From: Matt Felder
Subject: A weird garage door opener problem.

We have 2 cars: a Mazda Millenia Sedan and a Ford Windstar
When the Millenia is parked in the garage the garage door opener (for the other car) has to be ~10 feet from the door before it works.
When the Millenia is not in the garage (whether or not the Windstar is in the garage) the door opener works from ~100 feet away.

This happens regardless of:
1) which of the two remotes are used
2) whether you use the remote from inside the cars or outside the cars
3) which position (right or left) either car is parked in

I tried moving around the little antenna wire that hangs off the garage door opener- it didn't seem to help much.
Any idea why the Millenia parked in the garage would substantially reduce the remote range?  And why the Windstar doesn't?
Extra points for any testable theories... :)

Cheers,

Matt

The Millennia is emitting radio energy at the frequency of the garage door
opener.  In the U.S., the opener will operate around 303-307 MHz or at 433
MHz, depending on the manufacturer and date of manufacture.  You can
determine this with the spectrum analyzer that Jamie ordered, by bringing in
one of the transmitters.

It is not clear which system in the Mazda causes the radiation.  Several
could.  Furthermore, they can meet FCC specs for unintentional radiation and
still cause this level of interference.  It is unusual, in my experience,
for a vehicle to radiate this strongly when parked.  Ford automobiles had
poorly shielded ignition, until about 1985, and interfered with anything
they came close to when the engine was running...

Regards,

Lawrence

From: Billy Eno

Wouldn’t the noise need to be in the 390 MHz range to interfere with it?  For a car that old, I doubt that you would find a processor that was running much faster than 66MHz, though I suppose it could be a PLL or something.  But, they are concerned about power management too (nobody wants to come back from a 3 week vacation and find their battery dead), so most everything gets put to sleep when the car is off.

I would say find a similar Millenia at a used car lot and “test drive” it to you garage and see if it causes the same problem.  Then you can isolate it to whether it is your specific car or not.

Or, wrap your car in tinfoil.

Billy

Could be worth disconnecting the battery in the mazda.  That would narrow it down quite a bit.

Matt

Thanks for everyone's helpful suggestions and humor.
This was a fun one to debug and many of the suggestions were helpful- believe it or not even Billy's tin foil suggestion helped.
A ran a series of tests- each time rechecking the distance that the opener would work.
The initial problem is that with the Millenia in the garage the opener wouldn't work unless it was within ~15 feet of the door.
 
First Test- disconnect the Millenia battery
I was floored when this worked!  The opener consistently worked from >80feet away with the Millenia battery disconnected. 
I was really surprised that EMI from a powered-off car could cause this level of interference!
To double check I reconnected the battery and sure enough the opener range dropped to 15 feet.
 
Second Test- aluminium foil the front window to see if the EMI was coming from under the dash through the window.
I was surprised when this didn't help so I ended up adding foil to the driver window and the Sunroof too.
Unfortunately it still didn't help- this was very interesting.  How could the EMI escape from under all the metal?
 

The foil experiment.  You can see the small grey garage opener antenna wire in the picture below.

Third Test - start pulling fuses until I find what causes the interference.
I went through the obvious fuses with no luck. 
But during this process I realized/remembered that the Millenia has it's radio antenna embedded in the rear window!
I moved the aluminium foil to the rear window and surprise! the opener range doubled to ~30feet.
Now I knew it was either something connected to the embedded antenna or something in the trunk!
 
Fourth Test- Pull up the fabric covering in the trunk to look for suspicious components.
First I tried disconnecting the Bose amplifier with no luck.
Then I noticed a little box with a label "Keyless Entry"- I was pretty sure I found the culprit.
I pulled the plug on the keyless entry unit and gave the opener another try.  The range was >80 feet!
This was a great find because I never use the keyless entry- I don't like to carry the keyfob. 
So with one simple plug-pull the problem was solved.  
Of course finding the right plug took ~1 hour and a roll of aluminium foil, but it sure was fun.
 
This pciture shows the keyless entry system in the trunk- the small box with the yellow tag.

 
BTW.. the garage opener is a Sears Craftsman model and the keyless entry was original factory equipment on my 98 model.  I couldn't find any info on the garage opener or the Millenia keyless entry that specified the frequency range that they use.
 
If the keyless entry was important to me I would have tried instead to improve the antenna from the garage opener.
 
Thanks again for the help!

Matt