just brew it! wrote:
Have *you* acquired/moved/reconfigured any electronics in your house around the time when the trouble started? Just because something isn't a radio-frequency device doesn't mean it isn't radiating EMI. For example, it could be a PC with sub-standard EMI shielding that got moved closer to the carport or had its BIOS settings changed (tweaking clock settings or disabling "spread spectrum" can shift the frequencies of the emitted EMI).
The one possibility that I know of is that I rebuilt my main PC back in October, and then moved 80% of the old hardware into the media PC in the basement. However the media PC is normally shut down, and the new build upstairs -- which would be physically closest to the carport -- is in sleep mode whenever I'm not using it. From an RF perspective it is rather tightly sealed up inside an Antec Sonata (even have all of the spare slot bracket blanks installed). No funky acrylic windows or anything.
I can't believe they still install car alarm systems in that band. It has been a known issue that ~300-500MHz gets TONNES of interference from police/radio/etc.
Only thing you can do is the top gear trick of sticking the key to your head when you try to unlock it.
That one I did try but without much effect. Regarding the operating frequency, this alarm was probably installed by the original owner or the dealer when the car was new, back in 2002, so the technology may be old, or there may be FCC restrictions, who knows. Some garage door openers are also 300MHz, and in any case, the signal only has to transmit very briefly so interference is usually less of a concern than if the signal had to broadcast continuously (e.g. WiFi).