tmanson said:
The Fobs work at 315 MHz.
https://gullfoss2.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachment_id=170549&native_or_pdf=pdf
I have the same intermitent problem. Seams to happen at Walmart alot. In the usa, 315 MHz is a universal band for Industrial, Scientific and Medical uses (key fobs, garage door openers, theft control devices, etc...). It is also used for Aeronautical & Maritime Radionavigation.
Ok, so we've established 315 MHz as the normal frequency. Now, in order for something to interfer with the keyfob, it would have to be on that same spectrum (or extememly close to it), and would have to be putting out more power.
Microwave ovens, just for say, operate at 890 MHz and higher, so we can rule those out.
Based on input power, a radio signal can travel through the air a certain distance. Let's take a CDMA cellular site for example. CDMA takes around 20 watts of continuous power. Let's assume the power input never changes. A site running on the 800 MHz spectrum will have a larger footprint (the distance the signal will travel from the cell site antennas) than a site running on the 1900 MHz spectrum. Most sites are 1900 MHz, simply because the 800 MHz spectrum is full and no longer being issued new licenses. (Except in the snowy white north where I live, hehe)
So using the same idea, say your key fob is right next to your door lock. The footprint of the key fob is very small, and as many of us know, at a short distance away, the fob's don't work anymore. So let's use the example of a garage door opener. Again, not a very large footprint. Let me make a crude but quick diagram.
(((((((<Explorer>)))))))
Let's say that the curly brackets are the footprint where the key fob is usable. Let's say, for argument's sake, that distance is 10 feet. Now let's say your neighbour has a remote garage door opener.
(((((((<Explorer>))))))) ((((((((((((((<Garage>))))))))))))))
Now, important to note is the power input we looked at earlier in the post. Garage door openers and remote door lock key fobs don't use the same power input. The key fob usually runs on 1.5 volts, normally a watch battery (really depends, but this is average), and the garage door opener runs on AA batteries (again, depends), and I have seen two batteries required in garage door openers, so now you're looking at 3 volts for that.
So if the garage door opener is twice as powerful as the Explorer key fob, then the RF (radio frequency) footprint of the garage door opener would be 20 feet. So in theory, your Explorer would
have to be in that 20 foot radius in order to even have the possibility of the key fob being affected. And to add on top, the other device would have to be IN USE. A garage door opener doesn't emitt RF unless the button is being pressed.
So we can basically completely count out the small stuff like garage door openers, anti-theft devices, other car alarms and key fobs, etc. Government equipment like the above stated Aeronautical & Maritime Radionavigation, that could be a source of interference, but again, it would have to be on that exact same spectrum (or very close to it, we're talking like 315.11 and 315.15 being close together, 315.1 and 315.4 being completely independant).
If you want to test this out yourself, use your truck's FM radio. Tune to a station that's got good FM reception. Let's say it's 96.1 FM. Then tune to 96.3 FM. You can still hear the 96.1, but it's staticy and poor quality. This is when spectrum shifts / overlaps a bit. Now if there was another station that was licensed for that 96.3 FM, then the 96.1 would have to correct their issue so their RF doesn't overlap onto another spectrum (called isolating).
As I said in another thread, I'd grab a digital radio frequency scanner and go hunting on the 315 MHz band and see what else is out there. But interference shouldn't be THAT big of a concern, which would lead me to looking at other possibilities.