as being sarcastic.. But I'm really not sure what you're trying to say. Could you clarify please?


radio8z's picture


No problems at all here with what you have posted. I enjoy your thoughtful and pretty much right on view of things.

It is OK to take exception to my broadcasting comment for I don't think I expressed my thought very clearly. And even if expressed clearly, you may still take exception. When I wrote broadcasting I had in mind the idea to cover a community with a potential of many listeners. Perhaps that can be done depending on housing density and with attractive programming but I do believe it would be an exception. The OP in this thread with the power he mentioned could do this. Units such as the Rangemaster are not phono oscillators nor toys and with properly equipped listeners and signal conditions could provide a decent number of folks in the audience. Unforunately here the housing density and electrical noise are limiting factors for AM and building penetration due to the lack of outdoor antennas would be a factor for FM. The homes here have foil backed intermediate sheathing which, at least for my home, attenuates broadcast FM signals. Few provide a good signal on our indoor portable radios and the home theater receiver is on cable.

What I was doing in my opinionating, which is based mostly on what I observe in my home and those of neighbors, was to consider the odds of gaining listeners to my part 15 operation. Even if I could produce a usable signal I am not clever nor committed enough to deliver good programming that would induce anyone to listen. I realize that others are.

I don't want to discourage anyone from getting into part 15 activity but rather if they do so just advise them concerning realistic expectations. It is not just a simple matter of turning on a transmitter and having great coverage with many potential listeners. Could this happen? Yes. Does it? Rarely from my readings.

I think perhaps the original poster had expectations beyond what is reality for legal operation and I fear this is fairly common (based on the number on NOUOs reported weekly on the FCC site).


kc8gpd's picture

I would just like to note that in New Zealand you are allowed 500mW EiRP on select FM frequencies w/o a license or permit. you are allowed to have up to 2 Transmitters simulcasting within 25kM of each other.

units I Believe must be certified for this use.

now the FCC has a chance to do something like this with channels 5 and 6.

of course we can wish in one hand and crap in the other and see which happens first :)

I believe it will be the latter.

wdcx's picture

New Zealanders don't have to deal with the NAB

Druid Hills Radio AM-1610- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC. We do not censor free speech and hide public information.