How Far Can You Transmit FM Legally?

An FM transmitter is a radio transmitter with low power that broadcasts signals from portable audio devices to standard FM radio. Most of the transmitters plug into the device’s headphone jack and then broadcast signal over an FM broadcast band frequency so that it is picked up by any nearby radio.

This allows portable audio devices to make use of the louder or better sound quality of a home audio system or car stereo without a wired connection. Are used in cars in fixed locations like broadcasting from computer sound cards in the whole building.

Being low-powered, many transmitters have a short range of 100–300 feet, depending on the quality of receiver, obstructions, and elevation. They broadcast on an FM frequency from 87.5 to 108.0 MHz in most of the world.

Personal FM transmitters are used as a workaround for playing on car radios, portable audio devices, that don’t have an Auxiliary “AUX” input jack or Bluetooth audio connectivity. They are used to broadcast a stationary audio source, such as a computer or a television, around a home.

They are used for low-power broadcasting and pirate radio but only to a limited audience in near proximity. They can be used as a “talking sign” in real estate sales also.

The legality and maximum permitted power levels or field strengths of the devices varies by country. In 2006 these devices became legal in many countries in the European Union.

Industry Canada permits transmitters having output lower than 100 µV/m at 30 meters.

In the United States, Part 15 of the U.S. Federal Communications Commission rules specifies that no license is required if FM transmitters’ Maximum Effective Radiated Power (ERP) of 0.01 microwatts.

In Japan, no license is required for devices with the signal strength of less than 500 µV/m at 3 meters.

This is a stable, harmonic free, long-range FM transmitter circuit used for FM frequencies between 88 and 108 MHz. This can cover the 5km range. It has a stable oscillator because you use the LM7809 stabilizer which is a 9V stabilized power supply for T1 transistors and for frequency adjustment that is achieved by using the 10K linear potentiometer.

The output power of the long-range rf transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970, 2SC1971…
T1 is used as an oscillator stage to deliver low power stable frequency.

To adjust the freq. use a 10k linear potentiometer like this: if you trim down, towards the ground, the freq.drops and if you adjust it toward + it will rise. The potentiometer is used as a variable power supply for two BB139 varicap diodes.

These two diodes act as a variable capacitor when you adjust the pot. By changing the diode capacitance the L1 + diodes circuit makes a resonance circuit for T1. You may use transistors like BF199, BF214 but do not use BCs.

There are so many factors that determine how far the transmitter will go? A list of the factors to find out the range of FM transmitter:

1. The output power of the transmitter

2. The type of coaxial cable

3. The length of coaxial cable

4. The type of antenna

5. The height of antenna above average terrain

6. The amount and height of trees in area around antenna

7. The amount and height of buildings around antenna

8. The type of terrain around the antenna site

If You have 2 people using 100 Watt FM transmitters, one of them will get 15 miles whereas the other person gets 1 mile.

The reason could be that the one who gets 15 miles has a 2-Bay PCP2 antenna mounted on a 15 story office building located on top of a hill with downward sloping terrain in all directions whereas the other person has a cheap homemade antenna mounted on a 10-foot pole at ground level and terrain slopes upwards in all directions around him.

These are two examples in a myriad of possibilities. The best way to know what range of the transmitter will be is to set it up and try it out. Progressive Concepts help you determine what equipment you will require in order to achieve a certain range but remember that we can only estimate the range and not guarantee results until after a given system has been tested actually.

Can the existing power from, say, a 100,000 watt FM station burn a human if they’re working at a site with the antenna on? If a person is actually touching an AM or FM antenna, they can be electrocuted or get bad RF burns.

OSHA has determined that there is a level of FM radiation that is not permitted at ground level. So stations control the radiation of the antenna in a downward angle at the tower base and immediate vicinity.

OSHA rules will not allow a certain level of radiation on a tower if there is work being performed on it. The rules are specific to the frequency range.

More dangerous are microwaves.

Remember, for FM, the bays are the antenna and the tower is there to hold the bays up in the air.