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I would like to build an FM transmitter that transmits a signal to all fm frequencies simultaneously, so that any nearby radio tuned to any frequency can receive this signal. Is it possible to do this with gnu radio and it would be legal?

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    \$\begingroup\$ Very, very hard and most likely illegal, unless you used a very-low power transmitter. \$\endgroup\$
    – jaskij
    Jan 25, 2016 at 16:43
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    \$\begingroup\$ It will certainly not be legal. \$\endgroup\$
    – Chu
    Jan 25, 2016 at 16:44
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    \$\begingroup\$ Assuming you mean a standard FM receiver for listening to broadcast radio, that would be around 100 frequencies you would have to transmit on simultaneously. Since there won't be an adequate separation between channels, they will all interefere with on another and make a mess - squeaks, squeals, honks, distortion and other noises on receive. And, as already mentioned, it would be illegal since you want it to be powerful enough for other people to receive it. \$\endgroup\$
    – JRE
    Jan 25, 2016 at 16:49
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    \$\begingroup\$ This could turn into an interesting homework problem about bessel functions... \$\endgroup\$
    – pipe
    Jan 25, 2016 at 17:08
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    \$\begingroup\$ And guys, legal or not is decided by the laws where the user lives, and most places I know of (Europe, US, Canada, Japan, from reading wikipedia) allow unlicensed FM transmissions if it's low enough in power. \$\endgroup\$
    – pipe
    Jan 25, 2016 at 17:21

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In days long ago, this was done by transmitting a strong signal at the IF frequency of 10.7 MHz. Most radios in that era did not have sufficient shielding in the IF section and this signal was received easily.

I have no idea if this will work with the majority of modern FM broadcast-band receivers. You would simply have to try it.

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    \$\begingroup\$ Transmitting an unlicensed strong signal on any frequency will not be legal. \$\endgroup\$
    – Chu
    Jan 25, 2016 at 17:36
  • \$\begingroup\$ It wouldn't work universally. There are FM tuners with different IF frequencies. Leak Troughline for example, 12.5MHz. \$\endgroup\$
    – user207421
    Sep 18, 2016 at 0:00
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Don Lancaster came up with a novel theoretical solution a long time ago. In North America, the commercial FM broadcast range is from 88.3MHz through 107.9MHz, with 0.2MHz channel spacing. He theorized that you could take a 100kHz squarewave oscillator with very sharp edges to create such a transmitter. Since it's a squarewave, you'll only have the odd harmonics. And if the oscillator output is sharp enough, you'll have a lot of them. And lo and behold, the 881st harmonic is at 88.1MHz, the 883rd at 88.3MHz and so on, up to the 1079th harmonic at 107.9MHz.

I've never built such a thing, and I am not overlooking the technical problems involved in cleanly isolating and amplifying the very weak signals you're after, but it's definitely a novel approach, and one that seems to have caught the attention of at least one patent application.

His August 1995 Tech Musings article is located on his tinaja.com website.

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It's an old idea, and they actually exist now. They are used (or were being trialled) on emergency services vehicles in Australia.

To create the signal, you use a modulated Comb generator (with appropriate channel spacing) and filter the results to conform with the band.

They would be highly illegal for a private person to use of course.

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A comb generator is a good place to start.

Applications might be to provide announcements to vehicles traveling in tunnels or on emergency vehicles or trains to announce danger to passenger vehicles.

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Even if it were legal (which it MOST CERTAINLY IS NOT LEGAL!), it would be technologically and practically difficult/impossible. If you look carefully, you will see that licenses for broadcast stations (AM, FM, TV,) are NEVER issued for ADJACENT channels/frequencies. Because that would guarantee interference between the two adjacent signals. Regulating authorities use a complex variety of calculations to determine frequency allocation, allowed maximum power, and directional antenna characteristics to avoid interference with other channel users within range. And all of this is regulated not only on the national level, but internationally via the International Telecommunications Union (ITU).

Others have suggested various clever approaches as using comb filters and insanely high-order harmonics, etc. But it would be practically impossible to modulate any of those schemes with any information that could be reasonably demodulated to yield anything intelligible.

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    \$\begingroup\$ Not legally possible, if transmitting significant power. But, pulse generators with few-nanoseconds edges are impractible/impossible? No, it's 1970s technology. And, to FM modulate a pulse-generator is impossible? No, that's more like 1930s technology. The only slightly-difficult part would be in sharply limiting the pulse-gen harmonics to only cover FM broadcast band and nothing adjacent. \$\endgroup\$
    – wbeaty
    Sep 17, 2016 at 23:38
  • \$\begingroup\$ The reason for the impossibility is not the technology of the transmitter. The limiting factor is the performance of the cheap, mass-produced consumer receivers. They are not designed for the selectivity that would be required for even alternate-adjacent channel spacing, not to mention true-adjacent. Check the radio station frequency allocations in your area and see for yourself. \$\endgroup\$ Sep 20, 2016 at 3:46
  • \$\begingroup\$ I'm assuming a single audio channel, which would modulate all the carriers coherently in synch. Closed-spaced channels certainly interfere with each other when each carries a different audio signal. But what comes out of a receiver if all audio signals are the same, and all FM carriers move up and down in synch? "Selectivity" has a different meaning then (if not simply vanishing entirely.) \$\endgroup\$
    – wbeaty
    Oct 3, 2016 at 23:30
  • \$\begingroup\$ The issue isn't the modulated program signal. The issue is the interaction between the carriers of adjacent channels. Almost no receivers will handle this gracefully. Not that it is really a practical exercise anyway. \$\endgroup\$ Oct 4, 2016 at 0:56
  • \$\begingroup\$ This would be just as illegal as a cellphone jammer, and should not even be on this SE. \$\endgroup\$
    – SDsolar
    May 19, 2017 at 5:04

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