Slashdot Mirror
An FM Broadcast Transmitter For Your Home
Bruce Perens writes "I wanted to be able to listen to net audio and my ham station around the house and yard. Those iPod FM transmitters don't have enough range. So, I bought a digital controlled, 100 miliwatt transmitter that covered my whole block the first time I plugged it into what was, until then, a recieving antenna."
Just look in the DIY kit section of any electronics store, there's always a number of small FM transmitters, some of which get 2km range.
They usually have an electret microphone, but it wouldn't be very hard to modify one to take a line-level source.
It used to be. As a ham you SHOULD know this. The rules regarding unlicensed broadcast transmission in the FM band changed. You can, in fact, use any amount of power you want. That is, as long as you don't excced 250uv/meter measured at a range of three meters.
For most of the unlicensed stuff that translates to roughly 10 to 20 mw. This is governed by part 15 of the FCC code and you can read even more about it at http://www.fcc.gov/mb/audio/lowpwr.html#PART.
Going to the store page reveals that a 0.3W transmitter costs a wild $140. Yes, there's an LCD and menus, but no self-respecting budget-minded geek would lay down that much money to broadcast music over a short distance.
There's plenty of circuit designs at ePanorama that are relatively simple and powerful. DIY; save your money and your honor.
1: You might get surprisingly long range reception. From my experience on the 145 MHz Ham band, I can easily communicate 30 miles with less than 1 watt with a roof-mounted antenna (of course the receiving station also has a good high antenna). There are differences that will reduce your range (particularly the wide bandwidth of the signal), but in general 2 watts can potentially cover a very wide area..
2. Yes, particularly if the antenna is high up and your signal gets into the city, or if your transmission annoys anybody for any reason.
3. $10,000 a day fines and your equipment (and possibly other nasty things I don't know about!)
Ultra cheap FM transmitters use a tuned tank circuit to control frequency (ie - a variable capacitor). These are very difficult to get to the exact center of the frequency you choose, the result will more than likely be poor audio response.
Furthermore, your receiver is probably digitally controlled to an exact FM frequency and the variable cap system will change frequency slightly with temperature and humidity, so that the transmitter will slowly go in and out of channel. The receiver is phase locked to the correct channel, and the result is that the system will gain and lose quality over time. Digitally controlled frequency is a must for this application.
Kit systems are usually poor at doing audio; making a noise-free circuit is somewhat of a black art and it takes a lot of research and tinkering to get it right. For example, Ramsey Electronics makes an FM transmitter kit with digital frequency control and all that. The problem is that the board layout introduces an unacceptablt amount of hiss into the audio signal. No amount of adjustment will fix this, and the average hobbyist is unprepared to even figure out what the problem is.
Finally, Be very careful of high end ($300) transmitters used for gyms. Some of them are mono only (!) but do not specify this on the advert.
Your best bet is to look on the net for reviews of any transmitter you're thinking of buying. When people have good things to say and don't point out the obvious flaws, then the product might be OK.
I purchased several poor FM transmitters before I got one that worked well, and I use it for transmitting FM throughout the house.
That would be the C.Crane FM transmitter. You can get it for about $80, and Froogle lists several vendors.
On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters).
/ Documents/bulletins/oet63/oet63rev.pdf
source:
http://www.fcc.gov/mb/audio/lowpwr.html
see also:
http://www.fcc.gov/Bureaus/Engineering_Technology
Bottom line:
The transmitter described is not even close to complying with FCC regs. FCC compliance depends not only on transmitter power, but also on antenna gain and several other factors. You cannot simply say "X milliwatts is safe". Not only that, but 100mW into almost any antenna is going to be a violation. Something in the range of 1mW into an omni antenna would be more like it.
----
All of whose base are belong to the what-now?
In Europe it is not permitted to use any transmitter on broadcast frequencies, unless you are a licensed broadcaster. And it doesn't matter how low the output power is, so those Ipod thingies are not allowed either.
or how about anyone come over..
what he is doing is already in violation of the FCC rules. 100 miliwatts is the maximum allowed into the antenna provided in the kit. hooking it up to a gain antenna is illegal as he now went past the 100mw ERP.
all it will take is one phone call by a pseron that does not like him to get the FCC and a 10,000.00US fine on his arse.
Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice. On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range. For more information on Part 15 devices, please see OET Bulletin No. 63 ("Understanding the FCC Regulations for Low-Power, Non-Licensed Transmitters"). Questions not answered by this Bulletin can be directed to the FCC's Office of Engineering and Technology, Customer Service Branch, at the Columbia, Maryland office, phone (301) - 362 - 3000, e-mail LabHelp@fcc.gov.
PENALTIES FOR OPERATION WITHOUT A PERMIT OR LICENSE
The Commission considers unauthorized broadcast operation to be a serious matter. Presently, the maximum penalty for operating an unlicensed or "pirate" broadcast station (one which is not permitted under Part 15 or is not a Carrier Current Station) is set at $10,000 for a single violation or a single day of operation, up to a total maximum amount of $75,000. Adjustments may be made upwards or downwards depending on the circumstances involved. Equipment used for an unauthorized operation may also be confiscated. There are also criminal penalties (fine and/or imprisonment) for "willfully and knowingly" operating a radio station without a license. DON'T DO IT!
He also far exceeds the distance limit set by the FCC.
Nice way to publically admit you are violating FCC rules.
Do not look at laser with remaining good eye.
Program restrictions are not relevant for my purposes, which are not broadcasting. Those who want to open a broadcast station for their own neighborhood can potentially run into them.
Bruce
Bruce Perens.
Bruce
Bruce Perens.
Bruce
Bruce Perens.
Bruce
Bruce Perens.
Low power broadcasting is legal under FCC part 15 as long as you stay under radiated power limits that they set.
Bruce
Bruce Perens.
I am not plugging EITHER of those transmitters, because I'm not USING either of those transmitters.
I AM plugging the utility of having an FM transmitter in the home, because I'm doing it and it is truly useful... I rate it as highly as a fileserver or my wireless link.
Keep in mind, too, that not all transmitters have good quality sound. I'd be leery of the cheap ones.
Murphy was an optimist.