I hope to see more here about part 15 FM .

At what range are you broadcasting and still complying with Part 15?

My average range is about a mile to a car radio (autos produced in the past 12 years) or if a "boom box" is used as a receiver, my range is about 500 to 1000 ft.

The range really depends on the quality of the receiver used and other variables.

If a "walkman" is used as a receiver my transmit range is about 500 feet or less.

"My average range is about a mile to a car radio..."

Wow this really sound interesting!!!

Can you give more details about your transmitter, antenna, antenna heights, etc...

I am using a Ramsey FM25B adjusted to only 1mW of output power. Making some test with a simple dipole antenna (vertical polarize) I get about 1000 feet coverage with my car's radio. I really don't know is this complies with FCC Part 15. So I will ask the FCC because I don't want any trouble.

Mickey

I don't want any trouble either so I had a local radio company do a field strength test .

At 25 milliwatts and a 5/8 wave groundplane at 28 feet , the field strength test showed that I was within the limits of part 15 FM .

But a big part of getting good range is a clear frequency to transmit on and a good receiver to listen with. Car radios are the best recievers ! Especially when there are no strong stations nearby.

But you should know that at a mile away, my signal is not "booming" in , it is very choppy when in a moving vehicle and most people would not want to listen to such a weak signal . (except me)

Thanks!

I will see who can make this "field strenght test", or maybe I will buy a F S Meter...

Do you have an aproximate value of what must be your ERP (effective radiated power), considering transmission loss and antenna gain ?

Mickey

Buying your own field strength meter will not help unless you spend at least several thousand dollars on one. The equipment used to do this kind of testing is extremely expensive.

I don't know what my ERP is that is why I had a field strength test done. I figured it was the best thing I could do and I wanted to do things right .

Another way to get an aproximate idea of your output is ,...

Most modern car radios produced in the pst 12 years have a useful sensitivity of about .5 to .1 microvolt. So using the inverse square law, you can see that you can detect a signal with a 250 uVm at 3 M up to a distance of about 1 to 1 1/4 miles under good conditions before the signal dissapeares completely. At this distance the signal may not be very readable at all but will be detectable to some degree.

🙂

This looks like a great website, I am hoping to see more information about part 15 FM hobby broadcasting.

8) 💡
Some new FM info has been added in Library and Reviews, and be sure to check Part 15 resources and visit Howard Lute's Optimum Camel Part 15 FM site

At 25 milliwatts and a 5/8 wave groundplane at 28 feet , the field strength test showed that I was within the limits of part 15 FM .

Gorgo,

If I can ask, how long and what type of transmission line are you using? I'm trying to do some calculations on my FM and was curious.

Jerry 1290 AM

i'd be interested in multicasting fm if you can really do that.. i was expecting maybe 100-200ft max staying part 15..

that with the limitations of part 15 FM, that it is mostly a hobby and in house type of transmission. I read about FM antenna systems, but I'm wondering why go to that much trouble when you're so limited in range? Is over 200ft legal for FM?

I don't want any trouble either so I had a local radio company do a field strength test.
At 25 milliwatts and a 5/8 wave groundplane at 28 feet , the field strength test showed that I was within the limits of part 15 FM.

This comment goes back a few years, but now there will be new readers of it who could be left with an incorrect idea. A power of about 11 nanowatts radiated from a simple 1/2-wave dipole will produce the Part 15 FM peak field of 250 µV/m, 3 meters away from the antenna. The field 3 meters from that dipole radiating 25 milliwatts is nearly 370 millivolts per meter, which is 1,480 times greater than the legal limit. So whoever did those tests produced some invalid results for the hardware and power that is described.

It seems that with the limitations of part 15 FM, that it is mostly a hobby and in house type of transmission... Is over 200ft legal for FM?

The FCC doesn't limit Part 15 range, only the peak field strength produced 3 meters from the transmit antenna. So if your antenna is high, the radio path to the FM receiver is clear, the frequency is interference-free, and the FM receiver/antenna system is good, then greater range is possible. But even under the best conditions, the legal, free-space Part 15 FM field is down to about 1 µV/m at about 1/2 mile.
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You wrote:

At 25 milliwatts and a 5/8 wave groundplane at 28 feet , the field strength test showed that I was within the limits of part 15 FM .

Could you tell us what the field strength reading was and at what distance? It seems that if the antenna was at 28 feet it would be difficult to measure this at 3 meters.

