1.https://www.pjrc.com/teensy/td_libs_VirtualWire.html
These modules worked very reliably when sitting only close to each other on a table. When separated by about 20 feet with ordinary office furniture, and a 13 cm wire attached to each (in the middle of the 10 to 15 cm suggested), they were able to communicate, but approximately 20% of messages were corrupted.
Perhaps using better antennas could help, but each board has a loading coil that appears to be designed for relatively short antennas, and no other documentation seems to exist regarding best antennas.
These modules can work well for low performance, non-critical applications. For more demanding applications, more sophisticated (and more expensive) RF modules should be considered.
2.http://www.dipmicro.com/store/RF315PAIR
When using an external antenna a 1/4 wavelength is recommended. Ideally use 50 impedance ohm single-core wire, the length of the antenna 433M is about 17cm (1/4 wavelength). When locating the receiver antenna keep it as far away as possible from shielded areas, high voltages, and any other possible interfering frequencies.
3.https://www.sparkfun.com/products/retired/8947
1. Tune your antenna to the proper length. You can use this site to calculate a suitable length depending on frequency. (http://www.csgnetwork.com/freqwavelengthcalc.html). Full wave is most optimal, but ½ or ¼ is next best. You can experiment by adding/trimming length to the antennas. This will reduce your SWR. (http://en.wikipedia.org/wiki/Standing_wave_ratio)
4.http://www.seeedstudio.com/depot/images/product/VirtualWire.pdf
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