Then it's almost certainly illegal.
Unsupervised transmissions such as these are restricted. They cannot transmit continuously but for a limited time-slice out of every x number of seconds. It is detailed in FCC Part 15 regulations. It's been too long since I worked on this specifically and the recent loss of a HDD took my code and notes but I designed just this type of system a few years ago using battery operated transmitters as shown at...http://davehouston.org/adc.htm. I very recently discussed the choice of batteries and measuring battery charge in http://www.picbasic.co.uk/forum/show...9263#post99263.
In N. America most of the spectrum from ~300-500MHz can be used for this type of project but off the shelf transmitters/receivers are generally only available for 315MHz, 418MHz & 433.92MHz although other frequencies are available for volume orders. Most top shelf RF remotes (e.g. Philips Pronto) use 418MHz here.
In Europe, 433.92MHz is used for everything, fancy remotes, RF controlled lighting, RF controlled thermostats, RF controlled ceiling fans and much, much more.
Interference is not much of a problem because of the restricted time-slices and because the limited range of these low power devices. The FCC power limits are much lower than CE limits so that out-of-the-box commercial devices usually have only 30-40' range. It's quite easy (and legal) to improve the receiving side with a preamp and better antenna to extend this to hundreds of feet. It is not legal to mess with the transmitter side.
There are links to the FCC and other pertinent sites on my web page. http://davehouston.org/
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