I'm thinking about a project that wiressly connects two or more arduino boards to transmit small data packages, with a distance of around 20m between devices.
The devices will be placed on the floor (the boards will be protected from damage), and antennas would be obtrusive and break off, so I need something that connects them in an inobstrusive package.
The data to be transmitted will be small, a single byte representing an ASCII character.
So, what options do I have? Which is the cheapest option available?
XBees come with three different styles of antennas. I get good range on the wire antenna modules - the wire is about an inch long. The chip antennas are smaller - just a pad on the XBee,
I would try 433 MHz modules before spending the money on XBees.
I am able to get from one end of my house to the other with the 433 MHz modules. My antennas were 17 cm of wire.
Bluetooth 4 (BLE) is good for 30m and I'm sure they are less than $10 each these days. No antenna, and no harder to use than the serial monitor. Essentially, they are serial without wires.
There's no such thing as "antenna-less" wireless. BT, nRF24, etc, they all need antenna.
The difference is that the antenna for those high-frequency radios (2.4Ghz) can be very small and often are just a PCB trace. Other options for small antennas are chip-antennas (common on mobile phones) and little coils for lower-frequencies (433Mhz, 915Mhz). You normally can find the coil antennas on many garage remotes and other sub-Ghz devices.
Regarding the device, the nRF24 is a great option to start with. The RF24 needs to be powered by 3.3V but it's 5.V logic compatible and should work fine for the required distance.