My wife asked,
How does that work? Is it by radio? How come the various signals in the air don't bump into each other?
That's a tough bunch of questions. The short answer is yes, the remote sends a scrambled radio signal to the garage door opener/closer. The opener sees that signal. It recognizes one push as an instruction to open the door and two pushes as an instruction to close the door. It turns on the motor for a moment, and the door closes.
The signals don't "bump into each other" most of the time because government agencies make sure all manufacturers of transmitters put their radio signals into the right places "in the air".
Or in more technical terms, they allocate the frequency spectrum.
Here's the US allocation chart and also the UK allocation chart (be ready to ZOOOOM)
Sheesh. Can you believe those charts? A simple list was all I needed.
It seems like there are millions of frequency bands. Here are the main ones:
|ELF||extremely low frequency||3Hz to 30Hz||100'000km to 10'000 km|
|SLF||superlow frequency||30Hz to 300Hz||10'000km to 1'000km|
|ULF||ultralow frequency||300Hz to 3000Hz||1'000km to 100km|
|VLF||very low frequency||3kHz to 30kHz||100km to 10km|
|LF||low frequency||30kHz to 300kHz||10km to 1km|
|MF||medium frequency||300kHz to 3000kHz||1km to 100m|
|HF||high frequency||3MHz to 30MHz||100m to 10m|
|VHF||very high frequency||30MHz to 300MHz||10m to 1m|
|UHF||ultrahigh frequency||300MHz to 3000MHz||1m to 10cm|
|SHF||superhigh frequency||3GHz to 30GHz||10cm to 1cm|
|EHF||extremely high frequency||30GHz to 300GHz||1cm to 1mm|
A global group called the International Telecommunication Union manages these definitions.
Some frequencies are used by the military. Others are considered international because they are used by satellites, aircraft and other non-local, non-national devices. The rest of the frequencies are allocated by agencies (FCC in the US, Ofcom in the UK, etc.) who monitor usage and figure out ways to maximize use and revenue from each frequency band.
Ofcom recently did an experiment where they loaded antenna arrays on top of regular vehicles and drove all around the UK, monitoring how crowded the frequencies were. After scanning hundreds of pages, I almost gave up trying to find a simple way to explain the issues. But I will give it a try.
The regulatory agencies need to
- Decide what wireless services people are using
- Predict how people might use those or more services in the future
- Determine which technologies / frequencies each service uses
- Figure out how quickly the demand might grow for each kind of wireless
- Determine the geographic distribution of the users: rural wide open; cities full up
- Guess how quickly the technology might change and get more efficient at using its space
- Decide which frequencies will be full first
- Decide if other frequencies can give up space (military? satellite? aircraft?)
But in doing this research I found something that would be really cool to have - a full-spectrum jammin truck. You can drive it around and block virtually all frequencies across all spectrums. All electronic services come to a halt.
I can confidently make this prediction - They won't sell it to us!