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I was planning to interface an RF module to my PIC16F877A microcontroller and was exploring the web. I came across many modules which use 433 MHz as the standard frequency. Why is it so? Can't we change the frequency as per our requirements?

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The usage of frequencies is restriced by authorities in each country. The 433 MHz ISM band is free to use in many countries and thus very popular.

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    \$\begingroup\$ It is also worth noting that 433 MHz falls squarely within an amateur radio band, and you thus need to allow for the possibility of not just low-power interference (other ISM transmitters) but also high-power interference (amateur radio transmissions in that frequency range are commonly in the 10-100 W range). \$\endgroup\$ – a CVn Aug 14 '15 at 11:58
  • \$\begingroup\$ @MichaelKjörling Indeed. Car keyfobs tend to have issues with this, which is why it's nowadays relatively hard to get approval for a 70cm amateur radio repeater in an urban area. \$\endgroup\$ – Muzer Aug 14 '15 at 15:47
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This is because traditionally, the 433.050 - 434.090 MHz band could be used without license in many countries. For example it is license-free in all European countries. This is nice, since applying for a license is bureaucratic, has to be done nationally, involves a yearly fee, and is restricted to a geographic area.

However, the European Union has tried to harmonize the use of radio frequencies, adding some restrictions to the 433MHz band:

"Short range devices" (generic radio) using 433.050 to 434.040 MHz are not allowed to use an output power greater than 1mW (or 10mW in case they use 10% "duty cycle"). They are allowed to use 10mW from 434.040 to 434.900.

National exceptions to this still exist, and unfortunately radio amateurs are excluded from these power requirements.

And in other countries in North America and Asia, you can't use 433MHz at all. In some countries it is a restricted band for RFID container identification systems only.

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  • \$\begingroup\$ Why do you say unfortunately? \$\endgroup\$ – Felix Crazzolara Jan 23 '17 at 13:14
  • \$\begingroup\$ @Aresloom Because they are free to send with massive output power and thereby jam out all other radio use on the band. \$\endgroup\$ – Lundin Jan 23 '17 at 13:20
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    \$\begingroup\$ Hey if you cheap out on your front end design and filtering you deserve whatever happens to you during a contest. Also, Hams are actually secondary users, the MOD is primary at least in the UK. Seriously, the UK limit for hams is less then 60dB above the 1mW limit, and designing a front end to cope with that is not that hard (Just costs a few quid in LO drive power and filters). \$\endgroup\$ – Dan Mills Jan 23 '17 at 17:51
  • \$\begingroup\$ @DanMills The worst culprit of polluting the band is indeed not radio amateurs but various SRDs. But no design in the world is going to save you if someone else transmits on the very same carrier frequency as you do - it's all a gamble who wins in that case. \$\endgroup\$ – Lundin Jan 24 '17 at 7:29
  • \$\begingroup\$ Well yea, but no Ham is going to try to communicate there because of all the QRM.... The OOK am stuff should be really narrow band, you would have thought an IF bandwidth broadly comparable to CW, so a few hundred Hz or so would get it done, but the manufacturers do not I suspect shop at inrad or KVG for their IF filtering. FM kit will be wider of course, but capture effect should help there if you get the keyfob close to the rx. \$\endgroup\$ – Dan Mills Jan 24 '17 at 11:40
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Taking into account that whole automatic branch uses 433MHz I don't recommend You to use this frequency. I have done my engineering project with this modules: http://www.ebay.com/itm/433MHz-Radio-Transceiver-Transmitter-Sender-Module-Remote-Arduino-/252184498733?hash=item3ab75e122d:g:MyAAAOSw2xRYWe46 so I know what I'm talking about. If You want to save Your time, use any other frequency, for example 868MHz or 2.4GHz. 433MHz is full of noises. For instance if Your neighbor uses wireless thermometer be sure that You can receive nothing.

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    \$\begingroup\$ There is no difference between those public bands. There's as much noise on the 868 band, if not more, since the 1st overtone of 434 ends up there too. 2.4GHz is even worse, but it is a larger band. The reason your 433 modules didn't work well is more likely because they are cheap AM crap. The same solution would work poorly on any band. \$\endgroup\$ – Lundin Jan 24 '17 at 12:04

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