Also, someone mentioned the inverse square law. Just a reminder that this applies to the power in the field, the field strength in v/m is linear with distance.

Neil

that with the limitations of part 15 FM, that it is mostly a hobby and in house type of transmission. I read about FM antenna systems, but I'm wondering why go to that much trouble when you're so limited in range? Is over 200ft legal for FM?

Hi freestyle899.

I run on part15 FM, and though it's almost impossible to tell if one is running strictly legal, I figure I'm close since the range is quite short. To a "walkman" type reciever, reliable range is maybe one house up and down the street, having checked it out on the sidewalk. To indoor stereo recievers, one that's hooked up to an old tv antenna they weren't using that's now aimed our way with an old "fm booster" from a yard sale can now pick up my transmitter reliably from a whole 5 houses away. That'd be my "fringe range", I guess. LOL Haven't even bothered to check with a car, since by the time anyone driving by could tune it in, they'd certainly be out of range. I do have one neighbor that can recieve my station on his clock radio.. But it's in his bedroom window just across the yard, and as such is maybe 40 ft from where my antenna is in my kitchen.

Now, that might sound discouraging, but it's probably about the best one can do with FM without going pirate so far as I've been able to understand it. No way to check for sure since the the 250 microvolt at 3 meters into a calibrated 1 m antenna is not exactly "homebrew friendly" for being able to measure. But considering my antenna is inside the house (aluminum siding) at ground level and there are plenty of trees around, it's unlikely that it's causing unwanted interference.

Ok, on to "why?". Because I enjoy it, and the listeners I have in the neighborhood enjoy it. So, yup. Hobby. I already had a home recording studio, so adding a little transmitter wasn't much expense for something I and others like.

I run shows for two hours in the am and three hours pm every day, and my teenaged daughter (who has a couple friends in our little listening range) often does shows on Fri and Saturday nights.

So far as listeners, I'm sure that there are times when there are actually none. But most mornings I know I have 4. All in the building right next door. Our "record high" for listeners was 11, confirmed by "the great cookie giveaway". While reading children's stories on the air, the offer was made of one dozen homemade cookies to children listening who could come over and answer an easy question about the story I'd been reading. Limited to one dozen cookies per family. Got 3 children who came over where I was able to ask how many people in their house had actually been listening that evening (and also a dozen for the next door neighbor who is probably our best listener).

That may not seem like a lot.. But for us it's enough to make it feel worthwhile and to get to know the neighbors better which makes the neighborhood a nicer place. I think all of the neighbors in our "record high" evening of 11 listeners were well within that 200 ft range. It's a residential neighborhood, but near the center of town, mostly 2 or 3 family houses.

It's being fun, especially with the holiday season. Hobby? Obviously. I don't see any commercial potential to it even if we had any inclination to be doing it for money. No need for it to be "commercially viable" since the transmitter runs off 4 AA batteries and we use rechargables (which are good for about 2 days of shifts before switching to a fresh charged set). Oh, I've blown some bucks on a studio mic and a new mixing board and etc, but i have a home recording studio, so those are useful for other things anyway.

I am, however, still thinking hard of switching to AM, possibly via the "Talking House" route, since they're a commercial FCC approved transmitter that was designed for continuous use, and I've seen them go for as little as 50$ on ebay. But it's not so much looking for the added range at this point as being less uncertain about actual part15 compliance. Or even with homebrew gear, 100 milliwatts and a known antenna length is an achievable measurement. One would think that with the number of radio circuit sites and people that can rattle off the equations for antennas and such, that somebody would have come up with a reasonably simple way of being able to measure whether one is at least somewhere near actual compliance. But perhaps the very low level makes this too challenging to come up with a reliable way of doing it with common instruments. "11 nanowatts" or "250 microvolts at 3 meters with a 1 meter antena" are equally useless to people who own maybe a multimeter. But anyway..

But with a bit more legal range? Even within an 1/8 mile there's one nursing home and a small sr. citizen's center and a fair little number of homes that could potentially be new listeners. !/4 mile would reach all the way downtown, and a half mile would cover a significant chunk of town. Might not be able to afford to keep giving away cookies then, though. LOL

But mostly just to not have to worry as much about possibly being slightly over legal limits and to not have to touch up the tank circuit tuning due to drift.

Anyway, different people will have different reasons why they find it worthwhile. For some, my listening audience would probably be too small to bother with. But for me "hobby" works well as a reason.

Daniel