"That's also why automobile keyfobs don't have much range in a store parking
lot when you try to lock the doors....or unlock. Also a lot of X band radar
watching you....."
Those typically operate between 310 and 315 MHz, with some centered around
27 MHz, a few at 430 MHz, rarely at 900 MHz, and units going back to the
late 1950's from 200 - 400 MHz at random. Check the FCC ID# to verify
operating frequency. If having trouble with it, try a new battery, or check
the antenna at the receive end. You should be able to get a couple of
hundred feet from an average vehicle installation. I once monitored 315 MHz
daily, surprisingly little activity, even in a big city, and devices are
very low power, but you'll hear some thermometers, doorbells, tire pressure
monitoring systems, garage door openers, et cetera, but even in a high
density area, signals are few and far between, most having moved to 440 MHz,
900 MHz and 2.4 GHz. It is becoming popular again. When people moved due
to congestion, it was no clear again for use. Put "315 MHz" into ebay and
see what comes up. Try "434 MHz" as well. 418 MHz is another rarely used,
but higher power device frequency. I usually hear weather stations doing a
data transfer on that frequency, coverage is several square miles.
THe X-band systems are typically door openers, and due to size, were
switched to K-band/IR combo units about twenty years ago. There is still
some X-band equipment out there, but very few and far between. Look for a
box about the size of a 3-pack of VHS cassettes over a door. By comparison,
the K-band is about the soze of your fist, then the small integrated units
are within a black bar over the door. The microwave unit behind a bulge in
the center.
On the plus side, with the X and K band application specific openers being
phased out for many, many years, I've managed to accumulate a few bankers
boxes of them to play with. It' not like in the 80's and 90's when the cost
was comparable to them being cast in heroin and gold plated, even at
swapmeet prices, new-old-stock long-range units can be had for typically $10
and less.
Kurt
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