jpexg@hermes.ai.mit.edu (John Purbrick) (01/03/89)
I'm thinking about an intrusion alarm for automobiles which would send a radio signal if a door were opened. Easily-available transmitters are very low power--like Radio Shack's 50MHz walkie-talkies give "up to" 1/4 mile range, where "up to" can mean anything at all. CB radio would do it (I'd like a range of 5 miles) but existing traffic is heavy and unpredictable. At least the system's bandwidth would be minuscule--one bit every 5 minutes would be OK--say a coded digital signal modulated onto an audio-range tone, or use frequency-shift techniques. The problem is the radio carrier. Would it be possible (purely as a design exercise, since this would be blatantly illegal) to use a broadcast-band AM or FM signal? Then a cheap and highly portable receiver could be used, say a credit-card sized receiver with some additional decoding circuitry. Radio Shack and Heathkit both have "wireless mikes" which broadcast to an FM radio, but I assume that their range is almost zero. Could they be souped-up?
craig@hp-lsd.HP.COM (Craig McCluskey) (01/05/89)
Call the FCC and inquire about their booklet on Part 15, Unlicensed
Transmitters. You are actually allowed to transmit on those frequencies
(with a specified low field strength {which isn't as easy to figure out
as power}) as long as you don't disturb the licensed services.
I have a copy of the booklet (somewhere at home). It lists all the
applicable criteria (field strength, power, frequencies, etc., etc.).
Craig McCluskey
HP Logic Systems Division
Colorado Springs, CO
...!hplabs!hp-lsd!craig