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How far range is legal for FM transmitter?

dietermoreno

Dec 30, 2012
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How far range is legal for FM transmitter?

I bought a stereo FM transmitter kit from Amazon that doesn't say what the wattage is and it comes with a telescopic antenna like on a boombox. It is powered by a 9 volt battery. It doesn't use a phase locked loop. It has 2 audio transistors, the stereo modulation IC, the non linear mixer, and an RF transistor. I don't think it has a power transistor. I don't think a 9 volt battery would be much power for a power transistor anyway.

I heard that the commercially available FM transmitters designed for use in cars have very poor results, so I thought I'd build my own with a bigger antenna to both get better quality in the car and so it can have enough range to cover the entire house and entire yard so I can listen to a CD or file on both a standard clock radio in the kitchen that only has a radio while washing dishes and listen on a clock radio outside while doing yard work and also even have a home speaker system with simple radios without having to buy expensive ceiling mounted speakers and expensive hi fi stereo units.

The manufacturer of the kit is Ramsey Electronics.

I saw Chinese kits that were able to go from 0.5-15 watts (highly illegal) for almost the same price using a phase locked loop and use mains power.

If my transmitter doesn't use mains power, would it be safe to assume that it is less than 0.5 watts?

If I can drive 200 feet in my car and am not able to receive the signal farther than 200 feet, would it be legal?

Putting it on the 2nd floor or 1st floor would change the line of sight range, so I could test to see which range is less than 200 feet.


I wonder if in the future, FM radio will go to all digital like TV and you will no longer need a license for what are currently licensed as "FM-LP", perhaps becoming like AM radio and it will maintain a few country and oldies stations with no other commercial music programming.
 
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davenn

Moderator
Sep 5, 2009
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Hi Dieter

The legality issue isn't in the distance/area the transmitter will cover
The legality is in the power output which for most of those consumer products is ~ 10mW
If you are in the USA .... think you are ... then the FCC regulations would clarify that

Dave
 

KJ6EAD

Aug 13, 2011
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FCC regulations usually specify an ERP (effective radiated power) at a given distance (usually 1m) received by a standardized dipole antenna. From a practical perspective, if you can't receive the signal with your best receiver outside of your own property, it's unlikely to bother anyone else and generate a complaint. Beware of generating harmonics at unintended frequencies; use a spectrum analyzer to check for out of band transmissions.
 
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