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The Science, Technology, and Method of Operation to Broadcast to Other Vehicles for Purposes of Harassment

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About 2 months ago, I had an experience where I was driving my car in town and I heard someone talk through my radio. I actually know the identity of this person, who was in their car and in near physical proximity, approximately 10 to 50 yards, at the time I heard their communication. During their communication, I drove straight through a roadway intersection in town, while they made a left hand turn onto the perpendicular roadway. As I looked left out of my driver side window, I saw a young female sitting in the front passenger-side of the vehicle looking my direction and laughing, which offers more confirming evidence that my experience is indeed fact, rather than my imagination or some type of schizophrenic-related auditory hallucination.

I have yet to question this individual--they would deny it anyways, and currently, I possess the social advantage in that they are likely oblivious to my knowing of their actions--and/or take other action until I have all of my facts regarding how such communication can be carried out. I would appreciate any feedback in terms of products, scientific/engineering knowledge, and organizations related to this incident. My car radio is a stock AM/FM radio that includes 6-disc CD audio, Sirius XM with antenna installed, and mp3 capability.

From my current research and understanding, I see that one can purchase FM transmitters and broadcast locally from within their vehicle. However, in order for someone in another a vehicle to hear their broadcast, they must be in near proximity and have their radio station tuned to their FM frequency. Such transmissions can be considered "Pirate Radio" and are considered illegal by the FCC. Additionally, it also appears one can purchase transmitters to broadcast in the S-Band frequency, for which Sirius XM is broadcasted.

Here are some of my questions:
  • Is it possible to broadcast to my vehicle's radio without my action to specifically tune into their broadcast frequency? My thoughts are that they would need a method to broadcast to all possible radio MHz frequencies. This type of system is too complicated, but could work in theory.
  • Is it possible to determine the radio frequency that my car is currently set to? This includes if (1) the FM radio is currently turned ON, (2) the car radio is ON but another audio source (CD, Sirius XM, etc.) is in usage, and (3) the car radio is OFF. My thoughts are that a basic car radio 'receives' radio frequencies but does not 'transmit' them, but I wondered if there could be a 'default' or 'standardized' radio frequency for receiving transmissions that are occurring beyond basic radio functioning, where we must get really deep into the engineered design of the device, manufacturer's patent, etc., which isn't such common knowledge.
  • Is it possible to overload a radio frequency antenna/receiver with a transmitted signal? Here I would look to the Sirius XM antenna/receiver as the targeted component. Despite this, the circuitry within the radio would have to be overloaded to produce audio through the speakers, since in theory, I could have my radio in many different possible usage states besides Sirius XM: radio ON, (1) cd-player mode, (2) FM player mode, (3) AM player mode, and (4) radio OFF.

I myself have an interest in science and technology. However, there are so many avenues and specializations that one could pursue and have knowledge of, yet be highly ignorant of other avenues, such as I am here. It's unfortunate to see the possible actions of harassment and ill intent that could be carried out with such technology. I thank you for your time to review and respond to this post.
Most likely a case of RFI (radio frequency interference). A nearby transmitter, such as a CB or ham rig, can simply overload the audio section of a nearby receiver, be rectified, and come out the speaker.
my guess would be a CB radio with an illegal linear amplifier. it would need to be an AM transmitter (and a CB fits the bill). the RF gets picked up in your speaker wires, rectified by your radio's output transistors, which applies voltage across the speakers, and you get sound. this can happen with the radio on or off. Sirius/XM uses a digital streaming protocol, which as far as i know is proprietary and so difficult to reverse engineer for a prank. an FM transmitter has a constant amplitude, because fm works by wobbling the transmitter frequency to follow the audio waveform. if they had been transmitting FM you probably wouldn't have heard anything.

did this person's car have any antennas on it (other than the usual cellphone or car radio whip near the windshield on the passenger side)?
Thank you for taking the time with your responses.

Your response makes sense, because it has also happened when the radio is off. If it is a CB antenna, then I will know exactly what to look for. The transmission does not always sound like a person's voice, like it does when you listen to a radio station or are talking to someone nearby directly. Rather, the transmission sounds like it has an echo effect and that it also sounds hollow. Does that make sense?
It could just be a FM transmitter that people use to hook their phone up through their radio and / or play MP3 and DAB through. I've seen one of these transmit to 50+ meters before. I wouldn't have thought it would be too hard to modify for a slightly higher power output to transmit over a popular frequency that many people are tuned in to.
If this was a pirate radio broadcast, the maximum unlicensed RF output is 100 milliwatts ( 100 mW). At that close range, it is possible to interfere with your selected radio station. Other folks who tuned in may have called the radio station to inquire about the phantom voice(s) heard over the program. In our connectivity environment nowadays, it's possible to be locked out of your car, home and office by a determined cybercriminal - rolling code technology is passe.
If this was a pirate radio broadcast, the maximum unlicensed RF output is 100 milliwatts ( 100 mW).
If it was a "pirate" station, what makes you think they would follow the FCC maximum power limits? Criminals by definition do not follow the laws.
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an FM transmitter would not cause what you are describing, since FM signals are constant amplitude. it would have to be an AM transmitter. a CB radio would definitely be the most likely.
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