since I see you are in Italy, may not be applicable.  BUT....in the
states, pagers are dirt cheap and so is the service.  Radio Shack even
sells a limited range paging system.  Why re-invent the wheel, and you
can buy off the shelf product, unless of course its for fun, or you
really are trying to do something different that cannot be currently
implemented by these products.  Or cannot because of other limitations
being in a country that limits what is available here.

just my  .02 worth
> ----------
> From:         Giorgio Alboni[SMTP:rac1337@PROVINCIA.RA.IT]
> Sent:         Wednesday, September 24, 1997 1:42 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [OT] Radio transmission
>
> I need to make a people search system. I'd like tu use a 400 mW
> trasmitter
> (module at 433 MHz already made). The system must be made from more
> trasmitter/relay to assure a long distance trasmission. Some
> suggestion to
> resolve the trasmission conflict (all trasmitter with the same
> frequency)
> and  assure reception (receiver can't transmit).
> My idea is: the master trasmitter (link to a personal computer) send
> the
> message, all relay that receive the message wait fo some mS than send
> again
> the message.
> Each trasmitter must attach a self number for assure rebounds of the
> message.
> Can it work?
> Thanks in Advance.
>
>
> ===================================================
> Alboni Giorgio
> Faenza (Ra) ITALY
> E-Mail: rac1337@racine.ravenna.it
>
> http://www.geocities.com/SiliconValley/Heights/5444
> (Last Update: 19/06/97)
> ===================================================
>