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The RFM69HCW is an inexpensive and versatile radio module that operates in the unlicensed ISM (Industry, Science and Medicine) band, a set of frequencies set aside for low-power, short-range, license-free radios. It’s perfect for building inexpensive short-range wireless networks of sensors and actuators for home automation, citizen science and more.
This RFM69HCW module operates on either the 915MHz frequency or the 434MHz frequency (depending on the model you choose) and is capable of transmitting at up to 100mW and up to 300kbps, but you can change both of those values to fit your application. For example, you can maximize range by increasing the transmit power and reducing the data rate, or you can reduce both for short-range sensor networks that sip battery power. At full power and with simple wire antennas, we can get messages from one side of a large office building to the other through numerous internal walls. In open air you can reach 500 meters or more. With more complex antennas and modulation schemes, similar parts have successfully transmitted from space to the ground (by very smart amateur radio enthusiasts; your mileage may vary)!
The RFM69HCW uses an SPI (Serial Peripheral Interface) to communicate with a host microcontroller, and several good Arduino libraries are available. It supports up to 256 networks of 255 nodes per network, features AES encryption to keep your data private, and transmits data packets up to 66 bytes long.
Although the ISM band is license-free, the band itself is different in different areas. Very roughly, 915MHz is for use in the Americas, and the 434MHz version is for use in Europe, Asia and Africa. Check your local regulations for other areas.