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I'm having a 433 MHz doorbell which is too far away from the receiver (too many walls in between). Sometimes it works but other times it doesn't.

I thought that there could be a 433 MHz repeater (just as WiFi repeaters) and I found some indeed:

However these say that they are built for alarms systems and I'm not able to tell whether they could work as general purpose repeaters? I guess it depends on whether the 433 MHz communication is standardized? Reasons that make be believe so are the there is a Sonoff RF Bridge (https://sonoff.tech/product/smart-home-security/rf-bridge/) which pretends to be a generic 433 MHz RF module.

Am I on the right path? I don't know if anybody has heard about a generic 433 MHz repeater?

UPDATE: Photo of the receiver unit

Photo of the receiver unit

UPDATE: Photo of the sender unit

Photo of the sender unit

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    \$\begingroup\$ It seems like a difficult problem; from what I know of a single 433 module, there are several different data rates and even different encoding mechanisms that can be used. It would need a relatively smart relay to be able to identify these on the fly. A relay for a specific system would know exactly what to expect. Therefore I doubt that you’d be able to find a general-purpose relay. \$\endgroup\$
    – Frog
    Apr 25 at 20:09
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    \$\begingroup\$ I guess it depends on whether the 433 MHz communication is standardized? Which it is NOT. Many devices use OOK modulation and send data packets of various lengths. Other devices use more complex modultion schemes. If that alarm system repeater would work with your doorbell then I think you are extremely lucky. I would not rely on it and would consider to build a repeater myself or buy a new system for which you can buy a repeater. \$\endgroup\$ Apr 25 at 20:29
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    \$\begingroup\$ I acknowledge you're looking for a repeater, but it might be easier to just improve one or both antennas. I'd start with the receiving antenna at the doorbell -- that wouldn't violate FCC regulations. Open the case and post a picture if you need suggestions about where to connect an external antenna. \$\endgroup\$ Apr 26 at 0:16
  • \$\begingroup\$ Thank you everyone, all answers are clear and addressing my questions (I upvoted all). I also added a photo of the receiver unit. \$\endgroup\$ Apr 26 at 8:31
  • \$\begingroup\$ I'm sorry, now I realized @MarkLeavitt said the doorbell (not the receiver unit), I will make a picture of that, too. \$\endgroup\$ Apr 26 at 8:38
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The band isn't standardized and what equipment that's allowed on it differs from country to country. In EU where this band is most popular, you have a EU-wide directive that's regulating the 434MHz band, but individual countries are allowed to make exceptions. There are more specific rules for 433MHz than for 434MHz in EU, though in many countries these are regarded as the same band.

Outside EU it turns even more complicated, since every country has it's own rules for spectrum allocation. In North America and South-East Asia you can barely use the band license-free at all. For example several countries have reserved the band for cargo container tracking systems/RFID.

Apart from the lack of standards and international legislation, it is not possible to make a "generic repeater" simply because it is a license-free band, where any form of signal is allowed. Imagine the following scenario:

  • In a central location of a city, you have 434MHz transmitters for doorbells, thermometers, telemetry, car locks, garage doors, radio amateurs, "smart" homes etc etc. So the band is already quite crowded.

  • A new building site is put up nearby. This means numerous cranes, pile drivers, trucks and other machinery using this band too. And they all use continuous transmission since they are control systems. You'll have things like concrete trucks coming in at any time, each with a 434MHz remote. Overall some 10 new remote systems on the band, in addition to what was already there.

  • Now someone invents a generic repeater which just scans the band for anything that appears to be a digital signal, of any bandwidth, with any modulation, then repeats the incoming signal the best it can with the maximum allowed output power (generally 10dBm E.R.P), at the same bandwidth. It will have to be very smart because signals could be modulated as FSK or ASK or OOK or something else... Where each modulation technique comes in different flavours: 2-FSK, 2-GFSK, 4-GFSK- 6-GFSK etc etc...

  • If they somehow get this repeater working as intended, then that means all of the mentioned sources will have their signals amplified in whatever direction the repeater is set to point (either directional or omni-directional). This will lead to a massive increase of local signals on the band. Because normally all these transmitters may only have some 10 to 100m range or so, after which they wouldn't really interfere with each other much. But because of the repeater, we potentially have every device in the area interfering with each other suddenly. With the result that no communication in the area will be possible at all - turns out we haven't actually invented a repeater but a signal jammer.

  • And congratulations, this repeater has now not only blocked the 434MHz band but is also causing major disturbance on the 868MHz one where the first harmonic ends up.

You can design repeaters for the band but they have to be specific for a certain equipment, as is the case for Wi-Fi. All the repeaters you have linked are specialized for a certain kind of signals.

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  • \$\begingroup\$ Agreed, this is a good explanation, thanks! \$\endgroup\$ Apr 26 at 13:02

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