Part 15 (FM and AM)

Hello Paul,

I'll get this going by offering some information. Yes, this site is similar to a library and it can be overwhelming at times in terms of information.

Regarding the FM transmitter, the FCC rules for FM limit the field strength of the radiated signal and do not mention the power of the transmitter. Though the transmitter power can be set this does not mean that it is delivering the indicated power to the antenna since the power the antenna will accept depends on the antenna and feedline. When we operate on FM the best we have to go on for legal operation when using a non-certified transmitter is to adjust the power and the antenna length to give about a 200 foot range. If you set this to cover your apartment with a signal that gives reception just above the noise you should be close to the limit. Unfortunately, lacking the proper test equipment, this is the best you can do and this doesn't guarantee anything.

The rules for AM are easier to follow for compliance. A transmitter which is "FCC approved" doesn't mean that it is certified to be compliant with the rules but this can be interpreted to mean that if you set the length of the antenna, transmission line, and ground lead total less than 3 meters you will be in compliance with Part 15.219. This differs from the case for FM in that the rules for AM permit operation based on this length plus the input power to the final of the transmitter. The SSTRAN AMT-3000 meets the specifications regarding input power and out of band signals if you adjust it according to the instructions so the only thing you need to do is keep the antenna, transmission line, and ground (if used) total length less than 3 meters.

Just as is the case with a car which is capable of exceeding the speed limit is it up to the user to operate in such a manner that the limits are not exceeded.

Hope this is helpful and ask if you still have concerns.

Neil

Neil, thank you so much for your answer.  

Antena length - i think I get this... it comes with AM transmitter... 

ground line - not sure how to set it up, probably will not have one

What is transmission line?   Sorry if I ask idiotic questions.

I think the fact that balkyny is 9-floors up inside a 21-story building we can assume he will be in FCC compliance, when taking the following into account.

Being inside a commercial-grade building means that only a fraction of the signal will penetrate the thick walls, and even less signal will be evident way down on the ground.

An FCC inspection would be made at ground level, not inside the building nor 90-feet up in the air.

You can do your own test based on the "200-foot estimate" mentioned by Radio8Z...

Once you have the power level high enough to serve your indoor space, play some music on the air and go down on the street with a good portable FM radio.

Tune yourself in and make note of how strong the signal is near the building, then walk 200-feet away from the building and if your signal is weak or gone after 200-feet, you are being reasonably compliant.

The AMT3000 is a perfect indoor transmitter, I am running two of them and the indoor coverage is very strong. The manual has information about setting up the antenna.

The thing is - use an ordinary, around the house radio while testing for compliance.

I'm getting 1/2 to 3/4 km range (between 1,600 to 2500 feet) to a very good car radio with my FM transmitter - that's Canadian rules.  But if I take my sports walkman type radio outside, I'm lucky if I can hear the signal 500 feet (and that's pushing it).  Divide those numbers by 4 (which is the multiplication factor for Canadian field strength limits) and that's what you'll likely get in the U.S.  If you're on a balcony 90 feet up, your signal will barely reach the ground.

My FM signal here gets about 75 to 100 feet

to a portable radio or a walkman type radio -

something like that. 

However, my car radio is so sensitive, that

it can almost routinely hear a few FM stations

that are 75 to 120 miles away.  My FM transmitter

can be heard on the car radio about 600 to 800 feet

away from my house, but at that point - it is about

the same strength as the distant FM stations I can

hear in other cities. 

So - even though my FM transmitter can't be heard

on an ordinary kitchen radio across the street,

it goes much further on a car radio.

My FM transmitter is compliant with the

FCC rules mentioned above. 

People who are looking into Part

15 radio for the first time are very

surprised to learn that the permitted

FM power (or really field strength) is

so low. 

However, in apartment buildings, college

dorms, and even in neighborhoods with

the houses very close together, there

have been some people that have run

legal Part 15 FM and have actually had

some listeners.  I wouldn't expect anyone

to stumble onto such an FM transmission,

but a friend who is very nearby might be

interested in listening if the programming

is right.  (They would also have to have

a very sensitive FM radio.)

Anyway, I know because I was able to do that. 

But the houses here are very very

close together. 

When my AM transmitter was at it's best

it went a lot further.l  I hope to do that

again someday. 

Bruce, The Dog Radio Group

My editing audio is heard on a C.Crane Radio Plus located 1-foot from a C.Crane FM Transmitter. The same sound system is used when I watch video on the computer, such as a TV show or movie, and although the C.Crane FM Transmitter dies out in about 10-feet I am right next door to another house, and I wonder if the neighbors are tuned in.

During moments when a movie sound track features a woman's heavy beathing or a man's grunts, I hope not.

But of course, if the neighbors did hear my movie, they would have no idea where it was coming from.

Continuing on with the receiver theme, we speak as if all car radios are equal.  That is definitely not the case.

I have a really good one in my car, a Nissan Cube.  It's OEM.  However, just today I was in a Kia with an OEM radio, and the reception for my FM signal got spotty at the end of the street (about 800 feet away) - again, that's Canadian rules, which allow 4 times the field strength of the U.S.  My car radio's reception doesn't start getting spotty until about double that distance and until then it's nice and strong.

So you can't always count on car radios either.

Best Wishes, 

Bruce, The Dog Radio Group

I'm inventing a new kind of car tire 

that is shaped like a triangle.  

That way it will be safer to park your

car on steep hills.  

(Hey - there's one for your next Low 

Power Hour.)

Bruce, The Dog Radio Group

Triangular tires might be better than chains for driving on snow and ice, and would help chop it up.

Let's make you a test car.

The ride inside might be a little bumpy, but that would prevent falling asleep on the road.

If you've ever heard the old story "STONE

SOUP," where a guy says he is going to make

soup out of a big stone - and nobody beleives

him...  But then different people start bringing in things

to add - water, a fire to cook from, vegetables, meat,

etc.  So they started with NOTHING and then added

a bunch of things to make something useful.

Unfortunately or fortunately - depending on how

you look at it - Part 15 "broadcasting" is like that.

250 uV/M @ 3M is really nothing for FM.  And 100 mW

with a 3 meter max ant/gnd isn't much either for AM.

You start with nothing and then you keep adding

things until you get something that works (legally.) 

And a real lot of the time it isn't "plug and play."

Because UNLESS you buy an expensive AM transmitting

system you have to build stuff and figure things out. 

And that takes knowledge and experience in the radio

and electronics field.  That's not only stone soup, but

it's CATCH 22.  Especially for the people who jump in

here just to see what Part 15 is all about.  And oddly

enough, the 100 mW myth for FM is still out there.

Even after all of these years.

Bruce, The Dog Radio Group

The comparison of part 15 with stone soup is tremendous.

That's it.

All I have to say.

But really, it's very good.

Bruce's stone soup parable is an excellent addition to this thread in that it captures the adventure of Part 15 radio as a pursuit. Some don't want the adventure but for those who do it becomes a never ending process of building and learning. Fortunately, getting started is easy and results are immediate but it is predictable that most will want to pursue better sound and longer range and good programming. This is where the adventure continues.

I have been around long enough to remember the advent of "Hi-Fi" and then stereo. Equipment was available on the market but the best sound was achieved by those who designed and built their own systems, including speakers. Some of the great companies such as KLH and Acoustic Research grew from the inventions and efforts of audio hobbyists. There are many parallels to the state of Part 15 development today with that of audio in the 50s and 60s.

There was a atory about the capstone examination in engineering college where the final question was "Given earth, fire, and water, derive everything." I feel that Part 15 radio is like this sometimes.

Neil