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  #1  
Old 07-10-2012, 12:26 AM
albinali albinali is offline
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Default 312Mhz vs 915Mhz Radio

I noticed that all Personal Emergency Response Systems (PERS) devices seem to use 312Mhz for their transmission. The device is usually used by the elderly and has a button when pressed the user is connected with a call center through a base station. This 312Mhz band is only approved for alert type transmissions by FCC. In my case, I am trying to have regular messages as well which would not work for the 312Mhz range so I am considering a 915Mhz radio.

Does anyone know why the 312Mhz is better for emergency response? What are the implications/downside of using a 915Mhz radio? Can they be overcome?

Thanks and appreciate your response.
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Old 07-10-2012, 08:22 AM
WBahn WBahn is offline
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Take this with a grain of salt, but my very limited understanding is that what is meant by "alert-type transmissions" are short, occasional broadcasts as opposed to sending data or messages of any length. For instance, many garage door openers and car remotes use the 312MHz band and they have nothing to do with emergency response. My understanding is the restriction is intended so that all of these devices can coexist in a licence-free environment and still function reasonably reliably. For instance, if you try to open your garrage door at the same time that your neighber is trying to unlock her car, and if you happen to be using the same frequency (or sufficiently close frequencies) within the 312MHz band (I don't know to what degree it is channelized and whether or not that is regulated), then you will jam each other. But how will each of you react? You will both see that nothing happened and then both of you will press the buttons again, but in almost all cases you will press them this time at just slightly different times and both systems will work. But if her keyfob was uploading her day's itinerary to her car's navigation system and/or your's was downloading the picture from the garage security camera, then this method of deconfliction wouldn't work very well.
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Old 07-10-2012, 03:16 PM
K7GUH K7GUH is offline
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In the U.S., the 900 MHz band is overrun with everything from baby monitors to drug trafficking. This can be a major annoyance if you would like to use this band for some specific applications. If possible try out your application before putting any money into the project.
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