Hi-Fi Stereo FM Transmitter

[MrForbes]

There are many FM radio transmitters available for use in cars, typically to allow you to use an ipod or similar device to transmit the sound to the car's radio. They have small antennas, and very low power output. But if you add a larger antenna, you can get a bit of range out of them. The problem is you might be violating FCC regulations if you do this. So we'll wait to hear from Alan and Al about what you should really do

As for being Hi-Fi....hmmm.....

[Chris is me]

Originally Posted by squirrel

There are many FM radio transmitters available for use in cars, typically to allow you to use an ipod or similar device to transmit the sound to the car's radio. They have small antennas, and very low power output. But if you add a larger antenna, you can get a bit of range out of them. The problem is you might be violating FCC regulations if you do this. So we'll wait to hear from Alan and Al about what you should really do

As long as you stay under 3.2 miles, your low power transmitter is completely (edit: not) legal. You should be fine with adding an antenna to any of the commercial iPod FM transmitters. A few of them have ports for antennas ready to go.

You used to be able to purchase everything you'd need to do this at Radio Shack, but I don't think so now.

Be aware that FM sound quality will probably not be exactly what you're looking for if you can tell the difference between an MP3 and a CD, or other similarly degraded audio. Running wires or using a Wi-Fi solution would get you better results.

[Dave Flowerday]

Originally Posted by Chris is me

As long as you stay under 3.2 miles, your low power transmitter is completely legal.

Bill, I strongly recommend you do your own research with the FCC who holds the authority for these types of devices. Be wary of any advice you receive on a forum like this (including mine) as any of it can be misleading or wrong, and if you follow it and end up breaking the law by accident they will not accept that as an excuse. A prime example here is Chris' assertion that "as long as you stay under 3.2 miles it's legal" - this is flat-out wrong.

Here is the FCC website describing low-power FM stations, which is probably where Chris got his 3.2 mile limit from. Note that you would likely not qualify for a LPFM license (and yes, you still need a license):
http://www.fcc.gov/lpfm/

Choice quotes from that document:

This page contains information about the Low Power FM (LPFM) radio service, which was created by the Commission in January 2000. These stations are authorized for noncommercial educational broadcasting only (no commercial operation) and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain (HAAT). The approximate service range of a 100 watt LPFM station is 5.6 kilometers (3.5 miles radius).

A construction permit is required before a LPFM station can be constructed or operated.

LPFM stations are available to noncommercial educational entities and public safety and transportation organizations, but are not available to individuals or for commercial operations.

Ramsey has some good real-world information on the use of low-power FM kits or homebuilt devices here: http://www.ramseyelectronics.com/res...t.asp?page=fcc

One of the important limitation is probably this one:

The most specific FCC regulation of 88-108 MHz FM Broadcast band unlicensed operation is that the "field strength" of the signal must not exceed 250 microvolts/meter at a distance of 3 meters from the transmitter (FCC rule 15.239). If you have any concern about this emission limit, have your device checked by a technician with accurate measuring equipment. Remember that the "field strength" of a signal is determined as much by the antenna as by the RF output of the transmitter itself.

They also have a table which shows the field strength at various distances if you adhere to this rule. If you modify a kit in any way, though, you'd need to somehow verify compliance with this rule.

And, most importantly, you cannot interfere with any licensed broadcaster in the band. If you cause interference for any of your neighbors who are trying to listen to a licensed station, you are violating federal law. The FCC has been known to crack down on offenders too, so don't just assume that no one will notice and you'll get away with it.

[Chris is me]

I'm going to go ahead and fire my lawyer now...

Sorry about my post.

[MrForbes]

Originally Posted by Dave Flowerday

And, most importantly, you cannot interfere with any licensed broadcaster in the band. If you cause interference for any of your neighbors who are trying to listen to a licensed station, you are violating federal law. The FCC has been known to crack down on offenders too, so don't just assume that no one will notice and you'll get away with it.

This is the crux of the matter...and whether or not you'll be able to do this depends on how far away your neighbors are. I'm lucky, we live on 4 acres, and the nearest houses are about 500 ft away, so we could probably make an FM transmitter system for our house that would meet the interference requirements, even though it might exceed the allowable field strength. If you're in an apartment or high density housing tract, it might be impossible.

[DonRotolo]

The distance you'll be able to go with an unmodified kit depends on what is between you and the antenna. A bunch of metal, not so far. Nothing but air, a lot further. Antenna in the basement, not so far. In the attic, or on the ground floor (in the linen closet?) better.

Build it, try it, and only if it's not OK do you go further. 200 feet on a few microwatts is easy.

500 mW or 1 Watt is WAY in excess of legal limits. A typical NAL (Notice of Apparent Liability = a fine) for unlicensed FM broadcast exceeds $10,000. Oh, and the FCC can find such a transmitter in perhaps 10 minutes, if it goes looking.