What’s stopping another person/company from transmitting on the same radio frequency as an established radio channel?
In: Technology
Government and diminished usability, since interference would waste both the signals and distort both information if transmission is happening simultaneously.
Nothing physically. If you have a transmitter, and try to broadcast on the same frequency as someone else, the more powerful transmission wins. The signal will weaken the further you get from a transmitter so if your transmitter is weaker then theirs, then their will be a radius around your transmitter where your signal is more powerful and will win. You can observe this if on a long drive and have the radio on. Sometimes you lose the first station and another starts coming through.
Now, if you are interfering with a licensed broadcaster, depending on who that is, either they or the FCC have equipment to locate sources of interference. If your mucking around on a band that is safety critical, they will find you real fast. Think Air traffic control, police band, or railroad radios. Or if you are messing with communication infrastructure like cell towers where one improper transmission could mess up service for hundreds of simultaneous users.
A lot of good answers here about nothing really physically stopping you, and it’s more about enforcement, but I have a short story about how that enforcement can work:
I worked for an aircraft radio manufacturer. Part of our testing involved making sure that the emergency channels at 121.5 and 243 MHz worked correctly.
This involved actually transmitting at those frequencies in the lab with attenuators so it didn’t go far, but could be detected with lab equipment. The lab also had connections to proper antennas on the building for testing long range communication.
A new guy once accidentally connected to the outdoor antenna, and started testing the emergency frequencies. Since he was connected to the outdoor antenna, his equipment wasn’t detecting anything so he left the radio on while trying to troubleshoot.
About 5 min after he started, the lab manager comes in and starts shouting “TURN OFF ALL RADIOS, NOW”. He had gotten a call about an emergency transponder that wasn’t responding and was coming from our area.
That’s how fast the FAA heard and responded to the emergency transmission.
Usually practicality, both in owning the power amplification equipment and in erecting an antenna viable for such transmission. The power that regional FM stations use is very well into the thousands of watts so creating something to overpower such a device without being FAR closer to the latter source is unlikely. For convenience sake you can think of the barrier to transmission on an established frequency as a yelling match between you and the operators of these radio stations, only you’re restricted to a whisper and they can scream like banshees.
If someone gets past practicality then as others have said the FCC (or whatever spectrum regulating entity there is for a country, and I’m fairly certain all countries have one) stops them. Definitely look into a spectrum chart from the FCC sometime, it’s full to the brim with allocations to various entities and organizations.
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