In article ,
mm wrote:

Today on the news I heard that a big bunch of electronic garage door
openers weren't working in Churchville Maryland because the govt. at
the Aberdeen Proving Grounds was doing something with a satellite or
something. Tomorrow their going to do the same thing around Aberdeen.

People are paying techs to change the freqs, but some may have paid
for other repairs by mistake, one would assume.

Someone in charge admits he didn't get the word out well enough.

1) Don't they assign frequency ranges to things so that this sort of
thing doesn't happen?

Not really. The garage door openers are "Part 15" devices - they
don't have a reserved frequency allocation. Instead, they (and other
low-power devices) are allowed to use a wide range of frequencies that
are primarily allocated for other radio services.

2) How could the use of a frequency mess up the garage door openers?
Even if the govt. signal was stronger, why wouldn't the opener still
work? If the govt. signal was picked up by the opener, how come the
doors didn't open or shut. (Apparently they didn't since they would
surely have mentioned that.)

A lot of unlicensed (Part 15) devices such as garage door openers and
car-alarm keyfobs use frequencies around 433.920 MHz.

The primary usage allocation for this frequency band is government
echolocation (radar). Ham radio operators have a secondary allocation
(i.e. they can use it as long as they don't interfere with government
radar). Unlicensed users are tertiary, and have *no* legal protection
against interference from licensed, or other unlicensed users.

The transmitters for these Part 15 devices use very low power, by
design and law. The receivers for them are, well, let's say
"inexpensively made" - they tend to be reasonably sensitive (so that they
can pick up the weak signals from the transmitters) but are not at all
selective.

Strong signals from other transmitters, on the same or nearby
frequency bands, can overload (saturate) the RF front end circuitry in
these receivers - a phenomenon known as desensitization or "desense".
When this happens, a strong transmission can completely block the
weaker one, even if the actual frequencies of the two transmissions
don't overlap at all. It's sort of like trying to hear a low-pitched
voice speaking quietly in the next room, when somebody is blasting
your eardrums with a piccolo :-)

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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