Signal Strength - sparkfun RFM69HCW Hook-Up Manual

Running the Sketches
You now have two nodes that will send messages to each other, but, to use
them, we'll need to open two serial terminals.
One way to do this is to run two separate Arduino IDEs. You'll have to
actually start Arduino twice - you can't just open a "new" code window from
the first IDE.
Set one IDE to the COM port of the first node and the other to the COM port
of the second. Then, open serial monitor windows from both IDEs.
In each serial monitor window, you'll need to set the baud rate to 9600 and
make sure the "line ending" dropdown is set to "carriage return." (The
example code uses carriage returns as a signal to send a packet.)
Once both windows are up, you should be able to type messages in the text
entry box at the top of one window and press return to send the message to
the other window. Try it!
You can also try turning ACKs and encryption on or off, using the
"broadcast address" (255), etc.
PROTIP: Having two Arduino IDEs open can lead to confusion when
compiling and uploading code. If you load the example code into both
IDEs, save one of them as a new name to keep the compilation and
uploading separate.
You can also try this one of the many serial terminals mentioned in this
tutorial.

Signal Strength

You might have noticed the "RSSI" number that the example code prints
out. "RSSI" stands for "Receive Signal Strength Indicator;" it's a
measurement of how strong the transmission was when a message was
received.
This number is in decibels (dB), which means that when nodes are close to
each other this will be a low number like -25, and, when they're further
away, they'll be higher numbers like -50. Usually you'll ignore this number,
but you could use it to estimate how far away nodes are from each other,
provide a warning when the link is becoming weak, or, in sophisticated
applications, vary the transmit strength so that you don't waste power
unnecessarily.
Resources and Going Further
For this tutorial, we connected both of the nodes to the same computer to
make it easy to send messages from one node to another. But, remember
that when you do your own projects, your nodes might be attached to
weather stations, garage doors, Halloween decorations, drones, robots,
your cat... the sky's the limit!
The example code has comments that explain how messages are sent and
received. Feel free to use it as a basis for the code in your projects.
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