How to create FM Radio Mesh

Question

What (devices transmitters, receivers and transceiver) do i need to create a fm radio mesh, instead of using a single transmitter with big antenna, i want to build fm transceiver mesh radio to cover an area, whereby when you tune in to that channel and you are within the area you'll get to hear what is broadcasted in that channel. Lets say a coverage of 30kms

Answer 1

whereby when you tune in to that channel and you are within the area you'll get to hear what is broadcasted

That is impossible.

FM is pretty much the worst technology you can use for meshing:

• You can't put multiple transmitters on the same frequency; FM can't deal with that, and there's no mechanism to share the same frequency in FM
• FM doesn't allow for sensible equalization, meaning that with FM you cannot have a multiple, geographically distributed transmitters that forward the same signal. Such a network would be called Single-Frequency Network (SFN) (and it's not even meshing – it's just bringing the same signal to different transmitters and transmitting from there; which is what radio stations would like to do, and why radio stations all over the world are happy to get rid of FM and move to technologies that allow you to do SFNs. DAB is such a technology.)

You could come up with a cellular network, where you use at least 4 different frequencies, and then you have a central planner person, who assigns different frequencies to transmit the signal on. You need at least 4 different frequencies, because no two neighboring transmitters can use the same transmit frequency (because a receiver that can receive both can't work); the problem of "how many different frequencies do I need to ensure that no two neigboring transmitters have the same frequency" is the same problem as "how can I color any map, so that two neighboring countries don't have the same color". And this problem is solved - it's the famous Four Color Theorem. You need four colors.

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So, if meshing FM is impossible, and even transmitting the same signal from transmitters in different places is impossible, what could you do instead?

1. Option: Do not use Meshing. Use something with which you can cover 30 km directly. Do not use FM.
   Your use case sounds like a very classical use case for which Digital Radio Mondiale was invented. You don't need much power or a high mast to cover an area of 30 km in radius with DRM. Transmitter software is free. There's large installations of that – All India Radio uses it, BBC World, Vatican Radio… It can, but doesn't have to, work in the same band as FM (it's commonly deployed in 47 – 68 MHz, 87.5 – 108 MHz or 174 to 230 MHz).
2. Option: Instead of a Mesh, build a Single-Frequency Network. Do not use FM.
   Should your terrain be too mountainous to allow for direct line of sight connection or one with few reflections, but you can still establish a connection for the audio signal to multiple transmitters, you can build a SFN.
   Broadcasters all over the world do that, using DAB+, which replaces FM currently all over Europe, for example. DAB+ is much more robust at lower transmit power than FM, while delivering the same audio quality or better. It is capable of equalization, which makes it theoretically and practically possible to build SFNs. Transmitter software is free, but you will need multiple transmitters and a way to synchronize them (GPS clock, probably), and a way to get the signal to each transmitter. As far as I'm aware, there's no free transmitter software yet that does the synchronization, so this will need some development effort.
3. Option: Use existing mesh-capable networks.
   There's a few technologies that allow you to build wide-area mesh networks (as I explained above, FM is not one of them). The Wifi-adjacent IEEE 802.11s standard allows you to connect many Wifi routers to cover whole cities with wifi (or use B.A.T.M.A.N.). You can use that same technology to stream arbitrary data. However, since wifi is not a broadcast protocol, your network throughput would scale proportionally to the number of listeners. You might alleviate that with IPv6 multicast groups, but I'm not knowledgeable enough about these in cooperation with IEEE 802.11s in practice.
   To transport stereo sound that is about the same quality as broadcast FM (i.e., good enough for music, but not quite as good as spotify or CDs), you roughly need 24 kb/s of bandwidth (click on "raw" here to download an example) (for speech, you need much much less, think 8 kb/s). So, your worst-case network load is $N_{\text{listeners}}\cdot24\,\text{kb/s}$; with a mediocre Wifi link carrying say 12 Mb/s of usable payload, a single Wifi origin would allow for 500 concurrent listeners - whether they listen to the same broadcast program or different ones. If you can add cable connections between a few of the meshing access points, you get potentially in the order of a million subscribers.
4. Option: Use existing cellular networks.
   Same audio bandwidth consideration, but data distribution becomes the problem of the EDGE/UMTS/LTE/LTE+/5G carrier. Say you have someone sitting at home listening to 24 kb/s audio for 16 hours a day, 30 days a month. That amounts to 5.2 GB per month. You have conflicting information on where you are (you say "Africa, where regulations are low" in your comments, but your bio says "Virginia, USA", which is not very close to the parts of Africa with low regulation), but a lot of African countries have excellent mobile network coverage at affordable prices. This might be a surprisingly cheap option!